Alternative olfactory neuron fates are specified by the LIM homeobox gene lim-4

The Caenorhabditis elegans AWA, AWB, and AWC olfactory neurons are each req uired for the recognition of a specific subset of volatile odorants. lim-4 mutants express an AWC reporter gene inappropriately in the AWE olfactory n eurons and fail to express an AWE reporter gene. The AWE cells are morpholo gically transformed toward an AWC fate in lim-4 mutants, adopting cilia and axon morphologies characteristic of AWC. AWE function is also transformed in these mutants: Rather than mediating the repulsive behavioral responses appropriate for AWE, the AWE neurons mediate attractive responses, like AWC . LIM-4 is a predicted LIM homeobox gene that is expressed in AWE and a few other head neurons. Ectopic expression of LIM-4 in the AWC neuron pair is sufficient to force those cells to adopt an AWE fate. The AWA nuclear hormo ne receptor ODR-7 described previously also represses AWC genes, as well as inducing AWA genes. We propose that the LIM-4 and ODR-7 transcription fact ors function to diversify C. elegans olfactory neuron identities, driving t hem from an AWC-like state into alternative fates.