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 p rogenies will give rise to five different territories are segregated a t 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 embryo s. We report evidence that PlHbox12, a paired homeobox-containing gene , is expressed in the embryo from the 4-cell stage. The abundance of t he transcripts reaches its maximum when the embryo has been divided in to the five polyclonal territories-namely at the 64-cell stage-and it abruptly declines at later stages of development, Blastomere dissociat ion experiments indicate that maximal expression of PlHbox12 is depend ent on intercellular interactions, thus suggesting that signal transdu ction mechanisms are responsible for its transcriptional activation in the early cleavage embryo, Spatial expression of PlHbox12 was determi ned by whole-mount in situ hybridization, PlHbox12 transcripts in embr yos at the fourth, fifth, and sixth divisions seem to be restricted to the conditionally specified ectodermal lineages, These results sugges t a possible role of the PlHbox12 gene in the early events of cell spe cification of the presumptive ectodermal territories.