Exploring the evolutionary history of a novel trait in Sepsidae

The family Sepsidae shows a complex history of gain, loss, and regain in a novel abdominal appendage used by males during courtship and mating.  The appendage is sexually dimorphic and develops during pupation from the 4^th male larval histoblast nests into a modified sternite, bristles, joint, and associated musculature.  In other Dipterans, histoblast nests lack three-dimensional organization in the larval stage, and proliferate during metamorphosis to form the adult epidermis.  Production of this complex morphological trait demonstrates that such organization is unnecessary in some circumstances.  Our objective was to characterize the histoblast nests and identify patterns in nest morphology across the evolutionary history of sepsid flies.  We used fluorescent confocal microscopy to determine nest size and sexual dimorphism in 10 genera and 16 species across Sepsidae that represent gain, loss, regain, and one outgroup for the evolution of an abdominal appendage.  Species with primary gain show a sexually dimorphic nest morphology which returns to ancestral state in species with secondary loss.  Species that recover the appendage show an increase in nest size in both sexes but appendage development still occurs only in males.