Factors affecting the genetic load in Drosophila: Synergistic epistasis and correlations among fitness components

Two factors that can affect genetic load, synergistic epistasis and sexual selection, were investigated in Drosophila melanogaster. A set of Eve chrom osomal regions containing visible recessive mutations were put together in all combinations to create a full set of 32 homozygous lines fixed for diff erent numbers of known mutations. Two measures of fitness were made for eac h line: productivity (a combined measure of fecundity and egg-to-adult surv ivorship) and competitive male mating success. Productivity, but not male m ating success, showed a pattern of strong average synergistic epistasis, su ch that the log fitness declined nonlinearly with increasing numbers of mut ations. Synergistic epistasis is known to reduce the mutation load. Both fi tness components show some positive and some negative interactions between specific sets of mutations. Furthermore, alleles with deleterious effects o n productivity tend to also diminish male mating success. Given that male m ating success can affect relative fitness without changing the mean product ivity of a population, these additional effects would lead to lower frequen cies and lower fixation rates of deleterious alleles without higher costs t o the mean fitness of the population.