Appreciation of endosymbiosis, a type of symbiosis in which a microbial partner lives within its host cells, as an important source of evolutionary novelty has developed relatively recently. In this proposal, we investigate a fundamental evolutionary process influenced by bacterial endosymbionts: the mechanisms of sex determination of their eukaryotic hosts.In animals, the most common system of sex determination is genetic. It can also be affected by inherited bacterial endosymbionts. However, very few systems have been analyzed and there is no extensive empirical evidence of how endosymbionts can shape host sex-determining systems. In the isopod crustacean Armadillidium vulgare, genetic sex determination follows female heterogamety. However, many A. vulgare populations harbour Wolbachia bacterial endosymbionts which can invert genetic males into phenotypic functional females.Other sex-determining factors have been identified in A. vulgare: a feminizing f element which may be a Wolbachia genome fragment carrying feminization information inserted into the host nuclear genome, and a masculinizing gene which can restore the male sex in the presence of the f element, as a result of a genetic conflict. Thus, sex determination mechanisms in A. vulgare seem to be largely driven by Wolbachia endosymbionts. However, the molecular genetic basis and evolutionary history of all these sex-determining factors is unknown.The A. vulgare/Wolbachia model provides a unique opportunity for directly investigating the impact of endosymbionts on the evolution of host sex determination mechanisms at the molecular genetic level. We will address this issue using the latest developments of molecular genetics technologies, such as next-generation DNA sequencing and high throughput genotyping.