Expression of Snail superfamily members in the sea urchin embryo and their genomic organization

Snail transcription factors are key regulators of embryonic development and cancer progression, mainly acting as transcriptional repressors. During animal development, Snail and Snai2 are involved in cell movement and morphogenesis, and functional analyses demonstrated their main role in mesoderm formation and Epithelial-Mesenchymal Transitions (EMTs) (1, 2). Although the role of Snail in the embryonic EMT has been reported during sea urchin embryogenesis (3), some other aspects are not yet clear. By qPCR and in situ hybridization we determined the expression of the sea urchin Snail superfamily components Snail, ScratchX /Snai2, and Scratch along development. Our results show that Scratch is not expressed at all, while Snail and ScratchX/Snai2 mRNAs appear at early mesenchyme blastula stage in an overlapping domain at the vegetal plate. Later, Snail localizes to the ingressing PMCs, the ventrolateral PMC clusters of gastrula stage embryos, and the tips of the post-oral arms of plutei; ScratchX/Snai2 mRNAs become confined in cells corresponding to the coelomic pouches progenitors on either side of the developing archenteron. This pattern persists up to 44 h post fertilization (hpf). In some experiments, we detect ScratchX/Snai2 hybridization in a number of cells scattered along the arms and the apex of 3-days old plutei. Finally, we annotated Snail superfamily genes on the P. lividus genome, finding that all members lie on a 1.6 x 107 bp long DNA region with different orientations.