7023 - Laserfiche WebLink

Home Browse Search

Data on inbreeding effects in fishes are impressive, and include severe body deformities, growth reduction, behav- ioral changes, and reproductive failures in such forms as convict cichlids, Cichlosorna nigrofasciatunc (Winemiller and Taylor 1982), carp, Cyprinus carpio (Moav and Wolfarth 1963, ref. in Mrakovcic and Haley 1979), zebra fish, Brachydanio rerio (Mrakovcic and Haley 1979), brook trout, Salvelinus fon- tinalis (Cooper 1961), and rainbow, trout, Sahno gairdneri (Aulstad and Kittleson 1971; Gjerde et al. 1983; Kincaid 1983). Although species and populations differ markedlv in their resistance to inbreeding depression (Ralls et al. 1979; Frank- lin 1980), and magnitude of inbreeding effects cannot be predicted beforehand, "... the unavoidable conclusion is that relatively small amounts of inbreeding can do tremen- dous damage to the reproductive potential and productivity of a fish stock." (FAO/UNEP 1981, p. 10). A different conservation genetics problem is the imposi- tion, knowingly or othenvise, of artificial selection upon a wild stock, leading to a state of domestication (Brisbin 1974). That fish respond by genetic change to selection in captivity is well established in salmonids (Vincent 1960; Donaldson 1970; Kincaid et al. 1977) and other groups (Moav et al. 1978; Hynes et al. 1981). By relaxing natural selection, or replacing it with random or directional artificial selective forces, the fate of propagated populations may be jeopardized. Assum- ing that natural selection has already optimized most char- acter states of populations in their particular habitats, any deviation from those states, even those advantageous in captivity, would not be beneficial in nature. "... genetically based performance under one set of conditions (i.e., hatch- ery) may not be correlated with performance under a dif- ferent set of conditions (stream, lake, natural area). If the goal is to release stock in a different environment from that where they are bred, then the brood stock selection practices must be designed to avoid unconscious selection and in- breeding." (FAO/UNEP 1981, p. 25). Frankel and Soule (1981) discuss four types of selection in captivity that can affect genetics: selection for increased productivity, selection for a (perfect) type, selection for tract- ability, and non-selection. Selection for increased produc- tivity in fishes can occur when the most fecund females are continually used to develop a breeding stock. Although this is most efficient in a propagation program,.it can be dele- terious in nature, where optimal clutch size is a function of predation risks, growth rates, adult and juvenile mortality schedules, food availability, etc. (Stearns 1976, 1977). Since increased fecundity may be maladaptive in nature, this type of artificial selection should be avoided. It may also be tempting to select for a particular type, such as larger or smaller, more or less aggressive, more colorful, etc. This is analogous to directional or stabilizing selection and can reduce quantitative or qualitative gene pool varia- tion. The third type of artificial selection, for tractability, is really a subset of selection for a type. It occurs when less aggressive animals are used for breeding because they are easier to handle. The practice of non-selection occurs when the survival probability of sick or abnormal individuals is increased by the captive breeding program; this may in- crease the incidence of deleterious genes in the population. Effects of Variance Reduction Within Populations Thus far, I have presented sources of genetic variance reduction within populations, under the implicit assump- tion that loss of variance is undesirable. What specific ge- netic and phenotypic changes, however, will arise from re- duced variance, and what is the evidence that these changes have adverse effects? Loss of genetic variance has three ma- jor effects: increased homozygosity (leading to reduced fit- ness), loss of additive variance, and increase in deleterious recessive alleles. There is little doubt that increased homozygosity can lower an individual's fitness (Beardmore 1983) although actual mechanisms for this reduction are not known. With respect to relative fitnesses of hetero- and homozygotes, there exists a "heterozygosity consensus," which is "... the belief based on extensive laboratory and farm experience that fitness (variability, vigour, fecundity, fertility, etc.) is enhanced by heterozygosity, and that any decrease in genetic variation will be paralleled by a diminution of fitness" (Frankel and Soule 1981, p. 48). Numerous data sets directly or indirectly support the notion that reduction in genetic variance may lower fitness (Wheat et al. 1974; Vrijenhoek and Lerman 1982; Leary et al. 1984) and the interested reader is referred to Mitton and Grant (1984, and numerous references therein) for a thorough review. These papers in aggregate demon- strate that increased homozygosity can result in at least slower growth, reduced survivorship, and developmental instabil- ity. Perhaps the most striking example of effects of ho- mozygosity is in the South African cheetah (Acinonyx j. ju- batus), which has a paucity of genetic variance at over 200 structural loci and the major histo-compatibility complex, apparently due to a severe bottleneck (O'Brien et al. 1985). These animals have great difficulty breeding in captivity, high rates of juvenile mortality in wild and captive popu- lations, and an abnormally high susceptibility to an epizootic coronavirus, all attributed to genetic uniformity. With respect to the second problem, additive genetic var- iance is generally lost at the same rate as heterozygosity 1 (2Ne) per generation [Franklin 19k]) . The major cause is genetic drift, which of course may be powerful in small populations. The reader is referred to Fisher (1930), Franklin (1980) and Falconer (1981) for more detailed discussions of additive genetic variance. The third point, increase in del- eterious recessive alleles, has been discussed in the section on inbreeding. Among-Population Variance Now we approach a very different problem, that of genetic variation among populations. Spatially isolated populations of organisms, with little to no gene flow among them, will tend to genetically diverge either through different selective forces or random drift (Endler 1977); in its extreme form, this leads to speciation. Any isolated population of fishes is a potentially unique gene pool with characteristics that may differ from all other demes. This is particularly evident in salmonids, which demonstrate the potential for extensive genetic divergence of fish stocks (Donaldson 1971; Busack et al. 1979; Stahl 1981, 1983). Consequently, in cases where endangered fishes exist in more than one natural popula- tion, we have at our disposal a powerful management tool: the potential existence of genetically distinct groups with which to recover the species. This is not a trivial observation, for it allows the manager flexibility in building breeding stocks, reintroduction to field sites, and other management