Figure 4. Extensive Temporal Flexibility of Cell Fate Progression.
(A) Regulation of temporal identity. Black arrows represent progression of normal development. Germline cells are shown above with numbers indicating different generations. Somatic cells are shown below. Red arrows indicate the fate transformations and boxes below show genes that induced the corresponding phenotype. Black indicates known genes and red indicates new genes found in this study.
(B) Regulation of cell fate restriction. Boxes show detected cases of fate renewal (delayed restriction, left) and skipping of cell fate (precocious restriction, right).
(C) Differentiation of the AB lineage in normal development. In the left panel, colored tree represents expression patterns of tissue markers used to assay cell fate. Purple bar below the tree highlights the ABp fate. Right panel summarizes cell fate progression from P0 to the ABx generation in the wild type. Texts denote cell name and different colors denote cell fates.
(D) cdc-25.1(RNAi) induces precocious differentiation. Left panel shows lineage differentiation and right panel summarizes the fate progression. Tissue marker expression pattern for the AB cell is identical to that of normal ABp. The pattern highlighted by cyan box-3 is different from that of box 1 in Figure 4C, but identical to that of box-2. This is presumably a secondary phenotype caused by fate change of AB cell in cdc-25.1(RNAi), which causes the loss of the third Notch signaling that distinguishes ABpla and ABpra fates. In the absence of third Notch, both ABpla and ABpra cell adopt the ABpra fate (box 2) (Hutter and Schnabel, 1995).
(E) Schematic representation of ABp fate induction caused by Notch signaling (arrow) from the neighbor cell. Upper panel shows the process in the wild type that occurs at the 4-cell stage and lower panel shows a possible scenario of premature induction of ABp fate in the AB cell in cdc-25.1(RNAi). Text and color indicate cell and fates respectively.
(F) AB lineage adopts two “ABa” fates in glp-1(e2141). The hypodermis marker is used to assay differentiation. Bars below the tree highlight the “ABa fate”. The difference between the “ABa” fate and normal ABa fate (shown in Figure 4C) is due to additional function of Notch in ABalp and ABara sublineages (Hutter and Schnabel, 1994). Number shown above indicates penetrance.
(G) AB lineage adopts the “ABa” fate in glp-1(e2141); cdc-25.1(RNAi) suggesting AB skipping is not caused by to premature Notch induction. Hypodermis marker is used to assay differentiation and number shown above indicates penetrance. In cdc-25.1(RNAi) embryos, the position of ABal and ABar cells is not conventional.