Measuring and modeling the dynamics of the thermal memory of C. elegans*

The roundworm C. elegans learns from its experiences. When placed on a thermal gradient, worms perform thermotaxis to or away from the conditioned temperature, depending on food abundance during the conditioning phase. To quantify this behavior, we developed a novel assay that tracks single worms—each experiencing a Spatio-temporal thermal gradient with thermal precision ±0.01C—in a small (2.8ul) droplets of buffer, arrayed on hydrophobic-printed microscope slides. CCD cameras monitor many worms simultaneously in many droplets, each droplet at 20C midpoint with a thermal gradient of 0.5C/cm. A worm’s thermal preference is summarized as a thermotaxis index, and the index dynamics are tracked at a high temporal resolution for many hours. Initially, worms reared at 15C and 25C exhibit cryophilic or thermophilic tendencies, respectively, and starvation during conditioning or in the droplet reverses these tendencies. This reversal is non-monotonic indicating multiple dynamic processes for learning that operate on different time scales. We build a predictive model with multiple time scales and utilize mutants to detangle the various learning processes. The model predicts the behavior under various conditions.