Whole genome analyses reveal weak signatures of population structure and environmentally associated local adaptation in an important North American pollinator, the bumble bee Bombus vosnesenskii

Abstract

Studies of species that experience environmental heterogeneity across their distributions have become an important tool for understanding mechanisms of adaptation and predicting responses to climate change. We examine population structure, demographic history and environmentally associated genomic variation in Bombus vosnesenskii, a common bumble bee in the western USA, using whole genome resequencing of populations distributed across a broad range of latitudes and elevations. We find that B. vosnesenskii exhibits minimal population structure and weak isolation by distance, confirming results from previous studies using other molecular marker types. Similarly, demographic analyses with Sequentially Markovian Coalescent models suggest that minimal population structure may have persisted since the last interglacial period, with genomes from different parts of the species range showing similar historical effective population size trajectories and relatively small fluctuations through time. Redundancy analysis revealed a small amount of genomic variation explained by bioclimatic variables. Environmental association analysis with latent factor mixed modelling (LFMM2) identified few outlier loci that were sparsely distributed throughout the genome and although a few putative signatures of selective sweeps were identified, none encompassed particularly large numbers of loci. Some outlier loci were in genes with known regulatory relationships, suggesting the possibility of weak selection, although compared with other species examined with similar approaches, evidence for extensive local adaptation signatures in the genome was relatively weak. Overall, results indicate B. vosnesenskii is an example of a generalist with a high degree of flexibility in its environmental requirements that may ultimately benefit the species under periods of climate change.

Genomic variation in montane bumblebees in Scandinavia: High levels of intraspecific diversity despite population vulnerability

“…This could reflect a greater degree of habitat fragmentation, whereby subpopulations of this species have become geographically isolated at high elevation mountain peaks. By contrast, the common North American bumblebee B. vosnesenskii shows low population structure and no evidence of decline (Heraghty et al., 2023), similar to our observations in B. pascuorum .…”

“…In general, studies in both Europe and North America have found that relatively abundant bumblebee species such as B. terrestris do not exhibit population structure over thousands of kilometres (Colgan et al., 2022; Ghisbain et al., 2020; Heraghty et al., 2023; Lozier et al., 2011), whereas very rare and declining species (e.g. B. muscorum ) may show structure on a scale of only 10 km (Darvill et al., 2006).…”