Host specialization is ancestral in Torymus (Hymenoptera, Chalcidoidea) cynipid gall parasitoids

Abstract

The host range of parasitoids varies greatly among species: Some only parasitize one to a few hosts (specialists), while others parasitize multiple species or a va-riety of host types (generalists). The direction of most host range shifts in parasi-toid groups, that is from generalist to specialist or, alternatively, from specialist to generalist, is unknown. To explore the origin of host range shifts, we studied a clade within the genus Torymus(Hymenoptera, Chalcidoidea) that includes both generalist and specialist parasitoids of Cynipidae (Hymenoptera). We ana-lysed the phylogenetic relationships of the species of Torymuson the basis of two gene fragments (cox1and ITS2) of 246 specimens and performed an ancestral state reconstruction of the specialist/generalist trait. Our results revealed the fol-lowing: (a) The ancestral state of this group of Torymusis specialist, with the generalist state evolving through a loss of specialization. (b) The species Torymus cyaneusand Torymus flavipesboth have a strong genetic structure, suggesting the existence of different biological identities. (c) There has been a host plant shift in the lineage(s) leading to Torymus rubiand Torymus bedeguarisfrom galls on Quercusto those on Rosaceae. (d) The alien species Torymus sinensisand the native European species Torymus notatusare phylogenetically closely related. (e) Speciation within Torymuswas likely associated with the diversification of their cynipid hosts, which itself was driven by the dramatic changes in climate and vegetation that occurred during the Miocene.