Homeobox genes and sea urchin development

We describe the expression of three Paracentrotus lividus homeobox-containi ng genes of the dispersed class during sea urchin embryogenesis and discuss their possible roles in the mechanisms of cell specification and embryo mo rphogenesis. PIHbox12 represents the first regulator identified in sea urch in that belongs to the zygotic class of transcription factors. Its early an d transient expression and the localization of transcripts suggests that PI Hbox72 is involved in cell fate specification of the oral or aboral ectoder mal territories at the early cleavage stages. PIHbox9 is expressed just aft er the completion of gastrulation in a narrow stripe of cells at the ectode rm-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 th is regulator is conditionally activated in few cells of the oral ectoderm a nd 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 an d skeletal morphogenesis.