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 Apocrit a (Hymenoptera). Character-state reconstructions in both genes suggest ed that conversions between A and T (AT transversions) occurred much m ore frequently than any other type of change, although we cannot wholl y discount an underlying transition bias. Parsimony analysis of COT nu cleotide characters did not recover phylogeny; e.g., neither the Tenth redinoidea nor Apocrita were recovered as monophyletic, However, analy sis of COI amino acid characters did recover these relationships, as w ell as others based on fossil and morphological evidence. Analysis of 16S rRNA characters also recovered these relationships providing conve rsions between A and T were down-weighted. Analysis of the combined da ta sets gave relatively strong support for various relationships, sugg esting that both data sets supported similar topographies. These data sets, both separately and combined, suggested that the phytophagous Si ricidae were more closely related to the predominantly parasitic Apocr ita than were the ectoparasitic Orussoidea. This suggests that the was p parasitic lifestyle did not have a single origin, unless the Siricid ae have more recently reverted to phytophagy. Alternatively, parasitis m evolved twice independently, once in the Orussoidea and again in the Apocrita. The latter scenario is supported by the observation that th e evolution of parasitism was accompanied by a tendency for the larvae to develop inside plant tissues. Adaptations that accompanied the mov ement of wasps into a confined, wood-boring habitat may have preadapte d them to becoming ectoparasitic.