Fig 5. A reporter of POP-1-dependent activation shows stronger activity in double-posterior lineages.

A) POPTOP construct [59], a minimal promoter driven by 7 TCF binding sites. B) Expression of POPTOP in the embryo. Expression is limited to descendants of posterior daughter cells (right branches, histone-mCherry reporter is stable even after transcription ceases). C) Detail of expression of POPTOP in the ABpl lineage. We infer that expression initiated in the common ancestor of multiple expressing cells, but visualization lags by ~30 minutes, consistent with previous estimates of the time required for transcription, translation and mCherry maturation [60,83]. POPTOP activity for each cell is the average expression of that cell and its descendants (boxes). We then calculate differential activity between sister sublineages (e.g. ABplpppp HH sublineage minus ABplpppa “HL” sublineage). In the case marked, the SYS-1 High-High cell (HH) expresses POPTOP more strongly than its sister lineage (HL). D) POPTOP levels increase with number of previous divisions with high levels of nuclear SYS-1, as expected if it is specifically transcribed in posterior daughters. E) POPTOP sister asymmetry as outlined in panel C (SYS-1 high cells: red, SYS-1 low cells: grey) clustered by the time at which the division occurred. p-values test for significant differences between Xpp and Xap cells (Wilcoxon rank-sum test). POPTOP expression is more asymmetric in lineages of parent cells with high levels of nuclear SYS-1. Asymmetry is highest at the 100-cell stage, when most lineages initiate POPTOP expression. F) Enhancers with seven (POPTOP), six, and three TCF sites; cells with expressing daughters are marked by the number of consecutive high nuclear SYS-1 parents. A synthetic enhancer with a single TCF site showed no detectable embryonic expression (see S10 Fig). Fewer TCF sites leads to more restricted expression in multiply posterior lineages.