The complex sequence of inductive events responsible for the generation of the neural crest at the border between the neural plate and the epidermis,triggers a genetic cascade involving several families of transcription factors. Two members of the Snail family, Snail andSlug, have both been implicated in this cascade. In chick andXenopus, loss- and gain-of-function experiments have provided evidence that Slug plays a key role in neural crest development. However, in contrast to the chick, Snail rather than Slug is expressed in the premigratory neural crest in the mouse and, in Xenopus,Snail precedes Slug expression in this population. Thus, in order to study the function of Snail in neural crest development inXenopus, we have carried out conditional gain- and loss-of-function experiments using different Snail constructs fused to a glucocorticoid receptor element.

We show that Snail is able to induce the expression ofSlug and all other neural crest markers tested (Zic5, FoxD3,Twist and Ets1) at the time of specification. This activation is observed in whole embryos and in animal caps, in the absence of neural plate and mesodermal markers. We show that Snail is required for neural crest specification and migration and that it works as a transcriptional repressor. These functions have been previously attributed to Slug. However, Slug alone is unable to induce other neural crest markers in animal cap assays, and we show that Snail and Slug can be functionally equivalent when tested in overexpression studies. This suggests that, in Xenopus embryos, at least some of the functions previously attributed to Slug can be carried out by Snail. This is additionally supported by rescue experiments in embryos injected with dominant-negative constructs that indicate that Snail lies upstream of Slug in the genetic cascade leading to neural crest formation and that it plays a key role in crest development.