8. BENEFITS ARISING FROM THE USE OF GMOS

8.1 Aquaculture

Evidence of real benefit in terms of economically significant characters comes mainly from work on growth hormone (GH) (Table 4). The overall conclusion from the studies of several workers is that fish GH transgenics enjoy growth rates markedly superior to those in comparable (in some cases sibling) non transgenics. Studies have revealed enhancement of growth particularly in salmonids to an average of 3?5 times the size of non-transgenic controls with some individuals reaching as much as 10?30 times the size of controls (Devlin et al., 1994). The economic gains to be made from use of such GMOs are obvious and transgenics must therefore be considered as a route for providing superior strains along with selective breeding (Melamed et al., 2002). We should note that, not surprisingly, lines resulting from different transgenic events with the same construct in the same population may give different results and this has been confirmed in field trials (Dunham et al., 1992).

Table 4. Actual and potential benefits of GMOs to aquaculture.

The species involved include Atlantic salmon (Du et al., 1992), coho salmon (Devlin et al., 1995a), Nile tilapia (Rahman et al., 1998) and interspecific hybrid tilapia (Martinez, 1996). Work reported on carp (Chatakondi et al., 1995) and channel catfish (Dunham, 1996) shows less but still significant effect but, as indicated by Maclean and Laight (2000), this may be a consequence of 1) choice of promoter sequence and 2) a background of selective breeding in the strain used. In most cases the transgenics will be hemizygous for an unknown number of copies (possibly often one) of the transgene.

There is a most interesting suggestion from the work of Martinez et al. (1999) using tilapia GH in O. hornorum urolepsis that fish hemizygous for the transgene are superior in growth rate not only to wild type sibs, but also to transgenic homozygotes. This, if a real and general effect, may be of considerable significance for the use of GH transgenics in aquaculture and the maintenance of broodstock.

Considerable interest exists in making fish transgenic for the antifreeze protein genes found in some species such as winter flounder and if the difficulties involved in securing phenotypic expression of the antifreeze phenotype in a phenotype controlled by multiple loci can be solved (Hew et al., 1999; Hayes, Davies and Fletcher, 1991), the benefits would be very large.

There are also a number of other target phenotypes for which transgenics offer considerable potential. These include salinity tolerance, sterility, control of sexual phenotype, disease resistance to specific pathogens (Mialhe et al., 1995) and behavioural modifications. One particularly interesting possibility is that of modifying the genome to allow greater production of omega-3 fatty acids (Donaldson, 1997). There are, as yet, few concrete data which can be reported but clearly there are very promising areas of work which could bring substantial benefits to aquaculture.

The introduction of a transgene is intrinsically unlikely to have only one effect on the phenotype and possible pleiotropic effects need to be considered. These could in principle, be of two kinds:

i) genuine pleiotropy manifested through, for example, dose effects in the metabolic network; and

ii) apparent pleiotropy arising from disturbance in functioning of resident genes through integration of a transgene at a specific point in the genome. Such disturbances might be favourable or unfavourable.

It will not always be easy to distinguish between genuine and apparent pleiotropy. However, Chatakondi et al., (1995) and Dunham (1996) have reported favourable effects such as increased carcass yield, increased protein level, reduced fat and greater tolerance of low dissolved oxygen levels in common carp and channel catfish transgenic for rainbow trout GH. Dunham (1999) has argued, without an explicit rationale, that “disease resistance will likely be improved directly”.

Possible effects of other elements in the construct such as reporter genes or antibiotic resistance genes need mention. Such cotransgenes confer no benefits and may pose significant risks (particularly with antibiotic resistance genes). Best practice would certainly require removal of such elements before commercial use of the target transgenes is started (MAFF, 1994).

8.2 Other uses of transgenics in aquatic species

While the primary focus of this paper is on uses of transgenics is in improving production in aquaculture, it is worthwhile pointing out that there are several other potential uses with strong connections to aquaculture. These include living pollution monitors achieved by incorporating a pollution sensitive promoter in the transgenic animal.

A typical example would be a green fluorescent protein structural gene (GFP) driven by a metallothionin promoter. If the promoter is inactivated by heavy metal pollution the GFP is switched off and the colour change is readily visible. Another use closely related to aquaculture, is that of using fish as a production system for valuable gene products which can be extracted in a comparable fashion to similar production in mammalian species. Such products might include vitamins and work is underway to produce factor VII (one of the human blood clotting factors), in tilapia (Maclean, 2002).

Use of AFP of a tangential kind includes cases where it has been used to help protect membranes from cold and freezing damage by modification of the structure of membranes in vitro (Rubinsky et al., 1992; Rubinsky, Arav and DeVries, 1992). The ability of fish AFP’s to preserve sheep and pig embryos has been demonstrated (Arav et al., 1993; Baguisi et al., 1997). The use of AFP’s in cryopreservation of fish eggs and embryos still awaits further development (Melamed et al., 2002). However, some initial work has been carried out by (Lubzens, Rothbard and Hadani, 1993) who were able to cryopreserve spermatozoa from the ornamental Japanese carp (nishikigoi). Work exploiting AFPs generated by transgenic fish could become most useful in hatcheries in future in order to preserve transgenic lines and to supply new hatcheries and farms with suitable stocks.

8.3 Commercial significance

The demand for fish is increasing year on year and the yield from capture fisheries is declining. Thus, although aquaculture production is increasing the market for further expansion in aquacultural production is likely to be very good for many years to come.

An OECD (1995) view was that the time scale from 1995 for GMOs in salmon to be commercialized would be 15 years and that for tilapia would be five years. As matters stand at present the estimates for both species would lie between the two figures given. It is reported (Stokstad, 2002) that Atlantic salmon transgenic for a Chinook salmon GH gene are being considered for approval in aquaculture in the USA.

The data available on GH transgenics suggest that the monetary benefits to be obtained from use of these fish will be large. For comparison, the use of the single step genetic change represented by monosex genetically male tilapia (GMT) in Nile tilapia (though this is not a GMO) increased production by almost 30 percent and effectively doubled the net income, from this source, of Philippine farmers growing it (Mair et al., 1995; Mair and Abella, 1997). Nevertheless it is sensible to recognize that the benefits of use of GMOs are not always clear cut, at least in crop plants in the USA (Soil Association, 2002).