Phylogeny of the parasitic microgastroid subfamilies (Hymenoptera: Braconidae) based on sequence data from seven genes, with an improved time estimate of the origin of the lineage

Abstract

The microgastroid complex of braconid wasps is a widely recognized and biologically coherent lineage of endoparasitoids of lepidopteran larvae (caterpillars). The complex has received significant phylogenetic attention in recent years due in part to the taxons' association with mutualistic polydnaviruses, with which they compromise host immune systems. A number of previous attempts using a variety of morphological and molecular approaches have not unequivocally resolved relationships amongst the main subfamilies. This work represents a more extensive attempt to resolve the microgastroid relationships, using seven genes (16S rRNA, cytochrome oxidase I (CO1), 28S rRNA, arginine kinase (ArgK), long wavelength rhodopsin (Ops), elongation factor 1 alpha (EF1a) and wingless (Wg)) and a greater taxonomic representation. Bayesian, likelihood and parsimony phylogenetic reconstructions of this improved data set has determined that the chelonines diverged first from the remainder of the microgastroids, however the relationships amongst the other subfamilies are still unclear, suggesting a greater nucleotide sample is required to resolve them. Examination of the contribution of individual gene trees to the phylogeny demonstrates why the relationships between subfamilies are still unclear, with not all groups monophyletic for all trees. Filtered supernetworks demonstrate that monophyly of all subfamilies is only recovered when splits found in only one or two genes are excluded, but this also results in little remaining structure left in the deep nodes to resolve inter-subfamily relationships. By increasing the breadth of the study we were also able to re-evaluate previous attempts at dating the lineage and, therefore the origin of the polydnavirus association. Previous attempts used a much reduced data set and fewer fossil calibrations. Thorough literature searches have revealed a substantial increase in the fossil calibrations and these, combined with more sophisticated molecular dating analysis, have substantially increased the age of the microgastroid lineage from previous estimates of approximately 73MYA to approximately 100MYA. Examination of the resultant linearized clock tree also allows an insight into the evolution of the more species rich subfamilies. The chelonines appear to have had a steady rate of evolution, whilst the microgastrines and cardiochilines appear to have undergone a more significant "burst" of evolution. It is hypothesized that the different parasitism strategies of subfamilies (Chelonines are egg parasitoids and the remainder are larval parasitioids) may have influenced the evolutionary rates of the groups.