Landscape and climate-associated selection in the native and widespread bumblebee, <em>Bombus terrestris</em>

Anthropogenic land-use and climate change pose novel selection pressures on bees, yet their evolutionary responses in terms of morphological or physiological adaptations remain unclear. While adaptive responses are expected, these may be constrained by gene flow when changes in selection pressures are spatially heterogeneous. The buff-tailed bumblebee (Bombus terrestris) is a widespread species that copes well with anthropogenic land-use and climate change, suggesting high adaptive capacity or phenotypic plasticity. Here, we genotyped populations of native B. terrestris in south and central Sweden using RADseq to investigate genetic structure and local adaptation across a paired design of agricultural landscapes with high and low land-use complexity along a geographic climate gradient. We expected to find genetic structure reflective of regional barriers to gene flow, and molecular evidence for local adaptation to differing landscape and climate conditions. We found genetic structure separating southern Sweden from more northern regions, with a negative Tajima’s D indicating a potential population expansion, likely northwards and inland into forested areas, consistent with observational data indicating a range shift. We found weak but significant evidence for local adaptation to climate and land use, specifically to agricultural land cover, including genes under putative selection linked to insecticide resistance. Signatures of selection were also identified in relation to latitude, temperature, and urban land cover, with other candidate SNPs associated with olfaction and immune response. Our results suggest that B. terrestris successfully responded to anthropogenic land-use and climate changes, likely due to its generalist traits, enabling phenotypic adaptation to changing environments. Methods The study design was based on 19 study sites, organized into nine matched pairs based on landscape complexity (n=9), in addition to one unmatched study site. Within each pair, landscapes were characterized by either low or high availability of open semi-natural habitat (simple versus complex landscapes, respectively), and study site pairs were arranged along a temperature gradient spanning southern and central Sweden. Study site pairs were matched to represent similar temperatures across the temperature differences of the geographical gradient (from the southernmost to northernmost pairs). The design crossed three geographical regions in Sweden: Skåne (region 1), Västra Götaland (region 2) and Mälardalen (region 3). We collected B. terrestris workers during two time points at each study site in the summer of 2019 (once in June and once in July). Bombus terrestris workers were sexed both in the field and subsequently confirmed in the lab by counting the number of segments on the flagellum of the antennae under a dissecting microscope (males 11 segments, females 10 segments after the pedicel). Only female worker bees were retained in the study.