escargot

EVOLUTIONARY HOMOLOGS

escargot is most closely related to the Drosophila snail gene and its Xenopus homolog, xsnail. In the zinc finger region, including spacers, escargot has a 72% nucleotide identity with snail, and a 76% nucleotide identity with xsnail (Whitley, 1992). There is a mouse homolog as well (Smith, 1992). The Drosophila pan neural gene scratch also has a similar zinc finger domain (Roark, 1995).

The members of the Snail family of zinc-finger transcription factors have been implicated in the formation of distinct tissues within the developing vertebrate and invertebrate embryo. Two members of this family have been described in higher vertebrates, Snail (Sna) and Slug (Slu), where they have been implicated in the formation of tissues such as the mesoderm and the neural crest. The mouse homolog of the Slug gene has been isolated, enabling an analysis and comparison of the amino acid sequences and the patterns of expression of both Sna and Slu in the chick and mouse. Features in the sequences have been detected that allow the unequivocal ascription of any family member to the Sna or Slu subfamilies. A stretch of 29 amino acids immediately preceding the zinc-finger domain is exclusive to and highly conserved in vertebrate Slug proteins. The vertebrate Snail proteins appear to contain several amino acid stretches of variable size at conserved positions that are absent in the Slug protein sequences. The identity between each of these short sequences is not sufficiently conserved so as to enable these stretches of amino acids to be considered as diagnostic for Snail proteins. When compared to the three members of the Snail family in Drosophila (snail, escargot and scratch) it appears that all vertebrate members of the family show a slightly greater degree of identity to the product of escargot (between 50 and 59% identity) than to that of snail (between 44 and 55% identity), scratch being the most distant relative of the Drosophila genes. The region 5' to the zinc-finger domains in each of the Drosophila proteins is notably larger than that of the vertebrate homologs, as is the case for the only Snail family homologs described in sea urchin and ascidians. The vertebrate proteins of the Sna and Slu subfamilies contain sequences in this 5' region that are specific to both Esg and Sna. It has been observed that during early stages of development many of the sites of Slu and Sna expression in the mouse and chick embryo are swapped. Thus, an inversion in the expression of Snail family members appears to have occurred between the chick and mouse in the premigratory neural crest, early mesoderm and during early somite formation. These swaps appear to have taken place within the avian lineage. Later in development, the sites of expression of Slu and Sna are conserved between these two species. These data, together with the data available in other species, lead to a proposal that Slu and Sna arose as a duplication of an ancestor gene and that an extra duplication in the fish lineage has given rise to two Sna genes. This analysis of the Snail family may also shed new light on the origin of the neural crest. The neural crest first appears at the edges of the neural folds, precisely the region where the ascidian snail homolog is expressed. This raises the possiblity that these cells, that also appear in the cephalochordata, are the evolutionary precursors of the neural crest (Sefton, 1998).

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