Mosaic analysis of two genes that affect nervous system structure in Caenorhabditis elegans.

The mutation mec-4(e 1611), identified by M. Chalfie, leads to the degeneration and death of the six neurons, called the microtubule cells, that mediate the response of wild-type animals to light touch. The fates of two of these cells, PLML and PLMR, which are responsible for response to light touch in the tail of the animal, have been monitored in animals mosaic for the mec-4(e 1611) mutation. The results are consistent with the view that the mutation behaves cell autonomously in its killing effect; in particular, none of the neurons that make either chemical synapses or gap junctions to PLML or PLMR is responsible for the deaths of PLML or PLMR. The results of gene dosage and dominance tests suggest that the mec-4(+) gene product, which is required for wild-type microtubule cell function, is altered by the e 1611 mutation into a novel product that kills the microtubule cells. Mutation in the gene unc-3 leads to the derangement of the processes of the motor neurons of the ventral cord. Mosaic analysis strongly suggests that unc-3(+) expression is required only in the motor neurons themselves for normal neuronal development. In particular, the hypodermis surrounding the ventral cord is not the primary focus of unc-3 action (body muscle was excluded in earlier work). Finally, the mosaic analysis supports an earlier suggestion that a sensory defect caused by a daf-6 mutation is localized to a non-neuronal cell called the sheath cell.