Fish that breed without males 'could rewrite biology textbooks'

Scientists have discovered that female smalltooth sawfish are able to breed without a male partner, the first evidence of asexual reproduction in vertebrates in the wild.

The critically endangered fish normally reproduce by mating and the discovery could suggest that asexual reproduction may be a natural response when the species is threatened.

The process of parthenogenesis – when an unfertilised egg develops into an embryo – is rare in vertebrates and has only ever been witnessed in captivity.

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"This could rewrite the biology textbooks," said Kevin Feldheim, co-author of the report that outlines the discovery. "Occasional parthenogenesis may be much more routine in wild animal populations than we ever thought."

The discovery was made while ecologists were conducting routine tests on a dwindling sawfish population in a Florida estuary to see if the animals were reproducing with relatives because of their small population size.

"What the DNA fingerprints told us was altogether more surprising: female sawfish are sometimes reproducing without even mating," said lead author Andrew Fields, fish geneticist from the Stony Brook University in New York.

His team discovered that nearly four per cent of the fish in the estuary lacked any genetic contribution from a male. Reproducing in this way limits the genetic diversity of an animal, meaning they don't often survive, but the sawfish that were studied were "in perfect health and were a normal size for their age".

Though groundbreaking, scientists warn that this adaptation does not appear to be restoring population levels. "Parthenogenesis could help endangered species like sawfish dodge extinction for a little while, but it should also serve as a wakeup call that we need serious global efforts to save these animals," said Fields.