How does ageing affect life history traits and cold tolerance in summer- vs winter-acclimated invasive fruit flies?

All species exhibit functional senescence, a process related to ageing that is influenced by both abiotic and biotic factors. Ageing leads to a decline in function and drives phenotypic senescence, marked by a reduction in phenotypic performance over time. In temperate regions, overwintering species must survive and age for several months often facing challenging conditions with low food availability and chilling injuries. Yet, overwintering species might enter a state of dormancy, which, can extend longevity and enhance stress tolerance. Evaluating the performance of the overwintering survivors is crucial for predicting the population dynamics, especially for significant pests like invasive fruit flies. At the end of winter, the surviving fly populations, which are expected to rebuild new generations, are likely in a dormant state (i.e., reproductive quiescence) and, are likely senescent. However, their performance has not been thoroughly described. This study aims to decipher the effect of ageing on stress tolerance and reproductive capacity in both males and females of summer-acclimated (SP) vs. winter-acclimated (WP) phenotypes of D. suzukii. SP and WP flies were reared under normal and low temperature conditions, respectively, and four age categories were established: very young, young, middle, and old. SP flies, showed the typical marked age-related decline in stress tolerance and fecundity. In contrast, the cold-acclimated and likely dormant WP flies maintained high stress tolerance and exhibited lower but stable reproductive potential, despite ageing for months at low temperatures. Our study highlights the importance of distinguishing between overwintering and summer phenotypes, as ageing differentially affects both.