Female sharks 'can give birth without mating'
Female sharks can reproduce without having sex, scientists revealed today.
The discovery could have solved a mystery which has baffled experts studying the species in captivity.
An international team of researchers based in Northern Ireland and the United States made the breakthrough after a hammerhead shark gave birth without mating with a male.
No traces of any paternal DNA were detected in the offspring.
The researchers, from Queen's University Belfast, the Guy Harvey Research Institute at Nova Southeastern University in Florida, and Henry Doorly Zoo in Nebraska, found evidence that sharks can reproduce asexually by the unusual parthenogenesis method (wiki here
It is the first scientific report of such a development.
Dr Paulo Prodohl, of Queen's School of Biological Sciences, headed the university's research team and co-wrote the study.
He said: "The findings were really surprising because as far as anyone knew, all sharks reproduced only sexually by a male and female mating, requiring the embryo to get DNA from both parents for full development, just like in mammals.
"The discovery that sharks can reproduce asexually by parthenogenesis now changes this paradigm, leaving mammals as the only major vertebrate group where this form of reproduction has not been seen."
Researchers began their investigation following the unexpected birth of a baby hammerhead in an aquarium at the Henry Doorly Zoo in December 2001. Astonishingly, none of the three potential mothers in the tank - all caught in Florida waters as babies - had been exposed to any male sharks during their three years in captivity.
At first, it was thought the mother mated before capture, and then somehow stored the sperm for over three years before finally fertilising her eggs in the aquarium.
An alternative theory was that the hammerhead female had mated with a different species in the tank.
Using DNA profiling techniques to examine the genetic makeup of the baby hammerhead and the three candidate mothers, the researchers were able to identify which had given birth.
But the newborn's DNA only matched up with the mother's - there was none of any male origin.
This eliminated the possibilities of earlier mating or hybridisation.
Females of only very few vertebrate species can give birth to fully formed young without requiring their eggs to be first fertilised by a male's sperm, the researchers stressed.
This unusual reproductive ability, known as parthenogenesis, is only very occasionally seen in some vertebrate groups such as birds, reptiles and amphibians.
But it has never before been seen in other major vertebrate lines such as mammals or sharks.
Co-author Dr Mahmood Shivji, who led the Guy Harvey Research Institute team, said: "We may have solved a general mystery about shark reproduction - our findings suggest that parthenogenesis is the likely explanation behind the anecdotal but increasing observations of other species of female sharks reproducing successfully in captivity despite not having contact with males.
"It now appears that at least some female sharks can switch from a sexual to a non-sexual mode of reproduction in the absence of males.
"Unfortunately, this occurrence is not benign because it results in reduced genetic diversity in the offspring since there is no new genetic variation introduced from the paternal side."
The team established the most likely form of asexual reproduction that had occurred was a specific type called automictic parthenogenesis, which leads to less genetic diversity in the offspring compared to even the mother.
"During this process, the unfertilised egg, which contains about half of the mother's genetic diversity, is activated to behave as a normal fertilised egg by a small, genetically nearly-identical cell known as the sister polar body," lead author Dr Demian Chapman said.
"The resulting baby shark therefore gets a double-dose of genetic disadvantage."
The discovery has also raised concerns about the genetic and reproductive health of dwindling shark populations.
Dr Chapman, now head of shark research at the Pew Institute for Ocean Science, added: "Not only does it experience reduced genetic diversity because it has no father, but around half of the genetic variation present in the mother is not passed on to the offspring.
"Female sharks might reproduce like this more often when they have difficulty finding mates at low population densities.
"This could hasten the erosion of population genetic diversity and perpetuate the production of genetically disadvantaged offspring."