Temperature and food mediate long-term thermotactic behavioral plasticity by association-independent mechanisms in C. elegans

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Abstract
SUMMARY Thermotactic behavior in the nematode Caenorhabditis elegans exhibits long-term plasticity. On a spatial thermal gradient, C. elegans tracks isotherms near a remembered set-point (TS) corresponding to its previous cultivation temperature. When navigating at temperatures above its set-point (T>TS), C. elegans crawls down spatial thermal gradients towards the TS in what is called cryophilic movement. The TS retains plasticity in the adult stage and is reset by ∼4 h of sustained exposure to a new temperature. Long-term plasticity in C. elegans thermotactic behavior has been proposed to represent an associative learning of specific temperatures conditioned in the presence or absence of bacterial food. Here, we use quantitative behavioral assays to define the temperature and food-dependent determinants of long-term plasticity in the different modes of thermotactic behavior. Under our experimental conditions, we find that starvation at a specific temperature neither disrupts TS resetting toward the starvation temperature nor induces learned avoidance of the starvation temperature. We find that prolonged starvation suppresses the cryophilic mode of thermotactic behavior. The hen-1 and tax-6 genes have been reported to affect associative learning between temperature and food-dependent cues. Under our experimental conditions, mutation in the hen-1 gene, which encodes a secreted protein with an LDL receptor motif, does not significantly affect thermotactic behavior or long-term plasticity. Mutation in the tax-6 calcineurin gene abolishes thermotactic behavior altogether. In summary, we do not find evidence that long-term plasticity requires association between temperature and the presence or absence of bacterial food.