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Scientists say they documented the first virgin birth in a crocodile. And while the concept might sound bizarre to humans, this type of reproduction is not uncommon across the animal kingdom, according to researchers.
The crocodile in question, named Coquita, lived alone for 16 years at a Costa Rican zoo called Parque Reptilandia before laying a very special clutch in 2018. The fact that Coquita had spent virtually her entire life in isolation. There was almost no chance that she had consorted with male crocodiles.
It was clear evidence that crocodiles are capable of a mode of reproduction called parthenogenesis, in which unfertilized eggs can produce offspring – first presented in an article published June 7 in the journal Biology Letters.
It’s not uncommon for captive reptiles to lay eggs, but “given the period of isolation from mates, these would normally be considered non-viable and discarded,” the study says. But after experts shined a flashlight on the 14 eggs in Coquita’s clutch, they determined that seven of them might be viable and decided to incubate them artificially.
Ultimately, among these seven viable eggs, there was only one fully formed offspring. It never hatched and was considered stillborn.
But a scale from the fetus was donated from Costa Rica to Dr. Warren Booth, a co-author of the new study and a researcher at Virginia Polytechnic Institute and State University. Booth — who has studied parthenogenesis for more than a decade — is the person others in the reptilian world like to call when they suspect an animal has reproduced without mating.
Booth was able to sequence the stillborn crocodile’s DNA and confirmed that it was indeed formed by parthenogenesis and had a genetic makeup 99.9% identical to that of its mother.
“These results therefore suggest that eggs should be examined for their potential viability in the absence of males,” the study said.
Scientists have known for more than a century that some animals are capable of producing offspring without male fertilization. Booth said the first documented incident was seen in pigeons, but viable fetuses were not immediately identified.
Parthenogenesis has since been observed in a variety of different animal species, most notably snakes. And it has also been found in birds, lizards, turtles and sharks. Now, of course, there are crocodiles.
According to Booth, many of the offspring produced in this way are very sick or weak.
They are essentially “highly inbred individuals,” he said.
The genetic cards tend to be stacked against animals that result from this type of reproduction, but “it’s not that they don’t all survive,” Booth added. “Some of them certainly do.”
And offspring produced by parthenogenesis can reproduce — either sexually or through further parthenogenesis, Booth said, although he noted that not all of the research underlying these observations has been published.
Research on the subject evolved slowly before accelerating to exorbitant speed in the 21st century with the advent of DNA sequencing technology, Booth added.
It is very likely that thousands of bird, reptile and other animal species are capable of this type of reproduction. Many of the documented cases involved animals kept in captivity.
“The reason it’s not commonly documented in natural populations is because people don’t look for it,” Booth said. However, there are efforts to study the genes of wild animals to look for evidence of parthenogenic reproduction.
Parthenogenesis isn’t exactly a flawless conception. Each offspring produced in this way shares most of its DNA with the mother.
And the process can only happen in animals with certain types of chromosomes and the ability to pass genes in a certain way.
In short, this type of reproduction cannot occur in humans or other mammals because they use a type of genomic imprinting that “requires the turning on of a specific set of genes by the male and a specific set of genes by the female.” form an embryo, Booth said.
At least it can’t happen naturally in mammals, he noted.
Researchers have managed to create mice born through parthenogenesis in a lab, but they’ve had to undergo “pretty extreme gene editing — turning genes on and off at the right time,” Booth said.
Crocodiles have no sex chromosomes at all, Booth added. And Coquita’s parthenogenically produced offspring—which were female—probably formed that way simply because of the temperature at which the egg was incubated.
Observing parthenogenesis in crocodiles is a fascinating discovery, Booth said, because the method they use is strikingly similar to that used by birds and other reptiles.
“They all use exactly the same cellular mechanism for parthenogenesis,” Booth said. “It is very unlikely that such a complex mechanism would evolve independently.”
That said, he added, it’s likely that crocodiles and birds inherited this ability from their distant relatives – the dinosaurs.
“It’s very likely that the dinosaurs and pterosaurs also had the ability for parthenogenetic reproduction,” he said, joking that it’s all very reserved about Jurassic Park.
However, without dinosaur DNA, scientists will likely never be able to definitively prove this.