Parasitic Worms Rewire Thousands of Earwig Genes Before Driving Their Hosts Into Water
New Zealand researchers tracked gene activity in both the insects and the Mermis worms inside them, finding a coordinated 'molecular dialogue' as the infection pushes earwigs toward a fatal plunge.
Earwigs like damp places, but they normally stay out of open water. European earwigs infected with a parasitic roundworm called Mermis nigrescens are different: they head for it. The worm needs water to finish its life cycle, and once the insect reaches it, the parasite bursts out and the host dies. Scientists have now caught the genetic changes that happen in both animals along the way.
Researchers at the University of Otago in New Zealand, the New Zealand Institute for Bioeconomy Science, the University of British Columbia and Brown University reported the findings in Proceedings of the Royal Society B. They collected European earwigs and compared healthy insects with ones at early and late stages of infection, right up to the moment the worm emerged. Using RNA sequencing, which measures which genes are switched on, they read the activity of the insect and the parasite at the same time.
They detected activity in 12,876 earwig genes and 9,722 worm genes. Across the stages of infection, 673 earwig genes and 2,672 worm genes became more active, while 593 earwig genes and 2,293 worm genes quieted down.
The pattern was telling. As the manipulation took hold, the earwigs ramped up genes tied to sensing and signaling, the machinery an animal uses to perceive and respond to its world. The worms, meanwhile, switched on genes for transport and secretion, which could be how they send chemical signals into their host. "We identified coordinated changes in thousands of genes," said senior author Neil J. Gemmell of the University of Otago, calling it "a sophisticated molecular dialogue between host and parasite that ultimately drives the host's fatal journey to water."
The researchers are careful to say the data do not pin down the exact cause of the water-seeking behavior. What they provide is a list of suspects: the genes and pathways most likely to be involved, which future experiments can test one by one.
Gemmell said the project grew out of hikes in New Zealand, where he would sometimes find wētā, large native ground insects, drowned in puddles with long hairworms erupting from their bodies. "I was curious to know how the parasite exerted that power over its host that ultimately drove it to kill itself," he said. Earwigs turned out to be easier to study than wētā.
Parasites that hijack their hosts' behavior are found across nature, from fungi that steer ants to worms that make crickets leap into streams. How they do it has remained largely mysterious. Gemmell's team is now studying the wētā system, where several worms often share one host and the first to emerge can doom the others. The researchers want to know whether related worms cooperate or compete, and whether kinship decides which. He noted that funding for such fundamental work "remains a challenge."
