So you know the mind-controlling fungus that makes its victims climb trees and all that?

Plenty of other parasites alter their host’s behavior as part of their M. O. to complete their life cycle.

But the parasites known as hairworms are doing it with a genetic shortcoming we’ve never seen before. Hairworms are missing a huge portion of genes that are found in every other animal we know of. And the best part is it makes their name pretty ironic. [♪ INTRO] Parasites as a general rule lead strange lives that result in strange genetics, and hairworms fit that mold.

We know that most hairworms split their time between freshwater and land, though a few others are found in marine environments as well. Well, technically most of their time is spent inside other creatures in these habitats, doing things like forcing their insect host to go for a swim to let the adult worm emerge to look for a mate. But overall, we didn’t know a whole lot about them, so researchers sequenced the genomes of two hairworm species, one freshwater and one marine.

And they were surprised to find their genes were very sparse. Published in the journal Current Biology in August 2023, their analysis revealed that hairworms are flat out missing 30% of the basic set of genes we’d expect to find in any given animal. And one of the key missing pieces is the ability to produce a cell structure that is, as far as we know, universal among animals.

These parasites have no cilia, the hairlike structures found on cells. So yeah, hairworms have hairless cells. Now, given that hairworms spend most of their time snuggled up inside the bodies of other organisms, it’s not so strange to have a reduced genome.

After all, they’re not the ones doing the work navigating a complex outer world. When you’re a parasite, freeloading is the point. But still, no cilia is an extreme take, even for a parasite, because they seem so vital to, well, basically all other animals – and plenty of other groups too.

In single-celled organisms, these tiny hairs whip around to help with locomotion and feeding. And in multicellular organisms like us, they get our sperm from point A to point B, keep fluids flowing around the brain, and most importantly in this scenario, they’re heavily involved in how we sense our environment. So this key cellular feature plays a broad range of functions in cells.

And it’s not just that hairworms’ cells lack cilia. It’s that they couldn’t make them even if they wanted to. Their genome doesn’t have the instructions to do it, period.

These gene instructions were missing in both the marine and freshwater hairworm species that were studied, which implies that their common ancestor lost the ability to make cilia long ago. So they’re not only doing fine without cilia, but they have been for an incredibly long period of time! What’s even more odd about this discovery is that hairworms do have a free-living phase, so they’re not the type of parasite that never lives outside of their hosts.

And remember that cilia are crucial to animal sensory systems – at least as we know them. So those free-living hairworms are somehow finding mates and reproducing without any sensory mechanism we understand so far. Not to mention finding hosts, though once they’re inside, sensing becomes a bit less important.

However hairworms are finding their way around, they are not using cilia to do it. This study did find a bunch of unique genes whose functions haven’t been discovered yet, so maybe those play a key role in surviving life without cilia, but we really just don’t know yet! This discovery really makes us think twice about just how much genetic equipment an organism can afford to lose.

Somehow, hairworms are pulling off life without what we thought of as key genes for survival. But what we do know is that hairworms are smoother than a baby’s bottom, which makes their name kind of hilarious. And speaking of not-quite-babies, you parents and teachers out there might like to know that we have a spinoff channel: SciShow Kids.

You can join our main character Squeaks as he goes on fun adventures and learns about the world around him. And our episodes are classroom-ready, being built around the second grade Next Generation Science Standards for science education. We happen to think it’s a pretty great way for kids to learn and have fun at the same time.

So why not show it to the young people in your life? Thanks for watching, everyone, and see you next time. [♪ OUTRO]