This Trisexual Worm Bends The Rules Of Typical Genetics


Tom Hale

Senior Journalist

clockJan 19 2018, 17:08 UTC

A root-knot nematode (not the species from this study). William Wergin and Richard Sayre/Wikimedia Commons

Meet Auanema rhodensis, a species of nematode with three different sexes that bends the normal rules of genetics.

The reproductive strategy of A. rhodensis relies on three different sexes: males, females, and hermaphrodites that make both eggs and sperm. There’s nothing too unusual about that since many species of invertebrates, like the humble earthworm, are hermaphrodites.


However, these nematodes are no normal hermaphrodites, as shown by a new study published in the journal Current Biology. Unlike their other triple-sex buddies, this species is self-fertilizing and trisexual, so they are happy to breed with males, hermaphrodites, and females. 

In short, breeding between females and males only produces female offspring and hermaphrodites mating with males creates many male offspring. Finally, hermaphrodites can self-fertilize to produce females or other hermaphrodites.  

"What's not quite so common are self-fertile hermaphrodites. Think about earthworms: They're hermaphrodites, but it still takes two, because of the way the sex works, they're not self-fertile," Diane Shakes, lead researcher, said in a statement. "And in some organisms, when the 'leader of the pack' dies, another changes sex to be the new leader.”


"What we've figured out is that A. rhodensis has developed ways to stray from the genetics rulebook – specifically in regards to how it handles its X chromosome," she said.

Here’s how it “normally” works with most animals, such as humans. Biological sex is determined by pairings of the X and Y chromosomes. Females have an XX and males are XY. To ensure there’s a healthy 50/50 mix of males and females, female eggs only consist of a single X while male sperms are 50 percent X and 50 percent Y.

In A. rhodensis it’s a bit different because both the females and hermaphrodites are XX, while the males have a single X and no Y. Normally with hermaphrodites, they produce 1 X egg and 1 X sperm. However, this particular nematode produces sperm with two X chromosomes and eggs with none. So, when males mate with females, they produce only female offspring. 


"When hermaphrodites produce offspring through self-fertilization, they produce mostly XX females and XX hermaphrodites," said Shakes. "However, when hermaphrodites cross with males, the joining of 1X male sperm with no-X eggs yields a jackpot of male offspring!"

All of this oddness means the nematode's inheritance patterns confound the predictions of classical Mendelian genetics. Shakes added, "We are still figuring how exactly they do this, but this setup yields pretty interesting genetics."

It’s strange stuff, but it obviously has its advantages. Although it means self-fertilizing hermaphrodite offspring do not have the usual genetic diversity, it allows them to pass on their DNA without the need for a mate and fiddly courtship rituals. 

  • tag
  • genetics,

  • sex,

  • nematode,

  • hermaphrodite,

  • reproductive,

  • trisexual,

  • Mendelian genetics,

  • hermaphroditism