Parasitoidism, a specialized life strategy in which a parasite eventually kills its host, is frequently found within the insect order Hymenoptera (wasps, ants and bees). Within the Cynipoidea, a species-rich group of small to minute wasps, a parasitoid lifestyle is one of two dominant life strategies, with the other being phytophagy in the form of gall-making. Less commonly cynipoid wasps exhibit inquilinism, a strategy where some species have adapted to usurp other species galls instead of inducing their own. Using a phylogenomic data set of ultraconserved elements and extensive taxon sampling, we generated a solid phylogenetic framework and timescale for cynipoid systematics and the evolution of cynipoid life histories. Rooting our analyses with non-cynipoid outgroups, the Paralaucini, a group of lethal inquilines (which usurp a gall and kill its inducer), emerged as sister-group to the rest of Cynipoidea, rendering the gall wasp family Cynipidae paraphyletic. The families Ibaliidae and Liopteridae, wood-boring insect parasitoids previously thought to be the most ancestral cynipoids, were found nested as sister-group to the family Figitidae. Cynipoidea originated in the early Jurassic around 190 Ma. Inquilinism or parasitoidism are suggested as ancestral and dominant strategy throughout the early evolution of cynipoids, depending on whether a simple (three states: parasitoidism, inquilinism and galling) or more complex (seven states: parasitoidism, inquilinism and galling split by host use) model is employed.</p><p>Conclusions: Our study has significant impact not only on understanding cynipoid evolution, but highlights also the crucial importance of adequate outgroup sampling. Previous analyses on cynipoids have favored a parasitoid-first scenario, yet an inquiline-first scenario for Cynipoidea, as our simpler model suggests, is also appealing given that the earliest branching lineage Paralaucini exhibits lethal inquilinism, a strategy which could be regarded as intermediate stage on the transition between parasitoidism and gall-making.
