2024-03-17 11:18:47
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Charlotte is far from the first animal to have conceived alone.
This event seems to defy the laws of nature. In February 2024, a female stingray named Charlotte became pregnant in a small aquarium in Hendersonville, North Carolina (United States), after having not encountered a male in more than eight years. Scientists from Team Ecco Aquarium & Shark Lab were left perplexed.
How Charlotte conceived four baby stingrays, while floating in her aquarium without a partner, remains a mystery. One theory pointed the finger (or fin) at two white-spotted bamboo sharks that share the aquarium with Charlotte, due to suspicious bite marks found on the stingray’s body. Such markings may be a sign of mating behavior in sharks.
But it would result in an unusual shark-shagreen hybrid. Instead, scientists believe the pregnancy could be the result of a rare phenomenon called parthenogenesis. Coming from the Greek parthenos, which means “virgin”, and genesis, which means “creation”, this phenomenon occurs when an egg develops into an embryo without being fertilized by a sperm.
Charlotte is far from the first animal to have conceived alone. Parthenogenesis is quite common in insects, such as mayflies, but is rarer in vertebrates. Since a captive bonnethead shark gave birth in 2001, other cases have been reported in sharks and reptiles. Charlotte would be the first skate to have undergone parthenogenesis.
The exact reason for parthenogenesis remains a mystery. Some scientists suggest that this is a “last attempt” by females to pass on their genetic heritage to their offspring.
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It’s rare for vertebrates to undergo parthenogenesis, but a North Carolina stingray appears to have become pregnant without a male.
“The whole point of evolution is to pass on your genes,” says Kevin Feldheim, a molecular biologist who uses genetics to study shark populations and mating behavior at the Field Museum in Chicago. “A female isolated from males, who would normally give birth through sexual reproduction, simply does not have this possibility.”
Mr. Feldheim studied another case of parthenogenesis in zebra sharks at the Shedd Aquarium in Chicago in 2008. He first had to rule out the possibility of promiscuity among the aquarium inhabitants.
“There was no direct evidence that the males had mated with the females, but unfortunately the aquarium is not equipped with cameras twenty hours a day, seven days a week,” he explains.
According to Kevin Feldheim, determining a shark’s parentage can be very complicated, because some females can store sperm for months after mating.
Mr. Feldheim developed a paternity test that collects genetic markers called microsatellites. “These markers are used in cases of human paternity, such as in the series CSI and NCIS,” he explains. The results confirmed that the zebra shark’s offspring did not have paternal DNA, but only that of the female.
“The obvious question was how this could happen. The answer is parthenogenesis,” adds Feldheim.
In most animal reproduction, eggs are produced during a process called meiosis, in which cells divide, sharing genetic material and other cellular mechanisms. This process generates three cellular branches called polar bodies. Normally, these polar bodies are reabsorbed by the female, but in parthenogenesis, one of these polar bodies can fertilize the egg and form a viable embryo, thus mimicking sexual reproduction.
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It is common for mayflies to reproduce by parthenogenesis.
It’s a different process from cloning, which has drawbacks, adds Kady Lyons, a research scientist who studies sharks and rays at the Georgia Aquarium.
“The cells used are not a carbon copy of the mother,” she explains. However, because the egg and polar body only contain part of the mother’s genome, the offspring end up less genetically diverse than their mother, as can be seen in a “highly inbred individual,” adds- She.
For some species, asexual reproduction is advantageous. For example, some populations of whip-tailed lizards in Mexico and California have become exclusively female and reproduce asexually. The species has evolved an unusual way of maintaining its genetic diversity through parthenogenesis by doubling the number of chromosomes in the eggs of asexually reproducing females. This method has certain advantages: it allows the species to colonize new regions and avoid pitfalls such as sexually transmitted diseases.
But this also has a cost: their DNA undergoes more harmful genetic mutations through parthenogenesis than through sexual reproduction, due to the absence of natural selection.
Unfortunately, not all species are as well adapted to parthenogenesis as the whip-tailed lizard. In sharks, offspring resulting from parthenogenesis tend to live a short time, rarely reaching sexual maturity. “There is a lack of genetic variation in the offspring that could lead to what is called the expression of delusional recessive alleles,” says Feldheim.
In other words, despite their miraculous conception, vertebrates born via parthenogenesis can be short-lived.
Ms Lyons took part in a trial of artificial insemination in zebra sharks, during which scientists studied the chances of survival of young produced by sexual reproduction and those produced by parthenogenesis. The study found that parthenotes (individuals born through parthenogenesis) lived on average a year less, and many exhibited behaviors that reduced their chances of survival, such as sideways swimming, twirling, head movements and difficulty feeding.
Lyons says she wasn’t surprised to see evidence of parthenogenesis in rays, although Charlotte’s case didn’t solve all the mysteries surrounding the phenomenon. “We don’t know if there is a trigger for females to reproduce in this way. We just assume that when there are boys and girls together, they will do whatever they want.”
However, it is in these unique situations, such as animals cared for by humans, that parthenogenesis is most common. “It’s obvious that life finds a way,” she says.
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