Gastrulation in zebrafish: what mutants teach us.

A major approach to the study of development is to compare the phenotypes of normal and mutant individuals for a given genetic locus. Understanding the development of a complex metazoan therefore requires examination of many mutants. Relatively few organisms are being studied this way, and zebrafish is currently the best example of a vertebrate for which large-scale mutagenesis screens have successfully been carried out. The number of genes mutated in zebrafish that have been cloned expands rapidly, bringing new insights into a number of developmental pathways operating in vertebrates. Here, we discuss work on zebrafish mutants affecting gastrulation and patterning of the early embryo. Gastrulation is orchestrated by the dorsal organizer, which forms in a region where maternally derived beta-catenin signaling is active. Mutation in the zygotic homeobox gene bozozok disrupts the organizer genetic program and leads to severe axial deficiencies, indicating that this gene is a functional target of beta-catenin signaling. Once established, the organizer releases inhibitors of ventralizing signals, such as BMPs, and promotes dorsoanterior fates within all germ layers. In zebrafish, several mutations affecting dorsal-ventral (D/V) patterning inactivate genes functioning in the BMP pathway, stressing the central role of this pathway in the gastrula embryo. Cells derived from the organizer differentiate into several axial structures, such as notochord and prechordal mesoderm, which are thought to induce various fates in adjacent tissues, such as the floor plate, after the completion of gastrulation. Studies with mutants in nodal-related genes, in one-eyed pinhead, which is required for nodal signaling, and in the Notch pathway reveal that midline cell fate specification is, in fact, initiated during gastrulation. Furthermore, the organizer coordinates morphogenetic movements, and zebrafish mutants in T-box mesoderm-specific genes help clarify the mechanism of convergence movements required for the formation of axial and paraxial mesoderm.     

Zebrafish tbx-c functions during formation of midline structures.

Several genes containing the conserved T-box region in invertebrates and vertebrates have been reported recently. Here, we describe three novel members of the T-box gene family in zebrafish. One of these genes, tbx-c, is studied in detail. It is expressed in the axial mesoderm, notably, in the notochordal precursor cells immediately before formation of the notochord and in the chordoneural hinge of the tail bud, after the notochord is formed. In addition, its expression is detected in the ventral forebrain, sensory neurons, fin buds and excretory system. The expression pattern of tbx-c differs from that of the other two related genes, tbx-a and tbx-b. The developmental role of tbx-c has been analysed by overexpression of the full-length tbx-c mRNA and a truncated form of tbx-c mRNA, which encodes the dominant-negative Tbx-c. Overexpression of tbx-c causes expansion of the midline mesoderm and formation of ectopic midline structures at the expense of lateral mesodermal cells. In dominant-negative experiments, the midline mesoderm is reduced with the expansion of lateral mesoderm to the midline. These results suggest that tbx-c plays a role in formation of the midline mesoderm, particularly, the notochord. Moreover, modulation of tbx-c activity alters the development of primary motor neurons. Results of in vitro analysis in zebrafish animal caps suggest that tbx-c acts downstream of early mesodermal inducers (activin and ntl) and reveal an autoregulatory feedback loop between ntl and tbx-c. These data and analysis of midline (ntl-/- and flh-/-) and lateral mesoderm (spt-/-) mutants suggest that tbx-c may function during formation of the notochord.