Dunham, R.A., Majumdar, K., Hallerman, E., Bartley, D., Mair, G., Hulata, G., Liu, Z., Pongthana, N., Bakos, J., Penman, D., Gupta, M., Rothlisberg, P. & Hoerstgen-Schwark, G. 2001. Review of the status of aquaculture genetics. In R.P. Subasinghe, P. Bueno, M.J. Phillips, C. Hough, S.E. McGladdery & J.R. Arthur, eds. Aquaculture in the Third Millennium. Technical Proceedings of the Conference on Aquaculture in the Third Millennium, Bangkok, Thailand, 20-25 February 2000. pp. 137-166. NACA, Bangkok and FAO, Rome.
ABSTRACT: Genetic intervention has been used to enhance animal and plant agriculture production for many years. These techniques are now being applied to aquatic animals in an effort to overcome many different production challenges. As with agriculture, however, such advances are coming under increased scrutiny, thus the challenge facing geneticists and aquaculturists alike, is deciding which strategies are necessary, beneficial and acceptable in terms of social and environmental safety. Aquaculture genetics shows immense potential for enhancing production in a way that meets aquaculture development goals for the new millennium. This review covers the progress made to date, discusses the questions which need focussed research to answer and summarizes the areas where genetic knowledge can make a positive difference to aquaculture sustainability.
KEY WORDS: Genetics, Aquaculture, Breeding, Selection, Sustainability
With global population expansion, the demand for high-quality protein, especially from aquatic sources, is rising dramatically. Increased aquaculture production is clearly needed to meet this demand in the third millennium, because capture fisheries are at capacity or showing precipitous declines due to overfishing, habitat destruction and pollution. Further increases in capture fisheries are not anticipated under the current global conditions.
Increased demands for aquaculture production mean increasing pressure for development of more efficient production systems. Major improvements have already been achieved through enhanced management, nutrition, disease diagnostics and therapeutics, water quality maintenance and genetic improvement of production traits. A common theme through all these is genetics, which, actively and passively, has been used to meet many production challenges, such as disease resistance, tolerance of handling, enhanced feed conversion and spawning manipulation, i.e. all those areas to which wild animals must adapt for productive domestication.
Aquaculture genetics began with the advent of aquaculture in China more than 2,000 years ago, at about the same time as the Romans began to hold fish in ponds and learned how to breed them. Without realizing it, the first fish culturists who completed the life cycles of species such as the common carp, Cyprinus carpio, began changing gene frequencies and altering the performance of the fish they were domesticating. When the farmers noticed mutations in colour, body conformation and finnage, those with attractive traits were chosen as broodstock, and selective breeding was born! Fish culturists and scientists who compared and evaluated closely related species for their suitability for aquaculture over the past two millennia also unknowingly conducted genetic-based research. Closely related species, under wild circumstances, are reproductively isolated and have species status because of their genetic differences. Thus comparison for culture suitability is a genetic comparison.
Directed breeding programmes did not develop an intense or strongly focussed approach until the Japanese started to develop fancy varieties of koi carp in the 1800s. Fish genetics programmes became more prevalent in the 1900s with greater knowledge of breeding and inheritance (Mendelian principles).
|Genetic enhancement programmes began in the 1960s. Molecular-based
knowledge emerged in the 1980s and has continued to gain momentum. Efforts
are now well established in traditional selective breeding, biotechnology
and molecular genetics of finfish, and are rapidly developing for aquatic
| This can be attributed to use of wild broodstock
and postlarvae, a lack of understanding of shrimp reproductive biology for
domestication of the species and perceptions of low potential for genetic
improvement. Current reliance on wild broodstock is risky and negates the
opportunity to enhance disease resistance (as well as other production traits)
through selective breeding. Efforts to domesticate broodstock are now hampered
by endemic disease challenges; however, recent collaborative research between
the Commonwealth Scientific and Industrial Research Organisation (CSIRO)
in Australia and the shrimp culture industry has resulted in successful
captive breeding of Penaeus japonicus. Economic analysis has demonstrated
that domesticated broodstock are more cost-effective than wild broodstock
(Preston et al., 1999) and that reproductive performance of domesticated
P. monodon can match that of wild broodstock of a similar size (Crocos and
Use of established, high-performance domestic strains is the first step in applying genetic principles to improved aquaculture mana-gement. Strain variation is also important, since there is a strain effect on other genetic enhancement approaches, such as intraspecific crossbreeding, interspecific hybridization, sex control and genetic engineering.
Inbreeding and maintenance of genetic quality
It is as important to prevent production losses due to inbreeding as it is to increase production from genetic enhancement. This applies especially to species with high fecundity, e.g. Indian and Chinese carps, where few broodstock are necessary to meet demands for fry and broodstock replacement. The detrimental effects of inbreeding are well documented and can result in decreases of 30 percent or greater in growth production, survival and reproduction (Kincaid, 1976a, b, 1983a; Dunham, 1996b).
Intraspecific crossbreeding (crossing of different strains) may increase growth rate but heterosis (differences between offspring and parents) may not be obtained in every case. Increases of 55 percent and 22 percent in growth rate of channel catfish and rainbow trout crossbreeds, respectively, were achieved using this technique (Dunham and Smitherman, 1983; Dunham, 1996b). Chum salmon crossbreeds, however, have shown no increase in growth rates compared with parent strains (Dunham, 1996a).
Common carp crossbreeds generally express low levels of heterosis (Moav et al., 1964; Moav and Wohlfarth, 1974; Nagy et al., 1984; Wohlfarth, 1993; Hulata, 1995), however, those that exhibited positive heterosis are now the basis for carp aquaculture in Israel, Vietnam, China and Hungary.
The crossing of common carp lines in Szarvas, Hungary (Bakos and Gorda,
1995) is a good example of the relative success of crossbreeding. During
the last 35 years, more than 140 crosses have been tested. Three were
chosen, based on ~20 percent improvement in growth rate (and other qualitative
features), compared to parent and control carp lines, for culture purposes.
Now approximately 80 percent of common carp production comes from these
Szarvas crossbreeds. Production of gynogenetic female lines
and gynogenetic sex-reversed inbred male lines from common carp with the
best combining ability was an important part of the Hungarian crossbreeding
programme. A higher heterosis was expected from crossing inbred lines,
but the growth rate of F1 crossbreeds was only 10 percent higher than
controls (Bakos and Gorda, 1995).
In Indonesia, strain development using artificial gynogenesis and sex-reversal resulted in 10 common carp inbred lines, which were later used for crossbreeding (Sumantadinata, 1995). In Vietnam, eight local varieties of common carp, along with Hungary, Ukraine, Indonesia and Czech strains are maintained, with significant heterosis observed in F1 generations of crossbreeds. Double crosses among Vietnamese, Hungarian and Indonesian strains have subsequently been used for carp selection and crossbreeding programmes throughout Vietnam (Thien and Trong, 1995).
The Vietnamese x Hungarian common carp crossbreed is particularly popular, due to fast growth and high survival rates under different production conditions (J. Bakos, unpublished data). Under various rice-field conditions, growth rates of different strains of Nile tilapia and their crosses showed that the crosses were superior to pure Senegal strains (Circa et al., 1994). Breeding programmes are also under development in several countries for the Java or silver barb, Barbodes (formerly Puntius) gonionotus, another economically important fish species in Southeast Asia (Bentsen et al., 1996; Hussain and Islam, 1999). The Bangladesh programme used three strains: Bangladesh, Thailand and Indonesian. The growth rate of females from six crosses was 23 percent higher than the average growth rate of the parent strains. Even higher growth rates (35 percent improvement) were found in the three crosses using the Thailand strain as either the sire or dam. In the Vietnamese breeding programme, six different strains were used to produce a population ideally suited for culture (Bentsen et al., 1996).
Cross-breeds of different strains of European catfish, Silurus glanis, are characterized by outstanding adaptability under warmwater holding conditions and mixed diet feeding regimes (Krasznai and Marian, 1985). In addition, crosses with the walking catfish, Clarias macrocephalus, have shown improved resistance to Aeromonas hydrophila infections (Prarom, 1990). Studies on domestic a x domestic b channel catfish also showed greater heterotic growth rates than domestic x wild crosses (Dunham and Smitherman, 1983. The same was found in rainbow trout crosses (Gall, 1969; Gall and Gross, 1978; Kincaid, 1981; Ayles and Baker, 1983). Strain mating incompatibilities, however, can occur and can impede fry output.
Interspecific hybridization has been used to increase growth rate, manipulate sex ratios, produce sterile animals, improve flesh quality, increase disease resistance, improve tolerance of environmental extremes, and improve a variety of other traits that make aquatic animal production more profitable (Bartley et al., in press). Although interspecific hybridization rarely results in an F1 suitable for aquaculture application, there are a few significant exceptions.
|Channel catfish females x blue catfish, I. furcatus,
males (channel-blue) is the only cross (between 28 catfish hybrids
examined) that exhibits superiority for growth rate, growth uniformity,
disease resistance, tolerance of low oxygen levels, dressing percentage
and harvestability (Smitherman and Dunham, 1985). However, mating problems
between the two species have prevented commercial utilization.
The sunshine bass is a cross between white bass, Morone chrysops, and striped bass, M. saxatilis, and grows faster, with better overall culture characteristics than either parent species (Smith, 1988). In addition, crosses of the silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis), black crappie (Pomoxis nigromaculatus) and P. annularis (Hooe et al., 1994) and African catfish hybrids (Clarias gariepinus, Heterobranchus longifilis and H. bisorsalis)(Salami et al., 1993; Nwadukwe, 1995) all show faster growth than parent species. Numerous crosses of common carp with rohu, mrigal (Cirrhinus cirrhosus) and catla (Catla catla) (Khan et al., 1990); tambaqui (Colossoma macropomum) and Piaractus brachypoma and P. mesopotamicus (Senhorini et. al., 1988; green sunfish (Lepomis cyanellus) crossed with bluegill (L. macrochirus) (Tidwell et al., 1992; Will et al., 1994), and gilthead seabream (Sparus aurata) with red seabream (Pagrus major) (Knibb et al., 1998a), have also resulted in improved overall performance for aquaculture systems. In the family Sparidae, hybrids of P. major and common dentex, Dentex dentex, also grow faster than parent stocks (Colombo et al., 1998).
Hybridization in commercial Thai oysters (Crassostrea belcheri, C. lugubris and Saccostrea cucullata) was carried out to explore the possibility of producing hybrid oysters with superior traits. Hybridization of C. belcheri x C. lugubris was only successful to the spat stage, and growth rates of the hybrids and reciprocal crosses were significantly lower than their parents. Intergeneric hybridization was only successful with female C. lugubris and male S. cucullata. Growth rates of this hybrid were significantly higher than those of S. cucullata, but did not differ significantly from those of C. lugubris. Shell morphology of the hybrid was intermediate between the two parental types. Effects of intergeneric hybridization on the genetic diversity of natural oyster populations, however, require further investigation.
|Hybridization between some species, such as Nile tilapia
and blue tilapia, Oreochromis aureus, result in predominantly male
offspring (Rosenstein and Hulata, 1994). Other tilapia crosses, which produce
mainly male offspring, include Nile tilapia x O. urolepis honorum
or O. macrochir, and O. mossambicus x O. urolepis
honorum (Wohlfarth, 1994). Conversely, the cross between striped bass
and yellow bass (M. mississipiensis) produced 100 percent females (Wolters
and DeMay, 1996). This can be desirable for culture purposes where i) there
are: growth differences between the sexes; ii) sex-specific products such
as caviar are wanted; or iii) reproduction needs to be controlled (e.g.
overpopulation and stunting in tilapia production ponds).
Hybridization between species can also result in offspring that are sterile or have diminished reproductive capacity. As with monosex production, the production of sterile hybrids can reduce unwanted reproduction or improve growth rate by energy diversion from gametogenesis. Karyotype analysis can be used as a general predictor of potential hybrid fertility. For example, hybrid Indian major carps are generally fertile because they share similar chromosome numbers (2N = 50). When crossed with common carp (2N = 102), the hybrids are triploid and sterile (Khan et al., 1990; Reddy, 2000). A natural triploid results from the cross between grass carp, Ctenopharyngodon idellus, and bighead carp. Grass carp are commonly produced for aquatic weed control, but there is concern about spread to natural water bodies and the potential impact on desirable vegetation. The triploid hybrids have reduced fertility, but some progeny maintain diploidy and could be fertile (Allen and Wattendorf, 1987). Other exceptions to the chromosome number-fertility rule are crosses of some sturgeon species with different chromosome numbers that produce fertile F1 offspring (Steffens et al., 1990).
Dorson et al. (1991) investigated hybridization of coho salmon, (Oncorhynchus kisutch), which is considered resistant to several salmonid viruses. Disease resistance in the hybrids was improved, but overall viability was poor. Viability increased when hybridization was followed with triploidization. The same authors also reported that triploid hybrids from rainbow trout and char (Salvelinus spp.) were resistant to several salmonid viruses, but grew more slowly than their diploid counterparts. Similar results were found with rainbow trout and coho crosses.
|Hybrid triploids of Atlantic salmon (Salmo salar)
x brown trout (S. trutta) showed survival and growth rates comparable
to those of Atlantic salmon monospecific diploids. Triploid Pacific salmon
hybrids have also shown earlier seawater acclimation (Seeb et al., 1993).
Increased environmental tolerance may also be imparted to hybrids where one parent species has a wide or specific physiological tolerance or due to increased heterozygosity (Nelson and Hedgecock, 1980; Noy et al., 1987). Hybrid red tilapia, O. mossambicus, (high salinity tolerance) and Nile tilapia, O. niloticus, (low salinity tolerance) show enhanced salinity tolerance (Lim et al., 1993). Florida red-strain hybrids (O. mossambicus x O. urolepis hornorum) can reproduce in salinities of 19 ppt (Ernst et al., 1991) and O. niloticus x O. aureus hybrids also show enhanced salinity tolerance (Lahav and Lahav, 1990; Wohlfarth, 1994). Reciprocal hybrids of O. niloticus (N) x O. mossambicus (M) have different salinity tolerances (Thanakijkorn, 1997). The hybrid with an O. niloticus mother (NxM) had a higher survival rate after salinity challenges at 20 ppt than pure O. niloticus, but lower survival rates than those of the reciprocal hybrid (MxN). At 30 ppt salinity, a direct transfer killed all tilapia with O. niloticus maternal genes. Growth rates of NxM hybrids were comparable to those of Nile tilapia, while those of the NxM hybrids and O. mossambicus were comparable, but lower, than the first two groups. There were no statistical differences in carcass percentages of the four groups. Back-crosses were also evaluated. MNxN showed the highest salinity tolerance (comparable to that of O. mossambicus), but no significant differences in salinity tolerances were found in the remaining backcross (NxNM, NMxN, NxMN) or pure O. niloticus. Carcass percentage of the back-cross hybrids, however, tended to be higher than those of the parent species.
Hybrids between marine species, and between marine and freshwater spawning species, have produced surprising results. Hybrids between Sparus aurata and Pagrus major (both belonging to the Sparidae) developed vestigial gonads at two to three years and were sterile (Knibb et al., 1998a). Similar vestigial gonads were observed in offspring of the reciprocal crosses. No consistent growth or survival superiority, compared with the parent species, was detected until sexual maturity in the reciprocal crosses. Hybridization between European sea bass, (Dicentrarchus labrax) females and striped bass (Morone saxatilis) resulted in viable larvae; however 28 percent were triploids, and only the triploids appeared to survive to six months of age. At eight months, the survivors showed poor growth compared to diploid D. labrax.
|Such hybrids may be of commercial value where reproduction
needs to be restricted for ecological reasons.
Sometimes an interspecific hybrid does not exhibit heterosis for specific traits, but may still be important for aquaculture if it expresses other useful traits from the parent species. The main catfish cultured in Thailand is the hybrid between African (Clarias gariepinus) and Thai (C. macrocephalus) catfish. This combines the fast growth of the African catfish and the desirable flesh characteristics of the Thai catfish (Nwadukwe, 1995). Although it does not grow as fast as pure African catfish, overall production rates are improved and the flesh is still acceptable to Thai consumers. Likewise the rohu (Labeo rohita) x catla (Catla catla) hybrid grows almost as fast as pure catla, but has the small head of the rohu considered desirable in Indian aquaculture (Reddy, 2000). Catla catla x Labeo fimbriatus (fringe-lipped peninsula carp) hybrids have the small heads of L. fimbriatus, plus the deep body and growth rate of catla (Basavaraju et al., 1995). The sunshine bass has a suite of advantageous traits from the parent species (white and striped bass) including good osmoregulation, high thermal tolerance, resistance to stress and certain diseases, high survival under intense culture, and ability to use soy bean protein in feed (Colombo et al., 1998). Interspecific backcrossing has also been used to successfully introgress (merge) genes from one closely related species into another. Cold tolerance and colour genes have been transferred among tilapia in this manner.
Very little was done in this area prior to 1970, however, the field has
grown significantly in the past three decades and is now extremely active
(Dunham, 1996a). In general, the response to selection for growth rate
in aquatic species is very good compared to that with terrestrial farm
animals. Fish and shellfish often have higher genetic variance compared
to farmed land animals, e.g. Gjedrem (1997) notes the genetic variation
for growth rate is seven to ten percent in farmed land animals and 20-35
percent in fish and shellfish. Fecundity is also higher in aquatic organisms
than land organisms. This allows for higher selection intensity for aquaculture
production improvement, and over 200 heritability estimates have been
obtained for several traits of cultured fish and shellfish.
|There are few national breeding programmes in fish and shellfish
which aim at improved aquaculture production. In 1975, a National Breeding
Program for Atlantic salmon and rainbow trout was started in Norway, and
today this supplies about 75 percent of the Norwegian industry with improved
eyed eggs. In Canada, a similar breeding programme is operated by the Atlantic
Salmon Federation (Friars, 1993). In 1993, The Philippines National Tilapia
Breeding Program (PNTBP) was started with broodstock from the GIFT programme
(Genetically Improved Farmed Tilapia) and is now operated by the GIFT Foundation
(A.E. Eknath, pers. comm.). In Israel (Wohlfarth, 1983) and Hungary (Bakos,
1979), crossbreeding programmes with common carp (Cyprinus carpio) exist.
In addition, some private companies in several countries, including the
United States and Chile, now have their own breeding programmes.
Selection for body weight and disease resistance in salmonids has been particularly successful (Embody and Hayford, 1925; Gjedrem, 1979; Kincaid, 1983a). By 1925, three generations of selection of brook trout (Salvelinus fontinalis) survivors of endemic furunculosis (caused by Aeromonas salmonicida) improved survival from 2 percent to 69 percent. Ehlinger (1977) further increased resistance to furunculosis in brown trout and brook trout with selective breeding programmes. Okamoto et al. (1993) reported that an infectious pancreatic necrosis virus (IPNV)-resistant strain of rainbow trout showed 4.3 percent mortality compared with 96.1 percent in a highly sensitive strain.
With respect to body weight, a 30 percent increase in rainbow trout was achieved within six generations of selection (Kincaid, 1983b). In Atlantic salmon, an increase of seven percent was achieved within a single generation (Gjedrem, 1979) and an increased growth rate of 50 percent was achieved within ten generations in coho salmon (Hershberger et al., 1990). Body weight has also been improved in channel catfish, by 12-20 percent over one to two generations of genetic selection (Dunham and Smitherman, 1983; Bondari, 1983; Smitherman and Dunham, 1985). The best line grew twice as fast as nonselected lines (Burch, 1986). After three generations, the growth rate of channel catfish in ponds was improved by 20-30 percent (Rezk, 1993). Four generations of selection in a Kansas strain of channel catfish resulted in 55 percent improvement in growth rate (Padi, 1995). Four generations of selection also increased body weight by 50.5 gm and total length by 0.88 cm in walking catfish, C. macrocephalus (Jarimopas et al., 1989).
Genetic selection in gilthead seabream (Sparus aurata) has also been successful (Knibb et al., 1997a) despite early difficulties with producing single-pair offspring groups (full- and half-sibs). This led to the conclusion that family mating designs were inappropriate for group spawning of S. aurata (Gorshkov et al., 1997). Mass selection proved more effective and resulted in significant heritability estimates for growth (Knibb et al., 1997b; 1998a, b).
Different strains of common carp appear to possess varying amounts of additive genetic variation. Smisek (1979) estimated heritabilities for body weight of 0.15-0.49 in a Czechoslovakian strain of common carp. Vietnamese common carp also show significant heritability (0.3) for growth rate (Tran and Nguyen, 1993). Kirpichnikov et al. (1993) reported a successful selection programme, which started in 1965, against dropsy (spring viremia of carp) in common carp.
Responses can differ depending on the direction of selection. Body weight of common carp in Israel was not improved over five generations, but could be decreased in the same strain selected for small body size (Moav and Wohlfarth, 1976). Virtually identical results for Nile tilapia has also been reported.
Several authors reported that, in tilapia, mass selection improved body weight in Oreochromis mossambicus, red tilapia and O. aureus. However, selection for increased body weight in other red tilapia has been less successful. This is similar to the situation in Nile tilapia (Hulata et al., 1986; Huang and Liao, 1990). This may reflect a narrow genetic base in the founder stock or sole use of mass selection. Selection for increased growth in GIFT Nile tilapia gave much different results, with 77 percent to 123 percent growth improvement. The genetic gain was superior to results from crossbreeding experiments. The 11 percent genetic gain per generation in GIFT tilapia is better than that obtained in most other species of fish; the channel catfish (Padi, 1995) increased body weight by 14 percent per generation over four generations. However, a more typical genetic gain is five to seven percent per generation, as demonstrated by salmonids following approximately ten generations of selection (Gjedrem, 1979; Kincaid, 1983a; Hershberger et al., 1990). The only other exceptions that come close to the results with GIFT tilapia and the channel catfish study of Padi (1995) are the 13-14 percent increase per generation observed by Gjerde (1986), and five percent per generation for common carp after six generations of selection (Tran and Nguyen, 1993).
| Production trials and socio-economic surveys in five Asian
countries show that cost of production per unit fish is 20-30 percent lower
for GIFT strain tilapia than other Nile tilapia strains in current use.
Body weight of common carp appears unresponsive to selection; however, body conformation can be dramatically changed (Ankorion et. al., 1992). Selection for carcass quality and quantity has also been initiated for salmonids and catfish (Dunham, 1996a). In Thailand, selection results are not yet available for many species and traits, but numerous heritability estimates have been obtained, e.g. for growth and disease resistance in pangasiid freshwater catfish (Pangasius sutchi, syn. of P. hypophthalmus), rohu (Labeo rohita), Thai walking catfish (Clarias macrocephalus), Java barb (Barbodes gonionotus), bighead carp (Aristichthys nobilis) and Asian rock oyster (Saccostrea cucullata) (Table 1).
Selective breeding has also improved growth of the shrimp Penaeus japonicus in laboratory (Hetzel et al., 2000) and pilot-scale farm trials using offspring from CSIRO broodstock (Preston et al., 1999). In 1998/1999, comparative trials demonstrated significant improvements in the growth, survival and total yields in two selected lines (10-15 percent increase in mean yields) (Preston and Crocos, 1999). In a related species, P. vannamei, Fjalestad et al. (1997) estimated a response within one generation of selection of 4.4 percent for growth rate and 12.4 percent for survival of the viral disease Taura syndrome.
A genetic improvement programme was recently started for Pacific oysters, Crassostrea gigas, in Australia (Ward et al., 2000). This followed demonstrations that little genetic diversity had been lost since the Pacific oyster industry was founded in Australia with imports from Japan some 50 years ago (English et al., 2001). The improvement programme aims to combine family and mass selection with molecular genetics. Two generations of mass selection and two generations of family selection have been completed, with a growth rate improvement of about eight percent in the first generation from a mass selection. Based on work with a congeneric oyster species, C. virginica, Haley et al. (1975) reported that mass selection of adult oysters gave an apparent strong response to selection for growth rate.
| These authors concluded that, because of large environmental
variability, a combination of family and mass selection would be required
to achieve maximum response. Newkirk (1980) obtained considerable selection
response in growth rate of oysters after one generation of selection. He
concluded that 10-20 percent gain per generation in growth rate is a reasonable
expectation. Nell et al. (1999) reported a genetic gain of nine percent
increased growth rate in Sydney rock oysters (Saccostrea commercialis) and
similar results were obtained by Toro et al. (1996) in the Chilean oyster
(Ostrea chilensis). In the hard-shell clam or quahaug (Mercenaria mercenaria),
a genetic gain of nine percent per generation of selection for growth rate
has been estimated (Table 2).
Although selection for body weight has generally been associated with positively correlated responses (e.g. increased survival and disease resistance), there are examples of long-term selection resulting in decreased bacterial resistance, possibly due to genetic correlation changes or inbreeding. Increased fecundity, fry survival and disease resistance were correlated to selection for increased body weight in channel catfish after one generation of selection for body weight (Dunham and Smitherman, 1983; Smitherman and Dunham, 1985). Three and four more generations of selection resulted in increased dressout percentage, decreased tolerance of low oxygen and no change in body composition or seinability (Rezk, 1993; Padi, 1995). Progeny from select broodfish had greater feed consumption, more efficient feed conversion and greater disease resistance than controls (Dunham, 1981; Al-Ahmad, 1983).
|Progeny from selected broodstock grew faster during fingerling
trials in the first season than control populations in all strains examined
(Dunham and Smitherman, 1983). Two of the three select groups grew more
rapidly during winter, and all select lines grew slightly faster than controls
during the second season of growth.
Thodesen (1999) reported a correlated response in feed conversion when selecting for growth rate in Atlantic salmon. Wild salmon had a 17 percent higher intake of energy and protein/kg growth compared with fish from the 4th generation selected for growth rate. At the same time, they demonstrated eight percent lower retention of both energy and protein. This indicated that selected fish make better use of feed resources than wild counterparts.
Polyploidy has been well-studied in fish and shellfish. Triploid fish are generally sterile. Females produce less sex hormones and, although triploid males may develop secondary sexual characteristics and exhibit spawning behaviour, they are generally unable to reproduce. Triploidy can also be used to improve viability to nonviable interspecific hybrids.
Channel catfish triploids become larger than diploids at about nine months of age (90 gm) when grown in tanks (Wolters et al., 1982). This occurs slightly after the first appearance of sexual dimorphism in body weight. In tank experiments, the triploids converted feed more efficiently than diploids (Wolters et al., 1982), had six percent greater carcass yield at three years of age (Chrisman et al., 1983) and were darker than diploids.
| However, triploid catfish hybrids did not grow as rapidly
as diploids in commercial settings, such as earthen ponds (Dunham and Smitherman,
1987) and had decreased tolerance of low dissolved oxygen. This appears
to be related to genotype-environment interactions.
The flesh of triploid rainbow trout females is superior to that of diploid
females because postmaturation changes are prevented (Bye and Lincoln,
1986). In Scotland, a system of sex-reversal and breeding (see Sex Manipulation
etc.) is used to produce monosex female populations of rainbow trout.
There is also a smaller demand (<10 percent) for monosex triploid females,
where larger fish are required for aquaculture or restocking into angling
waters. This will produce fish with both superior growth rate and flesh
quality (Bye and Lincoln, 1986). Other European countries also produce
monosex females and triploids according to requirements. Triploid salmonid
hybrids show similar (Quillet et al., 1987) or slower growth than diploid
hybrids, but may grow faster than controls once they reach maturity (Quillet
et al., 1987). The rainbow trout x coho salmon triploid showed decreased
growth, but increased resistance to infectious haematopoietic necrosis
(IHN) (see also section on hybridization above).
Polyploidy in the Asian catfish Clarias macrocephalus was induced by cold shock and resulted in 80 percent triploidy (Na-Nakorn and Legrand, 1992). The effects on survival rate were not significantly different from diploid controls during the first two months, but in the third to fifth month, triploid fish showed lower survival rates and body weight compared to the diploid group. Diploids showed better food conversion ratios than triploids in the first month, but this evened out between the second and fourth month.
| Overall carcass percentages and resistance to haemorrhagic
septicaemia (caused by Aeromonas hydrophila) showed no difference between
the triploid or diploid catfish (Lakhaanantakun, 1992).
Triploidy has been induced in oysters, e.g. Crassostrea gigas, (Guo et
al., 1996) primarily to increase their size and flesh quality (Dunham,
1996a). Triploid oysters do not produce large gonads and are therefore
more marketable. This technique may or may not result in complete genetic
sterilization for oysters, as some triploids are able to revert a portion
of their cells back to the diploid state, thus creating mosaics
(S. Allen, pers. comm.).
Polyploidy is not commercially feasible for all species. Brämick et al. (1995) suggest that the use of triploid tilapia would reduce unwanted reproduction and stunting and would significantly increase yields from pond culture. However, mouth brooding of many tilapia, low numbers of eggs per batch and asynchronous spawning mean that it is not currently feasible to commercially produce triploid tilapia.
Sex manipulation and breeding
Various strategies utilizing sex reversal and breeding, progeny testing, gynogenesis and androgenesis can lead to the development of predominantly, or completely, male or female populations, or a super-male genotype (YY). The primary aim is to take advantage of sexually dimorphic characteristics (including flesh quality), control reproduction or prevent establishment of exotic species. All female populations have been successfully developed for salmonids, carps and tilapias. Populations of super males (i.e. fish with two rather than one Y chromosome) have been established for Nile tilapia, salmonids and marginally, for channel catfish (Dunham, 1996a).
Monosex populations may be produced by direct hormonal treatment; however, where the fish are destined for human consumption, some countries (e.g. the European Union (EU), the United States, India) may prohibit such treatment.
|Alternative methods require additional understanding of the
variety of mechanism(s) determining sex differentiation for each species
in question. While many commercially cultured families exhibit the usual
XX/XY sex determination mechanism (carps, salmonids), where XX are females
and XY are males, others may be sequential hermaphrodites (change sex as
they mature), such as gilthead seabream and groupers, or have temperature-controlled
sex determination in addition to an XX/XY mechanism (e.g. in Nile tilapia
(Mair et al., 1991) and hirame [Yamamoto, 1999]]. For others, such as the
European seabass, sex differentiation mechanisms have yet to be identified,
although temperature appears to be important (Dunham, 1996a; Blázquez
et al., 1998). Different mechanisms may also be found in closely related
species. The Nile tilapia has the XX/XY system with the female being XX,
while the blue tilapia has a WZ/ZZ system with the male being ZZ 2.
Sex reversal and breeding has allowed production of YY channel catfish males that can be mated to normal XX females to produce all-male XY progeny. Males that are XY can be turned into phenotypic females by use of sex hormones and can then be used as breeders (Goudie et al., 1985). The sex ratio of progeny from the mating of XY female and XY male channel catfish was 2.8 males: 1 female, indicating that most, if not all, the YY individuals are viable. All-male progeny are beneficial for catfish culture, since they grow 10-30 percent faster than females (Benchakan, 1979; Dunham and Smitherman, 1984, 1987; Smitherman and Dunham, 1985). YY males are also viable in salmonids, Nile tilapia, goldfish and channel catfish (Donaldson and Hunter, 1982). The channel catfish YY system has stalled, however, because YY females have severe reproductive problems, and large-scale progeny testing is not economically feasible to identify YY males (K. Davis, pers. comm.). A combination of sex-reversal and breeding to produce all-female XX rainbow trout is now the basis for stocking most of the culture industry in the United Kingdom (Bye and Lincoln, 1986), as is the case for the chinook salmon industry in Canada. All-female populations are desirable, in this case, because males undergo maturation at a small size, and have poorer flesh quality. Monosex chinook (O. tshchawystcha), and coho crossed with chinook have also been produced (Hunter et al., 1983).
YY male Nile tilapia were as viable and fertile as XY males, and capable of siring 95.6 percent male offspring (G. Mair un published data). YY genotypes can be feminized to mass produce YY males, eliminating the need for time-consuming progeny testing to discriminate XY and YY male genotypes.
| This has enabled the production of YY males and all male
progeny, XY (known as genetically male tilapia [GMT] to distinguish
them from sex reversed male tilapia), on a commercial scale. The YY male
technology provides a robust and reliable solution to culture problems with
early sexual maturation, unwanted reproduction and overpopulation (Mair
et al., 1995; Tuan et al., 1998, 1999; Abucay et al., 1999).
Sex ratios vary widely between spawnings of Nile tilapia, but at the population level, they maintain a normal distribution of around 1:1 males to females. Overall sex ratios vary, however, among strains of Nile tilapia (Shelton et al., 1983; Mair et al., 1991). Lester et al. (1989) observed greater heterogeneity in the sex ratios of families collected from a mix of strains, some of which were introgressed with O. mossambicus (Macaranas et al., 1986). YY males crossed with XX females produce 95-100 percent males, and Scott et al. (1989) observed no females from the mating of 285 progeny of a single YY male crossed to ten separate females.
YY-GMT technology has strong potential for commercial application, since YY Nile tilapia, unlike channel catfish, can be sex reversed to produce functional females. The progeny of the YY-GMT males increase yields by up to 58 percent compared to mixed sex tilapia of the same strain (Mair et al., 1995). This is also greater than yields from sex-reversed male tilapia. In addition, YY-GMTs have more uniform harvest size, greater survival and better food conversion ratios. GMT production is relatively environmentally friendly. No hormones are applied and hormone application to the broodstock is low. Species/strain purity is maintained and the fish produced for culture are normal genetic males. Although the development process is time-consuming and labour-intensive, once developed the production of monosex males can be maintained through occasional feminization of YY genotypes and existing hatchery systems without any special facilities or labour requirements. Additional costs for application of this technology at the hatchery level would be minimal. Research on YY male technology has been widely disseminated in the Philippines since 1995, Thailand since 1997 and, to a lesser extent, in a number of other countries including Vietnam, China, Fiji and the United States. In the Philippines and Thailand, broodstock are distributed from breeding centres to accredited hatcheries. This maintains quality control and, although limiting scale of dissemination, keeps it within financial viability - essential for long-term sustainability.
| Based on impending availability of further improved GMT,
along with increasing resistance to use of hormones in aquaculture, this
technique is likely to impact tilapia culture on a global scale.
A meiogynogenetic line of blue tilapia (O. aureus) was established and gynogenetically propagated for five generations at the Faculty of Life Sciences, Bar-Ilan University, Israel. Mitogynogenetic O. aureus were subsequently produced (Shirak et al., 1998) using third generation meiogynogenetic females from this stock. Three generations of gynogenetic O. niloticus were also produced, and males from this line were used for hybridization with gynogenetic O. niloticus females, resulting in consistent production of 100 percent male hybrids (R.R. Avtalion, pers. comm.).
In Israel, all-female common carp populations (Cherfas et al., 1996) have been established using sex-reversed XX gynogenetic females crossed to males (Gomelsky et al., 1994), and using these XX males for breeding. All-female offspring were released to commercial farms and resulted in 10-15 percent yield improvement over existing commercial stocks. Gynogenesis and sex-reversal have also successfully induced in Morone spp. to produce monosex populations to avoid limitations on introductions to areas where this species is exotic (Gomelsky et al., 1998, 1999).
Monosex female Java barb are another example of sex manipulation being adapted to a commercial scale over a relatively short period (eight years), and largely in Asia (Thailand and Bangladesh). Pongthana et al. (1995, 1999) found that gynogenetic Java barb were all female and showed it was possible to hormonally masculinize these fish. Most of the breeding of the resultant neomales produced all, or nearly all, female progeny. These gave greater yields in pond culture than mixed-sex batches and, perhaps surprisingly, had higher survival rates than the mixed-sex fish. Hatchery trials in Thailand showed that neomale broodstock performed satisfactorily. Monosex female fingerlings from neomale broodstock are now supplied on a commercial scale in Thailand. Similar research is ongoing elsewhere in the region.
Gynogenesis, androgenesis and cloning
Gynogenesis, and androgenesis are techniques to produce rapid inbreeding and cloned populations. Gynogenetic individuals (gynogens) produced during meiosis (meitoic gynogens) are by definition inbred, since all genetic information is maternal. Meiotic gynogens are not homozygous, since cross-overs and recombination during oogenesis produce different gene combinations in the ovum and second polar body. The rate of inbreeding through gynogenesis is roughly equivalent to one generation of full-sib mating. Mitotic gynogens are totally homozygous, but are more likely to die during embryonic development due to the higher frequency of deleterious genotypes found in 100 percent homozygous individuals.
Androgenesis, or all-male inheritance, is more difficult to accomplish than gynogenesis (Scheerer et al., 1986), since diploidy can only be induced in androgens at first cell division, a difficult time to manipulate the embryo. Also androgens are totally homozygous, so a large percentage with deleterious genotypes probably die (Scheerer et al., 1986).
Gynogenesis and androgenesis can be used to elucidate sex-determining factors in fish. If the male is the homogametic sex when androgens are produced, the androgens will be 100 percent ZZ (all-male). If the male is the heterogametic sex, XX and YY androgens will be produced, resulting in both sexes.
Fully inbred clonal lines have been produced in zebrafish, ayu, common
carp, Nile tilapia and rainbow trout (Komen et al., 1991; Sarder et al.,
1999) using both gynogenetic and androgenetic techniques. These should
have identical genotypes throughout their entire genome. However, the
performance of individuals within such clones is highly variable. Individuals
with extreme homozygosity appear to lose the ability to respond to environmental
variables in a consistent, stable manner, and even micro-environmental
differences affect performance among individuals (Komen et al., 1991).
Therefore, as genetic variation decreases, environmentally induced variation
increases, and at a more rapid rate than in heterozygous populations.
Interspecific nuclear transfer
Interspecific nuclear transfer has been accomplished, primarily for cyprinids in China, resulting in embryos with the cytoplasm and mitochondrial DNA of the host species and the nuclear DNA of the donor species. As a result, these fish may exhibit traits from both parental species. This technology may later prove key for the application of gene knock-out technology3.
Gene transfer and genomics
Compared to the thousand years of aquaculture and its genetic improvement programmes (deliberate and unintentional, see Domestication section), aquaculture genomics and gene mapping are truly in their infancy. Molecular genetics is less than thirty years old, although DNA was only discovered 47 years ago. However, the late 1990s have seen an explosion in genomics and gene mapping of aquatic organisms. Many fish genes and regulatory sequences have been identified and isolated, and the fish genome is now better understood (Kocher et al., 1998). Likewise, gene maps are also being generated for oysters (Gaffney et al., 1997) and some penaeid shrimp (Li et al., 2000).
The first successful form of gene transfer, genetic engineering, was accomplished in China in 1985 and has subsequently been achieved in other countries. Most of this work focussed on hormone enhancement of growth (size and rate), with results ranging from 0 percent up to an incredible 300 percent under some conditions. Due to the lack of fish gene sequences, initial transgenic research in the mid-1980s employed mammalian growth hormone (GH) gene constructs, which enhanced growth in some, but not all fish species examined (Zhu et al., 1986; Enikolopov et al. 1989; Zhu 1992; Gross et al. 1992; Wu et al. 1994). Salmonids showed no effect (Guyomard et al., 1989a, b; Penman et al., 1991), despite the fact that they are very responsive to growth stimulation by exogenously administered GH protein (McLean and Donaldson, 1993). Subsequent gene constructs using fish GH sequences have stimulated some growth enhancement (less than a doubling of weight compared with controls) in carp, catfish, zebrafish and tilapia (Zhang et al., 1990; Dunham et al., 1992; Chen et al., 1993; Zhao et al., 1993; Martinez et al., 1996), providing the first convincing evidence that growth enhancement in fish can be achieved by transgenesis.
|More recently, GH gene constructs have been developed that
are comprised entirely of fish gene sequences: ocean pout (Macrozoarces
americanus) antifreeze promoter driving a chinook salmon GHcDNA, or sockeye
salmon (Oncorhynchus nerka) metallothionein promoter driving the
full-length sockeye GH1 gene. When introduced into salmonids, these constructs
elevate circulating GH levels by 40 fold in some cases (Devlin et al., 1994;
Devlin, 1996) and induce 5-11 fold increases in weight after one year of
growth (Du et al., 1992; Devlin et al., 1994, 1995a). Precocious smoltification
(physiological adaptation from fresh water to sea water) was also noted
(Rahman and Maclean, 1999).
When a gene is inserted with the objective of improving a specific trait, it may affect another trait. Such pleiotropic effects can be positive or negative, thus it is important to evaluate all important traits in transgenic fish - not just the trait under active alteration. Transfer of growth hormone genes has been documented to affect body composition, body shape, feed conversion efficiency, disease resistance, reproduction, tolerance of low oxygen concentrations, carcass yield, swimming ability and even predator avoidance.
Rainbow trout growth hormone (rtGH) transgene reduces survival of common carp and the number of F2 progeny inheriting the transgene is less than expected. Differential mortality or loss of the recombinant gene during meiosis are likely explanations, since transgenesis was evaluated after the fish reach fingerling size. Remaining transgenic individuals, however, showed higher survival than controls when subjected to a series of stressors, such as low dissolved oxygen (0.4 ppm)(Chatakondi et. al., 1995b).
Increased growth rate in transgenic individuals may reflect increased food consumption, feed conversion efficiency, or both. Fast growing common carp containing the rtGH gene were found to have a higher feed conversion efficiency than controls (Chatakondi et al., 1995a). Various other transgenic common carp families demonstrated increased, decreased or unchanged food consumption, but had improved feed conversion efficiencies. Body composition of rtGH transgenic common carp differed from controls by having more protein, less fat and less moisture than nontransgenic full-siblings (about a ten percent change). Growth hormone promotes synthesis of protein over fat, thus the protein/lipid ratio is higher in transgenic fish with elevated growth hormone.
|Increased protein levels in the muscle of transgenic common
carp also increased levels of amino acids. However, amino acid ratios and
fatty acid ratios are virtually identical in control and transgenic common
carp (Chatakondi et al., 1995a). Fecundity or precocious sexual development
appear to be unaffected by insertion of rtGH in common carp; however, transgenic
male tilapia show decreased sperm production (Rahman and Maclean, 1999).
Body shape of common carp is also changed by insertion of rtGH genes. Transgenic
individuals have relatively larger heads and deeper and wider bodies and
caudal areas compared to controls. As growth differences increase, body
shape differences also increase, but then plateau. These morphological changes
do not affect condition factor, but do improve the dressing percentage (Chatakondi
et al., 1994).
Salmonids injected with somatotropins display improved feed conversion (Devlin et al., 1994b), an effect also anticipated in GH transgenic salmonids. In some GH transgenic salmon, however, endocrine stimulation could be elevated to pathological levels and excessive, deleterious, deposition of cartilage was observed (Devlin et al., 1995b), analogous to mammalian acromegaly. The effect can be severe enough to impair feeding and respiration, reducing growth and viability. Thus the fish with the greatest growth enhancement are those that were only moderately stimulated (Devlin et al., 1995a).
Although initiated later than for other agricultural animals, aquaculture genomics has seen dramatic progress over the last ten years (Kocher et al., 1998; Liu and Dunham, 1998; Waldbieser et al., 1998). This includes progress in construction of framework genetic linkage maps for catfish (Li et al., 2000), tilapia (Lee and Kocher, 1996; Kocher et al., 1998; Agresti et al., 2000; McConnell et al., 2000) , salmonids (Young et al., 1998; Hoyheim et al., 1998), penaeid shrimp (Li et al., 2000), and Crassostrea and Ostrea spp. oysters (Hubert et al., 2000)). Genomic mapping of these five phyletic groups was recently approved by the United States Department of Agriculture (USDA) as a regional project (NE-186). The P. monodon genome is being mapped in an International Shrimp Map collaborative effort, initiated in 1998 (Li et al., 2000).
|Much work is ongoing on production of framework linkage maps
with greater numbers of markers, particularly type I markers of known genes;
quantitative trait loci (QTLs) involved in determination of performance
traits important to aquaculture and marker-assisted selection, development
of mapping tools, i.e. radiation hybrid panels in tilapia , BAC libraries
in catfish (G.C. Waldbiser unpubl. data); and construction of normalized
cDNA libraries for EST analysis and functional analysis. Similar work is
being undertaken for significant pathogens of commercially important aquaculture
species especially viral, bacterial and protistan agents that are
difficult to detect, isolate and/or differentiate from benign relatives.
This work is focussed on improving the rapidity and accuracy of current
disease diagnosis (Subasinghe et al., this volume).
In the last few years, large numbers of molecular markers have been developed and evaluated for application in the culture of catfish, as well as other commercially important species. Of the several types of markers evaluated, microsatellites and AFLP (amplified fragment length polymorphisms) markers were most reliable, efficient and reproducible for genetic linkage mapping in catfish (Liu et al., 1998a, b, 1999a, b, c, d, in press). Although continuing efforts by several laboratories are producing more type II markers in catfish for linkage mapping, fine linkage mapping depends on availability of large numbers of ESTs and anchoring of well-ordered contigs of BAC clones to linkage maps.
In aquaculture species, much effort is devoted to QTL mapping. QTL markers for growth, feed conversion efficiency, tolerance of bacterial disease, spawning time, embryonic developmental rates and cold tolerance have been identified in channel catfish, rainbow trout and tilapias (LaPatra et al., 1993, 1996). Putative linked markers to the traits of feed conversion efficiency and growth rate have been identified for channel catfish.
| In trout and salmon, a candidate DNA marker linked to infectious
haematopoietic necrosis (IHN) disease resistance has also been identified
(Palti et al., in press). In shrimp, Dr Acacia Alcivar-Warren and several
researchers at the Oceanic Institute of Hawaii are evaluating for marker-assisted
selection for penaeid shrimp. Drs Gaffney, Guo, Allen, Hedgecock and others
are working toward QTLs for control of disease problems in oysters.
Combining genetic enhancement programmes
The best genotypes for aquaculture applications in the future will be developed by using a combination of traditional selective breeding and the new biotechnologies described above. Initial experiments indicate good potential for this combined approach, with examples using mass selection and crossbreeding, genetic engineering and selection, genetic engineering and crossbreeding, and sex reversal and polyploidy; all work more effectively in combination than alone.
Since the best genotype for one environment is not necessarily the best genotype for another environment, genetically improved animals that work well in a research environment may not necessarily be the best performers under commercial conditions. In general, genotype-environment interactions increase for aquacultured animals with increasing genetic distance and increasing environmental differences, especially associated with species such as carps or tilapias that can be cultured simply and low on the food chain or with complete artificial feeds.
Genetic improvement is an ongoing process with tremendous opportunity for sustainable aquaculture development. As current demands increase and wild stocks are overexploited, more management tools will be required to increase aquaculture production. Genetic enhancement is an increasingly important component of the management toolbox and, if used properly, has strong potential to enhance aquaculture production, efficiency and sustainability.
Constraints and limitations
Before any of the opportunities discussed above can be fully realized and genetics can achieve its maximum on aquaculture
development, a number of constraints and limitations need to be recognized and addressed, including:
The impact of aquacultured organisms, including domesticated strains, interspecific hybrids, polyploids and genetically engineered stocks on genetic variation, population numbers and fitness of conspecifics, as well as on the ecosystem in general, is currently under debate. Unfortunately, few scientific data exist on interactions between domestic and wild aquatic populations to enlighten the debate or assist policy and management decisions.
It is recognized that farmed species can interact with other species under open-culture systems. The degree of interaction constitutes the basis for determining the ecological hazard(s), if any. Likewise, interaction with conspecifics constitutes the genetic hazard. Physical containment has, historically, only been partially successful in containing aquaculture stocks. A mechanism that prevents breeding in exotic, highly selected or transgenic stocks is considered to be a better option. CSIRO has begun research on a transgenic technology that creates functional sterility, so stocks can only complete their life cycle under culture conditions and any escapees are unable to breed or produce viable offspring.
Where viable culture stock can escape, ecological hazards include alteration of predation, resource competition, or other behavioural dynamics, as well as establishment of the cultured stock in ecosystems outside the natural range of their species. The degree of interbreeding impact will depend on the fitness of the novel genotypes in the wild. Concerns about environmental hazards posed by genetically improved species are generally inferred on ecological principles (Kapuscinski and Hallerman, 1990a, 1991; Hallerman and Kapuscinski, 1992, 1993).
|Experimental evidence (Dunham, 1996a, b; Farrell et al., 1997;
Devlin et al., 1999; Guillen et al., 1999; Muir and Howard, 1999) supports
the view that some genetically improved fish could pose ecological risks,
e.g. infertile triploid male salmon undergo sexual maturation, seek matings
and can negatively impact brood production. This would pose a significant
demographic risk to small natural populations they might enter. Triploid
Pacific oysters, Crassostrea gigas, can exhibit reversion to diploidy (mosaics),
although it is not known if they can re-assume reproductive capability.
Since oysters are cultured, by necessity of access to natural feed in open
water, the use of triploidy to limit potential for interaction/establishment
needs further evaluation.
Depending in the extent of phenotypic and/or genotypic change due to transgenisis improvement, a genetically improved species can be considered to be analogous to an exotic species. For example, attempts were made to introduce antifreeze protein genes from winter flounder into Atlantic salmon to increase their cold tolerance (Shears et al., 1991), allowing the fish to be farmed in areas outside their natural distribution. In such cases, sterilization would reduce the risk of establishing wild populations, but methods such as triploidy decrease performance (Dunham 1996b) and fertile broodstock are still necessary, so risk is minimized but not eliminated. In many countries, it has also been common practice to introduce exotic species to address shortcomings in aquaculture performance of native species (Welcomme, 1988). Use of genetically improved organisms from indigenous aquaculture species is probably an environmentally safer means of addressing the aquaculture productivity shortcomings and is less likely to impact biodiversity or genetic diversity compared to introduction of exotic species.
With respect to escaped farmed species, there are three scenarios specific to genetic issues:
Article 9.3 of the FAO Code of Conduct for Responsible Fisheries (CCRF) addresses Use of aquatic genetic resources for the purposes of aquaculture including culture-based fisheries. This article calls for:
Article 9.2.3 advises, States should consult with their neighbouring States, as appropriate, before introducing non-indigenous species into transboundary aquatic ecosystems and the Technical Guidelines on Aquaculture Development state, Consultation on the introduction of genetically modified organisms should also be pursued. The definition of non-indigenous, in the broadest sense of the term, should include organisms that are the product of domestication, selective breeding, chromosome manipulation, hybridization, sex-reversal, and gene transfer.
The current lack of trained fishery and aquaculture geneticists is a constraint that needs to be addressed in order to effectively pursue research and accurate impact assessment protocols. Both are needed to ensure that genetic research and genetic material development are appropriate for the commercial sector, applied properly and disseminated efficiently to achieve maximum benefit. With respect to environmental risks, more research is needed on reproductive performance, foraging ability and predator avoidance as key factors determining fitness of transgenic fish. This should be standard data gathered for risk assessments prior to any commercial release or application.
Aquatic animal research in aquaculture is underway throughout Asia, Europe and the Americas. Organized development of these programmes is required to help ensure that environmental risk and fitness traits, as well as food safety issues, are well-addressed at the research stage. Collaborative networks to develop protocols for, and conduct sound and safe review of genetically improved aquatic animal research are needed to help ensure that results are beneficial. Cooperative learning from previous research oversights, rather than competitive secrecy, should also enhance future research economically and technologically.
Economic and consumer issues
Consumer education on the positive aspects of genetic biotechnology, as well as risk management, is particularly urgent. Concerns over consumption of transgenes, related proteins, toxic by-products, activation of viral sequences and allergenicity of transgenic products are all questions requiring science-based answers. Most have been analysed, and allergenicity appears to be the most critical concern with a data basis, making it one of the strongest arguments for enactment of some type of labelling (Weiss, 1999). A recent example of a transgenic soybean expressing a gene from Brazil nuts to increase its protein content was found to be allergenic to humans (Nordlee et al., 1996). Food safety issues posed by transgenic fish are discussed by Berkowitz and Kryspin-Sorensen (1994). Those from other aquatic animals are still under investigation.
|Bearing in mind collaborative vs. competitive issues mentioned
under research issues, another prime economic issue is related proprietary
rights. Ownership, in cases of international genetic material transfer,
is an ongoing issue, with clear examples emanating from human genetic studies.
Genetic research and breeding programmes require significant financial support.
Appropriate, equitable dissemination and ownership of genetic material developed
with tax money or donor funding, and aimed at improving economic development
in impoverished countries, is a complex and often controversial topic. The
issue is further complicated by the initiation of private biotechnology
companies that supply alternatives to government-mediated technology transfer.
The most cost-efficient dissemination strategies with the highest impact
have not yet been completely defined or evaluated.
Patenting and intellectual property protection are so complicated that international instruments dealing with the issue are in conflict. The World Trade Organization (WTO) and the United States allow patenting of living organisms, whereas the European Community (EC) does not (http://www.uspto.gov/web/offices/pac/doc/general/what.ht). Aquaculturists need to be aware of the controversies associated with patenting GMOs, and how these affect marketing, proprietary rights and trade in certain areas.
Worldwide, policies for research and marketing of transgenic food organisms range from non-existent to stringent, as in the European Union (EU). Government regulation of transgenic aquacultured species, based on sound scientific data, is lacking and much needed. Not surprisingly, global cooperation on issues of biotechnology is not unified. Countries party to the Convention on Biological Diversity (CBD) and involved in the WTO are divided on key issues such as:
|Recently, however, international legislation, guidelines and
codes of conduct have been, or are being, established to help address these
areas of concern.
International instruments, some legally binding and others voluntary, cover a broad range of issues associated with GMOs in aquaculture, including introduction (transboundary movements) and release into the environment, international trade, human health, labelling, intellectual property rights and ethics.
Kapuscinski et al. (1999) proposed a framework of adaptive biosafety assessment and management involving definition of goals, problem analysis and policy design, policy implementation, and monitoring for the effects of management actions. This recognizes the fact that our knowledge of the environmental and social systems into which GMOs will enter is always incomplete, and that unexpected effects of GMOs are inevitable. Biosafety regimes cannot simply be divided into sequential phases of research, policy design and implementation. Nor can they be reduced to a single passage through these different phases. They also need to be transparent to the general public, and to engage society at critical points in decision-making, in order to maximize the publics trust in policy implementation. Kapuscinski et al. (1999) propose that fisheries and aquaculture professionals press for adoption of an adaptive management framework by relevant national or international bodies. In terms of implementation, a truly comprehensive set of biosafety policies would include measures for risk management, capacity-building programmes, national permitting of trade and uses of GMOs, genetic marking for international trade of GMOs, and an international system of liability and compensation. Aspects of this framework are relevant at any level of political jurisdiction, from local to national to international.
The management framework should be based upon science. Biosafety assessment of transgenics has begun for aquaculture species such as channel catfish ,salmon and Nile tilapia. In general, the data from these initial studies indicates that these transgenic fish demonstrate fitness traits such as decreased predator avoidance , lack of enhanced growth under foraging conditions, decreased swimming ability and lower sperm counts(Dunham 1995,1999;Dunham et al. 1995,1999;Farrell et al. 1997) that would likely make their genotypes less competitive than wild genotypes in the environment.
|Since commercialization of aquatic GMOs will go forward within
a global market for fisheries products, it is appropriate to consider international
trade policies that will affect commercialization of GMOs. Multilateral
discussion of means for promoting international trade occurs within the
WTO. The mandate of the WTO, organized through the General Agreement on
Tariffs and Trade (GATT), is to promote international commerce. While this
mandate may not seem relevant to conservation of biodiversity or to environmental
and food safety issues posed by GMOs, certain decisions made by the WTO
have important bearing (Baker, 1998). The recent meeting of the WTO in Seattle
was aimed at setting the agenda for a new round of international trade negotiations.
There were concerns over commercialization of GMOs, and one of the key issues
that deadlocked talks between trade ministers at the meeting was biotechnology.
The United States sought formation of a WTO working group on genetically
modified goods, hoping to establish rules that would protect trade in these
goods. Europe resisted, arguing that the safety of such products had not
been proven (Kaiser and Burgess, 1999). General agreement ultimately was
reached on establishing a WTO group to study international trade in genetically
modified foods (Pearlstein, 1999).
Although debate focussed on genetically modified crops and related products, similar issues loom for aquaculture products. For example, starting in 1996, Otter Ferry Salmon in Scotland initiated a growth trial with transgenic Atlantic salmon in a closed system. The fish were grown for 18 months and destroyed. The Scottish Salmon Association distanced itself from the experiments, fearing a market backlash. There was an uproar in the United Kingdom in late July when it was revealed in the British House of Commons that the government had approved the privately funded experiment with transgenic salmon. Seafood Datasearch (1999) noted that although the technology worked the salmon grew at four times the rate of controls the extent to which the technology will be adopted will depend on market acceptance of genetically modified foods. They reported that nine salmon-growing countries agreed to ban use of genetically modified fish.
The challenge to aquaculturists, fisheries scientists, and policy-makers is to strike an appropriate balance between realizing the potential for economic development posed by aquaculture biotechnology while minimizing any risks to the environment and human health. Beneficial use of biotechnology in aquaculture development programmes will require sustained efforts aimed at deploying well chosen, sustainable applications.
|This will require active participation by a wide
range of aquaculturists, fisheries scientists, public policy specialists
and other professionals. As described above, decision support aids now exist
for assessing and managing any risks posed by the use of aquatic GMOs.
Genetic improvement of cultured fish and shellfish that increases productivity and turnover rate, results in better use of resources and reduces production costs, should be given higher priority by government, NGOs and commercial organizations. Such improvement methods include:
Specific development issues
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Agresti, J.J., Seki, S., Cnaani, A., Poompuang, S., Hallerman, E.M., Umiel, N., Hulata, G. & Gall, G.A.E. 2000. Breeding new strains of tilapia: development of an artificial centre of origin and linkage map based on AFLP and microsatellite loci. Aquaculture, 185: 43-56.
Al-Ahmad, T.A. 1983. Relative effects of feed consumption and feed efficiency on growth of catfish from different genetic backgrounds. Ph.D. Dissertation, Auburn University, AL. Allen, S.K.J. & Wattendorf, R.J. 1987. Triploid grass carp: status and management implications. Fisheries, 12: 20-24.
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Ayles, G.B. & Baker, R.F. 1983. Genetic differences in growth and survival between strains and hybrids of rainbow trout (Salmo gairdneri) stocked in aquaculture lakes in the Canadian prairies. Aquaculture, 33: 269-280.
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Bakos, J. & Gorda, S. 1995. Genetic improvement of common carp strains using intraspecific hybridization. Aquaculture, 129: 183-186.
Bartley, D.M., Rana, K. & Immink, A. (in press) The use of inter-specific hybrids in aquaculture and fisheries. Rev. Fish Biol. Fish.
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Benchakan, M. 1979. Morphometric and meristic characteristics of blue, channel, white, and blue-channel hybrid catfishes. M.S. Thesis, Auburn University, AL. Bentsen, H.B., Godrem, T. & Hao, N.V. 1996. Breeding plan for silver barb (Puntius gonionotus) in Vietnam. INGA Rep. No.3.
Berkowitz, D.B. & Kryspin-Sorensen, I. 1994. Transgenic fish: safe to eat? BioTechnology, 12: 247-252.
Blázquez, M., Zanuy, S., Carrillo, M. & Piferrer, F. 1998. Effects of rearing temperature on sex differentiation in the European sea bass (Dicentrarchus labrax). J. Exp. Zool. 281: 207-216.
Bondari, K. 1983. Response to bidirectional selection for body weight in channel catfish. Aquaculture, 33: 73-81.
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1. Dunham, Rex (Dr), Professor, Auburn
2. Majumdar, Kshitish (Dr), Scientist, Centre for Cellular and Molecular
Biology, Uppal Road, Hyderabad 500007, India;
3. Hallerman, Eric M. (Dr), Associate Professor, Department of Fisheries and Wildlife Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, USA. 24061-0321; Tel: 540/231-3257; Fax: 540/231-7580; e-mail: email@example.com
4. Bartley, Devin (Dr), Senior Fishery Resources Officer, Fisheries Dept,
FAO, Viale delle Terme di Caracalla, Rome 0100, Italy; Fax: +39 06 5705
5. Mair, Graham C. (Dr), Research Scientist, Asian Institute of Technology, PO Box 4, Klongluang, Pathumthani 12120,Thailand; e-mail: firstname.lastname@example.org
6. Hulata, Gideon (Dr.) Professor, Department of Aquaculture, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel. email: email@example.com
7. Liu, Zanjiang (John) (Dr), Associate
8. Pongthana, Nuanmanee (Dr), Director, National Aquaculture Genetics
Research Institute, Department of Fisheries,
9. Bakos, Janos (Dr), Senior Scientific Adviser, Fish Culture Research Institute, PO Box 47, Szarvas, H-5541, Hungary; e-mail: Info@haki.hu
10. Penman, David (Dr), Institute of Aquaculture, University of Stirling, Univ of Stirling, FK9 4LA, Scotland, UK; Fax: +44 1786 472133; e-mail: firstname.lastname@example.org
11. Gupta, Modadugu V. (Dr.), Director, International Center for Living
Aquatic Resources Management, ICLARM -
12. Rothlisberg, Peter (Dr.), CSIRO Marine Research, PO Box 120,Cleveland, Queensland 4163; Australia. e-mail: Peter.email@example.com
13. Hoerstgen-Schwark, Gabriele (Dr); Professor, Institut fuer Tiezucht, Universitaet Goettingen, Albrechty-Thaerweg 3, Goettingen, D-37075, Germany; e-mail: Ghoerst1@gwdg.de
2 Sex chromosomes are named differently in Nile tilapia (X and Y, with females being the homogametic sex) and blue tilapia (W and Z, with males being the homogametic sex) to indicate the different sex-determining mechanisms in the two species.
3 Cell lines would be transfected with knock-out constructs to disrupt specific genes. The cells would then be screened to identify transformed cells, and these cells grown. Nuclei from the transformed cell line would be transferred to activated enucleated embryos to make knock-out transgenic embryos with the targeted gene knocked-out or inhibited (Z. Liu, pers. comm.).