A role for the fusogen eff-1 in epidermal stem cell number robustness in Caenorhabditis elegans.

Developmental patterning in Caenorhabditis elegans is known to proceed in a highly stereotypical manner, which raises the question of how developmental robustness is achieved despite the inevitable stochastic noise. We focus here on a population of epidermal cells, the seam cells, which show stem cell-like behaviour and divide symmetrically and asymmetrically over post-embryonic development to generate epidermal and neuronal tissues. We have conducted a mutagenesis screen to identify mutants that introduce phenotypic variability in the normally invariant seam cell population. We report here that a null mutation in the fusogen eff-1 increases seam cell number variability. Using time-lapse microscopy and single molecule fluorescence hybridisation, we find that seam cell division and differentiation patterns are mostly unperturbed in eff-1 mutants, indicating that cell fusion is uncoupled from the cell differentiation programme. Nevertheless, seam cell losses due to the inappropriate differentiation of both daughter cells following division, as well as seam cell gains through symmetric divisions towards the seam cell fate were observed at low frequency. We show that these stochastic errors likely arise through accumulation of defects interrupting the continuity of the seam and changing seam cell shape, highlighting the role of tissue homeostasis in suppressing phenotypic variability during development.

Canalization of C. elegans Vulva Induction against Anatomical Variability.

It is a fundamental open question as to how embryos develop into complex adult organisms with astounding reproducibility, particularly because cells are inherently variable on the molecular level. During C. elegans vulva induction, the anchor cell induces cell fate in the vulva precursor cells in a distance-dependent manner. Surprisingly, we found that initial anchor cell position was highly variable and caused variability in cell fate induction. However, we observed that vulva induction was "canalized," i.e., the variability in anchor cell position and cell fate was progressively reduced, resulting in an invariant spatial pattern of cell fates at the end of induction. To understand the mechanism of canalization, we quantified induction dynamics as a function of anchor cell position during the canalization process. Our experiments, combined with mathematical modeling, showed that canalization required a specific combination of long-range induction, lateral inhibition, and cell migration that is also found in other developmental systems.

Long-term time-lapse microscopy of C. elegans post-embryonic development.

We present a microscopy technique that enables long-term time-lapse microscopy at single-cell resolution in moving and feeding Caenorhabditis elegans larvae. Time-lapse microscopy of C. elegans post-embryonic development is challenging, as larvae are highly motile. Moreover, immobilization generally leads to rapid developmental arrest. Instead, we confine larval movement to microchambers that contain bacteria as food, and use fast image acquisition and image analysis to follow the dynamics of cells inside individual larvae, as they move within each microchamber. This allows us to perform fluorescence microscopy of 10-20 animals in parallel with 20 min time resolution. We demonstrate the power of our approach by analysing the dynamics of cell division, cell migration and gene expression over the full ∼48 h of development from larva to adult. Our approach now makes it possible to study the behaviour of individual cells inside the body of a feeding and growing animal.

Cells change their sensitivity to an EGF morphogen gradient to control EGF-induced gene expression.

How cells in developing organisms interpret the quantitative information contained in morphogen gradients is an open question. Here we address this question using a novel integrative approach that combines quantitative measurements of morphogen-induced gene expression at single-mRNA resolution with mathematical modelling of the induction process. We focus on the induction of Notch ligands by the LIN-3/EGF morphogen gradient during vulva induction in Caenorhabditis elegans. We show that LIN-3/EGF-induced Notch ligand expression is highly dynamic, exhibiting an abrupt transition from low to high expression. Similar transitions in Notch ligand expression are observed in two highly divergent wild C. elegans isolates. Mathematical modelling and experiments show that this transition is driven by a dynamic increase in the sensitivity of the induced cells to external LIN-3/EGF. Furthermore, this increase in sensitivity is independent of the presence of LIN-3/EGF. Our integrative approach might be useful to study induction by morphogen gradients in other systems.