The nematode C.elegans shows behavioral plasticity in response to environmental stimuli. For instance, animals conditioned with NaCl and food migrate toward NaCl, while animals conditioned with NaCl and starvation do not migrate toward NaCl (Saeki S et al, 2001). This learning behavior is probably based on modulation by the integration of paired stimuli, food and chemical stimuli. However, only limited results have been obtained on the cellular and molecular events in the sensing of food and the integration of paired signals. Food and starvation also modulate sensory adaptation to an AWC-sensed odorant, benzaldehyde. Food inhibits sensory adaptation (Ishihara T and Katsura I, unpublished results; Bargmann CI, pers. comm.; Nuttley WM et al, 2002), while starvation enhances sensory adaptation (Colbert HA and Bargmann CI, 1997). On the basis of these observations, we developed an assay system to isolate mutants defective in olfactory learning behavior. In the assay system, wild-type animals were exposed to AWC-sensed odorants in the presence or absence of food, and then tested for their response to the same odorants. The results showed that animals conditioned with butanone and food exhibited enhanced chemotaxis to butanone, while those conditioned with benzaldehyde or isoamyl alcohol and food did not change the efficiency of chemotaxis to the same odorants. Using this assay system, we screened 5,000 genomes and obtained mutants defective in the olfactory learning behavior. Among these mutants,
ut305 and
ut306 showed defects in olfactory learning behavior induced by butanone and food. Namely, these mutants exhibited weaker chemotaxis to butanone after conditioning with butanone and food. Interestingly,
ut305 also showed defects in adaptation to benzaldehyde or isoamyl alcohol, while
ut306 showed normal adaptation to these odorants. This observation may indicate that a specific pathway for butanone exists in olfactory learning. We mapped
ut305 between R03A10 and C02C6 on chromosome X and
ut306 between F44C4 and VC5 on chromosome V.
ut305 was mapped to the region containing a single candidate gene C02C6.2, which encodes a novel protein with two or three transmembrane domains.