Applying molecular tools to investigate the species boundaries of North American Thoracobombus (Hymenoptera: Apidae, Bombus)

Bumble bees (Bombus L.) are one of the most conspicuous bee pollinators of the approximately 467 described bee genera in the world. Despite the relative ease of generic identification by both scientists and the general public, their identification to species remains difficult, and in some cases, controversial. Here we investigate the species boundaries of Bombus californicus F. Smith and B. fervidus Fabricius, sympatric species in western North America. While genetic support for two distinct species exists, some authors consider them conspecific, and the phenotypic ‘intergrades’ remain to be fully investigated. To assess the degree of genetic differentiation of the phenotypic intergrades in the two species, we initially applied 20 microsatellite markers to 163 individuals to estimate their population genetic structure and degree of gene flow among populations. Using principal component analysis (PCA) and population statistics (F_ST) we observed genetic isolation of B. californicus from B. fervidus, (PC1 > 0.60; F_ST > 0.20, P < 0.05), with B. californicus populations constituting three main phenotypes.  Within B. californicus, the phenotypes did not demonstrate any significant differences in heterozygosity (F_ST < 0.20). However, each B. californicus phenotype formed a distinct cluster in the PCA, suggesting a genetic association between population structure and color form. While the habitat requirements and ecology of both species are arguably different, they both are suspected of population decline. By assessing the species boundaries of B. californicus and B. fervidus, effective policy decisions can be made to conserve their habitat and resources.