Evidence for AT-transversion bias in wasp (Hymenoptera: Symphyta) mitochondrial genes and its implications for the origin of parasitism.

We inferred the incidence of nucleotide conversions in the COI and 16S rRNA mitochondrial genes of members of the Symphyta and basal Apocrita (Hymenoptera). Character-state reconstructions in both genes suggested that conversions between A and T (AT transversions) occurred much more frequently than any other type of change, although we cannot wholly discount an underlying transition bias. Parsimony analysis of COI nucleotide characters did not recover phylogeny; e.g., neither the Tenthredinoidea nor Apocrita were recovered as monophyletic. However, analysis of COI amino acid characters did recover these relationships, as well as others based on fossil and morphological evidence. Analysis of 16S rRNA characters also recovered these relationships providing conversions between A and T were down-weighted. Analysis of the combined data sets gave relatively strong support for various relationships, suggesting that both data sets supported similar topographies. These data sets, both separately and combined, suggested that the phytophagous Siricidae were more closely related to the predominantly parasitic Apocrita than were the ectoparasitic Orussoidea. This suggests that the wasp parasitic lifestyle did not have a single origin, unless the Siricidae have more recently reverted to phytophagy. Alternatively, parasitism evolved twice independently, once in the Orussoidea and again in the Apocrita. The latter scenario is supported by the observation that the evolution of parasitism was accompanied by a tendency for the larvae to develop inside plant tissues. Adaptations that accompanied the movement of wasps into a confined, wood-boring habitat may have preadapted them to becoming ectoparasitic.