Worm Breeder's Gazette 15(4): 26 (October 1, 1998)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

Different chemosensory pathways may converge on a single chemotaxis mechanism in C. elegans. 

Yvonne Bradford , Jonathan Pierce , Shawn Lockery

Institute of Neuroscience, 1254 University of Oregon, Eugene, OR 97403 USA.

C. elegans exhibits chemotaxis in response to gradients of two main classes of attractants: soluble compounds (cAMP, biotin, various amino acids, and inorganic anions and cations) and volatile compounds (alcohols, ketones, and esters). Chemotaxis to these two classes of compounds are mediated by two different sets of chemosensory neurons. We previously found that chemotaxis to soluable attractants involves a series of course corrections, called pirouettes, triggered whenever the worm heads down the gradient. Here we asked whether the course correction model also applies to chemotaxis to volatile compounds, and thus to stimuli mediated by a different sensory pathway.

We used an automated tracking system to record the position, speed, and turning rate of individual worms (n = 29) at high spatial and temporal resolution as they moved in well-defined gradients of the volatile attractant diacetyl established in agar plates. Stable, reproducible gradients were created by allowing the attractant to diffuse through the agar from a point source located at the center of the assay plate. Two 5 mL aliquots of a 1% aqueous solution of diacetyl were placed on the plate, one at 20 h and the other at 1 h before the experiment. Each worm was placed at a point 2.5 cm from the peak of the gradient and tracked for 20 min. As in previous assays involving the soluable attractants biotin and NH4Cl, data were analyzed by computing the worm's bearing with respect to the peak of the gradient immediately before and after each pirouette. Bearing was defined such that 0o represented movement directly up the gradient; + 180o represented movement directly down the gradient. Results were quantified by computing the angle B and amplitude A of the vector average of the bearings before and after pirouettes. Data were normalized so that the amplitude of the vector average varied from 0, reflecting a high degree of angular variation, to 1, reflecting no angular variation.

Three lines of evidence indicate that chemotaxis in volatile gradients involves a mechanism that is qualitatively identical to the mechanism for chemotaxis in soluable gradients. First, B before pirouettes was 172o (A = 0.53). This value agrees well with previous results in soluable gradients (biotin: B = 150o, A = 0.31; NH4Cl: B = 162o, A = 0.23) and suggests that pirouettes in volatile gradients are triggered when the worm heads down the gradient. Second, B after pirouettes was 16o (A = 0.30). This value agrees well with previous results in soluable gradients (biotin: B = 16o, A = 0.25; NH4Cl: B = 0o, A = 0.24) and suggests that pirouettes in volatile gradients correct the animal's course. Third, the change in bearing produced by individual pirouettes was positively correlated with the bearing before pirouettes (r = 0.212, n = 435, p < 0.01). This correlation indicates that bigger course corrections are made when the worm is further off course. The similarity between the course correction behavior of worms in gradients of soluable and volatile attractants suggests that two distinct sensory pathways may converge on a single chemotaxis mechanism.