Environmental and developmental conditions, but not thermosensation, alter cold hardening ability of Drosophila melanogaster

In temperate regions, an organism’s ability to rapidly acclimate to varying seasonal temperatures, particularly the subfreezing cold of winter, is essential for its survival. Although less famous than hibernating bears and migrating birds, the fruit fly, Drosophila melanogaster, must also overcome winter’s challenges. One of the mechanisms implicated in D. melanogaster’s overwintering is cold hardening, a process in which exposure to cool, but nonlethal, temperatures significantly increases the organism’s ability to later survive freezing temperatures. We measured the cold hardening ability of D. melanogaster populations kept in outdoor cages over the summer, fall, and winter. We bred outdoor-caught flies for two generations in the lab and matched each outdoor generation to an indoor control population of similar genetic background. To elucidate potential environmental, parental, and genetic effects over the seasons, we measured the cold hardening ability of indoor and outdoor-caught flies, their offspring, and their offspring’s progeny. We also explored the effects of age, developmental density, and developmental nutrition on cold hardening. Finally, we tested cold hardening ability of flies with physically and genetically altered thermosensory capacities. Our experiments indicate that while thermosensation is not required for the cold hardening process, environmental factors and developmental conditions do influence cold hardening. We observed an increase in cold hardening ability of outdoor-caught flies that correlated with onset of winter conditions, but our data did not show evidence of seasonally varying parental or genetic effects on cold hardening. We conclude that environmental conditions likely play an influential role in D. melanogaster’s overwintering strategy.