Progressively restricted expression of a homeo box gene within the aboral ectoderm of developing sea urchin embryos.

Abstract

A homeo box-containing gene, Hbox1 is expressed in an unusual and highly conserved spatial pattern in embryos of two different species of sea urchin, Tripneustes gratilla and Strongylocentrotus purpuratus. Hybridization in situ shows that this mRNA accumulates initially throughout the aboral ectoderm; however, between blastula and pluteus stages, the region containing Hbox1 mRNA retracts gradually until only a small area around the vertex is labeled in pluteus larvae. Aboral ectoderm appears cytologically uniform and also accumulates uniform levels of other tissue-specific mRNAs. Therefore, the Hbox1 pattern reveals a previously unsuspected heterogeneity of aboral ectoderm cells and a polarity within this tissue. In S. purpuratus, the Hbox1 gene product probably is not involved in initial specification of cell fate, as this message does not achieve a significant fraction of its peak abundance until almost hatching blastula stage, well after the time aboral ectoderm cells have initiated a tissue-specific program of gene expression. RNA blot and RNase protection analyses revealed low levels of Hbox1 mRNA in all adult tissues examined. However, this message was not detectable in mature eggs, suggesting that the Hbox1 gene does not have a maternal function. In addition to highly conserved spatial and temporal patterns of expression, the homeo box genes of these two urchin species also are conserved highly in sequences outside the homeo domain, despite the divergence of these two species (30-45 my). Two notable features of the protein shared with several vertebrate homeo proteins are a short conserved sequence encoded by an exon upstream of that encoding the homeo domain and a large region of high serine and proline content.

The dynamic gene expression patterns of transcription factors constituting the sea urchin aboral ectoderm gene regulatory network

“…Later, the complete sequence of Sp-Hox7 was determined, and it was confirmed to be a paralog of the Hox7 gene group (Martinez et al, , 1999. Sp-Hox7 is expressed in the AE of the blastula stage embryo , and its mRNA retracts to a small area around the apex of the pluteus larva (Angerer et al, 1989). The detailed in situ hybridization results showed that both Sp-Dlx and Sp-Hox7 were first detected in the AE near the vegetal plate of the 20-hpf embryo (Fig.…”

“…Further studies showed that Sp-Dlx is expressed in the AE and has a positive feedback relationship with SpIrxA . A Sp-Hox7 fragment was originally isolated from a genomic DNA library (Angerer et al, 1989) using the orthologous gene Hbox1, isolated from the sea urchin Tripneustes gratilla, as a probe (Dolecki et al, 1986). Later, the complete sequence of Sp-Hox7 was determined, and it was confirmed to be a paralog of the Hox7 gene group (Martinez et al, , 1999.…”

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.…”

A Sea Cucumber Homolog of the MouseT-Brain-1is 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.…”