Department of Biology, University of Florida, Gainesville, FL 32611, USA.
Comp Biochem Physiol A Mol Integr Physiol. 2011 Nov;160(3):400-11. doi: 10.1016/j.cbpa.2011.07.008. Epub 2011 Jul 23.
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.
脊椎动物的盐腺已经多次独立进化，但关于盐腺在分类群中趋同进化的潜在过程的假说却很少。在这里，我们比较了海洋蛇（Laticauda semifasciata）的特化盐分泌腺的形态和分子生物学，以及半咸水（Nerodia clarkii clarkii）和淡水（N. fasciata）水蛇的头腺，以寻找前者盐腺的证据，并提出关于蛇盐腺进化的假说。与 L. semifasciata 的盐腺一样，Nerodia 属的两种蛇的鼻腺和前/后舌下腺都具有复合管状形状，并表达基底外侧 Na(+)/K(+)-ATPase (NKA) 和 Na(+)/K(+)/2Cl(-)共转运蛋白 (NKCC)；然而，N. fasciata 中的 NKA 和 NKCC 的丰度似乎低于 N. c. clarkii。Aquaporin 3 (AQP3) 在两种 Nerodia 属的舌下腺中也是基底外侧的，并且富含中性粘蛋白；AQP3 和粘蛋白都不存在于 L. semifasciata 的盐腺中。因此，我们提出蛇盐腺的进化通过对现有腺的共同选择至少涉及两个步骤：（i）基底外侧分泌上皮中 NKA 和 NKCC 的丰度增加，以及（ii）这些上皮中 AQP3/粘蛋白分泌的丧失。
