SPATIAL AND TEMPORAL CONTROLS TARGET PAL-1 BLASTOMERE-SPECIFICATION ACTIVITY TO A SINGLE BLASTOMERE LINEAGE IN C-ELEGANS EMBRYOS

The early asymmetric cleavages of Caenorhabditis elegans embryos produ ce blastomeres with distinct developmental potentials. Here, we show t hat the caudal-like homeodomain protein PAL-1 is required to specify t he somatic identity of one posterior blastomere in the 4 cell embryo. We find that pal-1 activity is sequentially restricted to this blastom ere. First, at the 4 cell stage, it is translated only in the two post erior blastomeres. Then, its function is restricted to one of these bl astomeres. This second targeting step is dependent on the activities o f the posteriorly localized SKN-1 and asymmetrically segregated PIE-1 proteins. We propose that the segregation of PIE-1, combined with the temporal decay of SKN-1, targets pal-1 activity to this posterior line age, thus coupling the regulation of this conserved posterior patterni ng gene to asymmetric cell cleavages.