Data from: Distinct copulation positions in Drosophila pachea males with symmetric or asymmetric external genitalia

Left-right asymmetric genitalia have appeared multiple times independently in insects and have been associated with changes in mating positions. However, there is little experimental data on how the evolution of genital asymmetries may have affected the evolution of mating positions or vice versa. As opposed to its closely-related species, Drosophila pachea has a conspicuous asymmetry in its male genitalia external lobes, with the left lobe being 1.49 ± 0.08 (SD) times longer and thinner than the right lobe. In a laboratory stock, we found that 20% of the males possess fully symmetric lobes. To better understand how asymmetric genitalia may affect mating, we compared D. pachea copulation behaviour between these mutant males and wild-type males. We found that D. pachea wild-type males adopt a one-sided mating posture with the male always one-sided 8.55° ± 1.79° (SD) towards the female’s right side. Within 45-min recordings, all wild-type males did mate whereas 39% of symmetric mutants failed to form a stable mating complex and did not mate. In successful copulations, symmetric mutants also adopted a right-sided mating posture but the angle between male and female bodies was significantly more variable compared to wild-type males. Our results suggest that lobe size asymmetry is required for the formation of a stable mating complex and for the positioning of the male according to a precise angle on the female. However, lobe size asymmetry is not required for D. pachea right-sided mating posture.

左右不对称生殖器在昆虫类群中多次独立演化，并与交配姿势的改变存在关联。然而，目前关于生殖器不对称的演化如何影响交配姿势的演化，反之亦然的实验数据仍较为匮乏。与其近缘物种不同，帕氏果蝇（Drosophila pachea）的雄性生殖器外叶存在显著不对称性：其左叶较右叶长1.49±0.08（标准差，SD）倍，且更为纤细。在实验室种群中，我们发现有20%的雄性个体其外叶完全对称。为深入探究不对称生殖器如何影响交配行为，我们比较了帕氏果蝇对称突变体雄性与野生型雄性的交配行为。实验结果显示，帕氏果蝇野生型雄性会采取单侧交配姿势：其身体始终偏向雌性右侧，偏移角度为8.55°±1.79°（标准差，SD）。在45分钟的记录时长内，所有野生型雄性均成功交配，而39%的对称突变体无法形成稳定的交配复合体，未能完成交配。在成功完成交配的个体中，对称突变体同样采取右侧偏向的交配姿势，但雌雄个体间的身体夹角相较于野生型雄性存在显著更高的变异性。本研究结果表明，外叶的大小不对称性对于稳定交配复合体的形成以及雄性在雌性身体上的精准角度定位是必需的，但对于帕氏果蝇的右侧偏向交配姿势并非必需。