Virgin birth confirmed

Scientists have confirmed the second-ever case of a ‘virgin birth’ (scientifically known as parthenogenesis) in a shark, indicating once again that female sharks can reproduce without mating and raising the possibility that many female sharks have this incredible capacity. This study has been published in the Journal of Fish Biology.

DNA testing was used to confirm that the offspring of a female blacktip shark named Tidbit contained no genetic material from a father. Tidbit had lived at the Virginia Aquarium in the Norfolk Canyon Aquarium for eight years since shortly after her birth in the wild.

Tidbit was an Atlantic blacktip shark (Carcharhinus limbatus) whom Virginia Aquarium biologists believe had only just reached sexual maturity. Researchers had never observed her in reproductive behaviour or showing typical signs of having been bred. In fact, the scientists did not even know that Tidbit was pregnant until after she unfortunately died and an autopsy (called a necropsy for animals) was performed. During the necropsy, researchers were surprised to find that she was pregnant, especially as there had been no male blacktips in the tank for the past eight years.

The phenomenon of ‘virgin birth’ occurs when a baby is conceived without male sperm having first fertilised the female’s eggs, and has been proved in some bony fish, amphibians, reptiles and birds. In the type of parthenogenesis seen in these sharks, known as automictic parthenogenesis, the newly forming pup acquires one set of chromosomes when the mother's chromosomes split during egg development. But instead of uniting with similarly split chromosomes from sperm, as occurs in sexual reproduction, the mother’s set is paired with a copy of itself. This results in offspring of reduced genetic diversity who may be at a disadvantage for surviving in the wild.

The DNA-fingerprinting techniques used to prove both cases of virgin birth are identical to those used in human paternity testing.

The finding of parthenogenesis in blacktip sharks, which are close relatives of some of the larger predatory sharks in the ocean including the tiger, bull and dusky sharks, raises intriguing questions about how frequently parthenogenesis may occur in the wild in this group of heavily fished sharks. It is possible that parthenogenesis could become more common in these sharks if population densities become so low that females have trouble finding mates. Populations of all of these sharks have declined in the past 20 years due to overexploitation, mainly to supply the shark fin markets.

There have been nearly a dozen reports of suspected virgin births in sharks in recent years, but scientists largely assumed these cases were the result of long-term sperm storage by females after mating with males. Virgin birth is now the more probable explanation, and DNA testing is underway to confirm it in additional sharks.

Sharks’ ability to reproduce alone should not be viewed as an adequate replacement for normal sexual reproduction. For one, the blacktip and hammerhead sharks that reproduced without mating both only produced one pup, rather than an entire litter. Shark litters can contain anywhere from a few to more than a hundred shark pups, depending on the species.