| Summary: | This dissertation describes a study of the role of sexual selection in inbreeding avoidance and an extensive analysis of the genetics basis of inbreeding depression using species of the fruit fly <i>Drosophila</i> as model organisms. Using behaviour experiments and molecular genetic techniques I investigated the role of both female mate choice and polyandry in inbreeding avoidance. My results show that in the four <i>Drosophila</i> species analysed (<i>D. melanogaster, D. simulans, D. subobscura</i> and <i>D. littoralis</i>) female flies are not able to avoid inbreeding through mate choice. However, when polyandry was investigated in <i>D. melanogaster</i> and in <i>D. littoralis</i> I found that post-copulatory mechanisms can contribute to inbreeding avoidance. Specifically, females sometimes appear preferentially to use sperm from an unrelated rather than related partner. However, the pattern is complicated and the exact outcome appears to depend on the species, the remating frequency and on male mating order. Concerning inbreeding depression, two main genetic mechanisms have been invoked to explain the deleterious effects associated with inbreeding, one based on the exposure of deleterious recessive alleles and the other on heterozygote advantage. In order to investigate the relative importance of these two mechanisms, I analysed the relationship between heterozygosity at microsatellite markers and fitness in <i>D. melanogaster, </i>comparing inbred and outbred crosses under benign and stressed conditions. It was found that although the effects of deleterious recessive alleles have a significant effect, heterozygote advantage also plays an important role on inbreeding depression. Additionally, I show that the deleterious effects of inbreeding are at least partly environmental-specific.
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