Abstract
The emergence and diversification of morphological novelties is a major feature of animal evolution.[1][,][2][,][3][,][4][,][5][,][6][,][7][,][8][,][9] However, relatively little is known about the genetic basis of the evolution of novel structures and the mechanisms underlying their diversification. The epandrial posterior lobes of male genitalia are a novelty of particular Drosophila species.[10][,][11][,][12][,][13] The lobes grasp the female ovipositor and insert between her abdominal tergites and, therefore, are important for copulation and species recognition.[10][,][11][,][12][,][14][,][15][,][16][,][17] The posterior lobes likely evolved from co-option of a Hox-regulated gene network from the posterior spiracles[10] and have since diversified in morphology in the D. simulans clade, in particular, over the last 240,000 years, driven by sexual selection.[18][,][19][,][20][,][21] The genetic basis of this diversification is polygenic but, to the best of our knowledge, none of the causative genes have been identified.[22][,][23][,][24][,][25][,][26][,][27][,][28][,][29][,][30] Identifying the genes underlying the diversification of these secondary sexual structures is essential to understanding the evolutionary impact on copulation and species recognition. Here, we show that Sox21b negatively regulates posterior lobe size. This is consistent with expanded Sox21b expression in D. mauritiana, which develops smaller posterior lobes than D. simulans. We tested this by generating reciprocal hemizygotes and confirmed that changes in Sox21b underlie posterior lobe evolution between these species. Furthermore, we found that posterior lobe size differences caused by the species-specific allele of Sox21b significantly affect copulation duration. Taken together, our study reveals the genetic basis for the sexual-selection-driven diversification of a novel morphological structure and its functional impact on copulatory behavior.
