Phenotypic plasticity underlies seasonal and latitudinal variation in thermal tolerance in a native bee.

Climate change threatens biodiversity and ecosystem services around the globe. Despite the importance of native bees as pollinators, there is evidence of global declines, and we know very little about how climate shapes their distributions now and into the future. In the current study, we combined large-scale seasonal field sampling and experimental acclimation to examine whether populations of an Australian bee, Exoneura robusta, vary in their capacity to adapt to different climates. Collecting female bees across a latitudinal cline and examining heat and cold tolerance, we found populations did not vary in their heat tolerance along a latitudinal gradient. In contrast, bees from higher latitudes tended to be more cold-tolerant than bees from lower latitudes, but the relationship between cold tolerance and latitude differed between summer and spring (post-winter). Such seasonal variation suggests that phenotypic plasticity plays a role in shaping cold tolerance, as bees are likely to belong to the same generation from summer to spring. To untangle the roles of plasticity and genetic variation in shaping variation in thermal tolerance across seasons, we acclimated adult females from three populations spanning the species' distributional range to either 21 or 26°C in glasshouses (approximating summer and spring/autumn temperatures experienced throughout their range). We then estimated heat and cold tolerance. Contrasting acclimation responses observed in the glasshouses to those observed in the field point to phenotypic plasticity in cold tolerance rather than genetic variation underpinning population variation. In contrast, heat tolerance varied little in the field and in our glasshouse experiments. These results suggest bees may have little capacity to increase their heat tolerance, which is high at ~47°C, via genetic or plastic responses as climate changes.