Cheating the hangman: How worms escape a fungal noose

Classic rivalries of summer 2011: Harry verus Voldemort. Cap versus the Red Skull. Optimus versus Megatron. And now, worms versus fungus.

Normally, we think of fungi as decomposers that sit around and wait for something to die. Some fungi might infect the living. But there are are few have decided to screw all that and will kill for their sustenance.

Fungi are not mobile, so their technique is to create snares. They form a loop of cells that can inflate when their inner surface is touch, trapping anything within them in a matter of about one tenth of a second. Human reaction time is about two tenths of a second, just for comparison. Fortunately, the opening of these snares are about 10 to 25 millionths of a meter (µm). These are not traps that we need concern ourselves with.

But that is just the right diameter for small nematode worm, like Caenorhabditis elegans.

(The image has been coloured; nematode worms are not generally purple, not are the fungal snares red.)

The worms can avoid this when they are very young or fully grown, because they are too small and big to get stopped by or enter into the loop, respectively. The juveniles, however, are just the right width. The do have a plan, however: they can escape. When a worm is touched on its front half (but not the very frontmost tip), it will stop, stop moving it head side to side, and reverse. If you touch the very tio (the nose, so to speak), the side-to-side head movements don’t stop.

As it happens, the neural basis of this touch response – how it’s triggered, what neurons are active, and so on – was worked out before people were able to show what the function of the behaviour was.

Here, Maguire and colleagues provide a whole mess of evidence showing the relationship between the touch response of the worm and the hunting success of the fungus.

First, they show that because the worms are tapered, the fungus almost always traps the front half of the worm, explaining why touch to the rear does not trigger this response.

Second, they show that mutants that have defects in their sense of touch are trapped at much higher rates than those without the mutations, tying the presence of this behaviour with fitness consequences.

They are also able to show that if the animals have normal touch, but keep performing the side-to-side exploratory behaviour with their head after they get touched, they are still caught more often than animals with the normal touch response.

There are more experiments in this short paper, but those are some of the core findings. This short paper is wonderfully clear and logical in the design and presentation of its experiments. It’s an excellent example of neuroethology.

Reference

Maguire SM, Clark CM, Nunnari J, Pirri JK, Alkema MJ (2011). The C. elegans touch response
facilitates escape from predacious fungi. Current Biology: In press. DOI: 10.1016/j.cub.2011.06.063