Sanger Thomas J, Gredler Marissa L, Cohn Martin J
Department of Molecular Genetics and Microbiology, University of Florida, PO Box 103610, Gainesville, FL 32610, USA.
Department of Biology, University of Florida, PO Box 103610, Gainesville, FL 32610, USA.
Biol Lett. 2015 Oct;11(10). doi: 10.1098/rsbl.2015.0694.
The breadth of anatomical and functional diversity among amniote external genitalia has led to uncertainty about the evolutionary origins of the phallus. In several lineages, including the tuatara, Sphenodon punctatus, adults lack an intromittent phallus, raising the possibility that the amniote ancestor lacked external genitalia and reproduced using cloacal apposition. Accordingly, a phallus may have evolved multiple times in amniotes. However, similarities in development across amniote external genitalia suggest that the phallus may have a single evolutionary origin. To resolve the evolutionary history of amniote genitalia, we performed three-dimensional reconstruction of Victorian era tuatara embryos to look for embryological evidence of external genital initiation. Despite the absence of an intromittent phallus in adult tuataras, our observations show that tuatara embryos develop genital anlagen. This illustrates that there is a conserved developmental stage of external genital development among all amniotes and suggests a single evolutionary origin of amniote external genitalia.
羊膜动物外生殖器在解剖结构和功能上的多样性，导致了阴茎进化起源的不确定性。在包括喙头蜥（Sphenodon punctatus）在内的几个谱系中，成年个体没有用于交配的阴茎，这增加了羊膜动物祖先没有外生殖器且通过泄殖腔贴合进行繁殖的可能性。因此，阴茎可能在羊膜动物中多次进化。然而，羊膜动物外生殖器发育过程中的相似性表明，阴茎可能有单一的进化起源。为了解决羊膜动物生殖器的进化史问题，我们对维多利亚时代的喙头蜥胚胎进行了三维重建，以寻找外生殖器起始的胚胎学证据。尽管成年喙头蜥没有用于交配的阴茎，但我们的观察表明，喙头蜥胚胎会发育出生殖嵴。这说明所有羊膜动物在外生殖器发育过程中存在一个保守的发育阶段，并暗示羊膜动物外生殖器有单一的进化起源。
