Bizarre Animals: Nature’s Mind-Control Experts Lack Genes Present in Every Other Animal

Parasitic worms known as hairworms possess a peculiar characteristic that sets them apart from all other animals. These worms, notorious for their ability to manipulate the behavior of their hosts in what is commonly referred to as “mind control,” have been found to be lacking certain crucial genes.

A recent study published in the journal Current Biology sheds light on this intriguing aspect of hairworm biology: they are missing approximately 30% of the genes that are typically present in other animal species. Particularly notable is the absence of genes responsible for the development of cilia, the tiny hair-like structures found in the cells of virtually all known animals.

Hairworms are widely distributed across the globe and bear a resemblance to thin strands of spaghetti, measuring a few inches in length. Their simple anatomical structure hints at their parasitic way of life: devoid of excretory, respiratory, or circulatory systems, they spend a significant portion of their 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,” explains Tauana Cunha, lead author of the study and a postdoctoral researcher at the Field Museum in Chicago, who conducted the research in collaboration with Harvard University and the University of Copenhagen.

Freshwater hairworms, numbering a few hundred species, commence their life cycle when their eggs hatch in water, subsequently being consumed by minuscule predators dwelling in water, such as mayfly larvae. These predators are then consumed by larger, land-dwelling creatures like crickets. After reaching adulthood within their newly acquired hosts, hairworms manipulate their hosts’ behavior, inducing them to leap into water. Once in the water, the worms exit their hosts’ bodies and seek out mating partners, intertwining to initiate the cycle anew. (Five hairworm species also inhabit marine environments, parasitizing aquatic creatures like lobsters. However, it remains uncertain whether these species possess host manipulation capabilities, as there is no pressure for these worms to return to water since their hosts already reside there.)

Although hairworms’ behavioral peculiarities are indeed remarkable, Cunha’s research interest lies primarily in their DNA.

“We set out to sequence their genomes, because nothing like them has ever been sequenced before at that level,” Cunha explains, referring to the collaborative efforts with 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.”

To achieve this, Cunha and her colleagues collected DNA samples from two hairworm species—one from freshwater and the other from saltwater—and sequenced them. Yet, upon comparing the genetic codes of the hairworms with those of other animals, they made a surprising discovery.

“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,” Cunha reveals.

Such findings often lead scientists to question their accuracy. However, a link existed 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,” Cunha asserts.

Through an examination of the functions these absent genes fulfill in other animal groups, Cunha and her colleagues determined that they are responsible for the production of cilia.

Cunha and her colleagues determined that they are responsible for the production of cilia.

Cunha explains, “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.”

Cilia are present in numerous cells within the human body; for instance, sperm cells possess cilia in their tails, and cells in our retinas contain cilia as well.

Previously, scientists had noted the absence of cilia in hairworms where they are typically found. For instance, hairworm sperm lack tails. However, since no ciliated hairworm cells had ever been observed, it was inconclusive evidence that they lacked cilia. Proving something based on negative evidence is challenging.

“Without the genomes, this would require looking at all cells in all life stages in all species,” remarks Bruno de Medeiros, co-author of the study and Curator of Pollinating Insects at the Field Museum.

“Based on previous observations, it didn’t seem like hairworms had any cilia, but we didn’t really know for sure,” Cunha explains. “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.”

Moreover, the fact that both freshwater and marine hairworm species lack cilia genes suggests that this evolutionary modification occurred in their common ancestor in the distant past.

“It is likely that the loss happened early on in the evolution of the group, and they just have been carrying on like that,” suggests Cunha.

This discovery prompts several new questions. It remains unclear how the absence of cilia has impacted hairworms or whether the worms’ parasitic behavior is related to the absent 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,” Cunha adds. “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.”

Hairworms are not the only parasites capable of “mind control”; this behavior also manifests in protozoans like the organism responsible for toxoplasmosis, which diminishes rodents’ fear of cats, as well as 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. Although these organisms are only distantly related to hairworms, Cunha suggests that this study could help scientists identify common threads underlying the mechanics of such behavior.

“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,” Cunha concludes.

Image Credit: GONZALO GIRIBET