Leal Francisca, Cohn Martin J
Department of Biology, University of Florida, Gainesville, Fla., USA.
Sex Dev. 2015;9(1):6-20. doi: 10.1159/000363758. Epub 2014 Jun 20.
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.
在羊膜动物中，外部交配器官经历了广泛的形态多样化。最极端的例子之一是有鳞目动物（蜥蜴和蛇）的半阴茎，它是从泄殖腔的侧缘伸出的成对交配器官。在这里，我们描述了一种基干蛇——球蟒（Python regius）半阴茎的发育过程。蛇的半阴茎起源于泄殖腔尾部两侧的一对侧隆起，这些成对的突出物持续存在并形成左右半阴茎。在非有鳞目羊膜动物中，外部生殖器由泄殖腔前侧出现的成对隆起形成，然后向内侧融合形成单个生殖结节，即阴茎或阴蒂的原基。而在非有鳞目羊膜动物中，泄殖腔内胚层中表达音猬因子（Shh）的细胞形成尿道或沟上皮，并且是阴茎生长所必需的，有鳞目动物的半阴茎缺乏内胚层的贡献，并且沟中不表达Shh。因此，蛇的半阴茎在胚胎起源和至少部分模式形成机制上与非有鳞目羊膜动物的生殖结节不同，这增加了半阴茎可能不是不成对的羊膜动物阴茎的直接同源物的可能性。尽管如此，我们发现一些发育基因在蛇半阴茎芽和非有鳞目动物生殖结节中表现出相似的表达模式，这表明同源发育机制参与了整个羊膜动物外部生殖器发育的各个方面，即使这些结构可能有不同的发育起源，并且可能在进化过程中独立出现。
