Understanding patterns of genetic diversity in the oak gallwasp Biorhiza pallida: demographic history or a Wolbachia selective sweep?

The endosymbiont Walbachia can be responsible for selective sweeps on mitoc hondrial DNA variability within species. Similar signals can also result fr om demographic processes, although crucially the latter affect nuclear as w ell as mitochondrial loci. Here we present data on Wolbachia infection stat us and phylogeographic patterning for a widely distributed insect host, the oak gallwasp Biorhiza, pallida (Hymenoptera: Cynipidae;). Two hundred and eighteen females from eight European countries were screened for Wolbachia. All individuals from Hungary, Italy, France, U.K., Ireland, Switzerland, S weden, and northern and southern Spain were infected with a single group A strain of Wolbachia, while populations in central Spain were not infected. A mitochondrial marker (cytochrome b) shows low variation and departure fro m neutrality in infected populations, but greater variation and no deviatio n from neutrality in Wolbachia-free populations. This pattern is compatible with a Wolbachia-induced selective sweep. However, we also find parallel d ifferences between infected and uninfected populations for nuclear markers (sequence data for ITS1 and ITS2)All markers support the existence of a dee p split between populations in Spain (some free of Wolbachia), and those in the rest of Europe (all infected). Allelic variation for five allozyme loc i is also consistent with the Spain-rest of Europe split. Concordant patter ns for nuclear and mitochondrial markers suggest that differences, in the n ature and extent of genetic diversity between these two regions are best ex plained by differing demographic histories (perhaps associated with range e xpansion from Pleistocene glacial refugia), rather than a Wolbachia-associa ted selective sweep.