Novel Cloacal Glands in Snakes: The Phylogenetic Distribution of Ventral Urodaeal Glands in Thamnophiini

Abstract

Novel ventrally positioned extra-cloacal glands were discovered through examination of cloacal histology of female thamnophiine snakes. The glands discovered, termed ventral urodeal glands, are paired and empty into the caudal extremity of the urodeum, lateral to the opening of the urodeal sphincter, through primary ducts that resemble folds of the urodeal wall. In concordance with historical literature, these glands are the first cloacal glands described in snakes that definitively empty into the urodeum from a ventral position. Ventral urodeal glands are complex, with multiple branching ducts that empty the terminal alveolar glands. The epithelial linings of the branching duct networks are stratified (primary and secondary ducts), pseudostratified (tertiary ducts), or simple (quaternary ducts and terminal alveoli). The epithelia of the terminal alveoli stain weakly with hematoxylin, and an eosinophilic secretory material is common in the lumina of alveoli. The secretion produced in the alveoli stains positive with the periodic acid-Schiff's procedure, negative with brilliant blue, and positive with fast green, indicating that the majority of the secretion is composed of a neutral carbohydrate moiety with a minor protein component. Ventral urodeal glands were observed in all taxa of Nerodia examined, except N. cyclopion, and were not observed in any other taxa of Thamnophiini. Through optimization to Thamnophiini topologies, ventral urodeal glands either evolved once on the branch leading to Nerodia and were subsequently lost on the branch leading to N. cyclopion, or they evolved independently on the branches leading to N. floridana and all other species of Nerodia. Like many of the cloacal glands in squamates, the function of ventral urodeal glands is unknown, and further investigation is needed to elucidate the function of the secretions of these glands.