We propose to use the simple nematode C. elegans as a model system to study the genetic basis of how LEFT/right asymmetry is established within the nervous system. The nervous system of nearly every animal displays morphological bilateral symmetries that are often paralleled by symmetric patterns of gene expression and function. A wealth of information exists regarding dorsoventral and antero-posterior development, but virtually nothing is known with respect to the induction of left/right gene expression in the nervous system and how it is integrated into morphologically bilaterally symmetric structures in the adult brain. In order to define genetic components that are required to establish asymmetric lim-6 expression, we have constructed a transgenic line, otls6, that expresses a GFP reporter gene under the control of the lim-6 promoter exclusively in the ASEL chemosensory neuron, but not in its bilaterally symmetric partner, ASER. We have crossed the otls6 transgenic line into various mutant backgrounds and have determined that asymmetric lim-6 expression is conferred by calcium signaling which acts by an unknown pathway through the GTPase, ras. We have also mutagenized otls6 with EMS. After mutagenesis, we screened for mutants in which the asymmetry of lim-6 expression was disrupted. We have screened through 3,302 haploid genomes so far and have retrieved nine lim-6 asymmetry mutants. We plan to further characterize these mutants and to scale up the screening approach in order to identify more mutants. We hope to identify all of the genes that are involved in calcium-activation of ras. This pathway may yield significant insights into the establishment of left/right asymmetry in the nervous system of other or organisms as well.
