Expression of snail2, a second member of the zebrafish snail family, in cephalic mesendoderm and presumptive neural crest of wild-type and spadetail mutant embryos.

Transcripts of a newly discovered gene called snail2, encoding a zinc finger protein of the Snail family, first appear in rows of cephalic mesendodermal cells in gastrulating zebrafish embryos. At the end of gastrulation, snail2 RNA accumulates in a domain of ectodermal cells that mark the border between the epidermal epithelium and the neural plate and includes precursors of the neural crest. During somitogenesis, snail2 expression becomes restricted to neural crest. snail2 is thus one of the earliest genes yet known to be specifically expressed in neural crest in zebrafish embryos. Since snail2 is expressed in mesendoderm, a tissue layer whose convergence in the trunk is known to be altered in embryos homozygous for the spadetail mutation, we examined snail2 expression in spadetail embryos. In these mutants, the number of cephalic mesendodermal cells expressing snail2 is strongly reduced and the distribution of cells containing snail2 and no tail transcripts in the axial mesoderm is much broader than normal Moreover, the embryos are shorter than normal at the end of gastrulation. This shows that, in addition to the failure of paraxial mesoderm to converge normally in the trunk during gastrulation, spadetail also affects the elongation of the embryo and the convergence of axial and lateral mesendoderm in both trunk and head.