Tuesday, October 25, 2011

An Real Case of Sperm Storage in Eastern Diamondbacks

The New Scientist is carrying the following story. It is of interest because female snakes that have not been in contact with a male have produced offspring by parthenogenesis. Here is an apparent case of long term sperm storage. In most vertebrates sperm is thought to have a relatively short shelf life - unless of course the cells are placed in stasis, which some times happens in special sperm storage tubules.

FAMILY planning campaigners looking for a mascot should consider the eastern diamond-backed rattlesnake. A female of the species can store sperm in her body for at least five years before using it.

The rattlesnake (Crotalus adamanteus) in question was collected in Florida in 2005 and kept in a private collection for five years, with no contact with other snakes. In late 2010, she unexpectedly gave birth to 19 snakelets. To find out what had happened, Warren Booth of North Carolina State University in Raleigh took samples of DNA from the mother and her young.

Booth studies "virgin birth", in which a female produces young without any contribution from a male. But in this case the snakelets carried genes that their mother didn't, so she must have mated before she was captured and stored the sperm (Biological Journal of the Linnean Society, DOI: 10.1111/j.1095-8312.2011.01782.x).

Previous studies have hinted that reptiles can store sperm for several years, but this is the first case confirmed by genetics. Booth suspects other reptiles can store sperm even longer. "How long is anyone's guess," he says.It's becoming clear that snakes have unconventional ways of reproducing, including virgin birth and long-term sperm storage, says William Holt of the Institute of Zoology in London, though so far no one knows how they do it.

So much for the press release. In fact sperm storage in squamates has been relatively well studied. 

Oviducal sperm storage is known in females of all taxa of squamates except Amphisbaenia. However, in Rhynchocephalia and Crocodilia, sperm storage is poorly studied, and specialized sperm storage tubules (Ssts) are unknown. Sever and Hamlett (2002) used the molecular phylogenetic hypothesis [(Chelonia+Archosauria) (Squamata)] to trace evolution of sperm storage characters and found Ssts arose independently in Chelonia and Squamata. Turtles have albumen-secreting glands in the anterior half of the oviduct (the tuba or isthmus), and the most distal of these glands act as  Ssts; in addition, some turtles possess Ssts in the adjacent segment of the oviduct, the uterus. Squamates lack albumen-secreting glands, and the ancestral state is possession of Ssts in the posterior infundibulum (uterine tube). Secondarily, iguanids have evolved vaginal Ssts. They used ultrastructural observations on vaginal Ssts in lizards, with Anolis sagrei (Polychrotidae). Proximally, the neck of these simple tubular glands continues the alternation of ciliated and secretory cells lining the lumen of the vagina. However, the epithelial cells of the distal sperm storage area are neither secretory nor ciliated. The Ssts of Anolis are more similar to those of birds more than to infundibular receptacles in snakes and lizards.

Sever, D. M. and Hamlett, W. C. (2002), Female sperm storage in reptiles. Journal of Experimental Zoology, 292:187–199. doi: 10.1002/jez.1154

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