ACTIVIN-LIKE SIGNAL ACTIVATES DORSAL-SPECIFIC MATERNAL RNA BETWEEN 8-CELL AND 16-CELL STAGES OF XENOPUS

In many animals the dorsal-ventral axis forms by an initial localizati on of maternal molecules, which then regulate the spatial location of signals that directly influence the expression of axis-specific fates. Several recent studies have demonstrated that dorsal-animal blastomer es of the Xenopus morula (8-32 cells) are biased toward dorsal fates p rior to mesoderm inductive signaling. In this study we ask whether the dorsal bias is the result of autonomous expression of maternal molecu les specifically localized within dorsal cells or of early activating signals. It was found that although 16-cell dorsal-animal blastomeres (D1.1) can differentiate into dorsal tissues when cultured alone, the 8-cell mothers (D1) can not. likewise, although RNA extracted from D1. 1 can induce an extra dorsal axis when injected into vegetal blastomer es, RNA extracted from D1 can not. However, D1 does express dorsal tis sues if co-cultured with dorsal-vegetal cells or with culture medium c ontaining a mixture of activins (PIF-medium). Furthermore, short-term culture of D1 in PIF-medium enables the D1 RNA to induce an ectopic do rsal axis. Ventral-animal blastomeres also can express dorsal axial ti ssues when co-cultured with dorsal-vegetal blastomeres or in PIF-mediu m, but the RNA from the activin-treated ventral cells cannot induce ec topic dorsal axes. These studies demonstrate that there are maternal R NAs that, shortly after fertilization, are present only in the dorsal- animal region. They do not act cell autonomously, but require an activ in like signal. These RNAs may function by increasing the responsivene ss of dorsal-animal blastomeres to the mesoderm inductive signals pres ent in both the morula and the blastula. (C) 1996 Wiley-Liss, Inc.