Anticipating change: the impact of simulated seasonal heterogeneity on heat tolerances along a latitudinal cline.

An understanding of thermal limits and variation across geographic regions is central to predicting how any population may respond to scenarios of global change. Latitudinal clines, in particular, have been used in demonstrating that populations can be locally adapted to their own thermal environment and, as a result, not all populations will be equally impacted by an increase in temperature. But how robust are these signals of thermal adaptation to the other ecological challenges that animals commonly face in the wild? Seasonal changes in population density, food availability, or photoperiod are common ecological challenges that could disrupt patterns of thermal tolerance along a cline if each population differentially used these signals to anticipate future temperatures and adjust their thermal tolerances accordingly. In this study, we aimed to test the robustness of a cline in thermal tolerance to simulated signals of seasonal heterogeneity. Experimental animals were derived from clones of the Australian water flea, Daphnia carinata, sampled from nine populations along a latitudinal transect in eastern Australia. We then factorially combined summer (18h light, 6h dark) and winter (6h light, 18h dark) photoperiods with high (5 million algal cells individual-1 day-1) and low (1 million algal cells individual-1 day-1) food availabilities, before performing static heat shock assays and recording knockdown times as a measure of thermal tolerance. In general, higher food availably led to an increase in thermal tolerances, with the magnitude of increase varying by clone. In contrast, summer photoperiods led to rank order changes in thermal tolerances, with heat resistance increasing for some clones, and other decreasing for others. Heat resistance, however, still declined along the latitudinal cline, irrespective of the manipulation of seasonal signals, with northern clones always showing greater thermal resistance, and that this was most likely driven by adaptation to winter thermal conditions. While photoperiod and food availability can clearly shape thermal tolerances for specific clones or populations, they are unlikely to be used to anticipate future temperatures, and thus observed clines in heat resistance will remained robust to these forms of seasonal heterogeneity.