Wolbachia Feminises a Spider Host With Assistance From Co-Infecting Symbionts.

Arthropods commonly harbour maternally-transmitted bacterial symbionts that manipulate host biology. Multiple heritable symbionts can co-infect the same individual, allowing these host-restricted bacteria to engage in cooperation or conflict, which can ultimately affect host phenotype. The spider Mermessus fradeorum is infected with up to five heritable symbionts: Rickettsiella (R), Tisiphia (T), and three strains of Wolbachia (W1-3). Quintuply infected spiders are feminised, causing genetic males to develop as phenotypic females and produce almost exclusively female offspring. By comparing feminisation across nine infection combinations, we identified a Wolbachia strain, W1, that is required for feminisation. We also observed that spiders infected with both W1 and W3 produced ~10% more females than those lacking W3. This increase in feminisation rate does not seem to be due to direct changes in W1 titre, nor does W1 titre correlate with feminisation rate. Instead, we observed subtle titre interactions among symbionts, with lower relative abundance of R and T symbionts in strongly feminised infections. This synergistic effect of co-infection on Wolbachia feminisation may promote the spread of all five symbionts in spider populations. These results confirm the first instance of Wolbachia-induced feminisation in spiders and demonstrate that co-infecting symbionts can improve the efficacy of symbiont-induced feminisation.