Meeting Abstract
Amniote genitalia exhibit wide diversity in adult anatomical form with species possessing one phallus or two, an open sulcus or a closed urethra, and many additional differences in internal and external anatomy. Some amniote lineages, including the tuatara, Sphenodon punctuates, lack external genitalia altogether and mate using cloacal apposition rather than through penile copulation. It remains unknown whether external genitalia evolved only once at the origin of Amniota or independently in squamates, mammals, and archosaurs. We have surveyed genital development in six amniote species and found many similarities in early genital morphogenesis among species with distinct adult anatomies. This suggests a single evolutionary origin to the amniote phallus followed by later lineage-specific modification. However, the lack of a phallus in the tuatara remains difficult to reconcile because of its phylogenetic position between lineages possessing one midline penis and paired hemipenes. To further resolve the evolutionary history of amniote genitalia we performed three-dimensional reconstruction of Sphenodon embryos. These embryos were collected in 1898 and histologically prepared as serial sections for the Harvard Embryological Collection before being misplaced for the majority of the 20th century. Despite the lack of a phallus in the adult, Sphenodon embryos possess similar genital and cloacal swellings to other amniote species during embryonic stages. Therefore, it appears that the early stages of genital development are conserved across the whole of Amniota and lend greater support to the hypothesis of a single evolutionary origin of amniote external genitalia.
