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

The endosymbiont Wolbachia can be responsible for selective sweeps on mitochondrial DNA variability within species. Similar signals can also result from demographic processes, although crucially the latter affect nuclear as well as mitochondrial loci. Here we present data on Wolbachia infection status 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, Sweden, 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 from neutrality in infected populations, but greater variation and no deviation from neutrality in Wolbachia-free populations. This pattern is compatible with a Wolbachia-induced selective sweep. However, we also find parallel differences between infected and uninfected populations for nuclear markers (sequence data for ITS1 and ITS2). All markers support the existence of a deep split between populations in Spain (some free of Wolbachia), and those in the rest of Europe (all infected). Allelic variation for five allozyme loci is also consistent with the Spain-rest of Europe split. Concordant patterns for nuclear and mitochondrial markers suggest that differences in the nature and extent of genetic diversity between these two regions are best explained by differing demographic histories (perhaps associated with range expansion from Pleistocene glacial refugia), rather than a Wolbachia-associated selective sweep.