Species Phylogeny of the Bee Genus Exoneurella Michener (Hymenoptera: Apidae: Allodapini): Evidence from Molecular and Morphological Data Sets

Abstract

An hypothesis of genealogical relationships among taxa provides an historical map along which character evolution (e.g., social traits) can be traced to reveal patterns of common origin or repeated but independent evolution. To investigate the evolution of social traits in the allodapine bee genus Exoneurella, we have derived a species phylogeny based on 32 morphological features of adult and immature stages and sequences from 2 mitochondrial DNA genes, cytochrome oxidase I (COI) and cytochrome b (Cyt b). The morphological and COI data support the close relationship of Brevineura and Exoneurella in accord with previous hypotheses of phylogenetic relationships among allodapine bees. In contrast, the Cyt b data supported the sister-group relationship of Braunsapis and Exoneurella. This was most likely caused by long branch attraction or convergent patterns in the Cyt b data set. The morphological and COI data support the basal position of E. tridentata in the Exoneurella clade. Applying different weights to transversions versus transitions in the analyses of COI sequence data produced 2 trees with 1 totally congruent with the morphological data set. The other tree differed in terms of the arrangement of the 3 distal species of Exoneurella: E. lawsoni, E. eremophila, and E. setosa. Combined analysis of the 3 data sets yields the following species relationship in Exoneurella: (tridentata + [lawsoni + (setosa + eremophila)]). The A-T rich mitochondrial genome of bees affects the rate of nucleotide substitution leading to the predominance of transversions over transitions and effectively lowering the saturation threshold for transitions. This resulted in transversions accounting for almost all of the informative changes on the tree, even at the species level.