Bizarre “mind-controlling” parasitic hairworms are missing genes found in every other animal

Two individuals live entwining the freshwater hairworm Gordionus violaceus from Germany. Credit: Gonzalo Gerbet

Hairworms lack the tiny “hairs” responsible for cell movement, filtering and sensing that every other known animal has.

In a world teeming with exotic animals, hairworms are arguably among the strangest. These are parasitic worms known to manipulate the behavior of their hosts, a phenomenon sometimes called “mind control”.

In an interesting twist, a new study appears in the journal Current Biology I revealed that hairworm classify They share an odd trait: they lack nearly 30% of the genes the researchers expected to discover. Even more intriguing, the missing genes are linked to the development of cilia, the hair-like structures found in the cells of nearly all known animal species.

A live freshwater hairworm in the hands of Bruno de Medeiros at Muir Woods National Monument in California. Credit: Bruno de Medeiros

Hairworms, which look like thin strands of spaghetti and are a few inches long, can be found all over the world. Their basic body structure indicates their parasitic nature as they lack excretory, respiratory or circulatory systems and live almost entirely inside other organisms. Tawana Cunha, a postdoctoral researcher at the Field Museum in Chicago and lead author of the study, notes, “One of the coolest things, and perhaps what they’re best known for, is that they can influence the behavior of their hosts and make them do things they wouldn’t otherwise.”

Hairworm life cycle and host manipulation

Hundreds of hairworm species are found in fresh water. The life cycle begins when its eggs hatch in the water, and the larvae are consumed by small aquatic predators such as mayfly larvae. These, in turn, are preyed upon by larger terrestrial predators such as cockroaches. After maturing inside their host, the hairworms manipulate the host’s behavior, causing them to jump into the water. Once there, the worms writhe from their hosts and begin looking for mates, thus repeating the cycle.

While there are also five species of marine hairworm that parasitize aquatic creatures such as lobsters, their ability to manipulate hosts is unclear due to the lack of need to return to the water.

Freshwater hairworms live in the environment, at Muir Woods National Monument in California. Credit: Bruno de Medeiros

Genetic research on hairworms

As strange as hairworms’ behavior may be, Cunha’s research interest in animals has more to do with them DNA. “We set out to sequence their genomes, because nothing like hers had been sequenced at this level before,” she says of the study, conducted with her co-authors Bruno de Medeiros, Ariana Lorde, Martin Sorensen, and Gonzalo Gerbet. “The goal was to produce those genomes and eventually use them to understand the evolutionary relationships between hairworms and other types of animals.”

After obtaining DNA samples from two species of hairworm — one freshwater and one marine — and sequencing them, the team made a startling discovery when comparing the hairworms’ genetic codes with those of other creatures.

Curated photos of squatting (dead) lobster stewardess Monida S. , from Norway, infected with a marine hairworm. The image has now been taken as a true-scenario representation of how the worm was collected years ago, which was used for genome sequencing. Credit: Martin Sorensen

Discover missing genes

“What we found, which was very surprising, was that the hairworm genomes were missing about 30 percent of the set of genes that would be expected to be present in basically all groups of animals,” says Kuna.

Findings like these often lead scientists to wonder if they made a mistake. However, an association exists between genes that are absent in both types of worms. The vast majority of the missing genes were exactly the same between the two species. This was an unbelievable coincidence,” Cunha says.

Cunha and her colleagues discovered that these genes, missing in other animals, are responsible for the production of cilia.

“Cilia are organelles, small structures at the cellular level, found primarily in all animals and even more broadly, in protists and some plants and fungi. So they are present across a great variety of life on Earth,” Cunha says. They are present in many cells in the human body: for example, the tails of sperm cells are cilia, and cells in the retina also have cilia.

View image of squatting (dead) lobster host Monida S. , from Norway, infected with a marine hairworm. The lobster’s carapace has been opened to show where the worm was found. The image has now been taken as a true-scenario representation of how the worm was collected years ago, which was used for genome sequencing. Credit: Martin Sorensen

Implications for the loss of cilia

In previous studies, the scientists found that the hairworms appeared devoid of cilia where they would normally be observed. Hairworm sperm, for example, lack tails. However, the lack of visual evidence of cilia in hairworms was not considered definitive evidence that they were not present. “Without genomes, this would require looking at all cells at all stages of life in all species,” says Bruno de Medeiros, curator of pollinating insects at the Field Museum and co-author of the paper.

“Based on previous observations, hairworms don’t seem to have any cilia, but we didn’t really know for sure,” says Cunha. “Now with the genome, we’ve seen that they actually lack the genes that produce cilia in other animals — they don’t have a mechanism for making cilia in the first place.”

A live freshwater hairworm in the hands of Bruno de Medeiros at Muir Woods National Monument in California. Credit: Bruno de Medeiros

Understanding of evolutionary patterns and future directions

Furthermore, the fact that both freshwater and marine hairworm species have lost genes for cilia indicates that this evolutionary change most likely occurred in the deep past of their common ancestors. “It’s possible that the loss happened early in the group’s development, and they were just carrying it on,” says Kuna.

This revelation sets the stage for many new inquiries. It is unclear how the absence of cilia affects hairworms or whether parasitic behavior of hairworms is related to the missing cilia. “There are plenty of other parasitic organisms that aren’t missing these specific genes, so we can’t say that genes are missing because of their parasitic lifestyle,” Kuna says. But parasitic organisms in general are often missing many genes. It is assumed that because the parasites do not use certain structures and instead rely on their hosts, they end up losing those structures.”

Implications for future research

Hairworms aren’t the only parasites that exhibit “mind control” traits. Similar behavior is seen in the protozoa responsible for toxoplasmosis, which reduces the rodents’ fear of cats, and in fungi Ophiocordycepsmade famous by the video game and TV show The Last of Us, manipulates ants into dispersing fungal spores.

While these organisms are distantly related to hairworms, Cunha believes the new study can help scientists identify common patterns of how this behavior works. By performing this comparative analysis across organisms in the future, we may be able to look for similarities. Or, perhaps, these organisms evolved similar behaviors in completely different ways from each other.

Reference: “Rampant Loss of Global Metazoan Genes Detected by a Chromosome-Wide Genome Assembly of Parasitic Nematomorpha” By Tauana J.Cunha, Bruno AS de Medeiros, Arianna Lord, Martin V. Sørensen, and Gonzalo Giribet, 18 July 2023, Available Here. Current Biology.
DOI: 10.1016/j.cub.2023.07.003