Phenotypic plasticity in temperature stress resistance is triggered by photoperiod in a fly

Temperature variation poses a substantial challenge for individual survival and reproductive success, warranting effective means to counter negative effects of temperature extremes. Phenotypic plasticity offers a particularly powerful mechanism to cope with short-time temperature fluctuations. While temperature-induced plasticity in thermal tolerance has been widely explored, effects of other environmental factors have received much less attention. Using a full-factorial design we here show that variation in temperature stress resistance can be triggered by photoperiod (and temperature) in the fly Protophormia terraenovae, with shorter day lengths inducing more cold- and longer day lengths more heat-tolerant phenotypes. Such plastic changes were not related to different developmental pathways (reproductive activity or reproductive diapause), and can be induced during development but also in the adult stage (at least for cold tolerance). We suggest that short-term, photoperiod-mediated changes in insect thermal tolerance represent a mechanism of adaptive seasonal plasticity. Photoperiod further affected development time and body size, the significance of which is currently unclear.