Geographic distribution and inheritance of three cytoplasmic incompatibility types in Drosophila simulans

Wolbachia-like microorganisms have been implicated in unidirectional cytoplasmic incompatibility between strains of Drosophila simulans. Reduced egg eclosion occurs when females from uninfected strains (type W) are crossed with males from infected strains (type R). Here we characterize a third incompatibility type (type S) which is also correlated with the presence of Wolbachia-like microorganisms. Despite the fact that the symbionts cannot be morphologically distinguished, we observed complete bidirectional incompatibility between R and S strains. This indicates that the determinants of incompatibility are different in the two infected types. S/W incompatibility is unidirectional and similar to R/W incompatibility. A worldwide survey of D. simulans strains showed that type S incompatibility was found only in insular populations which harbor the mitochondrial type SiI. Both W and R types were found among mainland and island populations harboring the worldwide mitochondrial type SiII. Type S incompatibility could be involved in the reinforcement of the geographical isolation of SiI populations.