Abstract
While epigamic traits likely evolve via sexual selection, the mechanism whereby internal sexual dimorphism arises remains less well understood. Seeking clues as to how the internal sexual dimorphism evolved, we compared the abdominal musculature of 41 Drosophila montium group species, to determine whether any of these species carry a male-specific muscle of Lawrence (MOL). Our quantitative analysis revealed that the size of a sexually dimorphic MOL analog found in 19 montium group species varied widely from species to species, suggesting the gradual evolution of this sexually dimorphic neuromuscular trait. We attempted the ancestral state reconstitution for the presence or absence of the neuromuscular sexual dimorphism in the A5 segment; the neuromuscular sexual dimorphism existed in an old ancestor of the montium group, which was lost in some of the most recent common ancestors of derived lineages, and subsequently some species regained it. This loss-and-gain history was not shared by evolutionary changes in the courtship song pattern, even though both traits were commonly regulated by the master regulator male-determinant protein FruM. It is envisaged that different sets of FruM target genes may serve for shaping the song and MOL characteristics, respectively, and, as a consequence, each phenotypic trait underwent a distinct evolutionary path.
Highlights
While epigamic traits likely evolve via sexual selection, the mechanism whereby internal sexual dimorphism arises remains less well understood
As an attempt to understand how internal sexual dimorphisms evolve, we studied a male-specific muscle called the muscle of Lawrence or MOL4 in the Drosophila montium group[5,6], in which the male courtship display varies widely across s pecies[7,8], suggesting a unique evolutionary history of sexually dimorphic traits
Feret’s diameter was measured for the putative MOL (FA) and for the medial-most muscle running along the midline, which served as a control (FB) within the same hemi-segment for each fly
Summary
While epigamic traits likely evolve via sexual selection, the mechanism whereby internal sexual dimorphism arises remains less well understood. We attempted the ancestral state reconstitution for the presence or absence of the neuromuscular sexual dimorphism in the A5 segment; the neuromuscular sexual dimorphism existed in an old ancestor of the montium group, which was lost in some of the most recent common ancestors of derived lineages, and subsequently some species regained it This loss-and-gain history was not shared by evolutionary changes in the courtship song pattern, even though both traits were commonly regulated by the master regulator male-determinant protein FruM. FruM functions during the pupal stage as a master regulator of the formation of adult neural circuits underlying male mating behavior in D. melanogaster In this species, ~ 2000 FruM-expressed neurons distributed through the entire nervous system from the sensory to the central and motor systems are interconnected, forming a circuit (i.e., the fru circuit) that operates to generate courtship b ehavior[23,24]. We decided to use the MOL metrics as a convenient and reliable proxy of the FruM activity in the nervous system and compared them across species of the montium group to infer the evolutionary history of the fru circuit
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