A virgin birth is normally taken as a sign of divine intervention, but the phenomenon may be more common than we thought - at least in certain fish species.

Scientists have discovered that female sawfish appear to be routinely reproducing without any male input through an alternative form of reproduction known as parthenogenesis.

Asexual reproduction had been observed previously in various sharks, snakes and fish in captivity, when zookeepers were surprised to discover pregnant females that had not had any recent contact with males. But until now so-called “virgin births” were assumed to be incredibly rare and had never been observed in vertebrates in the wild.

Gregg Poulakis of the Florida Fish and Wildlife Conservation Commission, who led the fieldwork in the study, said: “There was a general feeling that vertebrate parthenogenesis was a curiosity that didn’t usually lead to viable offspring.”

In the latest study, DNA fingerprinting showed that about 3% of a sawfish population in Florida appeared to have been created through female-only reproduction, suggesting that parthenogenesis may play an important role in the survival of certain critically endangered species.

Although reproducing in this way depletes the genetic diversity of a population, it could help maintain numbers during critical periods, perhaps serving as a “bridging” strategy to get through a population bottleneck.

The smalltooth sawfish is a member of the ray family, distinguished by its studded saw-shaped nose-extension, which it uses to attack smaller fish. The fish, which grow to several metres in length, are found in southern Florida and have been driven close to extinction due to overfishing and habitat loss. The global population is thought to be around 1% of its level in 1900.

“We were conducting routine DNA fingerprinting of the sawfish found in this area in order to see if relatives were often reproducing with relatives due to their small population size,” said Andrew Fields, who led the study at Stony Brook University in New York. “What the DNA fingerprints told us was altogether more surprising: female sawfish are sometimes reproducing without even mating.”

During normal reproduction, the female egg cell matures and ejects half its chromosomes through a series of cell divisions, leaving a single set of chromosomes to combine with the single set that the sperm brings along. The resultant offspring end up with two sets of chromosomes in each of their cells, with half the genetic material coming from each parent.

Sawfish are close to extinction due to overfishing and habitat loss. It is thought that the ‘virgin births’ may be a survival strategy. Photograph: Florida Fish and Wildlife Conservation Commission (FWC)

In parthenogenesis, however, the mature egg is fertilised by a sister cell, known as a polar body, that contains an identical set of chromosomes. This means that while the resultant offspring will still have two sets chromosomes in each cell, the genes on each will be exactly the same.

In the study, published in Current Biology, the researchers captured 190 sawfish and in each case analysed 16 sites on the genome that were known to contain short sequences that are repeated multiple times in succession.

The same technique, known as Short Tandem Repeats, is used in human paternity testing: since half your genetic material comes from your father, the number of repeats on half of your chromosomes should match up with the number of repeats seen on his.

When applied to the sawfish, the paternity-style test revealed that some of the fish lacked a biological father altogether.

In these cases, the number of repeats on each chromosome was identical at each of the 16 sites, which could only be explained if they had inherited the entirety of their genetic material from their mother.

The survey identified two fish with different mothers, which both appeared to have been born through parthenogenesis, and a further five fish, which all shared the same mother.

Until now, scientists assumed that having two mirror image sets of genes would normally lead to serious health problems or be fatal, since it leaves individuals without any backup in the case of genetic flaws. Surprisingly, though, the seven parthenogens appeared to be in perfect health.



Dr Warren Booth, an evolutionary biologist at the University of Tulsa, who previously discovered an instance of parthenogenesis in snakes, said: “This is basically a very extreme form of inbreeding. Most people think of inbreeding as bad, but it could be helpful in purging deleterious mutations from a population.”

However, he added that it would also lead to populations losing genetic diversity, which is essential for a species to remain resilient to new threats.

All of the “virgin birth” fish were female, and the scientists believe that only female fish could be produced through this method since sawfish sex is determined through an XX/XY chromosome system similar to that of humans. Despite this, the population appeared to have a roughly 50:50 balance of male and female fish.

The researchers have not yet established whether the offspring were fertile themselves, but are tracking the population to investigate further. “It takes a very long time for sawfish to reach sexual maturity, so it could be up to ten years until we find out,” said Fields.

The authors are now trawling through publicly available genetic databases of other species to investigate whether parthenogenesis may be happening more widely.