SPHBOX7, A NEW ABD-B CLASS HOMEOBOX GENE FROM THE SEA-URCHIN STRONGYLOCENTROTUS-PURPURATUS - INSIGHTS INTO THE EVOLUTION OF HOX GENE-EXPRESSION AND FUNCTION

Hox genes, by virtue of their key functions in axial patterning, have long been thought to be pivotal players in the evolution of developmen tal mechanisms. Despite their potential importance in evolution, there is little information about Hox genes in animal groups that are most closely related to ancestral Chordates. Accordingly, we have taken the step of analyzing Hox gene expression and function in the sea urchin embryo, whose simple bilateral body plan is thought to resemble that o f a stem organism in the Chordate lineage. Here we describe the isolat ion, sequence analysis and spatiotemporal expression pattern of a sea urchin (Strongylocentrotus purpuratus) Abd-B-like gene, designated SpH box7. We show that this gene is one of at least two Abd-B-like genes i n the S. purpuratus genome, a result that argues against the simple hy pothesis that Hox gene duplications occurred only during the evolution of the chordates. SpHbox7 transcripts are first detectable in midblas tula stage embryos, increase in amount during gastrulation, decline sl ightly by the pluteus stage, and are not detectable in any tissue of t he adult. Whole mount in situ hybridization and antibody staining with an SpH-box7-specific antibody reveal that both SpHbox7 mRNA and prote in are present throughout the embryo in the blastula. Subsequently, th ey are localized in the invaginating archenteron, secondary mesenchyme , and oral ectoderm. By the pluteus larva stage, SpHbox7 protein and m RNA are present in the gut, larval arms, and portions of the oral ecto derm. This complex and dynamic expression pattern suggests that SpHbox 7 has a role in the patterning of the gut, the mesoderm, and the Oral surface. (C) 1996 Wiley-Liss, Inc.