Western Palaearctic phylogeography of an inquiline oak gall wasp, Synergus umbraculus

Insect-induced galls on plants comprise species-rich but self-contained communities of herbivores and natural enemies. In the present study, we focus on galls induced by cynipid gall wasps on oaks, and on the least-known trophic level that these galls contain: inquilines. These insects, also cynipids, feed on gall tissue and are an abundant but taxonomically poorly understood part of an otherwise well-studied system. We used DNA sequence data to examine spatial patterns in the genetic diversity of Synergus umbraculus Olivier 1791 (Hymenoptera: Cynipidae: Synergini), a widespread species attacking many host galls across the Western Palaearctic. Analysis of 239 cytochrome b sequences revealed eight haplogroups showing significant phylogeographic pattern across the Western Palaearctic, corresponding to putative glacial refugia in Iberia, Central Europe, Turkey, and Iran. There were significant genetic discontinuities across the Pyrenees and the Anatolian diagonal but no impact of the Alps, suggesting that significant discontinuities have biotic rather than physical causes. Detailed analysis of sites in the Carpathian Basin reveal a high diversity and low spatial structure, and identify Central Europe as the source of colonists for Quaternary colonization of Germany, France, and Britain. We found no evidence for host-associated differentiation of S. umbraculus lineages associated with the most common cynipid host galls, suggesting frequent shifts within the host gall assemblage by inquiline lineages.