Morphological and biochemical evidence for the evolution of salt glands in snakes

Vertebrate salt glands have evolved independently multiple times, yet there are few hypotheses about the processes underlying the convergent evolution of salt glands across taxa. Here, we compare the morphology and molecular biology of specialized salt-secreting glands from a marine snake (Laticauda semifasciata) with the cephalic glands from semi-marine (Nerodia clarkii clarkii) and freshwater (N. fasciata) watersnakes to look for evidence of a salt gland in the former and to develop hypotheses about the evolution of snake salt glands. Like the salt gland of L. semifasciata, the nasal and anterior/posterior sublingual glands in both species of Nerodia exhibit a compound tubular shape, and express basolateral Na+/K+-ATPase (NKA) and Na+/K+/2Cl−cotransporter (NKCC); however, the abundance of NKA and NKCC in N. fasciata appears lower than in N. c. clarkii. Aquaporin 3 (AQP3) is also basolateral in the sublingual glands of both species of Nerodia, as is abundant neutral mucin; both AQP3 and mucin are absent from the salt gland in L. semifasciata. Thus, we propose that the evolution of the snake salt gland by co-option of an existing gland involved at least two steps: (i) an increase in the abundance of NKA and NKCC in the basolateral membranes of the secretory epithelia, and (ii) loss of AQP3/mucus secretion from these epithelia.