Multipolar spindle assembly and mitotic slippage underlie symbiont-mediated asexual reproduction

Abstract

Maternally inherited bacteria can profoundly impact animal biology. A notable example is the induction of thelytokous parthenogenesis: the asexual production of female offspring. Despite decades of symbiont-mediated parthenogenesis documented across diverse arthropods, the underlying cell biological mechanisms remain poorly understood. In some species of parasitic wasps, Wolbachia causes a chromosome segregation failure during the first mitotic division in unfertilized haploid embryos. This diplodizes the embryo and facilitates asexual reproduction of females. To elucidate the mechanism of mitotic failure and understand how the animal continues development, we compared meiosis and early embryonic mitoses of the parasitoid wasp, Trichogramma pretiosum, with and without their parthenogenesis-inducing Wolbachia symbiont. We show that meiosis in these animals is anastral, and embryos undergo de novo microtubule organizing center (MTOC) biogenesis prior to the first embryonic mitosis, regardless of Wolbachia. During the first mitosis, embryos with Wolbachia had increased numbers of MTOCs associated with the oocyte nucleus and formed multipolar spindles in a modified metaphase, after which chromosome segregation failed. A restitution nucleus formed, and mitotic slippage enabled the diploid nucleus to transition to interphase and replicate. Subsequent mitotic divisions inherited supernumerary MTOCs, but formed pseudo-bipolar spindles and segregated normally. These results demonstrate that Wolbachia-mediated parthenogenesis stems from altered MTOC biology, provide a structural basis for mitotic failure and the subsequent restitution. Symbiont-mediated parthenogenesis offers unique opportunities to uncover the cell biology of asexual reproduction and a fascinating model for understanding mitosis more broadly.