Development of the Cloaca, Hemipenes, and Hemiclitores in the Green Anole, <b><i>Anolis carolinensis</i></b>

Abstract

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.

Evolution of External Genitalia: Insights from Reptilian Development

“…Most of the work regarding hormonal regulation of turtle development has examined development of the gonads, but androgen likely has a role in development of the phenotypically male phallus, as androgen exposure in turtles causes accelerated growth and hypertrophy of the phallus [Wibbels and Crews, 1992;Crews et al, 1995;Crespo et al, 2013]. The role of estrogen in regu- [Larkins and Cohn , 2014], Alligator mississippiensis [Gredler et al, 2014b], Gallus gallus [Herrera et al, 2013], Anolis carolinensis [Gredler et al, 2014a], Python regius [Leal and Cohn, 2014], and Mus musculus [Perriton et al, 2002] lating development of the phallus is not clear, although estrogen is required for development of the ovary, and estrogen production may be upregulated in response to female-producing temperatures [Wibbels et al, 1991;Bergeron et al, 1999]. Examination of the phallus in androgen-or estrogen-treated turtles will be important to determine the role of hormones in turtle external genital development.…”

“…The most comprehensive studies of early development of the hemipenes in squamates were done by Pieau [1970, 1985] in the European green lizard (Lacerta viridis) , slow-worm serpentiform lizard (Anguis fragilis) , and the dice snake (Natrix tessellata) . Little work has been done on hemipenes development since the publication of these classical descriptions, although 2 recent studies have characterized the embryology and the cellular and molecular development of hemipenes in the green anole, Anolis carolinensis [Gredler et al, 2014a] and the ball python, Python regius [Leal and Cohn, 2014].…”

“…Morphogenesis of the hemipenes and hemiclitores is generally conserved across different lineages of lizards and snakes but differs from development of the single amniote genital tubercle in a number of ways [Raynaud and Pieau, 1985;Gredler et al, 2014a;Leal and Cohn, 2014]. Major differences between squamate and nonsquamate amniote external genital development include the paired nature of the hemiphalluses, their embryonic origin relative to the hindlimb buds and cloacal membrane, and the molecular mechanisms underlying formation of the sulcus spermaticus [Raynaud and Pieau, 1985;Gredler et al, 2014a;Leal and Cohn, 2014].…”

“…Major differences between squamate and nonsquamate amniote external genital development include the paired nature of the hemiphalluses, their embryonic origin relative to the hindlimb buds and cloacal membrane, and the molecular mechanisms underlying formation of the sulcus spermaticus [Raynaud and Pieau, 1985;Gredler et al, 2014a;Leal and Cohn, 2014].…”

Resurrecting embryos of the tuatara, Sphenodon punctatus , to resolve vertebrate phallus evolution

“…11: 20150694 that expand towards the midline, where they merge to form the genital tubercle, the precursor to the phallus [12,14,15,19]. The squamate hemiphallus also forms from paired swellings, but rather than merging at the midline, they remain separate to form two discrete phalluses [9,16,17]. In reptiles (including birds), additional cloacal swellings emerge and fuse to form cloacal lips, although there are subtle differences in their number and position [10].…”