Who Needs Men? Not This Virgin Snake Mom

September 30, 2015 | Sarah Tse

Yellowbelly Water Snake (Nerodia erythrogaster flavigaster)
Photo credit: Greg Schechter/Flickr/CC BY 2.0

After eight years without any male attention, a captive female snake decided to take matters into her own hands. Tired of waiting for a mate to show up, she gave birth to a litter of baby snakes all on her own, without being fertilization by a male.

If that’s not impressive enough, this is the second year in a row that she has pulled off an immaculate conception at the Missouri Department of Conservation’s (MDC) Cape Girardieu Conservation Center. Last year’s miracle birth resulted in two healthy snakes that are still thriving at the center.

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The yellow-bellied water snake actually gives birth to live young encased in soft membranes instead of eggs. Intern Kyle Morton stumbled across the discarded membranes one morning and didn’t know what to think. “I thought, 'what joker put tomatoes in here for the snake?’” said Morton in an MDC news release. Upon closer examination, he realized what those “tomatoes” were and notified a naturalist at the center.

This type of “virgin birth” in which offspring develop from unfertilized eggs is called parthenogenesis, and it’s surprisingly common in the animal kingdom whenever females can’t find a worthy mate. It occurs in bees, wasps, stick insects, and many types of worms. Many vertebrate species can also reproduce via parthenogenesis in both captivity and the wild, including sharks, various amphibians, and even domesticated turkeys and chickens. In lizards and snakes, parthenogenesis has been observed in Komodo dragons, geckoes, boas, pythons, and rattlesnakes.

But the yellow-bellied water snake’s birth last year was the first documented case of parthenogenesis in this species. In fact, scientists initially doubted that captive animals giving “virgin birth” were actually pulling it off without fertilization. They suspected that the females stored sperm from a previous encounter with a male before entering captivity. But eight years is too long for the sperm to survive and produce viable offspring, and genetic analyses of these miracle offspring have revealed that all their DNA comes from their mother.

Unfortunately, none of the baby snakes from this year’s birth survived. MDC herpetologist Jeff Briggler speculates this could be a consequence of the genetic circumstances of parthenogenesis. In most reptiles, sex determination relies on a ZW chromosome system, where a set of ZZ chromosomes results in a male and a set of ZW chromosomes makes a female. A WW combination is inviable, in the same way humans can’t survive without an X chromosome.

So when the female produces eggs, each new cell receives either a Z or W chromosome when the egg precursor cell undergoes meiosis, or the separation of chromosomes. But without male sperm, the eggs have to clone the half set of DNA they received from their mother in order to restore the right number of chromosomes. The eggs that got a Z chromosome can double it to complete a ZZ set, and they turn into male snakes. But the eggs that receive a W chromosome can only become a WW set, which means they’re missing key genes for development that are located on the Z chromosome. It’s possible that this year’s hatchlings didn’t make it to term because they only received W chromosomes. But the snake’s male babies from last year are still doing well at the center.

Scientists are realizing that parthenogenesis is far less rare than previously thought, forcing them to reconsider the advantages of sexual reproduction. Although offspring of parthenogenesis have less genetic variation than those arising from sexual reproduction, it would be a shame to remain childless just because there aren’t any males around. It may be that these female snakes are so eager to pass on their genes that they’ll risk the severe inbreeding that comes from essentially mating with themselves. 

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