A parasitic worm sends earwigs into water, and genes offer clues

European earwigs normally stay away from open water. Earwigs that carry the roundworm Mermis nigrescens do the opposite: once the worm is fully grown, the insect heads for water and the worm crawls out into it. A team from the University of Otago in New Zealand has now tracked which genes switch on and off in both animals while this happens.

The worm needs water or very wet ground to leave its host, avoid drying out and reproduce. Biologists have written about infected insects seeking water for more than 140 years. How the parasite pulls it off is still poorly understood, even though two unrelated groups of parasites, these roundworms and horsehair worms, arrived at the same trick.

Catching the escape

The researchers collected earwigs in the Dunedin Botanic Garden and kept them in the lab. They sampled the insects at four points: uninfected, early infection, late infection and during the escape itself. In early infection the worm was under 2 millimeters long, in late infection 7 to 9 millimeters.

To catch the escape, they watched earwigs beside a small pool of water in the dark, under red light, for five to six hours at a time. When a worm came out into the water, both animals were frozen at once.

Thousands of genes shift

The team used RNA sequencing, a method that reads which genes are active in a tissue. It found 12,876 active genes in the earwig and 9,722 in the worm. Across the stages, 673 earwig genes became more active and 593 less active. In the worm, 2,672 went up and 2,293 went down.

During the escape, the earwig turned up genes for signaling and the senses. The worm turned up genes for transport and secretion, the ways cells move and release substances. In both animals, many of the genes that went up shared the same landing spot on the DNA for GATA proteins, which switch genes on.

Clues, not the cause

The authors present candidate genes and pathways, not a proven mechanism. The changes fit a worm steering its host, but they do not show what directly makes the earwig walk into water.

That step is still missing.