10 October 2008
Second virgin birth by shark confirmed
by Kate Melville
Scientists have confirmed the second-ever case of a "virgin birth" in a shark, raising the possibility that a relatively large number of female sharks may have this incredible capability. Authored by Dr. Demian Chapman, a shark scientist with the Institute for Ocean Conservation Science at Stony Brook University, the study that details the birth is published in the Journal of Fish Biology.
Chapman, working with Beth Firchau, Curator of Fishes for the Virginia Aquarium & Marine Science Center, and Dr. Mahmood Shivji, Director of the Guy Harvey Research Institute and Professor at Nova Southeastern University in Florida, has shown through DNA testing 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 most of her life.
Back in 2007, Chapman and Shivji were part of a team that made the first discovery confirming a virgin birth in a female shark. That shark was a hammerhead residing at an Omaha, Nebraska zoo and had not been in contact with male sharks for at least three years. That study was published in the journal Biology Letters and captured global media and scientific attention. The DNA-fingerprinting techniques used to prove both cases of virgin birth (scientifically known as "parthenogenesis") are identical to those used in human paternity testing.
"It is now clear that parthenogenesis occurs in sharks other than just hammerheads," Chapman said. "The first case was no fluke. It is quite possible that this is something female sharks of many species can do on occasion."
Interestingly, both the blacktip and hammerhead sharks that reproduced without mating 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 upon the species. "The revelation that female sharks can reproduce alone shouldn't stop us from worrying about driving shark populations to very low levels through overfishing," cautioned Chapman. "It is very unlikely that a small number of female survivors could build their numbers up very quickly by undergoing virgin birth."
The phenomenon of "virgin birth" occurs when a baby is conceived without male sperm having first fertilized the female's eggs, and has been proven 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 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," said Shivji. "It is possible that parthenogenesis could become more common in these sharks if population densities become so low that females have trouble finding mates."
Source: Stony Brook University