Mating behaviour in Drosophila melanogaster and its implication to genetic variation

Sammanfattning: Not much is known about the mechanisms affecting the genetic composition of populations of different species. To investi­gate one of these potential mechanisms, mating behaviour, the fruit fly Drosophila melanogaster, was chosen as an experimen­tal animal.To quantify mating behaviour in easily measurable parameters, it was subdivided into several distinct components; mating activity, mating time, mating competition ability and male mating capacity. As behavioural components to a great extent are influenced by environmental conditions all experiments were performed under controlled temperature and humidity. All components of mating behaviour were estimated by introducing females and males into mating chambers.Mating behaviour seems to be one of the major factors affect­ing the genetic composition of Drosophila melanogaster popula­tions. The experiments performed showed that differences in mating properties led to a substantial sexual selection among the genotypes. This selection was of a stabilizing type with regard to characters associated to bristle number and Y chromo­somal chromatin. This selection situation seems to warrant the retention of intermediate phenotypes in a population and will therefore contribute to the genetic variation present. Differences in mating properties were also shown to be able to maintain a balanced polymorphism for allozyme variants in populations. This maintenance was obtained through different forms of balancing selection as heterozygous superiority in sexual activity and balancing selection between female and male genotypes. Heterozygous superiority or overdominance in fitness always leads to balanced polymorphism through segre­gation of individuals with lowered fitness. The balancing selection between the female and male genotypes is best looked upon as a form of marginal overdominance, conferring the aver­aged highest fitness to the heterozygous genotype, thereby maintaining the polymorphism of the population.