Society for Experimental Biology Annual Meeting

Insect life cycles are adapted to a seasonal climate by expressing alternative voltinism phenotypes—the number of generations in a year. The problem is to understand how this phenotypic variation along latitudinal gradients is generated through the interactions between environmental factors, like temperature, and the traits of organisms, like development rate and dormancy. However, our current understanding is limited by how thermal responses are characterised, competing theories of thermal adaptation and an incomplete understanding of complex life cycles. Using the widely distributed grasshopper genus Warramaba as a model, we aimed to reconcile theories of thermal adaptation and tested their respective predictions. We hypothesised that the egg stage was a critical life stage for generating latitudinal patterns of voltinism in Warramaba. We described patterns of voltinism and thermal response of egg development rate within and among species of Warramaba along a latitudinal temperature gradient. We found a latitudinal pattern of univoltinism at high latitudes and multivoltinism at low latitudes that corresponded with remarkably strong divergence in egg dormancy patterns and thermal responses of egg development. We argue that the switch in voltinism along the latitudinal gradient was generated by the combined predictions of the evolution of voltinism and of thermal adaptation. We conclude that analyses of latitudinal patterns in thermal responses and corresponding life histories need to consider the evolution of thermal response curves within the context of seasonal temperature cycles rather than based solely on optimality and trade-offs in performance.