Do Wolbachia infections play a role in unidirectional incompatibilities in a field cricket hybrid zone?

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Abstract
Two closely related field crickets, Gryllus firmus and G. pennsylvanicus, hybridize along an extensive north–south zone in the eastern United States. Crosses between G. firmus males and G. pennsylvanicus females produce viable and fertile F1, but the reciprocal cross consistently fails to produce offspring. Wolbachia, a bacterial parasite of arthropods that causes unidirectional incompatibilities in a variety of insect species, has been suggested as the cause of the observed incompatibility between G. pennsylvanicus and G. firmus. We examine the presence/absence of Wolbachia strains, defined by sequencing the ftsZ gene, in four cricket populations from the north-eastern United States. Most G. firmus individuals are infected (100% in Guilford, Connecticut; 65% in Seaside Park, New Jersey) and > 95% of those infected harbour a single strain of Wolbachia. All individuals in G. pennsylvanicus populations (Ithaca, New York; Sharon, Connecticut) are infected; the majority of individuals carry a second strain of Wolbachia, but a significant fraction carry the same strain found commonly in G. firmus. The presence of an apparently identical Wolbachia strain in crickets of both species means that some crosses between G. pennsylvanicus males and G. firmus females should be compatible. We have no evidence of such compatibility. Furthermore, if Wolbachia infections are responsible for the observed incompatibility between species, then incompatibilities must also exist within G. pennsylvanicus, because this species harbours both Wolbachia strains. Although some single pair crosses within G. pennsylvanicus do fail to produce offspring, the proportion is lower than expected if Wolbachia were responsible. Therefore, Wolbachia is unlikely to be involved in reproductive isolation between the two cricket species.