By Carl Zimmer, The New York Times, February 5, 2018
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| The marbled crayfish is a mutant species that clones itself, scientists report. The population is exploding in Europe, but the species appears to have originated only about 25 years ago. Photo: Ranja Andriantsoa. |
Frank Lyko, a biologist at the German Cancer Research Center, studies the six-inch-long marbled crayfish. Finding specimens is easy: Dr. Lyko can buy the crayfish at pet stores in Germany, or he can head with colleagues to a nearby lake.
Wait till dark, switch on head lamps, and wander into the shallows. The marbled crayfish will emerge from hiding and begin swarming around your ankles.
“It’s extremely impressive,” said Dr.
Over the past five years, Dr. Lyko and his colleagues have sequenced the genomes of marbled crayfish. In a study published on Monday, the researchers demonstrate that the marble crayfish, while common, is one of the most remarkable species known to science.
Before about 25 years ago, the species simply did not exist. A single drastic mutation in a single crayfish produced the marbled crayfish in an instant.
The mutation made it possible for the creature to clone itself, and now it has spread across much of Europe and gained a toehold on other continents. In Madagascar, where it arrived about 2007, it now numbers in the millions and threatens native crayfish.
“We may never have caught the genome of a species so soon after it became a species,” said Zen Faulkes, a biologist at the University of Texas Rio Grande Valley, who was not involved in the new study.
The marbled crayfish became popular among German aquarium hobbyists in the late 1990s. The earliest report of the creature comes from a hobbyist who told Dr. Lyko he bought what were described to him as “Texas crayfish” in 1995.
The hobbyist — whom Dr. Lyko declined to identify — was struck by the large size of the crayfish and its enormous batches of eggs. A single marbled crayfish can produce hundreds of eggs at a time.
Soon the hobbyist was giving away the crayfish to his friends. And not long afterward, so-called marmorkrebs were showing up in pet stores in Germany and beyond.
As marmorkrebs became more popular, owners grew increasingly puzzled. The crayfish seemed to be laying eggs without mating. The progeny were all female, and each one grew up ready to reproduce.
In 2003, scientists confirmed that the marbled crayfish were indeed making clones of themselves. They sequenced small bits of DNA from the animals, which bore a striking similarity to a group of crayfish species called Procambarus, native to North America and Central America.
Ten years later, Dr. Lyko and his colleagues set out to determine the entire genome of the marbled crayfish. By then, it was no longer just an aquarium oddity.
For nearly two decades, marbled crayfish have been multiplying like Tribbles on the legendary “Star Trek” episode. “People would start out with a single animal, and a year later they would have a couple hundred,” said Dr. Lyko.
Many owners apparently drove to nearby lakes and dumped their marmorkrebs. And it turned out that the marbled crayfish didn’t need to be pampered to thrive. Marmorkrebs established growing populations in the wild, sometimes walking hundreds of yards to reach new lakes and streams. Feral populations started turning up in the Czech Republic, Hungary, Croatia and Ukraine in Europe, and later in Japan and Madagascar.
Sequencing the genome of this animal was not easy: No one had sequenced the genome of a crayfish. In fact, no one had ever sequenced any close relative of crayfish.
Dr. Lyko and his colleagues struggled for years to piece together fragments of DNA into a single map of its genome. Once they succeeded, they sequenced the genomes of 15 other specimens, including marbled crayfish living in German lakes and those belonging to other species.
The rich genetic detail gave the scientists a much clearer look at the freakish origins of the marbled crayfish.
It apparently evolved from a species known as the slough crayfish, Procambarus fallax, which lives only in the tributaries of the Satilla River in Florida and Georgia.
The scientists concluded that the new species got its start when two slough crayfish mated. One of them had a mutation in a sex cell — whether it was an egg or sperm, the scientists can’t tell.
Normal sex cells contain a single copy of each chromosome. But the mutant crayfish sex cell had two.
Somehow the two sex cells fused and produced a female crayfish embryo with three copies of each chromosome instead of the normal two. Somehow, too, the new crayfish didn’t suffer any deformities as a result of all that extra DNA.
It grew and thrived. But instead of reproducing sexually, the first marbled crayfish was able to induce her own eggs to start dividing into embryos. The offspring, all females, inherited identical copies of her three sets of chromosomes. They were clones.
Now that their chromosomes were mismatched with those of slough crayfish, they could no longer produce viable offspring. Male slough crayfish will readily mate with the marbled crayfish, but they never father any of the offspring.
In December, Dr. Lyko and his colleagues officially declared the marbled crayfish to be a species of its own, which they named Procambarus virginalis. The scientists can’t say for sure where the species began. There are no wild populations of marble crayfish in the United States, so it’s conceivable that the new species arose in a German aquarium.
All the marbled crayfish Dr. Lyko’s team studied were almost genetically identical to one another. Yet that single genome has allowed the clones to thrive in all manner of habitats — from abandoned coal fields in Germany to rice paddies in Madagascar.
In their new study, published in the journal Nature Ecology and Evolution, the researchers show that the marbled crayfish has spread across Madagascar at an astonishing pace, across an area the size of Indiana in about a decade.
Thanks to the young age of the species, marbled crayfish could shed light on one of the big mysteries about the animal kingdom: why so many animals have sex.
Only about 1 in 10,000 species comprise cloning females. Many studies suggest that sex-free species are rare because they don’t last long.
In one such study, Abraham E. Tucker of Southern Arkansas University and his colleagues studied 11 asexual species of water fleas, a tiny kind of invertebrate. Their DNA indicates that the species only evolved about 1,250 years ago.
There are a lot of clear advantages to being a clone. Marbled crayfish produce nothing but fertile offspring, allowing their populations to explode. “Asexuality is a fantastic short-term strategy,” said Dr. Tucker.
In the long term, however, there are benefits to sex. Sexually reproducing animals may be better at fighting off diseases, for example.
If a pathogen evolves a way to attack one clone, its strategy will succeed on every clone. Sexually reproducing species mix their genes together into new combinations, increasing their odds of developing a defense.
The marbled crayfish offers scientists a chance to watch this drama play out practically from the beginning. In its first couple decades, it’s doing extremely well. But sooner or later, the marbled crayfish’s fortunes may well turn.
“Maybe they just survive for 100,000 years,” Dr. Lyko speculated. “That would be a long time for me personally, but in evolution it would just be a blip on the radar.”
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