THE ULTRASTRUCTURE OF THE SPERMATOZOON OF THE NORTHERN WATER SNAKE, NERODIA SIPEDON (COLUBRIDAE, SERPENTES), WITH PHYLOGENETIC CONSIDERATIONS

The ultrastructure of the spermatozoon of Nerodia sipedon conforms clo sely to that of other described snake sperm: it is filiform; the acros ome vesicle is in the form of a hollow, concentrically zoned cone that basally overlies a subacrosomal cone which invests the tapered anteri or end of the nucleus; the putative perforatorium is a slender rod ext ending anteriorly from the subacrosomal cone; the midpiece contains de nse bodies and mitochondria; the axonemal fibrous sheath extends anter iorly into the midpiece (squamate autapomorphy); 9 peripheral dense fi bres 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 m embrane (an autapomorphy). In the squamates, only snakes, including N. sipedon, retain microtubules external to the plasma membrane of the m ature spermatozoon. Helically arranged zigzag mitochondria are shared (probably homoplasically) with iguanid sperm. A poorly developed ''sto pperlike'' putative perforatorial base plate in N. sipedon, unknown in other snakes, is questionably homologous with that of gekkonids. An e lectron-lucent space caps the nuclear point, as in the snakes Boiga ir regularis and Stegonotus cucullatus and in some other squamate orders.