An Endangered Fish Is Reproducing Asexually in the Wild
Researchers found smalltooth sawfish that were produced by parthogenesis—no sex required.
Juvenile smalltooth sawfish in the Charlotte Harbor estuarine system, Florida. Image: Florida Fish and Wildlife Conservation Commission (FWC)
Not all species need a man and a woman to explain where babies come from. Many plants reproduce asexually, and some animals, especially invertebrates, can reproduce without sex too.
This kind of "virgin birth," where only a female is needed to produce offspring, has now been observed in a critically endangered vertebrate: the smalltooth sawfish, or Pristis pectinata.
These sawfish usually reproduce sexually, but the new findings published in Current Biology suggest that, on occasion, males just aren't needed. Some sawfish (around three percent) were found to have only one parent: a mother.
The researchers claim it's the first time that live vertebrate offspring produced this way have been found in the wild. The phenomenon is called parthenogenesis, and it occurs when an egg develops into an embryo without fertilization (that is, no sperm required). Previous examples have been seen in animals kept in captivity, such as a hammerhead shark that was born at an American zoo in 2001, despite the tank containing only females.
Lead author Andrew Fields, from Stony Brook's School of Marine and Atmospheric Sciences, said in an email that the work showed parthenogenesis in vertebrates happens in nature, and not just captivity. "We have seen it in sharks, reptiles and birds in captivity, but never before have we seen a free living individual in the wild, therefore it is possible that this happens in more species be we just haven't been looking for it."
The researchers discovered the phenomenon when they set out to routinely test the DNA of a population of sawfish in Florida to see if they were breeding with relatives owing to their small number—the International Union for Conservation of Nature (IUCN) lists the species as "critically endangered," which is the last step before "extinct in the wild."
But they found instead that several DNA markers in some of the fish were "homozygous"; in other words, they shared the same DNA. Fields said this is rare in sawfish "so we screened a few more markers and the story held true that these fish are from parthenogenic origin."
This is how they think it happens in this case: During meiosis, cells divide and one becomes an egg, containing half the usual number of chromosomes (it is "haploid"). But the three other cells created in meiosis end up haploid too. In parthenogenesis, the egg cell is "fertilized by," or fused with, one of these other cells to produce a diploid organism (so it has the usual number of chromosomes you'd expect when egg and sperm combine, but less diversity in its DNA).
"It's possible that the fact that they are critically endangered makes this type of reproduction more likely."
The paper reports that the parthenogen fish, which were all females, were of normal size, though as they were young it was too early to tell if they'll be able to successfully reproduce or live as long as their two-parent peers.
It's unclear why the fish sometimes reproduce asexually, but Fields speculated that their endangered status might have something to do with it. "We don't know why they are reproducing like this," he wrote. "It's possible that the fact that they are critically endangered makes this type of reproduction more likely, simply because males and females fail to find one another when it's time to mate because they are so rare and then some of female's ova (eggs) develop without sperm."
He suggests that other researchers should check their data for similar occurrences in different species, to explore questions like whether endangered species are more affected or if it's just a natural unusual behaviour we're only now finding out about.
He was keen to stress the extinction threat faced by the sawfish; the fish caught were all tagged and released, so their movements can be studied.