Sperm morphology and function in passerine birds

The prime function of sperm cells is to fertilize eggs. Despite this common purpose, there is a tremendous diversity among animal taxa in sperm phenotypes. Part of the variation can be attributed to phylogenetic relatedness, and sperm are often used as di agnostic traits in taxonomy. But there is also increasing evidence that post-copulatory sexual selection, or sperm competition, is an important evolutionary force shaping sperm morphology in many taxa, including birds. Moreover, recent studies have reveal ed strong maternal genetic effects on particular sperm traits, presumably expressed by mitochondrial genes that are maternally inherited. A fascinating implication of the latter is the possibility that mutations that reduce sperm performance may not be se lected against unless they also have negative effects in females. The main goal of this project is to advance our understanding of the evolutionary processes that shape sperm morphology and diversity, and their function. We will use passerine birds as mod el organisms, as we already know a good deal about their mating systems and general biology. The project has three sub-goals. First, we will investigate the relative roles of phylogeny and sperm competition in the evolution of sperm traits across a number of passerine families and genera, using testes size as a proxy for the intensity of sperm competition. Second, we will study between-male variation in sperm morphology and its possible associations with mitochondrial and nuclear genotypes, and fertilizat ion success under a regime of intense sperm competition. These analyses will be carried out in well-studied model systems. Finally, we will estimate the occurrence of male infertility in selected species, in which we have prior indication that male infert ility has an evolutionary significance for the mating system. We will particularly seek to examine whether male infertility is related to testicular malfunction or sperm traits encoded by mitochondrial genes.