Meeting Abstract
As temperatures decrease, many insects increase the amount of melanin in their cuticle for a variety of purposes; including improved thermoregulation or desiccation resistance. However, melanin is also central to insect immunity, leading to the novel hypothesis that the thermal environment indirectly shapes immune function via direct selection on cuticle color. If true, then then insect immune investment may be significantly constrained in warm environments where melanin investment is expected to be low, but infection risk high. Here, we address this hypothesis in the cricket Allonemobius socius and in the fruit fly Drosophila melanogaster. We found that individuals reared in a “warm” environment (28°C) exhibited lighter cuticles and inferior bacterial defense compared with those in a “cool” environment (22°C; immune assays were conducted at 25°C to minimize treatment metabolic rate differences). Furthermore, we found that crickets from southern (warmer) latitudes exhibited lighter cuticles and inferior immune defense compared with their northern counterparts (all individuals reared in a common environment). These data suggest that seasonally or geographically distinct thermal environments directly shape cuticle color, which indirectly shapes immune function through pleiotropy. Our hypothesis may represent a widespread mechanism governing immunity in numerous systems, considering that most insects operate in variable thermal environments.
