Homeobox-containing gene transiently expressed in a spatially restricted pattern in the early sea urchin embryo

In the sea urchin embryo, the lineage founder cells whose polyclonal progenies will give rise to five different territories are segregated at the sixth division. To investigate the mechanisms by which the fates of embryonic cells are first established, we looked for temporal and spatial expression of homeobox genes in the very early cleavage embryos. We report evidence that PIHbox12, a paired homeobox-containing gene, is expressed in the embryo from the 4-cell stage. The abundance of the transcripts reaches its maximum when the embryo has been divided into the five polyclonal territories-namely at the 64-cell stage-and it abruptly declines at later stages of development. Blastomere dissociation experiments indicate that maximal expression of PIHbox12 is dependent on intercellular interactions, thus suggesting that signal transduction mechanisms are responsible for its transcriptional activation in the early cleavage embryo. Spatial expression of PIHbox12 was determined by whole-mount in situ hybridization. PIHbox12 transcripts in embryos at the fourth, fifth, and sixth divisions seem to be restricted to the conditionally specified ectodermal lineages. These results suggest a possible role of the PIHbox12 gene in the early events of cell specification of the presumptive ectodermal territories.