Worms that turn insects into “zombies” make hosts jump into water and drown

Threadlike worms that live in the guts of certain insects make their hosts jump into the water and drown.

What scientists knew about these spaghetti-like parasites was bizarre enough.

The larvae infest arthropods such as crickets and beetles and grow for several months inside their terrestrial hosts, but must return to the water to mate and lay eggs. To do this, worms manipulate a host’s behavior, forcing it to seek out water and dive. The parasites can then dodge the host’s rump and swim away.

But the worms got a lot weirder.

Genetic analysis revealed that horsehair worms – scientifically classified as the phylum Nematomorpha – made an evolutionary detour many millions of years ago that cost them about 30% of the genes found in all other animals.

The scientists discovered the loss when they sequenced the genomes of two nematomorph species: the freshwater worm Acutogordius australiensis and the marine species Nectonema munidae.

Which genes were missing?

Although the worms’ behavior has been well studied, little is known about their genetics. The researchers who sequenced the genomes of A. australiensis and N. munidae hoped to change that, said Tauana Cunha, lead author of the study published Tuesday in the journal Current Biology. Cunha is now a postdoctoral research scientist at Chicago’s Field Museum of Natural History, but began research as a doctoral candidate in the department of evolutionary and organismic biology at Harvard University.

However, when the scientists compared their horsehair worm genomes with genomic information from other animals, something was missing, Cunha told CNN.

See freshwater worm video

‘Mind controlling’ parasitic worms are missing genes found in every other animal

“There is a certain set of genes that you would expect to find in groups of animals,” she said. “It’s used as a metric for the quality of your genome.” The same cluster of genes – about 200 in all – were missing in both horsehair worm species, suggesting that this was a genomic quirk of the cluster rather than a data error.

Further analysis revealed that the genes controlled eyelash development, the study authors reported.

These genes, which also exist in single-celled organisms called protists and in some plants and fungi, regulate the development of short, hair-like structures at the cellular level called cilia. Cilia act as sensors, help cells move, and filter out microbes and debris. The tail of a sperm cell is a single specialized cilia; Cilia also help microscopic organisms swim and capture food. These structures line the surfaces of our lungs and respiratory tract and grow into cells in our retinas.

Mysteries remain about the movements of the worms

As useful as cilia are, horsehair worms seem to do just fine without them, the scientists reported. Previous studies of nematomorphs have noted that they lack cilia, but this has not been explored at the genetic level. The new study presents a new perspective on the anomaly, “combining genomic-scale molecular data and detailed morphological observation,” said biologist Keiichi Kakui, a professor in the department of biological sciences at Hokkaido University in Japan.

Kakui, who was not involved in the new research, was lead author of a study that identified the first known examples of juveniles of horsehair marine worms in deep-sea crustaceans called isopods. But how these parasites navigate their environment is unknown, particularly when they lack basic sensory structures that most animals have, Kakui told CNN via email.

“It’s hard for me to imagine how this species finds and enters its host in (the) vast deep sea,” Kakui said.

To date, scientists have identified several hundred species of freshwater mane worms and five species of marine worms. Sea horsehair worms spend their entire lives in water, but freshwater species are only aquatic as adults. Manipulation of host animal behavior is known only in freshwater species.

Because the marine and freshwater horsehair lineages diverged millions of years ago, the genetic loss likely happened in an ancestor, and both lineages inherited genomes without the cilia genes, Cunha explained. “By sequencing more caterpillar species, we would find even more evidence of this.”

According to Kakui, the work is “a great advance”, with the genomic data offering valuable information about the phylogenetic relationships and the evolution of parasitism in nematomorphs and their close relatives.

Other ‘Mind Controlling’ Parasites

Parasites that alter host behavior occur in many different groups. Ophiocordyceps – the fungal inspiration for the “cordyceps” infection in HBO’s “The Last of Us” – hijacks an ant’s brain, forcing it to climb onto high perches where it dies as the fungus erupts from its corpse. The protozoan Toxoplasma gondii suppresses rodents’ fear of predators and makes infected wolves more reckless when it comes to pack dynamics. The parasitoid wasps that lay their eggs inside the caterpillars also release chemicals that turn the hosts into bodyguards for the wasp’s young.

The video above, taken by entomologist Bruno de Medeiros, assistant curator of insects at the Field Museum of Natural History, shows freshwater horsehair worms at Muir Woods National Monument in California.

Scientists are still piecing together the molecular mechanisms that trigger these behavioral changes. More genomic data will allow researchers to compare genes and identify their roles in controlling behavior in various parasite species, Cunha said.

“Do they share some similarities in these mechanisms – in the genes they have or don’t have?” she said. “These are some of the questions we’ll be able to better address now that we’re building more genomic resources for different animals that weren’t represented before.”