Scientists have just recorded the first instance of the switch from sexual to asexual reproduction in a shark. A female zebra shark produced offspring after sexual contact with another shark but then gave birth again after several mating seasons had passed in which she had no access to males. One of her offspring gave birth after reaching reproductive maturity even though she had never mated. The details are in a paper that was just published in the journal Scientific Reports.
Parthenogenesis is a form of asexual reproduction that most commonly occurs in plants and small invertebrates. It’s a form of self-cloning in which an embryo grows and develops without fertilization. Very few vertebrates are capable of parthenogenesis and asexual reproduction is almost nonexistent among warm-blooded creatures. Parthenogenesis has been recorded in a few shark species but never in the zebra shark (Stegostoma fasciatum).
A zebra shark housed in Reef HQ, an Australian public aquarium in the city of Townsville, surprised researchers by suddenly producing eggs when she hadn’t been with a male for over three years. Further testing revealed that the pups only shared genes with the female, no other DNA had been contributed. In other words, the babies were identical clones of their mother. Although some sharks are capable of asexual reproduction, this was the first recorded case of a shark switching from sexual reproduction to asexual reproduction. This was also the first time a zebra shark reproduced asexually—something that might help the species, which are listed as endangered on the IUCN Red List.
Interestingly, one of the shark’s daughters that had been produced through sexual reproduction later underwent parthenogenesis, just like her mother. The daughter shark had never even lived with a male so could not have mated. Yet she produced eggs once she reached sexual maturity. The team believes that in both cases, the absence of available mates prompted the switch from sexual to asexual reproduction.
The team’s findings are potentially good news for zebra sharks, which are endangered and very rare in the wild. It appears that the removal of her mate triggered the first shark to switch to asexual reproduction and her daughter used the same strategy upon reaching maturity. The team will continue to track the sharks to see if the use of parthenogenesis helps or harms the population.
Dudgeon et al. Switch from sexual to parthenogenetic reproduction in a zebra shark. Scientific Reports (2017).