Biological systems constantly face perturbations both in the form of environmental changes and intrinsic stochasticity. Nevertheless, they are still capable of producing constant phenotypic outcomes. Development in particular is a remarkably robust process ensuring the reproducible generation of key phenotypic traits, such as tissues with the correct cell number. However, very little is known about the mechanisms governing developmental robustness. We use the epidermal stem cells of C. elegans, known as seam cells, as a model to identify factors modulating cell number variability. We employ a pipeline consisting of chemical mutagenesis, phenotype selection and mapping by next generation sequencing to identify genes modulating seam cell number variance. To establish a proof of concept, we present here that mutations in the Hes-related bHLH transcription factor lin-22 increase the variability of terminal seam cell number. Using time-lapse imaging of post-embryonic seam cell divisions, we pinned down the developmental basis of phenotypic variability. We show that stage and lineage-specific gain or loss of cell fate symmetry in stem cell daughters underlies the variable phenotype and that these developmental errors occur within the same animal or even a single cell lineage in a stochastic manner. We provide evidence by smFISH and genetics that LIN-22 acts in the epidermis to antagonise Wnt signalling. In contrast to the seam, lin-22 mutants also demonstrate loss of stochasticity when it comes to P3.p division frequency, emphasising the context-specific manifestation of phenotypic variability within a whole organism. Taken together, our work identifies a factor involved in phenotype construction as a modulator of developmental robustness and highlights the feasibility to map genes influencing developmental variability.

Authors: Katsanos, Dimitris, Koneru, Sneha, Mestek Boukhibar, Lamia, Gritti, Nicola, Ghose, Ritobrata, Appleford, Peter J., Doitsidou, Maria, Woollard, Alison, van Zon, Jeroen, Poole, Richard J., Barkoulas, Michalis

Affiliations:
- Centre for Translational Omics - GOSgene, Genetics and Genomic Medicine, UCL, Institute of Child Health, London WC1N 1EH, UK
- Department of Life Sciences, Imperial College, London SW7 2AZ, United Kingdom
- Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, UK
- Department of Cell and Developmental Biology, University College London, London WC1E 6BT, UK
- AMOLF, Science Park 104, Amsterdam 1098 XG, The Netherlands
- Centre for Integrative Physiology, University of Edinburgh, Edinburgh, EH8 9XD, UK