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Populations of cultured mouse embryonic stem cells (ESCs) exhibit a subfraction of cells expressing uncharacteristically low levels of pluripotency markers such as Nanog. Yet, the extent to which individual Nanog-negative cells are differentiated, both from ESCs and from each other, remains unclear. Here, we show the transcriptome of Nanog-negative cells exhibits expression of classes of genes associated with differentiation that are not yet active in cells exposed to differentiation conditions for one day. Long non-coding RNAs, however, exhibit more changes in expression in the one-day-differentiated cells than in Nanog-negative cells. These results are consistent with the concept that Nanog-negative cells may contain subpopulations of both lineage-primed and differentiated cells. Single cell analysis showed that Nanog-negative cells display substantial and coherent heterogeneity in lineage marker expression in progressively nested subsets of cells exhibiting low levels of Nanog, then low levels of Oct4 and then a set of lineage markers, which express intensely in a small subset of these more differentiated cells. Our results suggest that the observed enrichment of lineage-specific marker gene expression in Nanog-negative cells is associated with spontaneous differentiation of a subset of these cells rather than the more random expression that may be associated with reversible lineage priming.

Individual mammalian cells exhibit large variability in cellular volume even with the same absolute DNA content and so must compensate for differences in DNA concentration in order to maintain constant concentration of gene expression products. Using single molecule counting and computational image analysis, we show that transcript abundance correlates with cellular volume at the single cell level due to increased global transcription in larger cells. Cell fusion experiments establish that increased cellular content itself can directly increase transcription. Quantitative analysis shows that this mechanism measures the ratio of cellular volume to DNA content, mostly likely through sequestration of a transcriptional factor to DNA. Analysis of transcriptional bursts reveals a separate mechanism for gene dosage compensation after DNA replication that enables proper transcriptional output during early and late S-phase. Our results provide a framework for quantitatively understanding the relationships between DNA content, cell size and gene expression variability in single cells.

While many transcriptional regulators of pluripotent and terminally differentiated states have been identified, regulation of intermediate progenitor states is less well understood. Previous high throughput cellular resolution expression studies identified dozens of transcription factors with lineage-specific expression patterns in C. elegans embryos that could regulate progenitor identity. In this study we identified a broad embryonic role for the C. elegans OTX transcription factor ceh-36, which was previously shown to be required for the terminal specification of four neurons. ceh-36 is expressed in progenitors of over 30% of embryonic cells, yet is not required for embryonic viability. Quantitative phenotyping by computational analysis of time-lapse movies of ceh-36 mutant embryos identified cell cycle or cell migration defects in over 100 of these cells, but most defects were low-penetrance, suggesting redundancy. Expression of ceh-36 partially overlaps with that of the PITX transcription factor unc-30. unc-30 single mutants are viable but loss of both ceh-36 and unc-30 causes 100% lethality, and double mutants have significantly higher frequencies of cellular developmental defects in the cells where their expression normally overlaps. These factors are also required for robust expression of the downstream developmental regulator mls-2/HMX. This work provides the first example of genetic redundancy between the related yet evolutionarily distant OTX and PITX families of bicoid class homeodomain factors and demonstrates the power of quantitative developmental phenotyping in C. elegans to identify developmental regulators acting in progenitor cells., Author Summary Animals develop as one initial cell, the fertilized egg, repeatedly divides and its progeny differentiate, ultimately producing diverse cell types. This occurs in large part by the expression of unique combinations of regulatory genes, such as transcription factors, in precursors of each cell type. These early factors are typically reused in precursors of different cell types. The nematode worm Caenorhabditis elegans is a powerful system in which to identify developmental regulators because it has a rapid and reproducible development, yet it shares most of its developmental regulators with more complex organisms such as humans. We used state-of-the-art microscopy and computer-aided cell tracking methods to identify the developmental role of worm homologs of the OTX and PITX genes, whose human homologs play a role in the development of the brain, eye, and pituitary among other tissues. We identified broad roles for OTX in regulating development for many distinct cell types including muscles, neurons and skin, and found a redundant role for both OTX and PITX in a subset of cells. Future studies of these genes should address whether these genes also act redundantly in mammals.