Early photoperiodic cues modulate capacity for adult cold tolerance in the yellow fever mosquito <i>Aedes aegypti</i>

Climate change is driving the rapid range expansion of Aedes aegypti into temperate regions, presenting novel seasonal cues that can affect seasonal plasticity. Seasonal plasticity in Ae. aegypti is well studied in eggs but remains understudied in adults. Here, we investigated the effects of photoperiod and temperature on thermal plasticity, measured by the ability to cold acclimate, and other dormancy-related traits in female Ae. aegypti adults. Cold acclimation was done under a short-day (SD) and long-day (LD) photoperiod. LD photoperiod was introduced at three life stages: parental generation, egg state and adult acclimation, to determine if timing of LD photoperiod introduction affected thermal plasticity. LD photoperiod had a significant influence on both cold tolerance metrics assessed, chill coma onset temperature and survival after a cold stress. Introduction of LD photoperiod reduced thermal plasticity, with the magnitude of change in plasticity depending on when LD was introduced. At warm temperatures, LD photoperiod modified the rhythmic expression profile of timeless, one of the circadian clock genes analysed in this study, but not the other, period. In contrast, cold acclimation abolished the cyclical expression of both clock genes. Cold acclimation under both photoperiods suppressed blood-feeding behaviour, which resumed upon warming, but cold acclimation did not change relative transcript abundance of dormancy-implicated genes. This study presents the first evidence of an adult photosensitive seasonal phenotype in Ae. aegypti, with improved cold tolerance and reproductive quiescence. These findings contribute to a deeper understanding of how environmental cues may facilitate the continued expansion of this important disease vector into temperate environments.