IBED Seminar by Dr. Richard Cordaux
Title: Bacterial endosymbionts and the evolution of host sex-determination mechanisms
Dr. Richard Cordaux of the Université de Poitiers, France
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 they can differ even between closely related species, indicating that transitions occur frequently and independently in different groups of organisms. However, the evolutionary causes underlying sex chromosome turnovers are poorly known. I will present results supporting that genetic elements distorting host sex ratio can be powerful agents of transitions between sex determination mechanisms.
In the common pillbug Armadillidium vulgare, chromosomal sex determination follows female heterogamety (ZZ males and ZW females). However, many A. vulgare populations harbor maternally-inherited Wolbachia bacterial endosymbionts which can convert genetic males into phenotypic females, leading to populations with female-biased sex ratios. The W sex chromosome has been lost in lines infected by Wolbachia and all individuals are ZZ genetic males. The female sex is determined by the inheritance of Wolbachia by the A. vulgare individual, thereby leading to a shift from chromosomal to cytoplasmic sex determination. Surprisingly, some A. vulgare lines exhibit biased sex ratios despite the lack of Wolbachia. We identified a large piece of the Wolbachia genome recently transferred to the A. vulgare nuclear genome. Our results demonstrate that the Wolbachia insert is acting as a female sex-determining region in pillbugs and that the chromosome carrying the insert is a new W sex chromosome.
Overall, our results indicate that Wolbachia bacteria can drive shifts in sex determination mechanisms in A. vulgare. More generally, they emphasize that bacterial endosymbionts can be powerful sources of evolutionary novelty for fundamental biological processes in animals, such as sex determination.
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