XENOPUS AXIS FORMATION - INDUCTION OF GOOSECOID BY INJECTED XWNT-8 AND ACTIVIN MESSENGER-RNAS

In this study, we compare the effects of three mRNAs - goosecoid, acti vin and Xwnt-8 - that are able to induce partial or complete secondary axes when injected into Xenopus embryos. Xwnt-8 injection produces co mplete secondary axes including head structures whereas activin and go osecoid injection produce partial secondary axes at high frequency tha t lack head structures anterior to the auditory vesicle and often lack notochord. Xwnt-8 can activate goosecoid only in the deep marginal zo ne, i.e., in the region in which this organizer-specific homeobox gene is normally expressed on the dorsal side. Activin B mRNA, however, ca n turn on goosecoid in all regions of the embryo. We also tested the c apacity of these gene products to restore axis formation in embryos in which the cortical rotation was blocked by UV irradiation. Whereas Xw nt-8 gives complete rescue of anterior structures, both goosecoid and activin give partial rescue. Rescued axes including hindbrain structur es up to level of the auditory vesicle can be obtained at high frequen cy even in the absence of notochord structures. The possible functions of Wnt-like and activin-like signals and of the goosecoid homeobox ge ne, and their order of action in the formation of Spemann's organizer are discussed.