Host-microbe symbioses provide a unique opportunity to study the origins and stability of species interactions. Due to their distinct physiology, macrobes and microbes usually have environmental tolerances that do not overlap completely; when each partner is reliant on the other, the result is that overall niche breadth is constrained, and the system as a whole may be more vulnerable to disturbance. Additionally, symbionts often comprise multiple strains, and strain-level variability in responses to stressors may cause analogous variation among their hosts. Using bumble bees (Bombus sp.), I asked whether and how symbionts influence host tolerance of heat stress, a critical environmental control on macrobial physiology and species distributions. Bumble bees rely on simple, yet specialized and coevolved gut bacterial communities for nutrition and pathogen protection. Many species are declining, due in part to a combination of rising temperatures and disease. Furthermore, both bumble bees and their symbionts can be reared or cultured in the laboratory, separately from their partner, enabling controlled experimental tests of host-symbiont interactions. To examine these interactions under heat stress, I used a combination of culture-based assays and in vivo experiments with gnotobiotic bees.
Results/Conclusions
Using in vitro growth assays, I found that strains of Snodgrassella alvi and Gilliamella apicola, core members of the bee gut microbiome, show wide variation in their thermal tolerance. Given that strain diversity is widespread both within and between bee species, symbionts could thus influence host range limits and responses to environmental change. S. alvi and G. apicola responses to temperature also appear to correspond to the climatic niche of their host, suggesting local adaptation. Since the bee nest is thermoregulated and fairly constrained across hosts, this variation may arise from selection acting on foraging bees. An investigation of the potential genomic basis of strain-level variation in thermotolerance will also be presented.
Using reared bumble bees, I found that an apparently harmless exposure to 42°C causes accelerated mortality when bees are subsequently infected with the opportunistic bacterial pathogen Serratia. In vitro, this temperature drastically reduces population growth of most S. alvi and G. apicola strains, and prior work has shown that gut microbiota protect bees from Serratia and other pathogens. Follow-up experiments examining gut microbiota under heat stress and their subsequent interactions with Serratia are in progress. These findings illustrate how dependence on microbial symbionts, which often have highly distinct environmental niches as compared those of animals, may constrain host responses to global change.