Temperature niche shift observed in a Lepidoptera population under allochronic divergence

A process of adaptive divergence for tolerance to high temperatures was identified by using a rare model-system, consisting of two sympatric populations of a Lepidoptera (Thaumetopoea pityocampa) with different life-cycle timings, a "mutant" population with summer larval development, Leiria SP, and the founder natural population, having winter larval development, Leiria WP. A third, allopatric population (Bordeaux WP) was also studied. First and second instar larvae were experimentally exposed to daily-cycles of heat treatment reaching maximum values of 36, 38, 40 and 42ºC; control groups placed at 25ºC. A lethal temperature effect was only significant at 42ºC, for Leiria SP, whereas all temperatures tested had a significant negative effect upon Leiria WP, thus indicating an upper threshold of survival c.a. 6ºC above that of the WP. Cox regression model, for pooled heat treatments, predicted mortality hazard to increase for Leiria WP (+108%) and Bordeaux WP (+78%) in contrast with Leiria SP; to increase by 24% for each additional ºC; and to decrease by 53% from first to second instar larvae. High variability among individuals was observed, a population characteristic that may favour selection and consequent adaptation. Present findings provide an example of ecological differentiation, following a process of allochronic divergence. Results further contribute to a better understanding of the implications of climate change for ecological genetics.