Although humans and mammals must rely on sex to reproduce, many animals have other options. Sea stars, wasps, marble crabs, jellyfish and ants are only a few species that can reproduce asexually.
Among vertebrates or animals with a backbone, asexual reproduction is less common. However, California condors, New Mexico lizards, and some fish reproduce through a process known as parthenogenesis, a Greek word whose two roots translate to “virgin creation.”
Still, there’s a lot scientists don’t know about asexual animals. That’s why certain species, such as zebra sharks and killer fish, are changing what researchers know about asexual reproduction in nature.
The benefits of asexual reproduction
Why do these animals choose to forego sex and lay eggs containing essentially clones? According to Anne-Marie Dion-Côté, an evolutionary molecular biologist at the Université de Moncton in Canada, there’s a better question to ask: Why do so many animals choose to have sex?
“Sex is expensive for animals,” says Dion-Côté. “They have to look for partners in the environment.”
“You only pass on half of your genes to the next generation,” adds Anne Dalziel, an evolutionary physiologist at St Mary’s University. “You could make the full contribution and not have to pick up a hitchhiker like women usually do with men.”
Despite the trade-offs, vertebrates overwhelmingly choose to have sex over cloning. The reason for this is a fundamental unanswered question in evolutionary biology. Researchers who have thought long enough about the subject often find themselves looking for answers in organisms that break the mold – the asexual vertebrates.
“Scientists like to study oddities,” says Dalziel. “How does sex disrupt this system of making new embryos? And what does it tell us about how sex works?’
The Surprising Origin of the Zebra Shark
ST10 was the first zebra shark ever born in Shedd Aquarium in Chicago. Aquarium tour guides nicknamed her “Bubbles” after watching her go crazy in the emissions from staff masks, and the name stuck.
In 2016, Bubbles was housed with two unrelated male zebra sharks in the Wild Reef exhibit. When she laid eggs, Lisa Watson, the aquarium’s assistant director of habitats and operations, dived in to retrieve them. Then she called geneticist Jean Dubach and asked her to do a paternity test.
“If you’ve ever seen Maury Powish’s show when he says ‘you’re the father!’—we actually use the same types of genetic markers to do paternity tests and maternity tests in sharks,” explains Kevin Feldheim, a population biologist at Chicago Field Museum.
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The results were convincing. None of Bubbles’ roommates fathered her clutch. Eggs are clones produced by parthenogenesis. It was the first time anyone had seen a zebra shark reproduce asexually while coexisting with viable mates. accidentally another team at the Aquarium of the Pacific in Long Beach, California observed the same phenomenon at about the same time.
In November Fellheim, Dubach and Watson published their findings in Journal of Fish Biology, shortly after the California researchers published theirs.
Zebra sharks were previously thought to reproduce asexually only as a last-ditch effort to pass on their genes in the absence of a viable sexual partner. These findings punch a gaping hole in that theory.
“We thought we knew what was going on, and now it just raises more questions,” says Watson.
These questions may prove important to the survival of zebra sharks in the wild. Benthic predators are becoming less common in their range along the Indian and Pacific Oceans and IUCN Red List classifies the species as endangered. Unfortunately, zebra shark clones tend to die before reaching maturity. A better understanding of when (and why) these sharks switch from sexual to asexual reproduction could help conservationists successfully breed them in captivity.
Killifish and hybrid clones
While parthenogenesis appears to be a losing strategy for zebra sharks, not all asexual offspring suffer the same fate. Much further north, Dalziel and Dion-Côté are studying a hardy fish that has been successfully cloned for generations.
Mummichogs aka “muddy grits”, are a type of killer fish the size of a Twix bar. They tend to thrive along the Atlantic coast in salt water where rivers and wetlands border the ocean. Meanwhile, banded killer fish, close cousins of the mumichog, live primarily in freshwater in the eastern United States—and are among the few vertebrate species known to be able to reproduce asexually. However, there are areas not completely saline or completely saline where the species coexist.
More than a decade ago, a male mummified inseminated the eggs of a banded bloodfish. At least one of the eggs gave rise to a female hybrid. The daughter was sterile, but she was also quite adept at asexual reproduction. She gets cloned, her daughters get cloned, their daughters get cloned… Years later, clones of this fish are numerous inhabitants of Porter Lake in Nova Scotia, Canada.
“The hybrid offspring look pretty good,” says Anne Dalziel. “In some cases, they do better than a parent species.”
In fact, mummichog and banded killifish exchange genes somewhat regularly. Dalziel and Dion-Côté found several hybrids when they went fishing at Porter Lake. None of the clones are sexually viable, but some lines may be decades or even centuries old.
Hidden asexual animals
While asexual mothers avoid the dangers and cost of sex, asexuality comes with its own set of consequences. Chief among these are what Dion-Côté calls a “genetic dead end.” Apart from random mutations, asexual mothers pass on the same genome generation after generation. When the environment changes in a way that disadvantages the clones, everyone suffers the same consequences. In addition, deleterious mutations can accumulate over generations.
Read more: Sexual cannibalism: Why women sometimes eat their partners after sex
Dion Côté explains that conventional wisdom in evolutionary biology predicts that asexual lineages “will eventually become environmentally maladapted.” Yet some organisms, such as marine fish, challenge this assumption. A separate group of German researchers identified a type of mite that has survived without sex for millions of years.
In the past decade, scientists have identified asexual reproduction in a number of new species. Many of these discoveries happened by accident. Last year, conservationists at the San Diego Zoo collected genetic information on the California condor population and found that some of their birds were born by parthenogenesis.
Dalziel predicts that many more asexual species will be identified as genetic testing becomes cheaper and more readily available. For all you know, that songbird flying between the trees in the park could be a clone. By 2021scientists have sequenced the complete genomes of only 0.2% of known animal species.
“Unless you’re looking for DNA to see if they’re clones of a parent, you might not know,” says Dalziel. “It’s possible there are hidden asexuals out there somewhere.”