able in many instances to produce mature eggs lacking the gene product (Perrimon et al. 1989; Chou and Perrimon 1996). This provides the opportunity to determine the phenotype of embryos lacking the function of the gene in question. Similarly, somatic clones can be produced in embryos or larvae heterozygous for various mutations. These somatic clones, induced at high frequency by the FRT-FRP system, will be homozygous for the new mutations and show a phenotype (Xu and Rubin 1993). Thanks to these powerful methods, components of several of the signal transduction pathways have been identified first in Drosophila and C. elegans and later confirmed in vertebrate cell systems.
The zebrafish is a relative newcomer to the list of model animals. It has become important as the first vertebrate to be subjected to large-scale genetic screens, which were shown to be feasible by C. Nüsslein-Volhard and others in the 1980s (Haffter and Nüsslein-Volhard 1996). Such screens are possible be-