Parthenogenesis and The Spontaneous Sawfish Births That Are Saving an Endangered Species

Females of a critically endangered sawfish species have taken a drastic step toward survival: parthenogenesis, or asexual reproduction. This is the first time that a vertebrate that typically reproduces sexually has been observed engaging in parthenogenesis.

"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," says lead author Andrew Fields, a PhD candidate at the Stony Brook University's School of Marine and Atmospheric Science. "What the DNA fingerprints told us was altogether more surprising: female sawfish are sometimes reproducing without even mating."

The study, published in the June 1 edition of the journal Current Biology, was conducted by Demian Chapman of Stony Brook University in New York and his partners from the Florida Fish and Wildlife Conservation Commission and the Priztker Laboratory at the Field Museum of Chicago.

In the past scientists have seen certain vertebrates reproduce via parthenogenesis, but this was principally seen in captive animals such as sharks, reptiles, and birds. The Komodo dragon, for example, has reproduced via parthenogenesis, as have boa constrictors, pit vipers, and turkeys. Offspring created in this process are called parthenogens.

The question was whether this kind of reproduction happened in the wild. This study confirms that in fact it can happen.

"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 says. "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."

The team used DNA fingerprinting techniques to assess smalltooth sawfish one of five varieties of sawfish. They sampled DNA from 190 smalltooth sawfish between 2004 and 2013, and then tagged and released them as part of an ongoing study of sawfish movements. They discovered seven parthenogens among the 190, about three percent. Five were all siblings of almost the same age.

Smalltooth sawfish are large rays recognized for their notable, toothy rostrums that they use to capture prey fish. Like sharks, their skeletons are cartilage, and they can grow to be 25 feet long. Scientists believe that sawfish may be driven to extinction first among marine animals as a result of coastal habitat loss and overfishing. Although they were once common in the Atlantic ocean, today they are typically found in a small number of southern Florida areas including the Peace and Caloosahatchee Rivers.

"Sawfish are on the brink of extinction thanks to humans," Chapman says.

Parthenogenesis is rare in vertebrates although it is common among invertebrates. Vertebrate parthenogenesis happens when one unfertilized egg cell absorbs another cell which is genetically identical. This means that offspring produced in this way die frequently because they have only half of the genetic diversity of their parent.

"There was a general feeling that vertebrate parthenogenesis was a curiosity that didn't usually lead to viable offspring," says Gregg Poulakis, leader of the field collections process, of the Florida Fish and Wildlife Conservation Commission. "This suggests parthenogenesis is not a reproductive dead end, assuming they grow to maturity and reproduce."

However, each of the seven parthenogens studied were quite healthy. The researchers tagged each one and later released them in order to continue following their movements.

Fields and the team will watch for more parthenogens and track the seven existing subjects. "We're hoping to find out whether or not they can produce offspring," Fields says. The sawfish studied were born in 2011, so the team will have to wait until at least 2018 for these parthenogens to reach sexual maturity.

Experts believe that in small or dangerously shrinking populations parthenogenesis may sometimes occur. They are asking others in the scientific community to search DNA databases for other vertebrate parthenogens living out of captivity.

"Occasional parthenogenesis may be much more routine in wild animal populations than we ever thought," says Kevin Feldheim of the Field Museum of Chicago's Pritzker Laboratory, where the DNA fingerprinting took place. "This could rewrite the biology textbooks."

This may save the species in the short run, but over the long haul it is likely to be a dead end, limiting the genetic diversity of the species and leaving it even more vulnerable. Although parthenogenesis cannot save the species forever, the researchers hope these findings are taken as a desperate warning.

"This is a big warning sign," says Fields. "It's saying, 'hey, we need to pay attention to these guys'-or they're not going to survive that much longer."

"This should serve as a wake-up call that we need serious global efforts to save these animals," Feldheim says.

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