Normal Somitogenesis Despite the Absence of Snai1 in the Presomitic Mesoderm

During vertebrate somitogenesis, Snai1 is expressed dynamically in the presomitic mesoderm (PSM), oscillating in phase with core segmentation clock genes. Whether this reflects a functional requirement for Snai1 in clock activity has remained unresolved, largely because global or epiblast-specific loss of Snai1 causes early embryonic lethality. Here, we identify a conserved enhancer downstream of the Snai1 locus (SCDE) that recapitulates major domains of endogenous Snai1 expression, including the PSM, tailbud, cranial mesenchyme, branchial arches and forelimb buds. Functional dissection identifies two domains within this enhancer, B1 and B3, as partially redundant core regulatory elements that together account for most SCDE activity in the posterior mesoderm. Deletion of these modules strongly reduces Snai1 expression in the PSM while preserving its dynamic expression pattern and Lfng oscillations, without producing any detectable skeletal phenotype. To directly assess the requirement for Snai1, we generated conditional mutants lacking Snai1 throughout the posterior mesoderm, including the PSM. Despite the total absence of Snai1 expression in this area, these embryos show normal somite formation, compartmentalisation and segmentation clock activity. Unexpectedly, complete loss of Snai1 in this territory results in a fully penetrant gain of one pre-sacral vertebra, pointing to a role in fine-tuning axial segment number rather than in segmentation itself. Together, our findings identify SCDE as a bona-fide cis-regulatory element involved in the control of Snai1 expression in the PSM, uncouple its oscillatory expression from an essential role in the segmentation clock and suggest that this gene is dispensable for somitogenesis.