Unusual “Mind-Manipulating” Parasitic Hairworms Lack Genes Present in All Other Animals

Two intertwined living specimens of Gordionus violaceus, a freshwater hairworm, from Germany. Image provided by Gonzalo Giribet.

Contrary to other animals, hairworms do not possess the minuscule “hairs” crucial for cell movement, filtration, and sensory functions.

Among the myriad of peculiar creatures, hairworms arguably rank as one of the oddest. These parasitic worms are notorious for their capacity to alter their hosts’ behaviors, sometimes labeled as “mind control.”

A novel study featured in Current Biology has disclosed an intriguing characteristic shared by hairworm species: they are devoid of about 30% of the genes that scientists anticipated. Even more captivating, these missing genes are tied to the growth of cilia, hair-like formations discovered in the cells of almost every animal species known.

A live freshwater hairworm held by Bruno de Medeiros at the Muir Woods National Monument in California. Image provided by Bruno de Medeiros.

Resembling thin strands of spaghetti and only a few inches long, hairworms can be found worldwide. Their basic body structure exposes their parasitic nature since they do not have excretory, respiratory, or circulatory systems and reside almost exclusively within other creatures. Tauana Cunha, a postdoctoral researcher at Chicago’s Field Museum and the study’s lead author, states, “Their most intriguing trait, perhaps what they’re most famous for, is their ability to influence their hosts’ behaviors, compelling them to act in ways they normally wouldn’t.”

Life Cycle and Host Manipulation of Hairworms

There are hundreds of freshwater hairworm species. The lifecycle begins when eggs hatch in water, and larvae are consumed by small aquatic predators like mayfly larvae. These in turn are eaten by larger, land predators such as crickets. The hairworms mature inside their hosts, then alter the hosts’ behavior to make them jump into water. There, the worms escape from their hosts and start searching for mates, thereby continuing the cycle.

There are also five marine hairworm species that parasitize aquatic creatures like lobsters. However, it is unclear if they can manipulate their hosts due to the lack of a need to return to the water.

Live freshwater hairworms in their natural habitat at the Muir Woods National Monument in California. Image provided by Bruno de Medeiros.

Hairworm Genetic Research

As peculiar as hairworms’ behavior is, Cunha’s research interest extends to their DNA. “Our objective was to sequence their genomes, given nothing like them has ever been sequenced at that level before,” she says of the study carried out with her co-authors Bruno de Medeiros, Arianna Lord, Martin Sørensen, and Gonzalo Giribet. “Our goal was to generate these genomes and eventually use them to understand the evolutionary relationships between hairworms and other animal types.”

After extracting DNA samples from two hairworm species, one freshwater and one marine, and sequencing them, the team was astounded to discover major differences between the hairworms’ genetic codes and those of other creatures.

Staged photo of a deceased squat lobster host Munida sp., from Norway, with a marine hairworm. The picture was taken now to represent the original scenario when the worm was collected years ago for genome sequencing. Image provided by Martin Sørensen.

Discovery of Absent Genes

“To our surprise, both hairworm genomes lacked around 30% of a set of genes that are generally found across virtually all animal groups,” explains Cunha.

Such revelations usually lead scientists to question if they have made a mistake. However, a correlation was found in the missing genes in both worm species. “The large majority of the absent genes were identical between the two species. It was highly unlikely to be by chance,” asserts Cunha.

Cunha and her team found that in other animals, these absent genes are instrumental in generating cilia.

“Cilia are organelles, tiny structures at the cellular level, that are universally present in all animals and even more extensively, in protists, some plants, and fungi. They exist across a broad diversity of life on Earth,” Cunha explains. They are present in numerous cells in the human body: for instance, the tails of sperm cells are cilia, and our eyes’ retina cells also contain cilia.

Staged photo of a deceased squat lobster host Munida sp., from Norway, with a marine hairworm. The lobster’s shell was opened to show where the worm was located. The picture was taken now to represent the original scenario when the worm was collected years ago for genome sequencing. Image provided by Martin Sørensen.

Implications of Cilia Absence

Earlier studies showed that hairworms seemed to lack cilia where they would typically be seen. For example, hairworm sperm do not have tails. Yet, the absence of observable evidence of cilia in hairworms was not taken as definitive proof of their absence. Bruno de Medeiros, Curator of Pollinating Insects at the Field Museum and co-author of the paper, comments, “Without the genomes, it would require examining all cells in all life stages in all species.”

“Based on previous observations, it didn’t seem like hairworms had any cilia, but we didn’t really know for sure,” Cunha adds. “Now with the genomes, we discovered that they actually lack the genes that produce cilia in other animals — they don’t have the machinery to create cilia to begin with.”

A live freshwater hairworm in Bruno de Medeiros’s hand at the Muir Woods National Monument in California. Image provided by Bruno de Medeiros.

Understanding Evolutionary Patterns and Future Directions

Moreover, the discovery that both freshwater and marine hairworm species have lost the genes for cilia suggests that this evolutionary change probably occurred in their common ancestor’s distant past. “It is likely that the loss happened early on in the evolution of the group, and they have been continuing like that since,” Cunha explains.

This discovery raises numerous new questions. The impact of the absence of cilia on hairworms or if the hairworms’ parasitic behavior is linked to the missing cilia remains unclear. “Plenty of other parasitic organisms possess these specific genes, so we can’t claim that the genes are absent due to their parasitic lifestyle,” states Cunha. “However, parasitic organisms often lack many genes. The theory is that because parasites do not use certain structures and rely on their hosts instead, they end up losing those structures.”

Implications for Future Research

Hairworms are not the only parasites exhibiting “mind control” traits. Similar behavior is observed in protozoans responsible for toxoplasmosis, which diminish rodents’ fear of cats, and in the fungus Ophiocordyceps, popularized by the video game and TV show The Last of Us, which manipulates ants into dispersing the fungus’s spores.

While these organisms are only remotely related to hairworms, Cunha believes that the new study could assist scientists in identifying common patterns in how this behavior operates. “By doing this comparative analysis across organisms in the future, we might be able to look for similarities. Or perhaps these organisms developed similar behaviors in completely different ways from each other,” suggests Cunha.

Reference: “Rampant loss of universal

Frequently Asked Questions (FAQs) about Hairworms’ missing genes

More about Hairworms’ missing genes

• Mind Control Parasites
• Understanding Cilia
• Parasite Manipulation of Host Behavior
• Current Biology Journal
• The Last of Us and Parasitic Behavior