Mutant screens and whole genome sequencing reveal principles of generating neuronal diversity in Caenorhabditis elegans

Neuronal specification involves the progressive restriction of neural progenitor fate until the final step, terminal differentiation. Although much work has revealed extrinsic and intrinsic factors required for the early specification of the neural progenitor pool, the genetic logic that dictates terminal differentiation is only beginning to be discovered. Terminal differentiation diversifies neuronal types in sometimes surprising ways, for example, morphologically symmetric neurons across the left/right axis can display functional asymmetry. The work described here uses traditional and high throughput genetic approaches to identify factors that are required for the specification of the functionally lateralized gustatory neurons in C. elegans, ASEL (left) and ASER (right). I first describe a large-scale forward genetic screen that revealed mutants in novel genes that disrupt the lateralization of the ASE neurons. I then discuss the cloning and characterization of two genes resulting from this screen: nhr-67, a Tailless/TLX ortholog, and lin-59, an Ash1 ortholog and trithorax group member. nhr-67 plays reiterative roles in terminal differentiation while lin-59 seems to be required to maintain fate post-mitotically, after the initial specification event. I also describe the use of whole genome sequencing to identify a single, phenotype-causing mutation in a histone acetyl transferase that affects ASE lateralization. I used this study as a springboard for further work that allowed the analysis of a multi-gene mutant locus and helped describe chemically induced mutations on a genome-wide scale. High throughput genetics will allow the rapid identification of genes required for neuronal specification, and therefore help elucidate the genetic logic underlying terminal differentiation.