No abstract

The two common genetic variants of alcohol dehydrogenase in D. melanogaster, ADH-F and ADH-S, differ in substrate specificity and electrophoretic mobility. A third inherited variant, ADH-FCh.D., has a substrate specificity like ADH-S, an electrophoretic mobility like ADH-F, but much greater thermostability than either of the others. ADH-FCh.D. can be identified after post-electrophoresis heat treatment (15 s at 43°C) on cellulose acetate sheets. The Adh FCh • D • allele has been found in 19 of 34 natural populations in Australasia but its frequency in these populations does not exceed 0·06. The partial correlation between Adh FCh • D • frequency and a maximum temperature variable is significant and positive among the Australasian populations although different climatic associations are found for a thermostable form of ADH-F in North America.

The alcohol dehydrogenase (ADH) variant ADH-FCh.D. has a secondary alcohol/primary alcohol activity ratio characteristic of ADH-S although it has an electrophoretic mobility inseparable from ADH-F. ADH-FCh.D. is distinguished from these two common ADH variants by being much more thermostable.Genetic analysis suggests that ADH-FCh.D. is specified by an allele at the Adh locus. Biochemical comparisons show that ADH-FCh.D. has the same electrophoretic mobility, activity ratio and thermostability as the two other heat-resistant variants which have been reported, ADH-F71K in Europe and ADH-Fr in North America. The geographically widespread distribution of a thermostable ADH variant within the ADH-F electrophoretic class indicates that it should be considered in attempts to explain the Adh polymorphism in natural populations.

The distributions of five Drosophila species living in the vicinity of a domestic compost heap in Canberra (Australia) have been compared across traps baited with different fruits and vegetables. In both the adults trapped directly on the baits and those emerging from eggs laid on the baits, D, busckii made up about 2% of the sample, D. hydei about 1% and D. immigrans 7%. The overall frequency of D. simulans was 80% in the trapped adults but 53% in emergences, while D , melanogaster represented 9% of trapped adults and 37% of emergences. In both types of collection the frequency of D. busckii relative to the other species was highest on vegetables, the relative frequencies of D. hydei and D. immigrans were highest on melons, and the relative frequencies of D. simulans and D , melanogaster were highest on fruits. Overall the relationship between the frequency of each species and the ethanol contents of the baits was significant and negative for D. busckii and D. immigrans, non-significant and negative for D. hydei, non-significant and positive for D. simulans, and significant and positive for D. melanogaster. These differences were correlated with differences between the species in alcohol dehydrogenase activity.

Three experiments have been carried out which show that exogenous environments of ethanol impose selection on the alcohol dehydrogenase (Adh) locus of D. melanogaster. This locus is widely polymorphic for two alleles, AdhF and Adhs, and AdhF generally produces about twice as much alcohol dehydrogenase activity as Adhs. In the first experiment, AdhF IAdhF and AdhF/Adhs flies survived equally often and Adhs/Adhs flies less frequently after exposure for 7 days to medium impregnated with ethanol. The same pattern of survival differences was found in the second experiment in which flies were exposed for 1 day to an aqueous solution of ethanol and sucrose. In contrast, in the third experiment survival was scored after exposure for 45-min to ethanol fumes, and Adhs/ Adhs flies survived more often than AdhF/Adhs, both these genotypes surviving more frequently than Adh F / Adh F. We doubt whether anyone of the three experiments by itself adequately represents the ecology of natural populations of D. melanogaster exposed to ethanol. It is likely that mixtures of the three experimental conditions approximate more closely the natural environments and therefore we suggest that, overall, selection might favour intermediate levels of alcohol dehydrogenase activity, producing a net advantage for heterozygotes at the Adh locus.