No abstract

SUMMARYEgg-to-adult survival of Adh genotypes of Drosophila melanogaster on ethanol food appears to depend on the age at which the eggs are transferred from regular to ethanol-containing food. Young eggs give much higher viability than old eggs. This seems to be due to an increased ethanol tolerance in eggs exposed to ethanol medium. Additionally it is shown that ADH activity in ethanol-treated eggs has increased as compared with non-treated eggs. It is argued, however, that it is not this increase in ADH activity but other, still unknown, factors which are responsible for ethanol tolerance in eggs and larvae. The relation between ADH activity and fitness of Adh genotypes in environments containing alcohols is discussed.

The relative fitnesses of the different Adh genotypes under normal laboratory conditions and in the absence of alcohol stress were estimated in Drosophila melanogaster according to Prout's method. The larval component (viability) did not reveal fitness differences between the genotypes but for the adult component significant differences were observed. The female adult component (fecundity) showed an overdominant pattern: both homozygous genotypes showed a relative fitness significantly lower than the heterozygous genotype. For the male adult component (virility) also differences were observed. The homozygous SS genotype showed a lower relative fitness than the other two genotypes. Predictions for gene frequency changes based on the estimated fitness values do show a reasonably good correspondence with frequency changes actually observed in a number of experimental cage populations and indicate a globally stable equilibrium around a frequency of the F allele of 0.40-0.70. The relevance of these fitness estimates, obtained under conditions with no alcohol stress, for the explanation of the Adh polymorphisms observed in nature is discussed.