141. Jamieson, B.G.M. and Koehler, L. 1994k. The ultrastructure of the spermatozoon of the Northern Water Snake, Nerodia sipedon (Colubridae, Serpentes), with phylogenetic considerations. Canadian Journal of Zoology 72, 1648-1652.
The ultrastructure of the spermatozoon of Nerodia sipedon conforms closely to that of other described snake sperm: it is filiform; the acrosome vesicle is in the form of a hollow, concentrically zoned cone that basally overlies a subacrosomal cone which invests the tapered anterior end of the nucleus; the putative perforatorium is a slender rod extending anteriorly from the subacrosomal cone; the midpiece contains dense bodies and mitochondria; the axonemal fibrous sheath extends anteriorly into the midpiece (squamate autapomorphy); 9 peripheral dense fibres surround the distal centriole and the axoneme in the midpiece, of which fibres adjacent to 3 and 8 are enlarged; and the endpiece lacks peripheral fibres and the fibrous sheath. The midpiece is very long (a synapomorphy of the Serpentes) and is surrounded by a multilaminar membrane (an autapomorphy).
In the squamates, only snakes, including N. sipedon, retain microtubules external to the plasma membrane of the mature spermatozoon. Helically arranged zigzag mitochondria are shared (probably homoplasically) with iguanid sperm. A poorly developed "stopperlike" putative perforatorial base plate in N. sipedon, unknown in other snakes, is questionably homologous with that of gekkonids. An electron-lucent space caps the nuclear point, as in the snakes Boiga irregularis and Stegonotus cucullatus and in some other squamate orders.