RNA of AmVegT, the axolotl orthologue of the Xenopus meso-endodermal determinant, is not localized in the oocyte.

The transcription factor, VegT, is the meso-endodermal determinant in Xenopus laevis, and the localization of VegT RNA to the vegetal cortex of the oocyte is an important starting point for embryonic patterning. We have cloned the VegT orthologue from the urodele amphibian, Ambystoma mexicanum, the Mexican axolotl. Axolotl VegT (AmVegT) is expressed zygotically in the presumptive mesoderm and Rohon-Beard neurons as in X. laevis, and its expression persists at the tip of the tail as with zebrafish and chicken orthologues. Unlike in X. laevis, AmVegT RNA is not localized in the oocyte. This result highlights a major difference in the molecular organization of oocytes of these two amphibian orders.