Wolbachia infection in the terrestrial isopod oniscus asellus: sex ratio distortion and effect on fecundity

Maternally inherited Wolbachia bacteria are widespread in arthropods where they are responsible for various reproductive alterations. In terrestrial isopods (woodlice), Wolbachia may induce feminization or cytoplasmic incompatibility (CI), but their effect remains unknown in most host species. To increase our understanding of host/symbiont interactions in terrestrial isopods, the effect of Wolbachia was investigated in the oniscidean Oniscus asellus, mainly to discriminate between feminization and CI. The Wolbachia infection was not linked with a CI phenomenon, but females infected with Wolbachia produced female-biased broods compared with uninfected females. The fecundity of infected females was slightly lower than that of uninfected, but the number of young at the adult stage was similar between the two female categories. The experimental transfer of the symbiont into uninfected strains showed that Wolbachia was responsible for the feminization of a number of genetic males. In female-biased broods, Wolbachia were vertically transmitted to around 88% of the offspring, but the transmission rate was lower in the few male-biased progenies. The feminizing activity of these symbionts was not systematic, as many phenotypic males were infected. These results contrasted with what is known in another woodlouse species, and indicated that feminization has evolved in different ways in terrestrial isopods.