Virtually every species of insect harbors facultative bacterial endosymbionts that are transmitted from females to their offspring, often in the egg cytoplasm. These symbionts play crucial roles in the biology of their hosts. Many manipulate host reproduction in order to spread within host populations. Others increase the fitness of their hosts under certain conditions. For example, increasing tolerance to heat or protecting their hosts against natural enemies. Over the past decade, our understanding of insect endosymbionts has shifted from seeing them as fascinating oddities to being ubiquitous and central to the biology of their hosts, including many of high economic and medical importance. However, in spite of growing interest in endosymbionts, very little is known about the molecular mechanisms underlying most endosymbiont-insect interactions. For instance, the basis of the main phenotypes caused by endosymbionts, including diverse reproductive manipulations or symbiont-protective immunity, remains largely enigmatic. The goal of the present application is to fill this gap by dissecting the interaction between Drosophila and its native endosymbiont Spiroplasma poulsonii. This project will use a broad range of approaches ranging from molecular genetic to genomics to dissect the molecular mechanisms underlying key features of the symbiosis, including vertical transmission, male killing, regulation of symbiont growth, and symbiont-mediated protection against parasitic wasps. We believe that the fundamental knowledge generated on the Drosophila-Spiroplasma interaction will serve as a paradigm for other endosymbiont-insect interactions that are less amenable to genetic studies.