Genetic diversity in tropical tree species is being lost rapidly due to the process of deforestation, which has reduced the size of natural populations, eliminated local populations, or fragmented earlier continuous populations into at times non-viable fragments.
There are obvious risks to the genetic resources of tropical tree species. The situation is particularly grave with regard to some species of economic interest, such as the mahoganies. The present situation can only be remedied through obtaining detailed knowledge on the genetic variation and variation patterns of the species at risk, which should be utilized as the basis for their management, improvement, sustainable use and conservation.
Although some species such as S. macrophylla, C. odorata and C. fissilis are the most important tropical broadleaved tree species in the neotropics, little attention has been given to investigating the amount and distribution of their genetic variation in their natural ranges of distribution and among their populations. This knowledge is fundamentally important to determine their present status and strategies for conservation ex situ and in situ, for their sustainable management, and for the development of plantations and genetic improvement programmes.
The objective of this part of the present document is to explore the knowledge available about the genetic structure of populations and inter- and intra-specific genetic variation in neotropical Meliaceae species, and to examine the results obtained in genetic tests, including by the use of molecular marker techniques. Based on this, the document will discuss the implication of such results to the conservation, improvement and sustainable use of these important genetic resources.
The genetic structure of a species is defined by the type and magnitude of genetic variation and by the way that such variation is distributed between and within its populations. Genetic variation should be seen as the result of a series of processes in time and space, such as mutation, migration, selection, geographic isolation and geneflow between populations, all of which are strongly influenced by the breeding system of the species and by mechanisms of dispersal of pollen and seeds.
As stressed above, understanding of the genetic structure of a species is essential for defining strategies and action for its management and sustainable use, the establishment and management of plantations, and genetic improvement and conservation in and ex situ. Unfortunately for the majority of tropical tree species such information is either very limited or does not exist at all.
The distribution of genetic variation is influenced by various factors, including: effective population size, mechanisms for dispersal of pollen and seeds, and the type and successional stage of the plant community in which the species occurs. Roche (1978) notes that the genetic structure of a plant species and of its populations is determined in part by the characteristic ecological niche in which it is found, which defines the environmental conditions necessary for the survival of a given population.
It is important to also understand the effects that natural and man-made disturbances have on the structure of a population and on the diversity of its component species. For example, characteristic, cyclical disturbance patterns occurring in tropical forests are according to Sarukhán (1984), one of the forces which maintains the floristic diversity of these forests, and are the source of successional changes which determine the formation and structure of plant populations and, therefore, ultimately regulate the density and the age-structure of the individuals which form part of that community.
Pennington (1981) points out that in spite of the apparent lack of genetic stability, individual species in the tropical forest are relatively stable and maintain their identity due to pronounced morphological differences and varying ecological requirements.
The status of conservation of mahogany species in the neotropics has been the object of growing interest, due to the fact that mahogany populations have been severely reduced by deforestation and degraded by the lack of management and over-use. In response to this, some species have been included on Annexes of CITES (Convention on International Trade in Endangered Species), and the discussions on including others on Annexes II and III are continuing.
In terms of intra-specific genetic variation in mahogany species, Styles and Khosla (1976) report the presence of diploid (2n=54) and tetraploid (2n= 108) varieties of S. macrophylla; in addition, these authors mention the identification of a polyploid series for S. mahagoni in plantations in Fiji, with chromosome numbers varying from 2n = 12 to 2n = 60. Sareen et al (1980) mention that for S. mahagoni collected in India, the number of chromosomes was 2n =28. In the case of Cedrela odorata, Styles and Khosla (1976) indicate chromosome numbers of 2n =50 up to 2n =56.
In the last decade the use of molecular markers to study genetic diversity, and the systematics and the genetics of populations and species at the DNA and protein levels, has increased greatly. According to Haines (1994) these technologies have included in forestry research and application the following techniques: isoenzymes; Restriction Fragment Length Polymorphisms (RFLPs); Random Amplified Polymorphic DNAs (RAPD's); DNA cloning and, more recently, micro-satellites.
Haines (1994) indicates that genetic markers may have an immediate application in supporting the efforts of advanced tree improvement programmes for forest species of economic interest, principally in relation to quality control: for example, verifying clonal identification, determining levels of contamination by outside pollen of seed orchards established for the production of improved seed, and contamination in controlled crossing programmes.
At present, some genetic studies using molecular markets in natural populations of mahoganies are under way, with the objective of characterizing the genetic diversity of the Meliaceae species. Chalmers et al (1994) describe the application of polymorphic DNA based on the Polymerase Chain Reaction (PCR) to determine the relative amount of genetic variation in eight species of four genera of Meliaceae.
In the study quoted above, Chalmers et al observed large genetic differences between species and genera included. They noted a clear separation of C. odorata from the other species, finding that 95% of the variables used differed from those of other general, while Lovoa trichilioides, Khaya spp. and Swietenia spp. seemed more closely related in respect to these variables. These results are consistent with current taxonomic thinking. It was concluded that some genetic markers can be useful in the identification of species, and of great value for the determination of the composition of some presumed hybrids. The authors discuss the potential importance of the application of RAPDs in studies of genetic variation in the Meliaceae, particularly in the context of the utility of this tool in the planning of genetic conservation strategies.
Gillies (1995) developed markers to quantify the level of genetic variation within and between populations of C. odorata L. The author employed the RAPD technique to evaluate diversity and the effect of selective harvesting on variation in natural populations of the species. The populations sampled on the Pacific Coast of Costa Rica revealed profiles that were quite different from those of the Caribbean coast, and the samples from the Caribbean coast also differed from those from other Central and South American sites included in the analysis.
One apparent explanation to the above divergence could be the pressure, in very humid areas, for populations to adapt to an environment with high humidity throughout the entire year and competition with humid rain forest vegetation which is highly aggressive and more competitive than that in drier areas. The phenotypical frequency in each of the first 15 random primers was calculated and used in the present study to estimate the levels of genetic diversity within each population using Shannon's Diversity Index.
The study detected different levels of diversity within the population, for example the primer OPB-01 gave an index of 0.91 for the Upala population, while OPB-02 gave an index of 2.51 for the same provenance. The Shannon Diversity Index was also used to measure the diversity within and between populations; the results varied in accordance with the primer used, however, all the primers indicated that there was more variation between the populations than within them, with the average diversity within the populations ranging from 60.4% to 39.6%.
The above results contrast with those of Hamrick (1992), which indicated that, based on the variation at the molecular level, the taxa of woody and allogamic perennial plants, such as Cedrela odorata, showed greater diversity within than between populations. In this regard it should, however, be noted that most of the diversity in the 1995 studies on Cedrela odorata mentioned above can be explained by the considerable, overall differences between the populations on Costa Rica's Pacific and Caribbean coasts. In consequence, in considering conservation strategies for C. odorata, sampling must include populations, both from the Pacific and the Atlantic coasts, from both humid and dry areas, in order to represent the full range of diversity present.
Gandara (1995) reviewed genetic diversity at the molecular level employing iso-enzymes in a natural population of C. fissilis in primary forest in Brazil. The author reports an average heterozygosity of 0.222, considered significant, and an outcrossing rate of 0.033. Hamrick and Godt (1990) found an average heterozygosity equal to 0.149 for all the trees studied in another, similar study.
Ditlevsen (1980) indicates that many characteristics of interest for forest tree improvement are quantitative and inherited due to the interaction of many genes with an additive effect, each one contributing to the inheritance of the characteristic. Generally, in forest tree improvement, as a first step, trees with a good appearance in a given trait are selected. This phenotypical appearance can be the consequence of a good genotype, of a good environment, but generally is the result of the interaction between the genotype and the environment.
According to Zobel and Talbert (1988), the best way of determining the genetic value of a selected tree is through testing its offspring, which allows the indirect estimation of genetic value of the parents. This procedure permits the separation of the trees whose phenotypical superiority is the result of good site and environmental conditions, from those that are genetically superior. The progeny test thus forms the basis for calculating the coefficient of heritability for the characters under selection, and gives a quantitative index of the degree of genetic superiority for the character being analyzed.
Newton et al (1993b) carried out a review of the information on genetic variation and possibilities for its identification and use in genetic improvement of Swietenia species, and in programmes aimed at the conservation of genetic resources of species of this genus. They indicate that little information exists based on progeny trials of Swietenia species, while studies have been carried out in regard to many other tropical tree species. The scarcity of information may be a reflection of the difficulty of establishing plantations using species of Swietenia and Cedrela due to the attacks of the insect, Hypsipyla grandella Zeller.
In Puerto Rico the growth of S. macrophylla, S. mahagoni, S. humilis, and of natural and induced hybrids of these species has been studied in detail (Weaver, 1987; Weaver and Bauer, 1986; Newton et al 1993b); however, there is little published information on variation among provenances. Similarly, the National Research Council (1991) reported that there are no major activities in progress aimed at the genetic improvement of Swietenia species in advanced breeding programmes. Barres (1963) mentions a test of 41 provenances of S. macrophylla, seven of S. humilis and 17 of S. mahagoni, which has been established in 7 locations in Puerto Rico.
Geary et al (1973) report on preliminary results of provenance trials of Swietenia species at age 4.4. During 1964, seeds of S. macrophylla (11 seed sources), S. humilis (9 seed sources), and S. mahagoni (one naturalized source in St. Croix), were collected in Mexico, Central America and the Caribbean Islands; the material was planted on 11 sites in Puerto Rico and two in St. Croix, U.S. Virgin Islands, in very dry, dry and sub-humid tropical areas. On the humid sites, S. macrophylla was superior in growth, height and survival, but in the dry areas the performance of the three species was similar (Geary et al, 1973). Significant differences were found between provenances in growth and survival, especially in S. macrophylla and S. humilis.
Chudnoff and Beary (1973) analyzed the density of the wood of 12 progenies of S. mahagoni on four sites, testing one tree per site. The authors registered highly significant differences with values between 0.48 and 0.57 g/cm3. The same experiments were used by Glogiewics (1986) for examining differences in form and in growth of the progenies. There were differences among progeny of S. mahagoni, although these were not consistent over sites.
Newton et al (1995) carried out a study to determine the genetic variation in apical dominance of C. odorata; they utilized 30 different progenies of 5 provenances, removing the apical bud, leaving a stem of about 20 centimeters in height and with foliage reduced to one leaf with two folioles in each plant. They measured the length and number of lateral buds which formed at the root collor of the plants. The results indicated the existence of a potential for the selection of C. odorata genotypes with a relatively high degree of apical dominance, of potential use in breeding for tolerance to insect damage.
It is important to note that there are several studies in process in Latin American countries, including those carried out by the Centro Agronómico Tropical de Investigación e Enséñanza (CATIE) in Turrialba, Costa Rica. Noteworthy, are the studies established for the selection of genotypes resistant to the attack of the shoot borer. Navarro (1996) lists the following tests, established in order to clarify variation in growth and yield, and in resistance to Hypsipyla of mahogany species:
A test was established in Florencia Sur, Costa Rica in 1991 (Navarro, 1996), with five provenances: Guajataca, Puerto Rico; Juan Díaz, Puerto Rico; Honduras; Trinidad; and Haiti. Using three blocks of 25 trees per plot and 5 provenances, growth and resistance to Hypsipyla grandella was evaluated. The attack of Hypsipyla and phenology of the trees were evaluated in 1991 and 1992. The height was measured at 26, 56 and 88 weeks; and was evaluated at the age of 177 weeks (height to the first branch and number of bifurcations).
Another test was established in Bajo Chino, CATIE, in 1991 using an experimental design with six blocks, six trees in line per plot and 23 progenies per block. 14 progenies from Costa Rica and 9 from Honduras were included.
In both the above experiments there was evidence of presence of attack peaks. There were virtually no attacks during the first year after the establishment of the trials, but peaks of insect attack were noted from May to June 1992, in connection with the start of the rainy season on the Atlantic Coast of Costa Rica.
During the worst attack there were significant differences between provenances in the average number of attacks, number of bifurcations in the trees, the proportion of trees attacked, and the height of the first bifurcation. The San Juan, Puerto Rico provenance was markedly superior in the first three characteristics, although its height at 68 weeks was 16.7% less than that of the best provenance. Although these results are preliminary, and are based on studies including only a limited range of material and at only age four, they seem to indicate that there is ample genetic variation between and within the different provenances of mahogany in terms of growth and in insect resistance (Navarro, 1996).
In a test conducted in Trinidad, the growth and average height of the progenies included varied between 3.57 meters and 4.7 meters, at the age of 30 months. In the first study, the average of the best family was 192% better than that of the worst one, which can be used as an indicator of genetic variation present. The effect of origin, whether at provenance or progeny level, was statistically significant (Navarro, 1996).
The above experiments were established as part of a collaboration project between CATIE, the Institute of Terrestrial Ecology in Edinburgh, UK, the International Institute of Biological Control in Trinidad (IIBC) and the Trinidad Forest Service. (Newton et al, 1995). The tests were evaluated for height growth and incidence of attack of Hypsipyla grandella at the age of 30 months. In the CATIE test, average height ranged between 2.45 meters and 4.71 meters, at the age of 33 months.
In Mexico, Patiño et al (1996) reported on provenance/progeny of tests of S. macrophylla and C. odorata established in 1988, in which genetic parameters for 36 progenies of 3 provenances of Swietenia and 36 of Cedrela were assessed, in Campeche State. Growth in height as well as diameter were evaluated. The Swietenia seed from the three provenances in Campeche under study (Cayal, Escarcega and Zohlaguna) came from trees collected randomly in two natural populations and one plantation (Cayal). At the age of six, the coefficient of heritability for growth height of progeny from Escarcega was h2 = 0.038, Cayal h2 = 0.265, and for Zoh-laguna h2 = 0.164.
Outside of the natural distribution range of Swietenia and Cedrela the selection of plus trees from plantations is under way in a number of African and Asian countries, as well as in the Philippines (Zabala, 1978); in China (1970); and in Fiji (Shepherd, 1969).
The most important studies carried out to date in C. odorata are the international provenance trials coordinated by the Oxford Forestry Institute, United Kingdom. Results of these were reported by Chaplin (1980).
Within the framework of the above international trials, seed from 14 provenances was distributed to 21 countries in the tropics in 1967. Few tests were successful in the neotropics, due to serious problems with the Hypsipyla grandella borer and poor site selection for trial establishment (Whitmore, 1978). The same seed lots were tested in a number of countries in Africa, where the trials were generally more successful. The provenances presented marked differences in form as well as in growth. Low incidence of attack by the shoot borer was reported. The best growth usually corresponded to provenances from Belize and Costa Rica (Burley and Nikles, 1973; Chaplin, 1980).
In regard to C. odorata, the genetic improvement programme for this species developed in Cuba, which was started in 1973, should be noted. In this programme a clone collection containing remets of 250 plus trees has been established. Progeny trials were conducted between 1986–1990, and a seed stand of 10 hectares has been established (Lahera et al 1994).
There are two species of Hypsipyla in the neotropics: Hypsipyla grandella Zeller and Hypsipyla ferrealis Hampson; and one of importance in the old world, Hypsipyla robusta Moore (Entwistle, 1967; Newton et al, 1993b). H. grandella is found from tropical zones of Mexico and Central America into South America (except for Chile); it also occurs in the Caribbean Islands and in the southern parts of Florida, USA (Entwistle, 1967; Newton et al, 1993b). H. ferrealis is a species with a lesser range of distribution in tropical America, where it exclusively attacks the fruit of Carapa guianensis Aublet, however, there are also some reports of attack on Swietenia macrophylla. The two principle species of the shoot borer which cause damage to Meliaceae species are thus: H. grandella and H. robusta.
Some authors, including Tillmans (1964), Entwistle (1968), Gripjma (1970, 1973, 1976) Gripjma and Styles (1973), Whitmore (1976 a and b), and Newton et al (1993b), have reviewed and summarized much of the available knowledge on Hypsipyla, providing very complete information on this insect pest.
Grijpma (1976) considers that the attack of Hypsipyla is limited to members of the Swietenioideae sub-family. However, a number of reports indicate that the insect can attack any genus or species of the Meliaceae. According to existing reports, H. grandella attacks the following commercial species: Carapa guianensis, Cedrela odorata, Swietenia macrophylla and S. mahagony (De León, 1941; Martorell, 1943; Tillmans, 1964; Entwistle, 1968; Newton et al, 1993b). It should be noted that S. humilis is also attacked by H. grandella in its area of natural distribution.
As mentioned above Swietenia and Cedrela, species are susceptible to the attack of the shoot borer (Hypsipyla sp). The larva of this insect feed on the apical bud, destroying it, and causing deformation of the stem and bifurcation, considerably slowing the growth of the affected plant and, occasionally, causing death.
The attack of the shoot borer has rendered the establishment of Meliaceae plantations difficult in the neotropics. Numerous investigations for developing control methods to prevent damage have been carried out; however, few practical and effective methods have been developed.
On the other hand it is evident that there is an urgent need to establish plantations of Swietenia and Cedrela species to take pressure off natural populations. It is essential that the problem of Hypsipyla be resolved. Control of Hypsipyla is likely to necessitate the application of a combination of silvicultural, biological and perhaps chemical control methods (Newton et al, 1993; Newton et al, 1993b). Some authors consider that the insect could best be controlled through the identification and selection of individuals which are resistant to its attack, within the framework of a genetic improvement programme (Gripjma, 1976; Newton, 19990; Newton et al, 1993a).
Gripjma (1976) notes that resistance is expressed through three main mechanisms: lack of preference, antibiosis, and tolerance. In the first case, the insect is not attracted by the plant or is repelled from laying its eggs or feeding on the plant; in the second, the insect is damaged or killed and/or cannot complete its life cycle after feeding on the plant; and, in the third, the plant recovers after attack, to an acceptable degree.
Newton et al (1993a) review documented evidence of these resistance mechanisms in various Meliaceae species, however, little information is available relative to intra-specific variation of these attributes.
Newton et al (1994) comment that in spite of the economic importance of mahoganies, few attempts have been made to genetically improve them, mainly due to the high incidence of attack by shoot borer when grown in plantations in the neotropics. They suggest an improvement strategy for the mahoganies, which includes selection for insect resistance as part of the programme. They envisage the selection of genotypes and their vegetative propagation, and the use of these improved materials in plantations with appropriate silvicultural interventions, through which insect control can be optimized.
It has been shown that some mahogany species are less susceptible to Hypsipyla attack than others, for example Whitmore and Hinojosa (1977) point out that in Puerto Rico, S. mahagoni suffers less attack than S. macrophylla. The hybrid, S. mahagoni x S. macrophylla, shows a condition of intermediate susceptibility compared to its parent species.
On the other hand, C. odorata is more frequently and strongly attacked than S. macrophylla, S. mahagoni and S. humilis, and there is evidence that C. odorata is more palatable to the shoot borer than the Swietenia species (Dourojeanni, 1963; Grijpma, 1970; Gara et al, 1973; Schoonhoven, 1974; Menéndez et al, 1989).
The differences in susceptibility and in insect preference which influence the rate of growth and the form of the trees can potentially reflect, according to Grijpma (1976), variation in the production of attracting chemicals or of toxins. The same author states that there is a high probability that there are less-preferred individuals, which could be used with advantage in a genetic selection programme. However few studies have been carried out on intra-specific variation in the susceptibility to Hypsipyla.
Grijpma and Roberts (1975) review and discuss studies on the chemical causes for resistance to the shoot borer found in Toona ciliata, and give examples of antibiosis in this species which produces water-soluble compounds which have been shown to be toxic to Hypsipyla grandella, and which act as retardants to growth and interfere with the development of the insect pupas, producing high insect mortality rates.
C. odorata grafted on Toona cilata showed resistance to attack of H. grandella, and the chemical compounds toxic to the insect were translocated to it, conferring resistance. Certain mahogany species have been shown to produce resins which can hinder borer attack (Lamb, 1968; Whitmore, 1978; Wilkins, 1972). There is no information on intra-specific variation in the production of such resinous compounds.
Marquetti (1990) notes evidence from young plantations of a natural hybrid of Cedrela (C. odorata x C. cubensis), which was severely affected by shoot borer attack during its first two years but which, over the following four years, showed resistance to attack of Hypsipyla grandella. This suggests an acquired post-infestation resistance, in line with the tolerance reported by Grijpma (1976).
Recently, Watt et al (1996) reviewed the evidence of different forms of resistance in species of Meliaceae to Hypsipyla grandella, with particular reference to work carried out in Costa Rica with Cedrela odorata and Swietenia macrophylla. There is evidence of genetic resistance to the borer in these species. The basis for resistance seems to principally be tolerance, but there is also some evidence of variation in the mechanisms of preference and antibiosis in C. odorata. The authors conclude that the best option for the management of Hypsipyla seems to be the selection of resistant individuals and application of silvicultural measures to minimize attack.