Molecular Phylogeny and Revision of Species Groups of Nearctic Bombardier Beetles (Carabidae: Brachininae: Brachinus)

Abstract

Bombardier beetles of the genus Brachinus Weber are notorious for their explosive defensive chemistry; when threatened, they aim a 100°C cloud of benzoquinones directly at an enemy. Despite ongoing research on their defensive chemistry and ecology, the group lacks a robust molecular phylogeny. In this study, three loci from both the mitochondrial and nuclear genome (COI, CAD, 28S) were used to reconstruct the phylogeny of Nearctic Brachinus and to test hypothesized relationships based on a previous study based upon morphological characters (Erwin, 1970). All molecular phylogenetic analyses recovered all species groups proposed by Erwin as monophyletic except for the fumans species group. We find that Erwin’s fumans species group is polyphyletic. In its place, we erect the following five species groups: cinctipennis, fumans (revised), galactoderus, phaeocerus, and quadripennis. External morphological characters were traced on the molecular phylogeny to identify potential diagnostic characters. Apomorphic characters were identified for two new species groups, the fumans (revised) and phaeocerus groups. We also expand the cordicollis group to include B. medius and the americanus group.