DISRUPTION OF MESODERM AND AXIS FORMATION IN FISH BY ECTOPIC EXPRESSION OF ACTIVIN VARIANTS - THE ROLE OF MATERNAL ACTIVIN

Formation of mesoderm in Xenopus embryos is the result of an induction event in which peptides such as EGF or activins have been implicated. It was recently demonstrated, by the ectopic expression of a truncate d activin receptor, that activin receptor signaling pathways are invol ved in the processes of mesoderm and axis formation in vivo. However, this approach does not directly address the role of activin itself nor the involvement of activins in the formation of mesoderm in embryos f rom other vertebrates. In addition, activins are expressed maternally as a protein component of the egg as well as transcribed zygotically, and it is not clear which of the two forms is involved in mesoderm for mation. To address those three issues, we analyzed the role of activin s in the development of fish embryos by generating two activin dominan t negative variants. One of the variants behaves as an inhibitor of ac tivin protein. The second variant was found to deplete the activin poo l when cotranslated with wild-type activin. Injection of RNA encoding these variants into the two-cell embryo of the small teleost fish Oryz ias latipes (Japanese medaka) demonstrates that only the maternally pr ovided activin protein is required for mesoderm and axis formation in fish in vivo.