A fresh look at the myth of alcohol adaptation in Drosophila melanogaster
The results of the three series of experiments at the biochemical, organismic, and population levels suggest that stabilizing selection is at the basis of the maintenance of alcohol dehydrogenase (Adh) polymorphism in Drosophila melanogaster. At the organismic and biochemical levels, several genetically different strains showed adaptive responses to alcohol stress environments. However, this alcohol adaptation was not always associated with an increase in alcohol dehydrogenase (ADH) activity level. Strains with different starting ADH activities (some relatively lower and some higher) all reached an intermediate ADH activity after selection in alcohol environments. The results of further studies revealed the biochemical basis for the disadvantage of the high ADH activity. Relatively higher rates of secondary product buildup in flies with relatively higher ADH activities were shown to be the reason for the lower fitnesses of flies with high ADH activities. However, lower alcohol tolerance of flies with high ADH activities was environmentally dependent, i.e., occurred at high alcohol concentrations. Adh-F homozygotes (with relatively higher ADH activities) also showed lower alcohol tolerance (compared to other Adh genotypes) at low alcohol concentrations. Nevertheless, the biochemical basis for their lower alcohol tolerance at low alcohol concentrations was essentially different from that for high alcohol concentrations. The Adh-F homozygotes were the fittest Adh genotype at medium alcohol concentrations;These results were also confirmed at the population level. At this level the Adh-F allele became fixed in environments with medium alcohol concentrations, whereas in environments with high and low alcohol concentrations Adh gene remained polymorphic in the populations. Another result of the experiment at the population level showed that alcohol adaptation (in medium concentrations of different alcohols) was not just the simple process of the fixation of the Adh-F allele in the populations. Adh-F homozygotes of the populations fixed for Adh-F allele had relatively higher ADH specific activity than the original Adh-F homozygotes, before subjection to alcohol stress. In general, the results of the experiments presented in this dissertation reveal the complexity of the interaction of D. melanogaster and its alcohol environment.