Implications of Life History Changes for Genetic Polymorphisms

The effect of the alcohol dehydrogenase locus (Adh) on various life history traits in Drosophila mercatorum is discussed with respect to their potential ability to maintain polymorphism. In particular, developmental time, viability and fecundity are studied in different conditions to determine the impact of environmental change on these traits. A density dependent model of the effect of shifting developmental time patterns between genotypes is formulated under a continuum of competition for oviposition sites. The intensity of this competition can greatly affect the gene frequency, both quantitatively and qualitatively. It is evident from the model and some experimental work that in many cases mean developmental time can have less impact than the distribution of developmental times In vial monocultures there is strong heterosis for develomental time. Both egg to pupa and egg to adult developmental time exhibit this heterosis, and it is argued that with competition for oviposition sites or pupation sites this result can be responsible for polymorphism at the Adh locus. Fecundity is also affected by allelic substitutions at this locus, but a combination of this trait with developmental time shifts in a theoretical framework can confound the explanation of polymorphism unless competition is important. The possibility of extending these results to natural situations is cautiously optimistic When flies are raised in bottles instead of vials the above heterosis is not found. In fact, heterozygotes have significantly slower mean developmental times than either homozygote. This result does not hold in genetically mixed cultures; there heterosis is again true. Although significant regression relationships between culture density and mean developmental time hold for most genotypes, they fail to predict this dramatic change in relative rates of heterozygote development. The occurrence of this shift is consistent with earlier studies on Drosophila melanogaster and might be part of a preadaption to competitive ability mentioned in the literature. The sensitivity of developmental time to environmental changes makes it difficult to generalize about its consequences. Exposure to environments containing various concentrations of isopropanol affects both developmental time and viability. Under very low concentrations of the alcohol developmental time decreases slightly, but increases as the concentration does. Viability decreases consistently with increasing alcohol concentrations. The patterns of decline are genotypic specific with heterozygotes having generally higher survivorship. This result could be combined with the other indications of heterozygote superiority to strengthen the case for overdominance as a means of maintaining the Adh polymorphism in D. mercatorum