Love’s Labors Lost: Virgin Births from Komodo Dragons to Sharks

In 2005, on a seemingly normal day at the London Zoo, a small yet sensational event took place-a female komodo dragon gave birth in isolation, without a male in sight. Such is how Episode One, Season 2 of David Attenborough’s Natural Curiosities opens, with narrator David Attenborough delving into the unique process that has led these fantastic animals, along with some other types of lizards, snakes, birds, sharks and insects, to forego sexual reproduction whenever partners are not present. This process is facultative parthenogenesis (partheno-virgin, genesis-birth), the formation of embryos without the fertilization of an egg.

In komodo dragons, females produce half-clones of themselves by splitting and replicating their ZW sex chromosomes. ZZ and WW replicates are formed, but since only ZZ pairs form males, while WW forms neither male nor female, all the offspring turn out male [4]. Even with such an anomaly revealing out-of-the-norm birthing, scientists originally speculated long term sperm storage, though genetic markers have proven otherwise [2]. Such male-birth parthenogenesis is uncommon relative to female-birth parthenogenesis, which occurs in many species, such as the bonnethead sharks[1]. Interestingly, in the lab, these isolated female sharks produce female offspring, which cannot then mate themselves as no males exist. Such a discovery begs the questions: what evolutionary purpose does virgin birth serve, and why is such a trait so common across species?

Population isolation seems to be the main culprit, as sharks, snakes, and komodo dragons all isolated in labs have given birth parthenogenetically. Although such births lower diversityof an individuals offspring, such a trait may have been selected for because heavy storms have repeatedly displaced and transported komodos to isolated islands across the Indonesian archipelago, allowing these magnificent creatures to start new populations over the past 30-40 million years [3].

Even in male rich populations, Attenborough mentions how some cottonmouth snakes opt to forego mating and reproduce through parthenogenesis, since their standards are too high (Unfortunately to the dismay of many female readers, this is not a viable option, as sometimes you can’t always land a Bradley Cooper but have to find someone close enough). Thus, parthenogenesis is a back-up plan that has maintained itself in many species at very low rates, on the off-chance that either one can’t find a mate, or one can’t find a mate that they want to mate with.

by Aakash Doshi

David Attenborough’s Natural Curiosities, Season 2, Episode 1, starting at the beginning.

References

1. Edwards RG. The significance of parthenogenetic virgin mothers in bonnethead sharks and mice. 2007. Reproductive BioMedicine Online 15(1):12–15.
2. Field AT. Facultative parthenogenesis in a critically endangered wild vertebrate. 2015. Current Biology 25(11).
3. Van Der Kooi CJ & T Schwander. Parthenogenesis: birth of a new lineage or reproductive accident? 2015. Current Biology 25(15).
4. Watts PC. 2006. Parthenogenesis in Komodo Dragons. Nature 444(7122):1021-1022.