VERY EARLY AND TRANSIENT VEGETAL-PLATE EXPRESSION OF SPKROX1, A KRUPPEL KROX GENE FROM STRONGYLOCENTROTUS-PURPURATUS/

All endodermal and mesenchymal cells of the sea urchin embryo descend from the vegetal plate, a thickened epithelium of approximately 50 cel ls arising at the early blastula stage. Cell types that derive from th e vegetal plate are specified conditionally by inductive interactions with underlying micromeres, but the molecular details of vegetal-plate specification remain unresolved. In a search for regulatory proteins that have roles in vegetal-plate specification, a screen was performed to clone Kruppel/Krox-related genes from a Strongylocentrotus purpura tus embryo cDNA library. One newly identified clone, named SpKrox1, co ntained four zinc fingers and a leucine zipper domain. SpKrox1 express ion was low in unfertilized eggs, increased severalfold to the early b lastula stage and decreased between the early gastrula and pluteus sta ges. SpKrox1 mRNA was first seen in macromeres of 16-cell stage embryo s and was restricted to cells of the developing vegetal plate thereaft er. Vegetal-plate expression corresponded to a ring of cells around th e blastopore and overlapped the expression patterns of other genes wit h potential roles in vegetal plate-specification. As the vegetal-plate cells invaginated into the blastopore, SpKrox1 expression was lost, s uggesting that its role was not in endoderm differentiation per se but rather in the initial establishment of the vegetal plate.