The fundamental role of pirouettes in Caenorhabditis elegans chemotaxis

To investigate the behavioral mechanism of chemotaxis in Caenorhabditis ele gans, we recorded the instantaneous position, speed, and turning rate of si ngle worms as a function of time during chemotaxis in gradients of the attr actants ammonium chloride or biotin. Analysis of turning rate showed that e ach worm track could be divided into periods of smooth swimming (runs) and periods of frequent turning (pirouettes). The initiation of pirouettes was correlated with the rate of change of concentration (dC/dt) but not with ab solute concentration. Pirouettes were most likely to occur when a worm was heading down the gradient (dC/dt < 0) and least likely to occur when a worm was heading up the gradient (dC/dt > 0). Further analysis revealed that th e average direction of movement after a pirouette was up the gradient. Thes e observations suggest that chemotaxis is produced by a series of pirouette s that reorient the animal to the gradient. We tested this idea by imposing the correlation between pirouettes and dC/dt on a stochastic point model o f worm motion. The model exhibited chemotaxis behavior in a radial gradient and also in a novel planar gradient. Thus, the pirouette model of C. elega ns chemotaxis is sufficient and general.