Phylogenetic relationships and whole mount in situ hybridization of Snail and Slug genes in different vertebrates at mid-embryonic stages. Vertebrate phylogeny is based on recent studies that place turtles as a sister group of avians and not as basal sauropsids (43, 44).

Expression of Slug in the developing lens of vertebrates at early optic cup stages. ret, retina.

Whole-mounted embryos showing Snail and Slug expression in the cranial premigratory neural crest (pnc). Snail is the gene expressed in the pnc of zebrafish (snail2) and mouse embryos, whereas in lizard, turtle, and chicken embryos, Slug is the gene present in this tissue (yellow stars). mnc, migratory neural crest.

Sections through the trunk region showing expression of Snail: (A) dogfish; (C) zebrafish snail2; (G) mouse; and expression of Slug: (J) lizard; (K) turtle; and (L) chick in the pnc. Yellow stars indicate those genes expressed in this population in each species. n, notochord; ne, neural epithelium; nt, neural tube.

Close view of Snail and Slug expression patterns in the tails of whole-mounted embryos. Yellow stars indicate which gene is present in the tail bud mesenchyme in each species. tbm, tailbud mesenchyme.

Sections through the tail bud showing expression of both Snail and Slug in dogfish (A and H), but only Snail in zebrafish (snail1, B), lizard (D), turtle (E), and mouse (G), or Slug in chick (L) in the mesenchyme. Yellow stars indicate those genes expressed in this population in each species.