Two live tangled individuals of Gordionus violaceus, a freshwater hairworm, from Germany. Credit: Gonzalo Giribet
In
a world full of bizarre animals, hairworms are some of the strangest. Hairworms
are parasitic worms that manipulate the behavior of their hosts in what's
sometimes called "mind control."
A new study in the journal Current Biology reveals another strange
trait shared by different hairworm species—they're missing about 30% of the genes that
researchers expected them to have. What's more, the missing genes are
responsible for the development of cilia,
the hair-like structures present in at least some of the cells of every other
animal known.
Hairworms are found all over the world, and
they look like skinny strands of spaghetti, a couple inches long. Their simple
bodies hint at their parasitic lifestyle— they have no excretory, respiratory,
or circulatory systems, and they spend almost their entire lives inside the
bodies of other animals.
"One of the coolest things, maybe
the thing that they are most known for, is that they can affect the behavior of
their hosts and make them do things that they wouldn't do otherwise," says
Tauana Cunha, a postdoctoral researcher at Chicago's Field Museum and lead
author of the study done in collaboration with Harvard University and the
University of Copenhagen.
There are a few hundred species of
freshwater hairworms. Their eggs hatch in water, and the hairworm larvae get
eaten by tiny water-dwelling predators like mayfly larvae, which in turn get
eaten by bigger, land-dwelling predators like crickets.
After growing into adulthood inside of
their new hosts' bodies, the hairworms manipulate the hosts' behavior, causing
them to jump into water. There, the worms swim out of their hosts' butts and
seek out mates, knotting themselves together, to begin the cycle anew.
There are also five species of hairworms
that live in marine environments and parasitize water-dwelling creatures like
lobsters, but it's not clear if those ones also have host manipulation
capabilities— there's no pressure for the worms to get back to the water, since
the hosts already live there.
A live
freshwater hairworm in Bruno de Medeiros's hand in the Muir Woods National
Monument in California. Credit: Bruno de Medeiros
As
strange as hairworms' behavior is, Cunha's research interest in the animals has
more to do with their DNA. "We set out to sequence their genomes, because
nothing like them has ever been sequenced before at that level," she says
of the study conducted with her co-authors Bruno de Medeiros, Arianna Lord,
Martin Sørensen, and Gonzalo Giribet. "The goal was to produce those
genomes and eventually use them to understand the evolutionary relationships between hairworms and other kinds of
animals."
She and her colleagues took DNA samples
from two hairworm species— one freshwater and one saltwater— and sequenced
them. But when they compared the hairworms' genetic codes to those of other
animals, they found something striking.
"What we found, which was very
surprising, was that both hairworm genomes were missing about 30% of a set of
genes that are expected to be present across basically all groups of
animals," says Cunha.
Results like that often make scientists
wonder if they've made a mistake. But there was a connection between the
missing genes in the two worm species. "The large majority of the missing
genes were exactly the same between the two species. This was just implausible
by chance," says Cunha.
By looking at what functions these
missing genes are responsible for in other animal groups, Cunha and colleagues
showed that they give the instructions for producing cilia. "Cilia are
organelles, small structures at the cellular level, that are basically present
across all animals and even more broadly, in protists and some plants and
fungi. So they're present across a large diversity of life on Earth," says
Cunha. They're present in many of the cells in the human body: for instance,
the tails of sperm cells are cilia, and cells in the retinas of our eyes have
cilia too.
Previously, scientists had found that hairworms seemed to be missing cilia where they'd normally be found. Hairworm sperm, for example, do not have tails. But while no one had ever seen a ciliated cell from a hairworm, that wasn't considered definitive proof that they didn't have them. It's hard to prove something with negative evidence. "Without the genomes, this would require looking at all cells in all life stages in all species," says Bruno de Medeiros, Curator of Pollinating Insects at the Field Museum and co-author of the paper.
Live
freshwater hairworms in the environment, in the Muir Woods National Monument in
California. Credit: Bruno de Medeiros
"Based
on previous observations, it didn't seem like hairworms had any cilia, but we
didn't really know for sure," says Cunha. "Now with the genomes, we
saw that they actually lack the genes that produce cilia in other animals— they
don't have the machinery to make cilia in the first place."
What's more, the fact that both the
freshwater and marine hairworm species had lost the genes for cilia indicates
that this evolutionary change happened in the deep past to the two species'
common ancestor. "It is likely that the loss happened early on in the
evolution of the group, and they just have been carrying on like that,"
says Cunha.
The finding opens the door to several
new questions. It's not clear how the lack of cilia have affected hairworms, or
if the hairworms' parasitic behavior could be related to the missing cilia.
"There are plenty of other parasitic organisms that aren't missing these
specific genes, so we cannot say that the genes are missing because of their
parasitic lifestyle," says Cunha. "But parasitic organisms in general
are often missing lots of genes. It's hypothesized that because parasites are
not using certain structures and instead rely on their hosts, they end up
losing those structures."
Staged photos of the (dead) squat lobster host Munida
sp., from Norway, with a marine hairworm. The photo was taken now as a
representation of the real scenario of how the worm was collected years ago,
which was used for the genome sequencing. Credit:
Martin Sørensen
Hairworms
aren't the only parasites capable of "mind control"— it's a behavior
that's cropped up in protozoans like the organism responsible for
toxoplasmosis, which reduces rodents' fear of cats, and in the fungus
Ophiocordyceps, made famous by the video game and TV show "The Last of
Us," which manipulates ants into spreading the fungus's spores.
While these organisms are only distantly
related to hairworms, Cunha says that the new study could help scientists find
common threads for how this behavior works. "By doing this comparative analysis across organisms in the future, we might be able
to look for similarities. Or maybe these organisms evolved similar behaviors in
completely different ways from each other," says Cunha.
by Field Museum
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