The role of the yolk syncytial layer in germ layer patterning in zebrafish.

Formation of the three germ layers requires a series of inductive events during early embryogenesis. Studies in zebrafish indicate that the source of these inductive signals may be the extra-embryonic yolk syncytial layer (YSL). The characterization of genes encoding the nodal-related factor, Squint, and homeodomain protein, Bozozok, both of which are expressed in the YSL, suggested that the YSL has a role in mesendoderm induction. However, these genes, and a second nodal-related factor, cyclops, are also expressed in the overlying marginal blastomeres, raising the possibility that the marginal blastomeres can induce mesendodermal genes independently of the YSL. We have developed a novel technique to study signaling from the YSL in which we specifically eliminate RNAs in the YSL, thus addressing the in vivo requirement of RNA-derived signals from this region in mesendoderm induction. We show that injection of RNase into the yolk cell after the 1K cell stage (3 hours) effectively eliminates YSL transcripts without affecting ubiquitously expressed genes in the blastoderm. We also present data that indicate the stability of existing proteins in the YSL is unaffected by RNase injection. Using this technique, we show that RNA in the YSL is required for the formation of ventrolateral mesendoderm and induction of the nodal-related genes in the ventrolateral marginal blastomeres, revealing the presence of an unidentified inducing signal released from the YSL. We also demonstrate that the dorsal mesoderm can be induced independently of signals from the YSL and present evidence that this is due to the stabilization of (&bgr;)-catenin in the dorsal marginal blastomeres. Our results demonstrate that germ layer formation and patterning in zebrafish uses a combination of YSL-dependent and -independent inductive events.