VIABILITIES OF AMYLASE GENOTYPES IN DROSOPHILA PSEUDOOBSCURA ON STARCH AND MALTOSE FOOD

In order to study the relationship between selection on an enzyme and the environment, viability was measured for genotypes at the α-amylase (Amy) locus of D. pseudoobscura on food containing either starch as the primary source of carbohydrates or, as a control, maltose, the breakdown product of starch by amylase. These conditions were chosen to reveal possible differences among these genotypes in their abilities to survive on starch food. Counted numbers of larvae were cultured on food with low or high levels of starch, or with maltose, at both 20°C and 25°C. Only under the most stressful environmental conditions, on food with a low starch concentration at 25°C, did the genotypes differ significantly in viability. Four population cages maintained on food with either maltose or starch as carbohydrate sources were set up to assay differences in overall fitness among the Amy genotypes. No selective differences were observed over 10 generations, and we infer that components of fitness other than viability also differed little among the Amy genotypes on the two types of food. Thus our efforts to manipulate the environmental levels of starch, the substrate of amylase, led to measurable selective differences only under very stressful conditions.