Homeobox genes and sea urchin development

We describe the expression of three Paracentrotus lividus homeobox-containing genes of the dispersed class during sea urchin embryogenesis and discuss their possible roles in the mechanisms of cell specification and embryo morphogenesis. PIHbox12 represents the first regulator identified in sea urchin that belongs to the zygotic class of transcription factors. Its early and transient expression and the localization of transcripts suggests that PIHbox12 is involved in cell fate specification of the oral or aboral ectodermal territories at the early cleavage stages. PIHbox9 is expressed just after the completion of gastrulation in a narrow stripe of cells at the ectoderm-endoderm boundary. It probably organizes a novel spatial boundary which definitely separates the archenteron and the aboral ectoderm. Finally, the spatial and temporal expression of the PIOtp gene strongly indicate that this regulator is conditionally activated in few cells of the oral ectoderm and is involved in patterning of this territory at late stages. Furthermore, our data indicate that PIOtp acts upstream of signaling systems that lead to the activation of the primary mesenchyme cell gene expression program and skeletal morphogenesis.