Birth of a W sex chromosome by horizontal transfer of Wolbachia bacterial symbiont genome.

Sex determination is a fundamental developmental pathway governing male and female differentiation, with profound implications for morphology, reproductive strategies, and behavior. In animals, sex differences between males and females are generally determined by genetic factors carried by sex chromosomes. Sex chromosomes are remarkably variable in origin and can differ even between closely related species, indicating that transitions occur frequently and independently in different groups of organisms. The evolutionary causes underlying sex chromosome turnover are poorly understood, however. Here we provide evidence indicating that Wolbachia bacterial endosymbionts triggered the evolution of new sex chromosomes in the common pillbug Armadillidium vulgare We identified a 3-Mb insert of a feminizing Wolbachia genome that was recently transferred into the pillbug nuclear genome. The Wolbachia insert shows perfect linkage to the female sex, occurs in a male genetic background (i.e., lacking the ancestral W female sex chromosome), and is hemizygous. Our results support the conclusion that the Wolbachia insert is now acting as a female sex-determining region in pillbugs, and that the chromosome carrying the insert is a new W sex chromosome. Thus, bacteria-to-animal horizontal genome transfer represents a remarkable mechanism underpinning the birth of sex chromosomes. We conclude that sex ratio distorters, such as Wolbachia endosymbionts, can be powerful agents of evolutionary transitions in sex determination systems in animals.