(1) These features are tetrapod symplesiomorphies, (2) is an amniote synapomorphy; the fibers differ from those of reptiles in being uniform in size, (3) loss of the fibrous sheath is an apomorphy known elsewhere only in columbiforms, (4) are apomorphies relative to basal aminiotes (Chelonia, Sphenodon, and Crocodilia), (5) is an apomorphic condition shared with other nonpasserines (galliforms and the white-naped crane) and crocodilians, (6) the latter taxa differ from parrots in a plesiomorphic elongation of the distal centriole, and (7) is a unique apomorphy of parrot sperm relative to other nonpasserines and reptiles. The short midpiece of N. hollandicus distinguishes this cacatuine from the two psittacines.
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.
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