ZEBRAFISH WNT11 - PATTERN AND REGULATION OF THE EXPRESSION BY THE YOLK CELL AND NO TAIL ACTIVITY

This study analyzed the spatial and temporal expression pattern of zeb rafish wnt11 and the regulation of the expression during zebrafish ear ly development, focusing on the interaction with the no tail (ntl) gen e, a zebrafish orthologue of mouse Brachyury (T). Zygotic expression o f wnt11 was first detected at the late blastula stage in the blastoder m margin, a presumptive mesoderm region. wnt11 expression coincided wi th mesoderm induction, and the expression was induced by mesoderm indu cers such as the yolk cell (Mizuno, T., Yamaha, E., Wakahara, M., Kuro iwa, A., Takeda, H., 1996. Mesoderm induction in zebrafish. Nature 383 , 131-132) or fibroblast growth factors (FGFs), indicating that, like ntl, wnt11 is one of the immediate-early genes in mesoderm induction. Initial expression domains of wnt11 and ntl overlapped, and these gene s showed a similar response to mesoderm inducers. However, analysis of the ntl mutant embryos suggested that wnt11 and ntl are placed in dis tinct genetic pathways; the ntl mutation had no effect on wnt11 expres sion in the blastoderm margin. This was further supported by the resul t of RNA injection experiments showing that overexpression of Wnt11 di d not affect ntl expression in the margin. Thus, wnt11 and ntl express ion are induced and maintained independently in their initial phase of expression. In later stages, wnt11 was expressed in various organs, s uch as the somites, particularly in the developing notochord. Since no wnt gene has been reported to be expressed in the axial mesoderm, whi ch is known to act as a signaling source that patterns the neural tube and somites, zebrafish wnt11 is the first wnt gene expressed in the n otochord. Furthermore, in contrast to early expression, wnt11 expressi on in the notochord depended on Ntl activity. In the ntl mutant in whi ch somite patterning is severely affected, wnt11 expression was comple tely lost, while another signaling molecule, sonic hedgehog is express ed in the mutant notochord precursor cells (Krauss, S., Concordet, J.- P., Ingham, P.W., 1993. A functionally conserved homolog of the Drosop hila segment polarity gene hh is expressed in tissues with polarizing activity in zebrafish embryos. Cell 75, 1431-1444). wnt11 expression i n the somite also shows a characteristic pattern, correlated with the migration and differentiation of slow muscle precursors. These observa tions suggest a role for wnt11 in patterning the somites. (C) 1998 Els evier Science Ireland Ltd.