Patterns in the evolution of gall structure and life cycles in oak gall wasps (Hymenoptera : Cynipidae)

We present an analysis of the phylogenetic distribution of gall and life cycle characters within cynipid gallwasps of the genus Andricus (Hymenoperta: Cynipidae). Our intention is to identify which of these traits have proved evolutionarily labile during the radiation of oak cynipids, and so may have been associated with speciation events, and which (if any) are conserved within lineages. We show that:

1. While it is generally true that closely related species induce galls of similar structure, some gall traits have evolved repeatedly. One trait (an air space separating the larval chamber from the outer wall) has evolved independently in two entirely different groups of species in sexual and asexual generation galls.

2. Multiple transitions between alternative galling sites (acorn, bud, catkin, etc.) have occurred in both sexual and asexual generation galls. The most frequent shifts are from bud-galling to catkin-galling in the sexual generation. For species with alternation of generations, a shift from the ancestral galling sites in one generation is not correlated with a shift in the other.

3. There have been repeated evolutionary departures from an ancestral life cycle involving alternate sexual and asexual generations.

4. The transition from a life cycle involving two generations on oaks in the same section, Quercus, to a life cycle in which sexual generation galls develop on section Cerris oaks may have occurred only once in Andricus.