A NOVEL XENOPUS MIX-LIKE GENE MILK INVOLVED IN THE CONTROL OF THE ENDOMESODERMAL FATES

Here we describe a novel Xenopus homeobox gene, milk, related by seque nce homology and expression pattern to the vegetally expressed Mix.1, As is the case with Mix.1, milk is an immediate early response gene to the mesoderm inducer activin, milk is expressed at the early gastrula stage in the vegetal cells, fated to form endoderm, and in the margin al zone fated to form mesoderm, During gastrulation, expression of mil k becomes progressively reduced in the involuting mesodermal cells but is retained in the endoderm, suggesting that it may play a key role i n the definition of the endo-mesodermal boundary in the embryo. Overex pression of milk in the marginal zone blocks mesodermal cell involutio n, represses the expression of several mesodermal genes such as Xbra, goosecoid, Xvent-1 or Xpo and increases the expression of the endoderm al gene, endodermin. In the dorsal marginal zone, overexpression of mi lk leads to a severe late phenotype including the absence of axial str uctures. Ectopic expression of milk in the animal hemisphere or ectode rmal explants induces a strong expression of endodermin. Taken togethe r, we propose that milk plays a role in the correct patterning of the embryo by repressing mesoderm formation and promoting endoderm identit y.