Confirmation, consequences and causes of extensive polyphyly in the oak gall wasp genus Dryocosmus (Hymenoptera: Cynipidae: Cynipini)

Oak gall wasps are a species-rich group in which most genera induce galls on a single Fagaceae lineage (either a section within Quercus or a single non-oak genus). Preliminary data for the few exceptions (most notably Andricus, Callirhytis, Dryocosmus and Neuroterus) suggest that high apparent host plant diversity is an artefact of polyphyly resulting from inadequate morphological definition of genera. Here we demonstrate this issue in Dryocosmus, which (as currently defined) galls seven Fagaceae lineages, by reconstructing relationships among 35 species in the genus and 27 other Cynipini genera worldwide. Data for 1,871 base pairs of DNA sequence from one mitochondrial gene (cytochrome b) and three nuclear genes (28S D2, opsin, wingless) confirm that Dryocosmus comprises a polyphyletic assemblage of more host-specific lineages. Our analyses also strongly reject monophyly of Callirhytis. We survey the taxonomic issues underlying these problems in Dryocosmus and begin the taxonomic revision of this and closely related genera. We establish (i) D. asymmetricus as a junior synonym of Neuroterus asymmetricus (Kinsey), comb. nov., (ii) C. congregata as a junior synonym of Melikaiella congregata (Ashmead), comb. nov., (iii) C. clavula as a junior synonym of Andricus clavula (Osten Sacken), comb. rev., and (iv) reinstate Plagiotrochus kunugiphagus as Dryocosmus kunugiphagus Ide & Abe comb. rev.. We establish a new genus, Chrysolepicynips Stone, Nicholls, Melika & Tang, gen. nov. for D. castanopsidis, D. demartinii, D. juliae and D. rileypokei that gall chinquapins (Chrysolepis sp.). Finally, we identify alternating generations in two species: asexual C. furva is a junior synonym of sexual D. quercusnotha (Osten Sacken), and asexual Zopheroteras guttatum and sexual D. quercuslaurifoliae are junior synonyms of sexual D. quercuspalustris (Osten Sacken). Our results provide a backbone for revision of Dryocosmus and highlight the need for Cynipini-wide revision of polyphyletic genera.