Giant sawfish have virgin births, rewrite biology textbooks

© Florida Fish and Wildlife Conservation Commission (FWC)
The toothy snout of a juvenile smalltooth sawfish in Florida's Charlotte Harbor estuarine system.

    

To the surprise of scientists, giant endangered fish with sawlike snouts in Florida are experiencing virgin births, reproducing without sex. This is the first solid evidence of such asexual reproduction in the wild for any animal with a backbone, scientists added.

Asexual reproduction is often seen among invertebrates — that is, animals without backbones. It happens rarely in vertebrates, but instances are increasingly being discovered — only observed to survive in captivity previously. For example, the Komodo dragon, the world's largest living lizard, has given birth via parthenogenesis, in which an unfertilized egg develops to maturity. Such virgin births have also been seen in sharks, in birds such as chickens and turkeys, and in snakes such as pit vipers and boa constrictors. Such virgin-born offspring are known as parthenogens.

Until now, evidence of parthenogenesis in vertebrates came nearly entirely from captive animals, usually surprising their keepers by giving birth despite the fact that they had not had any mates. Scientists had recently found two female snakes in the wild that were each pregnant with progeny that developed via parthenogenesis, but it was not known if these parthenogens would have survived. As such, it remained uncertain whether virgin births happened to any significant extent in nature.

Now scientists find that among smalltooth sawfish, progeny of virgin births do regularly live in the wild. These fish are critically endangered relatives of sharks. [Watch the 'Virgin Birth' Baby Sawfish (Video)]

"Vertebrate animals that we always thought were restricted to reproducing via sex in the wild actually have another option that does not involve sex," study co-author Demian Chapman, a marine biologist at Stony Brook University in New York, told Live Science. "Rare species, like those that are endangered or colonizing a new habitat, may be the ones that are doing it most often. Life finds a way."

Smalltooth sawfish are one of five species of sawfish, a group of large rays known for long, tooth-studded snouts that the animals use to subdue small fish. Smalltooth sawfish are mainly found nowadays in a handful of locations in southwest Florida. These fish, which possess skeletons made of cartilage just like sharks do, can reach lengths of up to 25 feet (7.6 meters).

The researchers noted that sawfish could be the first entire family of marine animals to be driven to extinction, which is occurring due to overfishing and loss of the animals' coastal habitats. "Sawfish are on the brink of extinction thanks to humans," Chapman said.

Smalltooth sawfish have already disappeared from most of the places in the Atlantic where they were common a century ago.

"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," lead study author Andrew Fields, also at Stony Brook University, said in a statement. "What the DNA fingerprints told us was altogether more surprising — female sawfish are sometimes reproducing without even mating."

Between 2004 and 2013, the researchers sampled DNA from 190 smalltooth sawfish. All of the fish were tagged and released back into the wild as part of an ongoing study of sawfish movements.

The scientists found seven parthenogens, representing about 3 percent of the sawfish the researchers investigated. Five of these seven appear to all be siblings of about the same age, probably members of a single brood.

Sawfish and many other creatures carry out meiosis, in which cells divide to form sex cells, each of which only possesses half the material needed to make offspring. In the female sawfish the researchers investigated, pairs of sex cells likely fused to generate offspring. However, these progeny are not clones of the mother or each other; sex cells are not perfectly identical to each other, and neither are the parthenogens resulting from these sex cells.

Since virgin birth is essentially an extreme form of inbreeding, "there was a general feeling that vertebrate parthenogenesis was a curiosity that didn't usually lead to viable offspring," study co-author Gregg Poulakis of the Florida Fish and Wildlife Conservation Commission, who led field collections of the sawfish, said in a statement.

However, the seven parthenogens the researchers discovered appeared to be in perfect health and were the normal size for their age.

"This suggests parthenogenesis is not a reproductive dead end, assuming they grow to maturity and reproduce," Poulakis said in the statement.

Parthenogenesis may occur mainly in small or dwindling populations, perhaps when females cannot find males during mating season. The researchers are now encouraging other scientists to analyze their DNA databases of birds, fish, snakes, lizards, sharks and rays for other examples of vertebrate parthenogenesis in the wild.

"This could rewrite the biology textbooks," study co-author Kevin Feldheim of the Pritzker Laboratory at the Field Museum of Natural History in Chicago, where the DNA fingerprinting was conducted, said in the statement. "Occasional parthenogenesis may be much more routine in wild animal populations than we ever thought."

The scientists cautioned that parthenogenesis alone was not enough to save the critically endangered smalltooth sawfish from extinction.

"It would be great to use this interesting finding to inspire conservation action for sawfish," Chapman said.

The scientists detailed their findings online today (June 1) in the journal