Abstract
Bilateral asymmetry in Caenorhabditis elegans arises in part from cell lineages that differ on the left and right sides of the animal. The unpaired MI neuron descends from the right side of an otherwise left-right symmetric cell lineage that generates the MI neuron on the right and the e3D epithelial cell on the left. We isolated mutations in three genes that caused left-right symmetry in this normally asymmetric cell lineage by transforming MI into an e3D-like cell. These genes encode the proneural bHLH proteins NGN-1 and HLH-2 and the Otx homeodomain protein CEH-36. We identified the precise precursor cells in which ceh-36 and ngn-1 act, and showed that CEH-36 protein is asymmetrically expressed and is present in an MI progenitor cell on the right but not in its bilateral counterpart. This asymmetric CEH-36 expression promotes asymmetric ngn-1 and hlh-2 expression, which in turn induces asymmetric MI neurogenesis. Our results indicate that this left-right asymmetry is specified within the two sister cells that first separate the left and right branches of the cell lineage. We conclude that the components of an evolutionarily conserved Otx/bHLH pathway act sequentially through multiple rounds of cell division on the right to relay an initial apparently cryptic asymmetry to the presumptive post-mitotic MI neuron, thereby creating an anatomical bilateral asymmetry in the C. elegans nervous system.