STUDIES OF ESTERASE 6 IN DROSOPHILA MELANOGASTER. V. PROGENY PRODUCTION AND SPERM USE IN FEMALES INSEMINATED BY MALES HAVING ACTIVE OR NULL ALLELES.

Despite more than a decade of research, the evolutionary significance of molecular variants of enzymes in Drosophila and other organisms is far from clear. Recently several workers have stressed that a new research strategy is needed which will integrate findings on the biochemistry and psysiology of enzyme variants with observed changes in allele frequencies in natural or artificial populations (Clarke, 1973; Koehn, 1978). Our laboratory is engaged in a study of the extent and adaptive significance of molecular variants at the autosomal, esterase 6 (Est 6) locus in D. melanogaster (Cochrane, 1976; Cochrane and Richmond, 1979a, 1979b). Recent work (Sheehan et al., 1979) has shown that the specific activity of esterase 6 (EST 6) reaches its maximum in adult males. (The notation Est 6 will be used throughout to denote the genetic locus; the notation EST 6 refers to the esterase 6 enzyme.) Activity shows a gradual increase through larval until early pupal stages, then remains constant until 12 h post eclosion. At this young adult stage, near the beginning of reproductive fertility, males have about twice the EST 6 activity of females. EST 6 activity remains at a constant low level throughout female adulthood, but rises to five times this level in males by three days post eclosion, and is maintained at these high levels in virgin males. The increase in esterase activity in adult males corresponds to the period when fertility and the frequency of mating is rising in males (Stromnaes and Kvelland, 1962; Fowler, 1973). Further support for the hypothesis that EST 6 has a role in male reproductive fitness is provided by its location in males. Sheehan et al. (1979) and Aronshtam and Kuzin (1974) have shown that the anterior ejaculatory duct (a secretory organ connecting the testes and accessory glands to the ejaculatory bulb) contains EST 6 in concentrations nearly as great as found in whole fly homogenates. Current studies indicate that this enzyme is depleted in males and transferred to females during copulation (Richmond et al., 1980). Does this polymorphic enzyme, transferred to females in significant amounts at copulation, have a clear adaptive role in Drosophila male and/or female reproductive fitness? The availability of D. melanogaster stocks homozygous for null variants of Est 6 provides an excellent tool for examining this question. Many components of the male reproductive phenotype are potentially affected by a reproductive enzyme. These include aspects of courtship, copulation, sperm transfer and storage, sperm viability and use by the female, as well as remating and sperm displacement among males. The absence of an enzyme which contributes to reproductive fitness could be expected to lower the productivity of matings, through an effect on one of these components. Comparisons of the single tnating productivity (progeny per female) of females inseminated by males homozygous for an active Est 6 allele and males homozygous for a null allele were undertaken to test this expectation. Females having a genetic background similar to the two male types, but homozygous for the sex-linked mutation forked, were used to control for female influence on productivity. The complex nature of daily productivity led us to develop a mathematical model