Development of the cloaca, hemipenes, and hemiclitores in the green anole, Anolis carolinensis.

In most amniotes, the intromittent organ is a single phallus; however, squamates (lizards, snakes, and amphisbaenians) have paired hemiphalluses. All amniotes studied to date initiate external genital development with the formation of paired genital swellings. In mammals, archosaurs, and turtles, these swellings merge to form a single genital tubercle, the precursor of the penis and clitoris; however, in squamates, the paired genital buds remain separate, giving rise to the hemiphalluses (hemipenes in males and hemiclitores in females). Although the molecular genetics and sexual differentiation of the genital tubercle have been investigated in mammals and birds, little is known about hemiphallus development. Here we describe development of the cloaca and hemiphallus in the green anole, Anolis carolinensis. Each hemiphallus originates as a protuberance that emerges at the ventral base of the hindlimb bud. Development of the hemipenes resembles penis development; however, differences exist in their tissue composition, morphogenesis, and gene expression patterns. These findings reveal aspects of phallus development that appear to be evolutionarily labile, both within squamates and more broadly among reptiles, and identify features that are conserved across amniotes. Our results, together with parallel studies in other reptilian taxa, suggest potential mechanisms for the diversification of external genital form.