SpHbox7, a new Abd‐B class homeobox gene from the sea urchin Strongylocentrotus purpuratus: Insights into the evolution of hox gene expression and function

Abstract

Hox genes, by virtue of their key functions in axial patterning, have long been thought to be pivotal players in the evolution of developmental 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 of a stem organism in the Chordate lineage. Here we describe the isolation, sequences analysis and spatiotemporal expression pattern of a sea urchin (Strongylocentrotus purpuratus) Abd-B-like gene, designated SpHbox7. We show that this gene is one of at least two Abd-B-like genes in the S. purpuratus genome, a result that argues against the simple hypothesis that Hox gene duplications occurred only during the evolution of the chordates. SpHbox7 transcripts are first detectable in midblastula stage embryos, increase in amount during gastrulation, decline slightly by the pluteus stage, and are not detectable in any tissue of the adult. Whole mount in situ hybridization and antibody staining with an SpHbox7-specific antibody reveal that both SpHbox7 mRNA and protein are present throughout the embryo in the blastula. Subsequently, they are localized in the invaginating archenteron, secondary mesenchyme, and oral ectoderm. By the pluteus larva stage, SpHbox7 protein and mRNA are present in the gut, larval arms, and portions of the oral ectoderm. This complex and dynamic expression pattern suggests that SpHbox7 has a role in the patterning of the gut, the mesoderm, and the oral surface.

Hbox1 and Hbox7 are involved in pattern formation in sea urchin embryos

“…The expression territories of Hbox1 and Hbox7 are complementary. The expression pattern of Hbox1 and Hbox7 during development of H. pulcherrimus embryos is quite similar to that of SpHbox1 ) and SpHbox7 (Dobias et al 1996) in S. purpuratus embryos.…”

“…Several homeobox sequences have been found in different echinoid species (Dolecki et al 1986;DiBernardo et al 1994;Ruddle et al 1994;Dobias et al 1996;Popodi et al 1996). However, the function of these homeodomain proteins is not well understood in sea urchin development.…”

“…TgHbox1 (Dolecki et al 1986) and its ortholog, SpHbox1 ) are known to be expressed exclusively in the aboral ectoderm cells, and it has been suggested ) that Hbox1 genes could be involved in establishing the irreversible determination of aboral ectoderm. SpHbox7 transcripts are localized in oral ectoderm, endoderm, secondary mesenchyme cells (SMC; Dobias et al 1996). The expression territories of Hbox1 and Hbox7 are complementary.…”

A Sea Cucumber Homolog of the Mouse T-Brain-1 is Expressed in the Invaginated Cells of the Early Gastrula in Holothuria leucospilota.

“…3a). However, in the embryo, only two Hox genes (Hox7 and Hox11/13b) are expressed (Angerer et al, 1989;Dobias et al, 1996). The expression of Hox7 protein is restricted to the aboral ectoderm, and Hox11/13b expression is restricted to oral ectoderm, endoderm and SMCs in sea urchin embryos after the gastrula stage (Fig.…”

Origin and Evolution of Chordates

“…Sea urchins also possess a single Hox gene complex containing 10 genes (Martinez et al, 1999). However, in the embryo, only two Hox genes (Hox7 and Hox11/13b) are expressed (Angerer et al, 1989;Dobias et al, 1996). The expression of Hox7 protein is restricted to the aboral ectoderm, and Hox11/13b expression is restricted to oral ectoderm, endoderm and secondary mesenchyme cells in sea urchin embryos after the gastrula stage.…”