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Sunday, September 22

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Who’s your daddy?

A few years ago, when I was the Zoo’s Curator of Herpetology, I walked into the old World of Reptiles and the keepers told me that we had a newborn cottonmouth (Agkistrodon piscivorus). This was strange, I thought, because we had only a single female cottonmouth in the population. Female snakes are known to sometimes store sperm for a few years and suddenly become gravid without any recent mating activity. We checked the records for this cottonmouth and were sure she had not been with a male in well over a decade. Hmm…

Then there was the issue of the newborn baby. There was just one, plus one full-term stillborn snake, and two half-formed non-viable snakes. Cottonmouths usually have rather large clutches of offspring. We decided that there is chance we were experiencing something once thought to be very rare—parthenogenesis, or offspring created by a female with no genetic input by any male. This had never been documented in cottonmouths, nor even their close relative the copperheads (Agkistrodon piscivorus).

We needed input from expert snake geneticists, so we contacted Dr. Warren Booth of the University of Tulsa and his collaborator Dr. Gordon Schuett. Per their instructions, we simply raised mom and baby until we had shed skins from each; the shed skins are a harmless, non-invasive source of DNA. They confirmed this to be a parthenogenetic event, and meanwhile mom and son continued to thrive.

Wait…..her son?! How is that possible, I thought? I was familiar with the whiptail lizards (Aspidoscelis spp.) in the southwestern deserts that exist as all-female species (i.e., there are no males) and they reproduce via offspring that are genetic clones of themselves (and thus females). Clearly, I had not been following Warren and Gordon’s work very well, as they had been slowly working their way across the diversity of snakes and repeatedly documenting this peculiar pattern of mother→son reproduction in many, but not all types of snakes. In fact, our accidental coincidental timing was such that they were wrapping up a groundbreaking large-scale study of wild copperheads and had documented that this phenomenon was actually quite common in the wild. What they lacked was any evidence from cottonmouths, which is what we just mailed to them. They were delighted, and we collaborated to report these exciting results in the journal Biology Letters in 2012.

How had herpetologists gotten this so wrong for a couple of centuries? It turns out that, while the phenomenon can be quite common in the wild, it takes the years of meticulous work that Warren and Gordon had devoted to document it. Otherwise, it certainly had been seen, but wrongly dismissed, in zoos, occasionally, for many years. The fact that snakes can store sperm for extended periods led to the dismissal, with no evidence, that these events simply were long(er than likely possible) instances of sperm storage. In cases where sperm storage could be ruled out—female snakes born/hatched into lifelong isolation—then herpetologists and evolutionary biologists dismissed the cases as extremely rare, aberrant situations of reproductive malfunction in females without access to males in zoological settings. Importantly, we all assumed that this phenomenon only occurred in zoological settings and so was of no evolutionary or natural importance in the real world. Our colleagues Warren and Gordon turned that entire tall-tale on its head with their breakthrough work, and it was exciting that Zoo Atlanta was able to contribute in a small manner.

Oh, you want to know how exactly a female can produce a son (that obviously cannot be a clone of herself)? Next time you see me at the Zoo, I’ll sketch out their genes for you on a scrap of paper. Hope to see you soon!

The original publication:
Booth, W, C. F. Smith, P. Eskridge, S. K. Hoss, J. R. Mendelson III, and G. W. Schuett. 2012. Facultative parthenogenesis discovered in wild vertebrates. Biology Letters doi:10.1098/rsbl.2012.0666
Joe Mendelson, PhD
Director of Research

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