Male nematode worms have an extra pair of brain cells -- and scientists think the neurons exist to help them remember to have sex. In a study published Wednesday in Nature, researchers describe -- and demystify -- what they've dubbed MCMs (mystery cells of the male).

It's hard out there for a male nematode. See, the species Caenorhabditis elegans -- the first animal ever to have its genome sequenced -- doesn't come in "male" and "female." There are males, who carry sperm and need to have sex in order to pass along their genes, and then there are hermaphrodites -- who can make baby nematodes using their own sperm. The male nematodes exist to inject some genetic diversity into the population, which makes the species less fragile. But since hermaphrodite nematodes have no reason to want sex, the male has to be very proactive about seeking a mate.

That's where the MCMs come in.

Lead study author Arantza Barrios of University College London found the mysterious cells of the male when she was studying how certain nematode genes express themselves.

"I found it was expressed in a bed of cells that didn't correspond to any previous description," Barrios told The Post. That surprised her, because C. elegans has been studied quite extensively.

But they were indeed new cells. Barrios soon realized that they were unique to the male worms (most researchers focus on hermaphrodites, she explained) and that they only showed up when the worms were sexually mature.

That specificity explained how they'd escaped notice for so long -- and it also gave a big clue as to their purpose.

"The reasoning was that they must have something to do with male sexual behaviors," Barrios said. But none of the classic male mating behaviors seemed to be affected when Barrios and her colleagues switched the neurons off.

But then they tested associated learning -- the ability of the worms to remember that one thing was related to another.

When nematodes -- hermaphrodites, males, and males with their MCMs deactivated -- were kept hungry in areas with lots of salt, the purpose of the MCMs finally made themselves known. When just hunger was associated with a salty area, all of the nematodes would avoid higher concentrations of salt when moves to a new enclosure. But when a salty enclosure brought a mate within reach -- along with a lack of food -- male nematodes would scurry right over to the salty corner of their new enclosure. Male nematodes who lacked MCMs showed the same behavior as hermaphrodites, choosing to avoid hunger instead of choosing to seek out sex.

The working hypothesis, Barrios said, is that the male worms need to be able to prioritize mating opportunities over other impulses -- and these MCMs help them do so. The researchers believe that these extra neurons change the way information is processed in that region of the nematode brain. 

Barrios's lab will continue to test the function of the cells, trying to figure out how and why they tweak brain function in the worms. Meanwhile, her co-author Richard Poole (also of University College London) and his lab will try to puzzle out the details of their origin.

Poole has already found that the cells develop from glial cells -- the non-neurons that make up most of the brain. Scientists used to think that glial cells were just taking up space in the brain, but now they know that they can act as stem cells, and form new neurons. This is the first case of that seen in an invertebrate, so the researchers are excited to learn more.

Because nematodes are so easy to study in the lab, this could allow researchers to study a potentially therapeutic process -- the formation of brand new brain cells -- on a tiny, easy to control scale.

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