Reversible Thermal Gradient Device to Control Biased Thermotactic Response of C. elegans

C. elegans exhibits a directional migration toward a remembered temperature setpoint (T_s) by activating thermo-sensorimotor neurons. While cryophilic thermotaxis is well reproduced, thermophilic thermotaxis requires very stringent temperature regulations - otherwise, worms exhibit random migration in colder side of T_s. Here, we introduce a thermal stimulus device developed to control worms with different thermotactic behaviors on both colder and warmer sides of the T_s. On a linear gradient, the worm population displayed a Gaussian distribution near T_s but in a skewed shape with a peak shifted to the colder side due to their atactic motion in colder temperature than T_s. By repetitive application of thermal gradient-reversals, we found that their population density became higher near T_s because the speed at which the worms accumulate toward T_s was much faster than that of the dispersion by diffusion to the cold side, resulting in forced aggregation of worms at the desired temperature.