Temperature and Sex-Specific Differences in Reproduction Related to Genotypic Variation at the Esterase-6 Locus in Drosophila melanogaster

One question that continues to challenge evolutionary biologists is how polymorphic loci are maintained in populations. The two basic explanations for the existence of polymorphic loci are that the alleles of a particular locus are neutral with respect to natural selection or that selection may act in one of several ways to maintain the polymorphism (Kimura 1983). Natural selection can maintain two or more alleles at a locus if, for example, (1) the reproductive success of heterozygotes is greater than that of homozygotes, (2) there is frequencydependent selection on certain genotypes, (3) some environmental heterogeneity favors different genotypes under different conditions, or (4) the reproduction of the sexes differs with respect to their genotype at that locus (Nei 1987). The autosomal esterase-6 locus (Est-6 = locus, EST 6 = enzyme) of Drosophila melanogaster is one of the most polymorphic loci known (Oakeshott et al. 1989) and thus presents a particularly strong challenge to explain how a polymorphism is maintained. Est-6 specifies the synthesis of a carboxylesterase, and males produce 3-5 times more than females, primarily in the anterior ejaculatory duct (Sheehan et al. 1979). The enzyme is transferred to females in the seminal fluid during copulation (Richmond et al. 1980). The question of how the Est-6 polymorphism is maintained remains unanswered, but several pieces of evidence indicate that the locus is not neutral with respect to natural selection. Gilbert and Richmond (1982) found that at 18?C females mated to EST 6-null males produced fewer adult offspring (= number eclosed) than females mated to males that produced active EST 6, but there was no such difference in the production of offspring at 25?C. In addition, evidence from a study of the worldwide distribution of the "fast" and "slow" EST 6 electromorphs (Anderson and Oakeshott 1984) suggests that selection favors the fast and slow alleles at high and low temperatures, respectively. The above results lead to the hypothesis that selection may influence the Est-6 polymorphism through a sex-related function of the alleles at different emperatures. The purpose of our experiments was to determine whether the Est-6 genotypes of wild males and females influenced their production of adult offspring differently at 18? or 25?C. We used flies recently derived from a wild stock rather

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