9707470 Fry Natural populations of animals and plants often harbor heritable or genetically-based variation for traits closely related to reproductive fitness, such as juvenile survival or egg production. The existence of this variation presents a puzzle, because natural selection is expected to eliminate variants with low fitness. According to the environmental heterogeneity hypothesis, genetic variation for fitness traits is maintained because different genotypes have highest fitness at different times or in different places. Although much indirect evidence supports this hypothesis, fully evaluating the hypothesis has proven difficult, because to do so requires knowledge of how different alleles (variants of individual genes) affect fitness in different environments, information that is seldom available. In particular, for environmental heterogeneity to maintain variation, "trade-offs" must be present, such that alleles that increase fitness in some environments decrease fitness in others. In the proposed work, the chromosomal locations of genes affecting fitness on different diets, food with and without ethanol, will be determined in the fruit fly Drosophila melanogaster, in order to determine whether trade-offs are present. Ethanol is a natural component of breeding sites of D. melanogaster, and variation in ethanol levels has been proposed to maintain genetic variation in populations of this species. The proposed work makes use of large laboratory populations that are adapted to food either containing or lacking ethanol; on each type of food, the two populations maintained on that food type have substantially higher fitness than the populations maintained on the other food type. Chromosomal regions responsible for these fitness differences will be mapped, in order to determine whether regions causing higher fitness on one food type give rise to lower fitness on the other. If this work produces evidence for trade-offs, an important assumption of the environmental heterogene ity hypothesis will be supported. Whether environmental heterogeneity maintains genetic variation for fitness traits has implications for a number of important biological issues, including how new species are formed, and what determines the degree of ecological specialization of populations. In addition, a full understanding of the factors maintaining genetic variation for fitness traits is important for analyzing the potential for pest species to adapt to new crop varieties or pesticides, and for designing strategies for maintaining the long-term viability of populations of rare, endangered species. The proposed work will therefore make contributions on both basic and applied fronts.
