Characterization of the nutritional endoderm in the direct developing frog Eleutherodactylus coqui

Unlike Xenopus laevis, Eleutherodactylus coqui develops without a tadpole. The yolk-rich vegetal region of the embryo forms a transient nutritive tissue, the nutritional endoderm (NE). The definitive endoderm (DE) in E. coqui comes from cells closer to the animal pole in contrast to its vegetal origin in X. laevis. RNA important for initiating the endoderm specification network is absent in presumptive NE cells, raising the question whether signaling occurs in them. We explored the nature of NE and asked how differences between NE and DE cells arise. We identified differences between NE and DE that first become evident at gastrula, when NE cells become multinucleated. Nuclear β-catenin, an essential cofactor of sox 17, important for endoderm formation in X. laevis, is present in NE and DE at gastrula but remains in NE long after it is not seen in DE. We cloned E. coqui homologs of TGFβs activin b and derriere and provide evidence for their maternal expression. We also detected activin b and derriere RNAs in NE at gastrula and show that NE possesses some mesoderm-inducing activity, but it is delayed with respect to DE. Our findings indicate that altered development of NE begins at gastrula. RNAs important for mesendoderm induction and some mesoderm-inducing activity are present in NE.