Secondary Sexual Trait Melanization in “Black” Scavenger Flies: Nutritional Plasticity and Its Evolution

AbstractThe black scavenger fly Sepsis thoracica exhibits polyphenic development resulting in alternate small black and large amber male morphs. Although the behavior, ecology, and physiology of both morphs are being scrutinized, the evolutionary origins of the nutritional polyphenism remain poorly understood. I here use a comparative approach to study variation in the degree of melanization of the forefemur-a secondary sexual trait. Melanization showed nutritional plasticity in all species, and character mapping suggests polyphenic development to represent the ancestral character state that was lost repeatedly. That is, interspecific variation among the studied species is mainly caused by the loss and not the gain of polyphenic development. Coevolution between male melanization and mating system differences further implicates sexual selection in the evolution of male melanization. These findings highlight the usefulness of comparative and natural history data in shedding new light on the evolution of phenotypic variation.