Rapid spread of Spiroplasma defensive endosymbionts in Drosophila hydei under high parasitoid wasp pressure. Jialei Xie, Lauryn Winter, Caitlyn Winter, Mariana Mateos. Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX.

   Maternally transmitted endosymbionts of insects are ubiquitous in nature and play diverse roles in the ecology and evolution of their hosts. To persist in host lineages, many of these symbionts manipulate host reproduction to their advantage (e.g. Cytoplasmic incompatibility and male-killing), or confer fitness benefits to their hosts. Among the benefits conferred, a growing number of studies report symbiont-mediated defense against natural enemies of their hosts. Recent studies suggest that two non-male-killing bacteria strains (genus Spiroplasma) protect their host fly against natural enemies. Spiroplasma strain hy1, which is naturally harbored by Drosophila hydei, confers partial protection to its host against mortality induced by the parasitoid wasp, Leptopilina heterotoma. The Spiroplasma-conferred protection is partial and flies surviving a wasp attack have reduced adult longevity and fecundity. Therefore, it is unclear whether protection against wasps alone can counter Spiroplasma loss by imperfect maternal transmission and any possible fitness costs to harboring Spiroplasma. To address this question, we conducted a population cage study comparing Spiroplasma frequencies over time (host generations) under conditions of high wasp pressure and no wasp pressure. Our preliminary results indicate that Spiroplasma frequencies increase over time under high parasitoid pressure, whereas Spiroplasma frequencies fluctuate slightly in the absence of wasps. The prevalence of Spiroplasma reaches approximately 97% after three consecutive generations from 50% of starting frequency, and the population dynamics of both fly and wasp were largely influenced by the presence of Spiroplasma. The rapid spread of Spiroplasma under high parasitoid-pressure confirmed its strong of host-protection effects and may explain the high prevalence of Spiroplasma hy1 in natural D. hydei populations.