Game Theory Explains Fertility Mystery in Nematodes: Why Are 9% Males?

In Mesorhabditis, the girl worm uses the boy worm for sex, to 'fertilize' her eggs, but the kids are clones of herself. New study explains how and why the males are still around

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Fossil of Eophasma jurasicum, an extinct nematode
Fossil of Eophasma jurasicum, an extinct nematode, which is less upsetting than a live one.Credit: Ghedoghedo

Men are nothing but ambulatory sacs of sperm, some haters say. That's not nice. But it's positively complimentary compared with the role to which the male Mesorhabditis belari nematode has been reduced.

In a unique form of reproduction, as far as science knows, the female Mesorhabditis worm uses the male sperm to fertilize her eggs – as do we all – but then isolates his DNA in the fertilized egg. The "fertilized" egg – should we say "activated" – becomes an embryo that is a genetic clone of the mother. The male's DNA sits there uselessly twiddling its thumbs, and is not passed onto "his" daughters.

"M. belari [is] a unique case in which males make no genetic contribution, and can instead be seen as a simple extension of females by helping them initiate the development of their eggs," writes the international team from the CNRS, l'ENS de Lyon, l'Université Claude Bernard Lyon, and the Muséum national d'Histoire naturelle.

Yes, lady Mesorhabditises clone themselves and thus perpetuate the female of the species.

Which begs the question of how male Mesorhabditis are created. And they are. About 9 percent of these roundworms are male.

M. belari nematode embryos as they divide: cytoskeleton is green, female DNA is magenta, and male DNA is white. Male DNA is not included in the nucleus of embryonicCredit: Marie Delattre/LBMC/CNRS Photo library

It turns out, the team explains in their seminal paper in Population Biology, that a boy Mesorhabditis ensues in the rare cases where the egg does not isolate male DNA after fertilization, but uses the sperm's genetic material. Ta da! So a male may pass his genetic material on - but only to his sons.

From the perspective of evolutionary logic, if males do not transmit their mother's genes, then at least they should spread their mothers' genes by helping their daughters have as many descendants as possible. That way the clan will live long and prosper. But: "This is possible only if the sons produced by a female, help her daughters produce a large number of embryos. In other words, if the males preferentially fertilize their sisters," they write.

Okay. Fine. But: Why are 9 percent of nematodes male as opposed to 50 percent or 2 percent? To answer that, the researchers tapped game theory, and showed that sustaining a proportion of 9 percent males is a stable evolutionary strategy.

"This quantity is enough to ensure that a maximum number of female descendants are produced, without wasting too many resources in the production of males whose genes have no future." Those are the researchers' words.

Other fun facts about the nematode:

They are a vast family of worms that can and will infect everything from elephants to ant larvae to corn and tea. We will spare you a picture of a nematode inside an ant larva. Google it if you like.

Every single species on earth has its own nematode that preys on it, or more than one. That isn't to say that nematodes can only infect one species, mind you.

Many nematodes are thankfully microscopic but others can reach 8 meters in length. Placentonema gigantissima live in sperm whales. That alone sounds like a reason to ban whale-hunting: they have enough trouble.

According to Scientific American, four out of every five animals on Earth is a nematode.

They do not all consist of self-cloning queens who use males basically as a reproductive crutch: Most have "normal" sex. Or some sex. Some are hermaphrodite.

Not all nematodes are parasites! Some eat bacteria. Some are scavengers. Some eat other nematodes.

And here is a helpful hint, if you reached this far. Don't look at your dinner under a microscope. The end.