Unique homeobox codes delineate all C. elegans neuron classes

It is presently not known whether neuronal cell type diversity, defined by cell type-specific anatomical, biophysical, functional and molecular signatures, can be reduced to relatively simple molecular descriptors of neuronal identity (1). Examination of the expression of all conserved homeodomain proteins encoded by the Caenorhabditis elegans genome (2) reveals that the complete set of 118 C. elegans neuron classes can be described individually by unique combinations of homeodomain protein expression, thereby providing the simplest currently known descriptor of neuronal diversity. Computational as well as genetic loss of function analyses corroborate that homeodomain proteins not only provide unique descriptors of neuron type, but also play a critical role specifying neuronal identity. We speculate that the pervasive employment of homeobox genes in defining unique neuronal identities reflects the evolutionary history of neuronal cell-type specification.