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Larval nematode captured by sticky fungal nets, by George Barron

Worm-Eating Fungi Eavesdrop on the Chemicals of Their Prey

Fungi are fantastic at breaking down living tissues. They discolour pieces of bread and fruit, cause outbreaks of thrush and athlete’s foot, and wipe out harvests of rice and species of frog. All of these examples rely on fungal spores landing somewhere they can grow. But not all fungi are so passive. More than 200 species have evolved into predators that ensnare and devour their own meat.

Their victims are nematodes—small worms that are some of the most common animals on the planet. It’s said that four in every five animals is a nematode, which means there’s plenty of food for carnivorous fungi that can trap them.

Such meat-eaters evolved at least 100 million years ago, and they have since developed a wide variety of traps. Some fungi use sticky nets. Others use microscopic lassos made of single coiled cells, which can constrict round a blundering nematode. The fungi then penetrate the immobilised worms with root-like projections called hyphae, which break down their bodies from the inside out.

But these killers aren’t always so murderous. One of the most common and best-studied species—Arthrobotrys oligospora—usually feeds on decaying matter. It only manufactures traps (this one uses sticky nets) when nematodes are around. Only when there’s food does it become a predator. And that means it has some way of sensing its prey.

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Three nematode (arrows) captured by Arthrobotrys oligospora, by George Barron

In a way, this is very old news. Back in 1959, David Pramer and Norman Stoll grew nematodes in a liquid and showed that this broth, minus the worms, could encourage Arhtrobotrys to grow its traps. The worms must secrete some sort of chemical that the fungus can detect. Pramer and Stoll called this mysterious substance “nemin”.

Now, more than five decades later, Yen-Ping Hsueh from the California Institute of Technology has worked out what nemin actually is. It’s not actually one chemical, but an entire family of them, known as ascarosides. They’re an inextricable part of nematode life. The worms rely on these molecules to coordinate their growth, control their behaviour, and find their mates.

Hsueh suspected that the fungi might be attuned to ascarosides, especially those that are used by a diverse range of nematodes. He applied ten of these to cultures of A.oligospora, and sure enough, some of them triggered the creation of traps.

Other species of nematode-eating fungi have the same eavesdropping ability, but each one seems to respond to a subtly different blend of ascarosides. Nematodes also vary in which ascarosides they use, so perhaps the fungi have evolved to recognise the chemical signatures of their preferred prey, or the most common ones in their environment. Either way, it’s a great strategy. The nematodes can’t very well stop using ascarosides, any more than we could do away with food or oxygen. They cannot hide their presence from any fungus that can eavesdrop on these chemicals signals.

Hsueh also found that A.oligospora only kills nematodes when it has to. The worms provide a valuable source of nitrogen, which is missing from the decaying wood that the fungus otherwise digests. If Hsueh grew the fungus on a nitrogen-rich jelly, it didn’t respond to ascarosides and never bothered making traps. After all, it takes nutrients to grow the traps. Usually, the fungus gets enough back from the worms to repay its investment, but if there are plenty of nutrients in the environment, why make that initial outlay?

Bonus: Check out this wonderful site that lists the many ways in which these fungi, and others, kill their prey.

Reference: Hsueh, Mahanti, Schroeder & Sternberg. 2013. Nematode-Trapping Fungi Eavesdrop on Nematode Pheromones. Current Biology