Evolutionary trends in feminization and intersexuality in woodlice (Crustacea, Isopoda) infected with Wolbachia pipientis (alpha-Proteobacteria)

Sex ratio distortion (SRD) and intersexuality are common phenomena among Isopoda (Arthropoda, Crustacea), caused by the feminizing action of the alpha-Proteobacterium Wolbachia (F) and/or an F DNA segment (f) integrated into the host genome, probably as a transposon. A dominant autosomal masculinizing allele (M) overrides the primary sex determinants (WZ: females; ZZ: males) and f but not F. The latter can be counterbalanced by the transmission suppressor polygenic system (R). The present study pursued a double object. (i) SRD was found in naturally isolated populations of three out of seven Belgian woodlouse species: Oniscus asellus, Armadillidium vulgare and A. pulchellum. They were (inter)sexed based on external and internal morphology, and specimens of the former two underwent PCR and/or microscopic detection of F. (ii) A deterministic but flexible model was set up to describe SRD changes in an initially F-infested A. vulgare population sampled during the last 25 years. Observed and predicted sex ratios fit best if natural transmission rates (i.e., in the absence of M and R) approach 100\%, a condition fulfilled if f is carried on a multicopy transposon. Most often, such a population will then gradually lose its invader to the benefit of f. The stable end situation is a host population consisting of ZZ+f individuals of which 44\% are neo-females (having mm), and 56\% are reversed males (owing to Mm or MM). The model also explains the accelerated SRD evolution under variable degrees of F/f transmission.