Reproductive compensation favours male-killing Wolbachia in a live-bearing host

Wolbachia are maternally inherited, cellular endosymbionts that can enhance their fitness by biasing host sex ratio in favour of females. Male killing (MK) is an extreme form of sex-ratio manipulation that is selectively advantageous if the self-sacrifice of Wolbachia in males increases transmission through females. In live-bearing hosts, females typically produce more embryos than can be carried to term, and reproductive compensation through maternal resource reallocation from dead males to female embryos could increase the number of daughters born to infected females. Here, we report a new strain of MK Wolbachia (wCsc2) in the pseudoscorpion, Cordylochernes scorpioides, and present the first empirical evidence that reproductive compensation favours the killing of males in a viviparous host. Females infected with the wCsc2 strain produced 26 per cent more and significantly larger daughters than tetracycline-cured females. In contrast to the previously described wCsc1 MK Wolbachia strain in C. scorpioides, wCsc2 infection was not accompanied by an increase in the rate of spontaneous brood abortion. Characterization of the wCsc1 and wCsc2 strains by multi-locus sequence typing and by Wolbachia surface protein (wsp) gene sequencing indicates that the marked divergence between these two MK strains in their impact on host reproductive success, and hence in their potential to spread, has occurred in association with homologous recombination in the wsp gene.