Analysis of the effects of turning bias on chemotaxis in C. elegans

C. elegans advances up a chemical gradient by modulating the probability of occasional large, course-correcting turns called pirouettes. However, it remains uncertain whether C. elegans also uses other behavioral strategies for chemotaxis. Previous observations of the unusual, spiral-shaped chemotaxis tracks made by the bent-head mutant unc-23 point to a different strategy in which the animal continuously makes more subtle course corrections. In the present study we have combined automated tracking of individual animals with computer modeling to test the hypothesis that the pirouette strategy is sufficient on its own to account for the spiral tracks. Tracking experiments showed that the bent-head phenotype causes a strong turning bias and disrupts pirouette execution but does not disrupt pirouette initiation. A computer simulation of disrupted pirouette behavior and turning bias reproduced the spiral tracks of unc-23 chemotaxis behavior, showing that the pirouette strategy is sufficient to account for the mutant phenotype. In addition, the simulation reproduced higher order features of the behavior such as the relationship between the handedness of the spiral and the side to which the head was bent. Our results suggest that the pirouette mechanism is sufficient to account for a diverse range of chemotaxis trajectories.