Development of Hemipenes in the Ball Python Snake <b><i>Python regius</i></b>

Abstract

Within amniotes, external copulatory organs have undergone extensive morphological diversification. One of the most extreme examples is squamate (lizards and snakes) hemipenes, which are paired copulatory organs that extend from the lateral margins of the cloaca. Here, we describe the development of hemipenes in a basal snake, the ball python (Python regius). Snake hemipenes arise as a pair of lateral swellings on either side of the caudal part of the cloaca, and these paired outgrowths persist to form the left and right hemipenes. In non-squamate amniotes, external genitalia form from paired swellings that arise on the anterior side of the cloaca, which then fuse medially to form a single genital tubercle, the anlagen of the penis or clitoris. Whereas in non-squamate amniotes, Sonic hedgehog (Shh)-expressing cells of the cloacal endoderm form the urethral or sulcus epithelium and are required for phallus outgrowth, the hemipenes of squamates lack an endodermal contribution, and the sulcus does not express Shh. Thus, snake hemipenes differ from the genital tubercles of non-squamate amniotes both in their embryonic origins and in at least part of patterning mechanisms, which raises the possibility that hemipenes may not be direct homologs of the unpaired amniote penis. Nonetheless, we find that some developmental genes show similar expression patterns in snake hemipenes buds and non-squamate genital tubercles, suggesting that homologous developmental mechanisms are involved in aspects of external genital development across amniotes, even when these structures may have different developmental origins and may have arisen independently during evolution.

Evolution of External Genitalia: Insights from Reptilian Development

“…Historically, it has been unclear whether the ancestral condition for amniotes is a single midline phallus, a paired phallus, or whether the last common ancestor lacked external genitalia all together. Tree reproduced from Leal and Cohn [2014].…”

“…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].…”

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].…”