Over the past three years I've told you about virgin female sharks that have been giving birth to seemingly healthy pups. This was documented in 2007 and then again in 2008.
(A bonnethead shark, born in a virgin birth, is shown swimming. Analysis of the shark confirmed it had no father.
Research also confirmed it carried only half the genes of its mother. Image credit: Lee G. Simmons)
Now a new study, published in the Journal of Heredity, concludes that sharks born to virgin mothers can survive over the long-term. Two daughters of a white-spotted bamboo shark virgin, for example, are now over five years old.
Scientists are still puzzled by this phenomenon, so Kevin Feldheim, manager of the Pritzker Laboratory for Molecular Systematics and Evolution at the Field Museum, also studied the shark sisters' genetic material to ensure that daddy wasn't just absent from their records, but that he never existed in the first place.
"Examination of highly variable sections of the genome prove that these young sharks had no father," Feldheim said. "These findings are remarkable because they tell us that some female sharks can produce litters of offspring without ever having mated with a male."
"We compared several sections of the genome between two of the young sharks and their mother," he added. "It turned out that all the genetic material in each of the young ones came from the mother, proving there was no father."
Although the shark mother was kept in a tank at the Belle Isle Aquarium in Detroit where only another female of a different but related species resided, genetic testing was needed to rule out the possibility that the female shark could have encountered male sperm earlier in her life.
Another DNA study performed by Queen's University, Belfast, scientists, found that the virgin offspring didn't share all of their mother's genes. Feldheim instead calls the sharks "half-clones."
Virgin birth, known as parthenogenesis, happens when an egg or ovum fuses with a cell called a sister polar body, a byproduct of ova production, rather than with male sperm, to promote cell division. According to Demian Chapman of the Institute for Ocean Conservation Science at Stony Brook University, co-author of the current study, the sister polar body is nearly genetically identical to the ovum.
He now believes, "parthenogenesis may not be as much of a dead-end mode of reproduction as we thought for these sharks."
Douglas Sweet, who formerly worked at the Detroit aquarium and is now superintendent of the London State Fish Hatchery in London, Ohio, agrees with the new findings. He theorizes that virgin births first happened in isolated, non-captive female sharks, as a way to keep the population going even when mating wasn't possible.
"Sharks have been around for hundreds of millions of years," Sweet said. "I suspect they have some pretty interesting survival strategies that we are only now becoming aware of."