No sex please, we're sawfish!

Are males truly essential for reproduction? Not if you’re a sawfish, it seems.

Scientists have discovered that three percent of the critically endangered sawfish population living in a Florida estuary are the result of 'virgin births'.

Female birds, reptiles and sharks living in captivity have sometimes surprised their keepers by giving birth even though they have never been housed with a male. These offspring are produced by asexual reproduction, a process called parthenogenesis, or “virgin birth.” Although these events have captured tremendous public interest, it was unknown if this ever occurred in wild populations of these animals.

Now scientists at Stony Brook University, Florida Fish and Wildlife Conservation Commission, and The Field Museum have discovered that around three percent of the sawfish living in a Florida estuary are apparently the products of this type of reproduction, the first evidence of this in the wild for any vertebrate animal. These outwardly normal "parthenogens" live alongside other sawfish produced through normal sexual reproduction, suggesting that occasional virgin births may be more common in natural populations than ever suspected.

The Smalltooth sawfish, Pristis pectinata, is one of five species of sawfish, a group of large rays known for their long, tooth-studded rostrum that is used to subdue small fish. Sawfish may be the first entire family of marine animals to become extinct because they are all critically endangered as a result of overfishing and coastal habitat loss.

Parthenogenesis is thought to be triggered by an unfertilised egg absorbing a sister cell called the polar body that is nearly genetically identical to the egg. This results in an offspring that has roughly half the genetic diversity of its mother. In many cases these offspring are malformed or die early. But the seven sawfish parthenogens the scientists found looked to be in perfect health and were a normal size for their age. This suggests parthenogenesis is not a reproductive dead end, assuming they grow to maturity and reproduce.

The research team speculates that since Smalltooth sawfish are so rare, females might sometimes fail to find a male during the mating season, inducing the parthenogenetic process.

The paper, entitled 'Facultative parthenogenesis in a critically endangered wild vertebrate,' is published in the June 1, 2015 issue of the Cell Press journal Current Biology.

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