Impact of Wolbachia on the male stem cell niche biology. Stephanie M. Pontier¹, François Schweisguth^1,2. 1) Departement de developement, Institut Pasteur, Paris, France; 2) CNRS URA 2578.

   Stem cells are pivotal to tissue homeostasis. Understanding the biological elements controlling stem cell biology is hence essential to the development of efficient strategies allowing their control in vivo. One of these elements has been identified as the stem cell niche, which describes a particular environment insuring both the stem cell maintenance as well as the control of their proliferation. The most precisely identified stem cell niches are found in invertebrates, in both female and male gonads. In fly testis, a group of ten post-mitotic cells forming a hub at the very tip of the testis are believed to play a niche function. This hub was shown to fulfill all the prerequisite of a niche by controlling the position, the maintenance and the proliferation of two populations of stem cells, the germinal (GSC) and the somatic stem cells (SSC). While GSC differentiation takes place all along the gonad and gives rise to sperm, SSC can differentiate into two types of somatic cells: the cyst cells essential to sperm maturation and the hub cells that SSC can replenish upon their initial embryonic specification. Few years ago, a mutualistic and endosymbiotic alpha-proteobacteria, Wolbachia pipientis (Wolbachia), was found to invade the fly gonad niches with a strong tropism. While this bacteria was recently shown to increase germ cells proliferation in ovaries and improves female egg laying, its impact on the biology of the male stem cell niche remains poorly understood. Our study aims to describe these impacts and shows that Wolbachia affects both the establishment of the niche biology as well as its evolution in either stressed or physiological conditions. Importantly, Wolbachia impairs the role of the hub as a niche and promotes a novel equilibrium of the stem cell niche that correlates with a more robust interaction between somatic and germinal stem cells and their better maintenance. We are now trying to decipher the underneath signaling network supporting Wolbachia impacts on the male stem cell niche biology.