Let's Not Throw the Baby Out with the Bathwater: A Comment on Management for MHC Diversity in Captive Populations

Abstract

Genetic management of captive populations focuses on the design of breeding programs that minimize loss of genetic diversity (Rails & Ballou 1986). Because severe spatial and economic constraints limit the population sizes that can be maintained in most breeding programs, we agree with Hughes' (1991) assertion that breeding plans which focus strictly on the maintenance of heterozygosity will, nevertheless, lose a substantial proportion of the rare alleles (see also Allendorf 1986; Fuerst & Maruyama 1986; Nei et al. 1975). However, we disagree strongly with Hughes' assumption that most loci loss of diversity should not be a cause for concern, because the of genetic polymorphisms are selectively neutral. Based on this questionable assumption, Hughes proposes that we should breed individuals in captive populations to maintain allelic diversity for the subset of genes that is known to be under balancing selection, e.g., the major histocompatibility complex (MHC) of vertebrates. We do not dispute the role that MHC diversity plays in the ability of vertebrates to respond to a broad variety of infectious agents; however, Hughes' willingness to discount the role of diversity at the vast majority of other loci is comparable to throwing the baby out with the bathwater. Hughes' assertion that most polymorphism is neutral is questionable. For example, 15 years of genetic surveys and debates have not resulted in a consensus on the adaptive significance versus neutrality of allozyme polymorphisms (Kimura 1983; Lewontin 1974). Yet, careful physiological studies have revealed substantial evidence for an adaptive role of several biochemical polymorphisms, such as aminopeptidase-I in the mussel Mytilus edulis (Koehn et al. 1983), and several enzymes in Drosophila (Zera et al. 1985). Furthermore, while the adaptive significance of many allozyme variants may be unknown, there are other types of genetic polymorphisms which appear to have fitness consequences. For example, many visible polymorphisms, such as melanism in the moth Biston betularia and color spot intensity in the guppy Poecilia reticulata, are known to have strong effects on fitness (Endler 1980; Kettlewell 1973). A comprehensive review of evidence for adaptive polymorphisms in nature is provided by Endler (1986). Apparently Hughes also discounts variation in continuous traits, such as fecundity, growth rate, and foraging behavior. Though it is difficult to determine the adaptive value of quantitative variation at any point in time, most evolutionary biologists agree that such variation provides the basis for rapid responses to physical and biotic challenges (Fisher 1930). Thus, we do not see the immediate health of captive populations as the only issue at stake in captive breeding. Maintaining evolution-