Transcription factors (TFs) are the main players in transcriptional regulation in eukaryotes. However, it remains unclear what role TFs played in the origin of all of the different eukaryotic multicellular lineages. In this paper, we explore how the origin of TF repertoires shaped eukaryotic evolution and, in particular, their role into the emergence of multicellular lineages. We traced the origin and expansion of all known TFs through the eukaryotic tree of life, using the broadest possible taxon sampling and an updated phylogenetic background. Our results show that the most complex multicellular lineages (i.e., those with embryonic development, Metazoa and Embryophyta) have the most complex TF repertoires, and that these repertoires were assembled in a stepwise manner. We also show that a significant part of the metazoan and embryophyte TF toolkits evolved earlier, in their respective unicellular ancestors. To gain insights into the role of TFs in the development of both embryophytes and metazoans, we analyzed TF expression patterns throughout their ontogeny. The expression patterns observed in both groups recapitulate those of the whole transcriptome, but reveal some important differences. Our comparative genomics and expression data reshape our view on how TFs contributed to eukaryotic evolution and reveal the importance of TFs to the origins of multicellularity and embryonic development.phylotypic stage | Holozoa | LECA T ranscription factors (TFs) are proteins that bind to DNA in a sequence-specific manner (1) and enhance or repress gene expression (2-4). In response to a broad range of stimuli, TFs coordinate many important biological processes, from cell cycle progression and physiological responses, to cell differentiation and development (5, 6). Thus, TFs have a central role in the transcriptional regulation of all cellular organisms, being present in all branches of the tree of life (bacteria, archaea, and eukaryotes). There appears to be a correlation between elaborate regulation of gene expression and the complexity of organisms (7), such that the amount (as a proportion of an organism's total gene content) and diversity of TF proteins is expected to be directly correlated with this complexity (8). Indeed, TFs play a crucial role in multicellular eukaryotes. For example, TFs are the master regulators of embryonic development in embryophytes and metazoans (9), and analyses of their embryonic transcriptional profiles support the presence of a phylotypic stage in both lineages (10-14). These studies have also shown that evolutionarily younger genes tend to be expressed at earlier and later stages of development, whereas the transcriptomes of the middle stages (the phylotypic stage) are dominated by ancient genes (10, 13). It remains to be investigated how the evolutionary age and the expression patterns of the different TFs shift throughout the ontogeny of these lineages and whether TF expression profiles correlate with the general transcriptome profiles.Previous studies have analyzed the evolutionary ...

Intestinal organoids capture essential features of the intestinal epithelium such as crypt folding, cellular compartmentalization and collective movements. Each of these processes and their coordination require patterned forces that are currently unknown. Here we map three-dimensional cellular forces in mouse intestinal organoids grown on soft hydrogels. We show that these organoids exhibit a non-monotonic stress distribution that defines mechanical and functional compartments. The stem cell compartment pushes the ECM and folds through apical constriction, whereas the transit amplifying zone pulls the ECM and elongates through basal constriction. The size of the stem cell compartment depends on ECM stiffness and endogenous cellular forces. Computational modeling reveals that crypt shape and force distribution rely on cell surface tensions following cortical actomyosin density. Finally, cells are pulled out of the crypt along a gradient of increasing tension. Our study unveils how patterned forces enable compartmentalization, folding and collective migration in the intestinal epithelium.

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Correct nuclear position is crucial for cellular function and tissue development. Depending on cell context, however, the cytoskeletal elements responsible for nuclear positioning vary. While these cytoskeletal mechanisms have been intensely studied in single cells, how nuclear positioning is linked to tissue morphology is less clear. Here, we compare apical nuclear positioning in zebrafish neuroepithelia. We find that kinetics and actin-dependent mechanisms of nuclear positioning vary in tissues of different morphology. In straight neuroepithelia, nuclear positioning is controlled by Rho-ROCK–dependent myosin contractility. In contrast, in basally constricted neuroepithelia, a novel formin-dependent pushing mechanism is found for which we propose a proof-of-principle force generation theory. Overall, our data suggest that correct nuclear positioning is ensured by the adaptability of the cytoskeleton to cell and tissue shape. This in turn leads to robust epithelial maturation across geometries. The conclusion that different nuclear positioning mechanisms are favored in tissues of different morphology highlights the importance of developmental context for the execution of intracellular processes.