THE C-ELEGANS NEURONALLY EXPRESSED HOMEOBOX GENE CEH-10 IS CLOSELY-RELATED TO GENES EXPRESSED IN THE VERTEBRATE EYE

We describe the homeobox gene ceh-10 from the nematode Caenorhabditis elegans. The homeodomain of ceh-10 is closely related to the homeodoma ins of two genes recently cloned from the vertebrate retina, Chx10 fro m mice and Vsx-1 from goldfish. We show that the sequence conservation extends well beyond the homeodomain and includes a region (named the CVC domain) of roughly 60 amino acids immediately C-terminal to the ho meodomain. As assayed in transgenic worms, the promoter region of ceh- 10 directs expression of a lacZ reporter gene to a small number of neu rons. We draw a parallel between the bipolar cells of the inner nuclea r layer of the vertebrate retina, which express Chx10 and Vsx-1, and a n interneuron in C. elegans called AIY, which expresses ceh-10. AIY re ceives synaptic input from a sensory cell, just as do bipolar cells of the vertebrate retina. In C. elegans, the sensory cell AFD is not kno wn to be photosensitive but is known to be thermosensitive; moreover, a cell with similar position in the amphids of other nematodes has bee n suggested indeed to be photosensitive. Our results emphasize the hig hly conserved nature of sensory regulatory mechanisms and suggest one way in which photosensitive organelles might have originated in evolut ion.