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Sex ratio distorter and evolution

Final Activity Report Summary - SEX RATIO DISTORTER (Sex ratio distorter and Evolution)

Some of the most important evolutionary transitions of life, from as early as the advent of eukaryotes from prokaryotes, have involved symbiosis. Numerous infections have evolved towards mutualistic symbiosis, while some inherited microorganisms are reproductive parasites. Such organisms spread by manipulating the reproduction of arthropods, e.g. by forcing asexuality on its host, feminising hosts, inducing conditional sterility or killing males. The single inherited parasite wolbachia has received much attention in recent literature. It has been found to be widespread and is now regarded as a very important cryptic driver of host ecology and evolution. However, wolbachia is just one among bacteria known to act as reproductive parasites, and our knowledge on most other bacteria remains desperately weak. In this project, we filled this void for one male-killing bacterium, arsenophonus.

Our project developed a global view on the range and nature of the arsenophonus symbiosis. At least a third of arthropod species were infected by a diverse assemblage of maternally inherited bacteria like arsenophonus that were likely to strongly influence their hosts’ biology. The question on how new infections were founded had increased in importance, as had happened in important infections. In past thinking, new infections of reproductive parasites were rarely acquired, mostly on evolutionary rather than on ecological timescales. In this project, we established an unusual high rate of transmission of a male-killing arsenophonus strain through a community of parasitoid wasps. Contrary to the other reproductive parasites, arsenophonus displayed high capacities to cross the barrier of host species. Importantly, arsenophonus underwent efficient maternal transmission in all the newly infected species following initial infection, and killed males in all cases.

There were few studies on the evolution of other reproductive parasites than wolbachia, and none on a microorganism distorting sex ratio, that were both vertically and horizontally transmitted at high frequency. The reason of this omission was that the systems in question were rare, making arsenophonus an exceptional model.