Thermoregulatory strategy may shape immune investment in Drosophila melanogaster

As temperatures change, insects alter the amount of melanin in their cuticle to improve thermoregulation. However, melanin is also central to insect immunity, suggesting that thermoregulatory strategy may indirectly impact immune defense by altering the abundance of melanin pathway components (a hypothesis we refer to as thermoregulatory-dependent immune investment). This may be the case in the cricket Allonemobius socius, where warm environments (both seasonal and geographical) produced crickets with lighter cuticles and increased pathogen susceptibility. Unfortunately, the potential for thermoregulatory strategy to influence insect immunity has not been widely explored. Here we address the relationships between temperature, thermoregulatory strategy and immunity in the fruit fly Drosophila melanogaster. To this end, flies from two separate Canadian populations were reared in either a summer- or autumn-like environment. Shortly after adult eclosion, flies were moved to a common environment where their cuticle color and susceptibility to a bacterial pathogen (Pseudomonas aeruginosa) were measured. As with A. socius, individuals from summer-like environments exhibited lighter cuticles and increased pathogen susceptibility, suggesting that the thermoregulatory-immunity relationship is evolutionarily conserved across the hemimetabolous and holometabolous clades. If global temperatures continue to rise as expected, then thermoregulation might play an important role in host infection and mortality rates in systems that provide critical ecosystem services (e.g. pollination), or influence the prevalence of insect-vectored disease (e.g. malaria).