The nematode Caenorhabditis elegans is a useful model for studying a behavior or set of behaviors, because its entire sensorimotor circuit has been defined and can easily be manipulated genetically. New research takes advantage of this fact, using nematodes to study the mechanisms underlying the evolution of escape behaviors.

In the natural nematode environment, certain types of fungi prey on nematodes as their main food source and have developed distinctive trapping devices to catch them. One device utilizes a constricting ring mechanism: when a nematode passes through the ring, the friction induces the cells of the ring to rapidly inflate inwards and trap it. Unless the nematode can quickly escape from the constricting ring, it will be captured and digested by the fungi.

Mark J. Alkema and colleagues at the University of Massachusetts Medical School in Worcester, MA, examined the motor pattern required for C. elegans to escape from the fungi once caught (Curr. Biol. doi:10.1016/j.cub.2011.06.063; published online 28 July 2011). Wild-type worms that were able to escape used a behavior commonly seen in response to light touch on the nose or the front half of the body: the animal quickly moves backwards while suppressing its usual exploratory side-to-side head movements.

In order to investigate the individual components of this escape behavior, the researchers analyzed the trap encounters of several mutants with impairments in one or multiple aspects of the touch response. The ability to escape from the traps was markedly reduced in mutants with deficiencies of the gene mec-4 compared with wild-type animals. Other mutants with deficits in backwards locomotion also escaped less frequently. A mutation inactivating the gene lgc-55, which caused failure to suppress head movements, also decreased the animals' likelihood of escape. The results of these experiments show that successful escape requires a touch-induced coordination of backward movement with the suppression of side-to-side head movements.

The researchers also carried out competition experiments between wild-type and lgc-55 mutants to determine whether the suppression of head movements provides a selective advantage. They found that the mutants were more frequently captured by the ring mechanism. This raises the possibility that the touch-induced suppression of head movements may be an evolved means of preventing the activation of the ring cells, thereby increasing the likelihood of escape. This research not only provides an evolutionary explanation for the complex escape behavior of C. elegans but also highlights their usefulness as a research tool.