Totipotent germline blastomeres in Caenorhabditis elegans contain, but do not respond to, factors that promote somatic differentiation in other embryonic cells. Mutations in the maternal gene pie-1
result in the germline blastomeres adopting somatic cell fates. Here we show that pie-1
encodes a nuclear protein, PIE-1, that is localized to the germline blastomeres throughout early development. During division of each germline blastomere, PIE-1 initially associates with both centrosomes of the mitotic spindle. However, PIE-1 rapidly disappears from the centrosome destined for the somatic daughter, and persists in the centrosome of the daughter that becomes the next germline blastomere. The PIE-1 protein contains potential zinc-finger motifs also found in the mammalian growth-factor response protein TIS-11/NUP475 (refs 4-7). The localization and genetic properties of pie-1
provide an example of a repressor-based mechanism for preserving pluripotency within a stem cell lineage.