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This Worm Hasn’t Had Sex in 18 Million Years

By fusing its chromosomes, the creature could essentially clone itself while still maintaining genetic variation

Diploscapter pachys hasn't had sex for 18 million years, and is doing just fine (Image courtesy of Karin Kiontke and David Fitch)
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Researchers recently sequenced the DNA of a species of small translucent worm, discovering it has not had sex for more than 18 million years. And it's survived just fine, reports Rae Ellen Bichell for NPR

Sexual reproduction evolved more than a billion years ago on Earth, and it provides significant genetic benefits to the many animals that have adopted it. Along with increasing the rate of natural selection and evolution, sexual reproduction also allows for easy mixing of genes, which keeps the natural random mutations that commonly occur in DNA from building up over time. That buildup can result in a "mutational meltdown" that can severely impact the health and continued survival of the remaining animals, writes Bichell. (A famous example of this among humans is the severe disorders suffered by some Egyptian pharaohs as a result of inbreeding of their ancestors.) 

Unable to adapt to these mutations quickly, many asexual species often end up going extinct. But not all meet that fate.

"It has been a longstanding mystery in biology how some asexual animals have survived for so many generations," biologist David Fitch of New York University says in a statement. Fitch is an author of the new study published in the journal Current Biology that documents an exception to that trend: the roundworm Diploscapter pachys.

The creature has exclusively practiced asexual reproduction since it parted from its parent species roughly 18 million years ago. The researchers wondered: how has this creature avoided the buildup of genetic mutations? It turns out, it has developed a method to near perfectly clone itself.

"Somehow, the worm fused its ancestors' six pairs of chromosomes into one pair of huge chromosomes. It did away with a major step of meiosis — the part of the reproductive process where chromosomes reshuffle before splitting into two cells," writes Bichell. That means that they can still copy themselves while maintaining fairly high genetic diversity.

It's an unusual condition for complex critters, the researchers write in the study. Only two other organisms are know to have just a single pair of chromosomes: the nematode Parascaris univalens and the ant Myrmecia croslandi.

Why would a creature go through the effort of creating its own asexual reproduction method? Sex is an expensive biological process, notes Bichell. Organisms must spend time and energy competing for mates, and only one half of the population is actually capable of creating new organisms, limiting the potential growth of a species. Asexual organisms meanwhile can just focus on cloning themselves, giving themselves roughly double the potential birth rate of sexual organisms. So that's the path D. pachys took 18 million years ago.

D. pachys isn't alone in practicing some unusual reproductive techniques, however. Unisexual mole salamanders, for example, steal sperm from other, related species to help kickstart their own reproduction for some still-unknown reason. Meanwhile, females from several shark species can reproduce on their own by either storing sperm for years at a time, or even copying their own genetic material like D. pachys. And water fleas can actually switch between reproducing asexually and sexually when times are rough.

It turns out, humans are pretty tame when it comes to reproducing in the animal world.

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