A genomic and morphometric analysis of alpine bumblebees: Ongoing reductions in tongue length but no clear genetic component

Abstract

Over the last six decades, populations of the bumblebees Bombus sylvicola and Bombus balteatus in Colorado have experienced decreases in tongue length, a trait important for plant‐pollinator mutualisms. It has been hypothesized that this observation reflects selection resulting from shifts in floral composition under climate change. Here we used morphometrics and population genomics to determine whether morphological change is ongoing, investigate the genetic basis of morphological variation, and analyse population structure in these populations. We generated a genome assembly of B. balteatus. We then analysed whole‐genome sequencing data and morphometric measurements of 580 samples of both species from seven high‐altitude localities. Out of 281 samples originally identified as B. sylvicola, 67 formed a separate genetic cluster comprising a newly‐discovered cryptic species (“incognitus”). However, an absence of genetic structure within species suggests that gene flow is common between mountains. We found a significant decrease in tongue length between bees collected between 2012–2014 and in 2017, indicating that morphological shifts are ongoing. We did not discover any genetic associations with tongue length, but a SNP related to production of a proteolytic digestive enzyme was implicated in body size variation. We identified evidence of covariance between kinship and both tongue length and body size, which is suggestive of a genetic component of these traits, although it is possible that shared environmental effects between colonies are responsible. Our results provide evidence for ongoing modification of a morphological trait important for pollination and indicate that this trait probably has a complex genetic and environmental basis.

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

“…We were interested in determining whether the greater resolution afforded by WGR data would provide additional evidence for gene flow barriers across the sampled regions, but no major differences emerged compared to prior studies. Low genetic differentiation over large spatial scales, is often observed in bumble bee species in the absence of obvious physical dispersal barriers (Christmas et al, 2022;Heraghty et al, 2022;J. B. Koch, Looney, Sheppard, & Strange, 2017;Lozier et al, 2011).…”

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

“…We were interested in determining whether the greater resolution afforded by WGR data would provide additional evidence for gene flow barriers across the sampled regions, but no major differences emerged compared to prior studies. Low genetic differentiation over large spatial scales is often observed in bumble bee species in the absence of obvious physical dispersal barriers (Christmas et al, 2022; Heraghty et al, 2022; Koch et al, 2017; Lozier et al, 2011). Although long‐distance dispersal of individual reproductives (gynes or drones) may be rare (Williams et al, 2022), stepping stone dispersal through suitable habitat is likely in bumble bees (Williams et al, 2018; Williams et al, 2022) and would contribute to weak genetic structure when species inhabit continuous geographic ranges.…”

Genetic variation and microbiota in bumble bees cross-infected by different strains of C. bombi

“…So far, there seems to be no GWAS study that analyses long-term infection intensity as we do here. Rather, several GWAS studies in bumble bees address morphological traits, such as colouration [ 88 , 89 ], or tongue length [ 90 ]. A direct comparison with other studies is therefore not yet feasible.…”