Ancestral and derived functions of the Snail gene superfamily

Abstract

The Snail family of zinc-finger transcription factors plays an important role in early embryonic development of vertebrates and invertebrates, with an evolutionarily conserved function in gastrulation and mesoderm patterning, and further roles in neural differentiation and vertebrate neural crest formation. Several observations make the study of this gene family extremely interesting from an evolutionary point of view. Firstly, the neural crest is believed to have been crucial in the formation of the “new head” of vertebrates. In addition, we have reported a unique interchange in the expression patterns and the roles of the two vertebrate family members (Snail and Slug) between avian and mammals. Thus, we have carried out the study of the distribution and the evolutionary history of the Snail family. Until recently, four members of the Snail family had been described in Drosophila (snail, scratch, escargot and worniu) and up to three in vertebrates (Snail, Slug and Smuc), whereas only one had been found in nematodes, echinoderms and non-vertebrate chordates (ascidians and amphioxus). This led to the suggestion that a unique gene was present before the protostome/ deuterostome divergence and subsequent independent duplication events within the arthropod and vertebrate lineages gave rise to the present situation. We have identified new family members from different model organisms that has led us to define a new group of scratch-related genes present in all metazoans. Thus, we have proposed the existence of the Snail superfamily that groups the Snail and Scratch families and have made new hypotheses about common ancestor states and functional changes during evolution. These distinct duplication events and the subsequent divergence of the duplicated genes raise interesting questions as when and how the inversion in expression sites, observed for Slug and Snail between chicken and mouse embryos, occurred during evolution. We have isolated Snail family members from lizards and turtles and analysed their expression pattern during embryonic development. We will discuss whether the regulation of these genes is a derived feature of the avian lineage or a general characteristic of all the non-synapsid lineage, that includes turtles, avians and reptiles. This approach will offer a glimpse on how duplication and divergence within gene families during evolution correlate with early development and patterning of vertebrates.