Rapid spread of the defensive endosymbiont Spiroplasma in Drosophila hydei subjected to high parasitoid wasp pressure

Maternally transmitted endosymbionts of insects are ubiquitous in nature and play diverse roles in the ecology and evolution of their hosts.  These symbionts employ a variety of strategies to persist in host lineages, both actively through reproductive parasitism and passively by conferring a fitness benefit to their host. A growing number of studies have recently suggested that one way in which these symbionts may benefit host fitness is through symbiont-mediated defense against natural enemies.  Previously, our lab detected that bacteria strain Spiroplasma hy1, which is naturally harbored by Drosophila hydei, confers partial protection to its host against mortality induced by the parasitoid wasp Leptopilina heterotoma. Flies surviving a wasp attack, however, were noted to have reduced adult longevity and fecundity.  Therefore, it is unclear whether protection against wasps alone can explain the persistence of Spiroplasma infection despite imperfect maternal transmission and possible fitness costs associated with Spiroplasma infection. To address this question, we conducted a population cage study comparing Spiroplasma frequencies over time under conditions of high wasp pressure and no wasp pressure.  Our preliminary results indicate that Spiroplasma frequencies increase over time under high parasitoid pressure, reaching approximately 97% after three generations (starting at 50%), whereas Spiroplasma frequencies fluctuate slightly in the absence of wasps. The rapid spread of Spiroplasma under high parasitoid pressure confirms the existence of strong fitness benefits to the host and may explain the high prevalence of Spiroplasma hy1 in natural D. hydei populations.