Evolutionary genetics of mimetic coloration in bumble bees

       The ~250 species of the cold-adapted bumble bees are exceptionally color diverse. One of the main factors driving such variation is Müllerian mimicry – taxonomically diverse species in the same geographic region have converged on similar color patterns as selection favors shared advertisement of their toxicity (their sting) to predators. Bumble bee color diversity provides extensive replicates for understanding genomic targets of evolution (evolutionary genetics) (Hines and Rahman, 2019), for understanding how ecological factors influence the formation of mimicry complexes over time (evolutionary ecology), and for understanding how developmental genes such as segmentation genes control phenotypic variation (evo-devo).

Through an NSF funded project, we are studying mimetic bumble bee color diversity to understand the genetic regulation of adaptive phenotypic variation. We determined that a late developmental homeotic shift in Hox genes induced mimetic color variation in one species (B. melanopygus) (Tian et al., 2019) and are studying whether other species that attain these same patterns do so using similar mechanisms using comparative genomic, transcriptomic and developmental genetic approaches.

Photo credits: J. Cnaani (upper right), Sam Droege, USGS Survey (upper middle)

Bee pathogens and pollinator conservation

Bumble bee diversity has changed dramatically in the last 30 years due to a combination of habitat loss, pesticides, pathogens, and climate change. We are studying several of the major factors threatening these bees. We are currently studying:

• Habitat needs and resilience to anthropogenic change across bumble bee species and the physiological factors driving these preferences.
• The environmental factors driving pathogen loads in wild bees. Using landscape modeling, we are examining the landscape factors, such as habitat, climate, and disturbance, that most impact bumble bee pathogen loads, and are examining how pathogens fluctuate across time and by bee species.

Mechanisms of gall induction by gall wasps

Gall wasps oviposit into developing plant tissues where larvae induce the formation of elaborate and predictable gall morphologies. The ways in which both adults and developing larvae induce this developmental change remain largely unknown. With an NSF funded grant, we are combining comparative phylogenomics, comparative gland morphology, and transcriptomic and metabolomic approaches to explore potential mechanisms of gall induction. These analyses are comparing across species, sexual/asexual generations, and larval/adult generations of gall wasps.

Just a sampling of some of the incredible diversity of gall morphologies induced by gall wasp species. From Left: Wooly catkin gall wasp (Callyrhytis quercusoperator) including both asexual and sexual galling generations, Hedgehog gall (Acraspis erinacei), Honeycomb leaf gall (Callyrhytis favosa), Clustered midrib gall (Andricus dimorphus), Wooly oak gall (Callyrhytis lanata). Images by collaborator Andy Deans, available on iNaturalist.

Role of nectar metabolites in pollinator visitation bias and health

We are examining the role of plant defense compounds in nectar, focusing on milkweed toxins (cardenolides), in bumble bee visitation bias. (Villalona et al., 2020)

The evolution of social parasitism in bumble bees

Most bumble bees have a life history that starts with nest initiation by a lone queen who then produces several generations of workers that ultimately will rear some of their sisters to become new reproductive queens.  Some bumble bees, those in the subgenus Psithyrus, use a different strategy. They invade the young nests of other species, take over – either through killing or coinhabiting with the resident queen – and coerce the workers of this young colony to rear their reproductive offspring instead. Recent research on European species has revealed some of the strategies by which they manage to evade their hosts. We are seeking to understanding this strategy better by studying strategies of chemical and behavioral control in North American Psithyrus, including mechanisms of host evasion, differences in reproductive success by hosts, mechanisms inducing larval feeding, and potential for host imprinting.