Genome-wide markers test the status of two putative species of North American bumblebees and detect mito-nuclear discordance

Authors: ROHDE, ASHLEY - New Mexico State University; STRANGE, JAMES - The Ohio State University; MOCK, KAREN - Utah State University; Branstetter, Michael

Submitted to: Conservation Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2025
Publication Date: 2/13/2025
Citation: Rohde, A., Strange, J.P., Mock, K.M., Branstetter, M.G. 2025. Genome-wide markers test the status of two putative species of North American bumblebees and detect mito-nuclear discordance. Conservation Genetics. https://doi.org/10.1007/s10592-025-01674-6.
DOI: https://doi.org/10.1007/s10592-025-01674-6

Interpretive Summary: Bumbles bees are critical pollinators of flowering plants in agricultural and natural areas and many species are facing population declines. The western bumble bee, Bombus occidentalis, used to be widespread and used to be used in managed pollination of agricultural crops, but since the 1990s it has declined greatly and is now being considered for listing under the Endangered Species Act. Despite it's importance uncertainty exists as to whether the species comprises one or two species, with the most recent molecular study suggesting two species. To address this uncertainty, a group of researchers sequenced genome-scale molecular data for over 100 specimens, sampling across the range of the western bumble bee, and applied a diversity of species delimitation methods to assess species status. Overall results confirmed the existence of two species: Bombus occidentalis in the south and B. mackayi in the north, with some disagreement between nuclear and mitochondrial molecular markers. These results help improve understanding of bumble bee diversity and will shape on-going efforts to list

Technical Abstract: Accurate species delimitation is a critical first step to identifying the conservation status of species. Developments in molecular methods to delimit species have revealed previously unrecognized cryptic species across the taxonomic spectrum. However, studies vary in the molecular markers selected, the analytical approaches used, and taxon sampling, and this sometimes results in conflicting conclusions. One example of such a conflict is seen in the species delimitation analyses of the western bumble bee, Bombus occidentalis. This species was once an abundant insect pollinator in western North America, but has declined severely since the mid 1990’s and is predicted to continue to diminish under even optimistic future climate scenarios. Complicating this conservation crisis, the species status of B. occidentalis has varied over time, with most recent studies considering there to be one or two species: B. occidentalis and B. mckayi. In 2012 and 2020, two studies used mitochondrial cytochrome oxidase I (COI) barcode sequencing and different automated species delimitation methods to clarify the species status of the complex, but the study results disagreed. The primary author of both studies determined the taxa to represent separate species, trusting the results of one method over another. We expanded on the previous analyses using nuclear (ultraconserved elements) and mitochondrial (COI) markers to infer maximum likelihood and Bayesian phylogenies of the relationship between the taxa within the B. occidentalis complex. Here, we present our results and conclusions from eight species delimitation methods. We discuss the strengths and limitations of each genetic dataset and delimitation method, make recommendations for best practices when using speciation analyses to inform conservation policies and actions, and highlight opportunities for equitable knowledge and technology development for phylogenomics in conservation biology.