Molecular data support Bombus sonorus and Bombus pensylvanicus (Hymenoptera, Apidae) as distinct species

Abstract

Despite their distinctive banding patterns, setal coloration, and geographic ranges, the Sonoran bumble bee (Bombus sonorus Say, 1837) and the American bumble bee (Bombus pensylvanicus De Geer, 1773) are often treated as conspecific, with some authorities ranking B. sonorus as a subspecies of B. pensylvanicus. This lack of taxonomic clarity creates challenges, particularly for population monitoring and conservation initiatives. In this study, we used genetic analyses to assess the potential for clinal variation, ongoing hybridization, and historical introgression between B. pensylvanicus and B. sonorus within a broad area of sympatry across the state of Texas. Double digest restriction associated DNA sequencing (ddRADseq) was performed on 166 specimens (58 B. sonorus and 108 B. pensylvanicus), and a portion of the mitochondrial COI gene was sequenced for a subset of the specimens from Texas (46 B. sonorus and 32 B. pensylvanicus) and eight specimens of B. sonorus from California. An additional five sequences of B. pensylvanicus from Georgia and Florida were obtained from Genbank and BOLD along with one each of B. transversalis (Olivier, 1789), B. mexicanus Cresson, 1879, B. medius Cresson, 1863 and B. fervidus (Fabricius, 1798), and B. mesomelas Gerstäcker, 1869. For both genetic datasets (nuclear ddRADseq and mtDNA COI), individuals formed two distinct groups concordant with species identification based on setal coloration. We found no evidence supporting a clinal pattern of variation, ongoing hybridization, or historical introgression within the study area and conclude that B. sonorus should be recognized as a species under the Biological Species Concept.