Updated: Jul 21
The zebra shark (Stegostoma fasciatum), like all other species of sharks, breeds via "sexual reproduction". This means that both a male and a female are required to mate in order to produce offspring. After copulation, the female lays fertilised eggs, which eventually hatch into fully-formed offspring (aka "oviparity"). However, a female zebra shark, named Leonie, housed in an aquarium in Australia, made scientific history by being the first individual of this species to switch from sexual to asexual reproduction and there has only been three examples of any female shark switching between sexual and asexual reproduction! This was an incredibly rare event! So how did it happen? And why?
A Captive Zebra Shark Performed the 'Impossible'
Leonie was housed with a male between 2006 and 2013 and during this time, the pair mated
and produced several litters of offspring. After 2013, the male was removed from the aquarium and instead, one of their daughters from a previous litter was moved into the tank to keep Leonie company. This daughter was sexually immature, meaning she was not yet of a size or age to produce offspring of her own.
The keepers were absolutely flabbergasted when, in the next breeding season, both Leonie and her daughter laid eggs! Initially, only Leonie's eggs were viable and actually produced offspring, but later the daughter's eggs also successfully hatched!
Scientific researchers analysed the event, in order to determine how this was possible... They concluded that it was possible that Leonie reproduced via "sperm storage". This means that the female, after mating, will store sperm internally, which can be used to fertilise her eggs at a later date. This strategy can be especially beneficial in fragmented populations, where the likelihood of running into a mate regularly is particularly low (Dudgeon et al, 2017).
DNA Showed Zebra Sharks can Have Virgin Births
However, this did not explain how Leonie's daughter also came to breed; she was never exposed to a male at all. So, they used genotype testing to assess the DNA of the progeny. In sexual reproduction half of the offspring's DNA will come from the mother and half from the father, but in asexual reproduction, DNA only comes from one parent. Therefore, it was possible to analyse the DNA of Leonie and her daughter and grandchildren, to determine where their DNA came from.
They found that all of Leonie's progeny and all of her daughter's offspring only had DNA which matched Leonie's, suggesting that there was no father. This meant that Leonie's offspring were not produced via sperm storage. In fact, both of these sharks were reproducing via "parthenogenesis"; when females are able to reproduce using only their eggs, not requiring a male's sperm for fertilisation (Dudgeon et al, 2017).
Leonie only skipped one breeding season between when the male was removed and when she laid her next batch of eggs; switching from sexual reproduction to parthenogenesis. Whatsmore, her daughter immediately commenced reproduction via parthenogenesis only one year after she became sexually mature! She never bred sexually (Dudgeon et al, 2017).
So why did this happen?
Zebra Sharks Have Evolved to Breed When it Should Not be Possible
There have been other incidences when the removal of the mate has triggered female sharks in captivity to begin breeding asexually. Therefore, the researchers concluded that the switch to parthenogenesis was triggered by an environmental cue; the removal of the male. They hypothesised that this might be an evolutionary adaptation which allows the sharks to continue reproducing when sexual reproduction is not possible; at a critical low density of sharks, the females begin breeding, in order to bolster the population size. This strategy would allow for increased down-stream opportunities for mating success (Dudgeon et al, 2017).
However, it is also possible that parthenogenesis in the zebra shark has evolved as a holding-on mechanism; whereby the females are able to produce viable female offspring, until sexual reproduction becomes possible again. In fragmented habitats, where these sharks become isolated from each other, this strategy would mean that the population could remain healthy until a male immigrated into the area (Dudgeon et al, 2017).
As parthenogenesis is so rare in sharks, it may be a while before we can conclusively say what cue triggers it or why it has evolved, but whatever the mechanism and the cause... it is seriously cool that these sharks are capable of virgin births!
Dudgeon CL, Coulton L, Bone R, Ovenden JR & Thomas S (2017). Switch from sexual to parthenogenetic reproduction in a zebra shark. Scientific Reports, 7. Access online.