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Abstract Biogeographic clines in morphology along environmental gradients can illuminate forces influencing trait evolution within and between species. Latitude has long been studied as a driver of morphological clines, with a focus on body size and temperature. However, counteracting environmental pressures may impose constraints on body size. In montane landscapes, declines in air density with elevation can negatively impact flight performance in volant species, which may contribute to selection for reduced body mass despite declining temperatures. We examine morphology in two bumble bee (Hymenoptera: Apidae: Bombus Latreille) species, Bombus vancouverensis Cresson and Bombus vosnesenskii Radoszkowski, across mountainous regions of California, Oregon, and Washington, United States. We incorporate population genomic data to investigate the relationship between genomic ancestry and morphological divergence. We find that B. vancouverensis, which tends to be more specialized for high elevations, exhibits stronger spatial-environmental variation, being smaller in the southern and higher elevation parts of its range and having reduced wing loading (mass relative to wing area) at high elevations. Bombus vosnesenskii, which is more of an elevational generalist, has substantial trait variation, but spatial-environmental correlations are weak. Population structure is stronger in the smaller B. vancouverensis, and we find a significant association between elevation and wing loading after accounting for genetic structure, suggesting the possibility of local adaptation for this flight performance trait. Our findings suggest that some conflicting results for body size trends may stem from distinct environmental pressures that impact different aspects of bumble bee ecology, and that different species show different morphological clines in the same region. 

Bumble bees are ecologically and economically important insect pollinators. Three abundant and widespread species in western North America, Bombus bifarius, Bombus vancouverensis, and Bombus vosnesenskii, have been the focus of substantial research relating to diverse aspects of bumble bee ecology and evolutionary biology. We present de novo genome assemblies for each of the three species using hybrid assembly of Illumina and Oxford Nanopore Technologies sequences. All three assemblies are of high quality with large N50s (> 2.2 Mb), BUSCO scores indicating > 98% complete genes, and annotations producing 13,325 - 13,687 genes, comparing favorably with other bee genomes. Analysis of synteny against the most complete bumble bee genome, Bombus terrestris, reveals a high degree of collinearity. These genomes should provide a valuable resource for addressing questions relating to functional genomics and evolutionary biology in these species. 

Understanding evolutionary responses to variation in temperature and precipitation across species ranges is of fundamental interest given ongoing climate change. The importance of temperature and precipitation for multiple aspects of bumble bee (Bombus) biology, combined with large geographic ranges that expose populations to diverse environmental pressures, make these insects well‐suited for studying local adaptation. Here, we analyzed genome‐wide sequence data from two widespread bumble bees,Bombus vosnesenskiiandBombus vancouverensis, using multiple environmental association analysis methods to investigate climate adaptation across latitude and altitude. The strongest signatures of selection were observed inB. vancouverensis, but despite unique responses between species for most loci, we detected several shared responses. Genes relating to neural and neuromuscular function and ion transport were especially evident with respect to temperature variables, while genes relating to cuticle formation, tracheal and respiratory system development, and homeostasis were associated with precipitation variables. Our data thus suggest that adaptive responses for tolerating abiotic variation are likely to be complex, but that several parallels among species can emerge even for these complex traits and landscapes. Results provide the framework for future work into mechanisms of thermal and desiccation tolerance in bumble bees and a set of genomic targets that might be monitored for future conservation efforts.

 

Identifying drivers of dispersal limitation and genetic differentiation is a key goal in biogeography. We examine patterns of population connectivity and genetic diversity using restriction site‐associatedDNAsequencing (RADseq) in two bumble bee species,Bombus vosnesenskiiandBombus bifarius,across latitude and altitude in mountain ranges from California, Oregon and Washington, U.S.A.Bombus vosnesenskii, which occurs across a broader elevational range at most latitudes, exhibits little population structure whileB. bifarius, which occupies a relatively narrow higher elevation niche across most latitudes, exhibits much stronger population differentiation, although gene flow in both species is best explained by isolation with environmental niche resistance. A relationship between elevational habitat breadth and genetic diversity is also apparent, withB. vosnesenskiiexhibiting relatively consistent levels of genetic diversity across its range, whileB. bifariushas reduced genetic diversity at low latitudes, where it is restricted to high‐elevation habitat. The results of this study highlight the importance of the intersect between elevational range and habitat suitability in influencing population connectivity and suggest that future climate warming will have a fragmenting effect even on populations that are presently well connected, as they track their thermal niches upward in montane systems.