Figure 1. Asymmetric cell divisions produce blastomeres with distinct fates. Posterior is to the right. (a) The invariant division pattern leads to a precise arrangement of cells and the birth of six founder cells. The founder cells and the germline precursor cells (P cells) are labelled, and cells derived from each founder are indicated by shading. For simplicity, the 16-cell embryo at bottom does not show the daughter cells of the fourth AB cleavage, which occurs at about the same time as the P₃ division. (b) Diagrammatic representation of the early lineage, showing the major cell types produced by each founder cell. Horizontal lines indicate cell divisions, and the lengths of the vertical lines indicate relative differences in cell cycle rates of each blastomere.

Figure 2. Establishment of asymmetries in the early embryo. Posterior is to the right in all embryos. Just after fertilization, PAR-3/PAR-6/PKC-3 are initially localized uniformly at the cortex and contractions occur throughout the cortex. (a) The close association of the sperm with the cortex results in a local cessation of cortical contractile activity, but cortical contractions (small indentations) continue in the anterior. Cortical cytoplasm (arrows), NMY-2, and anterior PARs begin to flow towards the anterior, and PAR-2 localizes to the posterior cortex. (b) Asymmetric PAR domains are established, internal cytoplasm and P granules flow towards the sperm pronucleus, while more cortical cytoplasm flows away. As ruffling culminates in a medial pseudocleavage furrow, the female pronucleus migrates towards the sperm pronucleus. (c) P granules are localized to the posterior pole by the time of pronuclear meeting. (d) First division is intrinsically asymmetric resulting in an anterior cell AB and a posterior cell P₁ that differ in cytoplasmic inheritance and PAR protein domains; PAR-3 is present around the AB cortex while PAR-2 is present around the P₁ cortex. (e) As the P₁ cell cycle progresses, PAR protein asymmetry and cellular polarity are reestablished in this cell.

Figure 3. Localized cell fate regulators and signalling pathways in the early embryo. (a) The establishment of polarity in the one-cell embryo leads to the localized expression of cell fate regulators. The expression of various regulators at different stages is shown; for simplicity, only GLP-1 protein is shown in the 12-cell embryo. Arrows indicate the direction of cell–cell signalling events. Note that SKN-1 is enriched in the nucleus of the P lineage cells beginning at the two-cell stage. SKN-1 levels peak at the four-cell stage, then gradually disappear by the 12-cell stage. However, PAL-1 is present in the P lineage nuclei beginning at the four-cell stage and persists after SKN-1 disappears. Nuclear PIE-1 is shown; PIE-1 is also present in the cytoplasm and on P granules in the P cells. GLP-1 protein is present on the membranes of AB lineage cells. MEX-5 protein is present in the cytoplasm of somatic cells. (b) Model for how Src, Wnt signalling and MAPK signalling converge to establish the E cell fate. Arrows show positive interactions, while barred lines show inhibitory interactions. The mom genes (for more muscles) are required for downregulating POP-1 levels in the E cell, and thus allowing the endoderm fate. MOM-1 (Porcupine) is required for the production of the MOM-2 (Wingless/Wnt) signal in the P₂ cell. The MOM-5 (Frizzled) receptor acts in EMS to receive the signal. Based on mutant phenotypes in C. elegans, and Wnt signalling in other systems, MOM-5 is proposed to act through three partially redundant proteins DSH-2, MIG-5 and DSH-1 (Dishevelled family members), and SGG-1 (Shaggy/Gsk-3) and WRM-1 (Armadillo) to negatively regulate levels of nuclear POP-1 (TCF/LEF). The MOM-4 (MAPKKK) and LIT-1 (Nemo/MAPK) proteins also converge at WRM-1, and the MES-1 tyrosine kinase and SRC-1 kinase act in a parallel pathway that has been postulated to converge at WRM-1 as well. Only some of the Wnt pathway genes are required for spindle orientation, indicating a branch in the pathway. Drosophila and/or vertebrate homologues are indicated in parentheses.

Figure 4. Division patterns are determined by centrosome migrations and nuclear rotation. All embryos are left views, except (g) which is a dorsal view. Posterior is to the right. The small black circles represent centrosomes, lines represent astral microtubules and arrows indicate directions of centrosome movements. The number of astral microtubules drawn is reduced for clarity. Open circles represent interphase or prophase nuclei, black bars are chromosomes in metaphase or anaphase. (a) The female pronucleus meets the sperm pronucleus in the posterior. (b) The pronuclei centre and rotate so that the first spindle (c) is aligned on the long axis of the embryo. In two-cell embryos (d), centrosomes migrate until they are on opposite sides of the nuclei. In P₁, there is a subsequent 90° nuclear rotation (e) to align the spindle on the anterior/posterior axis (f). At the four-cell stage, centrosomes migrate on to the left/right axis (g) and the spindle forms on this axis in ABa and ABp (h, only the spindle pole is visible). In EMS, the nucleus then rotates 90° on to the anterior/posterior axis (h, i). A 45° nuclear rotation orients the P₂ spindle towards the site of contact with EMS, but division is dorsal/ventral due to constraints of cellular arrangements and the eggshell (not shown).

Figure 5. Model for PAR-dependent spindle positioning. G/GPR signalling is required for forces that pull on microtubules from the cortex (+), while LET-99 inhibits force (–). Note that both LET-99 and GPR localize to the entire cortex, but only the regions with highest levels are indicated for simplicity. (a) During nuclear rotation, GPR is uniformly low at the cortex, but LET-99 localizes to the cortex in a posterior band pattern in response to PAR-3. Inhibition of G/GPR signalling in the band would result in reduced force on lateral and posterior microtubles compared to the force acting on microtubules contacting the anterior cortex. As a result, one centrosome would experience a higher net force (large arrow) towards the anterior, than towards the posterior (small arrow). (b) At metaphase and anaphase, GPR becomes enriched at the posteriormost end of the embryo in response to PAR-3 and LET-99. The resulting increase in force at the posterior, possibly coupled with inhibition of force in the LET-99 band, would result in greater net force acting on the posterior spindle pole and thus posterior spindle displacement. Similar mechanisms are proposed to function in the P₁ cell.