Weird Sex: Tri-Sexual Worms Use Bizarre Genes to Help Them Survive

nematode kennedy space
C. elegans nemotodes, or round worms, undergo examination by project scientists May 1, 2003 at the Kennedy Space Center in Florida. Volker Kern/NASA/Getty Images

A type of roundworm are bending the rules of genetics, according to a paper published in December in Current Biology. These worms appear to be manipulating the sex ratios of their offspring using an unusual chromosome arrangement.

The worm featured in the study, Auanema rhodensis, is the only animal that gets an X chromosome passed down from their father, not their mother, study author and University of Warwick biologist Andre Pires-daSilva told Newsweek.

Of course, words like "father" and "mother" may be difficult to apply to these worms. This particular species has three possible sexes: male, female and hermaphrodite. That's not unusual. Here's the weird part: when hermaphroditic worms and male worms mate, only more male worms are produced.

In humans, and most other animals, a child has a 50 percent chance of being male or female because of the way chromosomes are inherited. A mother has only an X chromosome to give, but the father can pass down either an X or a Y chromosome. There are only two options, with an equal chance of either happening.

Not the case for these worms. Instead, the number of X chromosomes varies. Hermaphrodites and female worms have two X chromosomes; males have just one X chromosome. But Pires-daSilva and his collaborators found that some hermaphroditic worms will produce sperm with two X chromosomes and eggs with no chromosomes.

"In humans this happens once in a while as well—that oocytes have no X chromosome," he said. But it's rare that eggs with chromosomal abnormalities survive. It's far, far more common in worms, however; something must be happening that allows these eggs to survive.

Regardless, the end result is that when an egg and a sperm come together, the only possible result is XO—a male. Why on earth would worms want to do that? "That's a good question," Pires-daSilva said.

Our newest paper, on the only animal (as far as we know) in which sons inherit the X chromosome from their father:

— Andre Pires (@andre_evol) December 21, 2017

One possible explanation is that male worms may actually be important for the species' genetic diversity. Having kids with your cousins or siblings is genetically risky. Remixing the same genes may increase the likelihood that an odd mutation found in a family will actually show up in a person's offspring. Bringing in some fresh genes—by, say, bothering to create a ton of new male worms—could help the gene pool stay diverse.

This paper also underscores something a bit more philosophical. "Sexual identity is super important. That's the first thing you talk about when someone is pregnant," Pires-daSilva noted. "And we think it is so important that it's difficult to change in evolution, because if anything goes wrong, you might have someone with a mixed sex or someone who cannot reproduce."

But that's not actually the case, he said. There are many, many different ways of determining sex, and they seem to evolve very rapidly; some house fly populations in Japan have a different sex determination system than their relatives in Europe. "We are trying to understand the why question—why the sex determination evolves so fast, even though it's so important," he said.

What does this mean for humans? Nothing, at the moment. But in theory, he said, understanding how some animals recognize a mistake in an egg and let it slide could shed some light on how our own system works to shut down genetic glitches.