SPHMX, A SEA-URCHIN HOMEOBOX GENE EXPRESSED IN EMBRYONIC PIGMENT-CELLS

We describe the first complete sea urchin member of the H6 subfamily o f homeobox genes. A cDNA encoding the complete protein was recovered f rom a Strongylocentrotus purpuratus library, and sequence comparisons demonstrate that this gene belongs to the same family as the Nkx-5.1, Nkx-5.2 and Hb genes of mammals, and the GH6 and SOHo-1 genes of chick en. In accord with current nomenclature this gene is named SpHmx. The gene is present in a single copy per haploid genome. Single-strand pro be excess hybridization demonstrates that a rare maternal SpHmx mRNA i s present in unfertilized eggs. Strong zygotic expression begins durin g the blastula stage, and the transcript is present at moderately high levels until late in development. Whole mount in situ hybridization r eveals a remarkable pattern of expression: In midgastrula-stage embryo s SpHmx expressed throughout the archenteron, but particularly strongl y in delaminating secondary mesenchyme cells. This more-general expres sion resolves to a dramatic pigment cell-specific pattern in prism and pluteus stages. SpHmx apparently encodes a cell-type-specific transcr iption factor of embryonic pigment cells. (C) 1997 Academic Press.