Supplementary Material for: Development of Avian External Genitalia: Interspecific Differences and Sexual Differentiation of the Male and Female Phallus

Avian genitalia, particularly in waterfowl, are extremely diverse. Penis morphology varies among species, and penis length and elaboration are associated with the frequency of forced extra-pair copulations, yet the developmental mechanisms responsible for this variation are unknown. In addition, females have a small phallic structure that is homologous to the male phallus, but little is known about when or how sexual differentiation takes place. To determine whether species-specific genital morphologies and sexual differentiation occur during duck embryonic development, we characterized development from the onset of genital tubercle initiation through stages of sexual differentiation in 3 species. Pekin and Laysan ducks have long, thick penises, whereas those of Mandarin ducks are shorter and thinner. Development of the genital tubercle is similar throughout the pre-hatching period across the 3 species, suggesting that differences in penis morphology arise post-hatching. We observed that male and female phallus development is similar at early stages, but the female phallus later regresses. Then, we compared male and female genital development between ducks and chickens, which develop a non-intromittent penis. We found that external genital development in male and female chickens resembles that of female ducks, which raises the possibility that male phallus development became feminized during galliform evolution.

鸟类生殖器，尤其在水禽类群中，呈现出极为丰富的多样性。不同物种的阴茎形态存在显著差异，阴茎长度与复杂程度与强迫性额外配对交配（forced extra-pair copulations）的频率密切相关，但导致这种形态差异的发育机制仍不明晰。此外，雌性个体拥有与雄性阴茎（phallus）同源的小型阴茎状结构（phallic structure），但学界对该结构的性别分化（sexual differentiation）发生时机与具体调控方式仍知之甚少。为探究鸭类胚胎发育过程中是否存在物种特异性生殖器形态及性别分化现象，我们对3个鸭类物种从生殖结节（genital tubercle）起始阶段到性别分化各阶段的发育过程进行了系统表征。北京鸭与莱桑鸭拥有长而粗壮的阴茎，而鸳鸯的阴茎则更短更纤细。3个鸭类物种在孵化前阶段的生殖结节发育模式均保持一致，这表明阴茎形态的差异是在孵化后才逐渐形成的。我们观察到，雌雄个体的阴茎发育在早期阶段并无明显差异，但雌性的阴茎后续会发生退化。随后，我们将鸭类与阴茎不具备插入功能的鸡类的雌雄生殖器发育过程进行了对比。研究发现，鸡类雌雄个体的外生殖器发育模式与雌性鸭类高度相似，这提示在鸡形目（Galliformes）的演化过程中，雄性阴茎的发育可能出现了雌性化现象。