Seasonality of Wolbachia infection rate in two closely related sympatric species of terrestrial isopods (Isopoda: Armadillidae) in Okayama, Japan, with effects on sex ratio

Wolbachia are ubiquitous endosymbionts that infect many invertebrates and often manipulate their hosts' reproduction. Although a bias in the sex ratio of the host species due to infection with Wolbachia has been reported in the field, few studies have investigated the seasonal change in rates of infection by Wolbachia. Examining seasonal changes in Wolbachia infection is important because many parasitic infection agents, such as bacteria or viruses, usually show seasonal dynamics. In the present study, we examined the seasonal abundance and sex ratio of two closely related pill bug species that sympatrically inhabit Okayama, Japan. Phylogenetic and morphological analyses identified the two closely related species as Spherillo sp. sensu Karasawa et al. 2014 and Spherillo sp. shi-1 sensu Karasawa and Kawano 2014; they cohabit in fallen leaves, i.e., litter, on a mountain in Okayama City. An obvious peak in emergence of the two pill bug species was not observed. Both sympatric species were infected by Wolbachia, but no seasonal trends were found in the infection rate of Wolbachia. In Spherillo sp., females had higher infection rates than males, while the rates were almost 100% in both sexes in Spherillo sp. shi-1. The results suggest that the two pill bug species are infected by different Wolbachia strains with dissimilar manipulations of the sex ratio.