The Allonemobius-Wolbachia host-endosymbiont system: evidence for rapid speciation and against reproductive isolation driven by cytoplasmic incompatibility.

Evidence for the evolution of fertilization incompatibilities and rapid speciation can be biased by the occurrence of hybridization and reproductive endosymbionts such as Wolbachia. For example, patterns of mitochondrial DNA (mtDNA) variation can be obscured by mitotypes hitchhiking on extrachromosomal elements like Wolbachia, while such endosymbionts can also induce phenotypes that mirror the operation of intrinsic fertilization incompatibilities between species. Therefore, before strong inferences can be drawn concerning the rates and processes of speciation in arthropod systems, we must first assess whether extrinsic endosymbionts obscure patterns of speciation. Here, I use the Allonemobius fasciatus-socius species complex to determine what role Wolbachia has played in the presumed rapid divergence of this complex by analyzing patterns of mtDNA and nuclear DNA variation in conjunction with sequence and cytoplasmic incompatibility data on Wolbachia. Data on molecular variation suggest that Wolbachia has not induced a strong selective sweep of the mitochondrial genome; nor does Wolbachia appear to induce cytoplasmic incompatibility. Preliminary evidence indicates that a third species identified within this complex, A. sp. nov. Tex, is partially reproductively isolated from A. socius, its closest relative, via conspecific sperm precedence or some form of postzygotic isolation. Moreover, shared mitotypes between A. sp. nov. Tex and A. socius may indicate the occurrence of a hybrid zone between these species near the border of Texas and Louisiana, although they may also represent shared ancestral polymorphisms. Molecular data also indicate that all three species in this complex diverged from a common ancestor as recently as 3000-30,000 years ago. Finally, the radiation of this complex from its ancestral population likely occurred in the presence of one strain of Wolbachia, thus suggesting a minimal role for Wolbachia during this burst of speciation. In total, barriers to gene flow do appear to have evolved very rapidly in this group of crickets.