Pervasive homeobox gene function in the male-specific nervous system of <i>Caenorhabditis elegans</i>

We explore here how neuronal cell type diversity is genetically delineated in the context of the large, but poorly studied, male-specific nervous system of the nematode Caenorhabditis elegans. Mostly during postembryonic development, the C. elegans male adds 93 male-specific neurons, falling into 25 cardinal classes, to the predominantly embryonically generated, sex-shared nervous system, comprising 294 neurons (116 cardinal classes). Using engineered reporter alleles, we investigate here the expression pattern of 40 of the 80 phylogenetically conserved C. elegans homeodomain proteins within the male-specific nervous system. Our analysis indicates that each individual neuron class is defined by unique combinations of homeodomain proteins and that the male-specific nervous system can be subdivided along the anterior/posterior axis in HOX cluster expression domains. Using a collection of newly available terminal fate markers, we undertake a mutant analysis of five homeobox genes (unc-30/Pitx, unc-42/Prop, lim-6/Lmx, lin-11/Lhx, ttx-1/Otx) and identify defects in cell fate specification and/or male copulatory defects in each of these mutant strains. Our analysis expands our understanding of the importance of homeobox genes in nervous system development and function.