The strange 'virgin births' of the smalltooth sawfish

The strange tale of seven smalltooth sawfish who appear to be the product of a virgin birth

Desperate times call for desperate measures, which may explain why some critically endangered smalltooth sawfish are reproducing asexually in the wild. 

The smalltooth sawfish is a member of the ray family, and under normal circumstances it reproduces sexually like most other vertebrates. However, in a study published in Current Biology, scientists report seven instances of juvenile sawfish that appear to be the product of a virgin birth -- no dad involved.

The researchers were able to tell that these fish were produced asexually because routine DNA tests showed there was little to no variability in the genetic material of the seven specimens.

Asexual reproduction in a sexually reproducing species is called facultatitve parthenogenesis. Scientists think it occurs when an unfertilized egg absorbs a genetically identical sister cell called a polar body.

They do not yet know what might trigger that reaction, however, according to Andrew Fields, a PhD candidate at Stony Brook University and the lead author of the study.

It has been observed before in birds, reptiles and sharks, but only when they are living in captivity.

Researchers recently found a copperhead and cottonwood snake that were pregnant with one parthenogenetic baby each, but the babies were never born, so it is unclear if they would have survived in the wild.

The smalltooth sawfish is a large ray that is born at about 2 feet in length and can grow up to 12 feet in length. It lives mostly in southern Florida, and researchers estimate that its population has declined to just 1% to 5% of what it was in 1900.

The decline has been due to overfishing and loss of habitat, Fields said.

Between 2004 and 2013, scientists took DNA samples from the fins of 190 smalltooth sawfish to see if their limited population was leading to high instances of inbreeding. They found that 85% of the specimens were the offspring of non-relatives -- a good sign. However, 3% of the fish sampled were the offspring of just a single parent.

"It's not a majority, it's not even a quarter," said Fields. "It is still rare in this case." 

Smalltooth sawfish are a long-lived species that take years to develop into sexual maturity, so it is still unclear how well these virgin-birth fish survive and if they will be able to reproduce sexually. 

In the meantime, Fields is hoping other researchers who study endangered sharks, rays, lizards, snakes and birds will look for evidence of asexual reproduction that they may have missed.

"We don't know if this commonly happens when species are at a very low density," Fields said. "Because it has been seen in captivity that would lead to the conclusion that it is not so unusual."

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