Zebrafish: genomics on the fast track.

The zebrafish (Danio rerio) has proven its worth as an excellent model system for the study of vertebrate development. In 1996, in an issue of the journal Development, the results of the characterization of almost 2000 mutations derived from two screens of ENU-mutagenized zebrafish (Driever et al. 1996; Haffter et al. 1996) were reported in 36 articles (Vol. 123, Dec. 1996). The same issue, however, included only one article describing the generation of reagents for genomic analysis that will be required for the positional cloning and characterization of these newly identified developmental anomalies (Knapik et al. 1996). Since then, remarkable progress has been made in zebrafish genomics, based largely on technology that has been developed for mammalian genetic analysis. The rapid implementation of these techniques, which will enable the characterization of the large set of zebrafish mutations, speaks to the utility of research that aims to develop resources for a community of investigators rather than to test a specific hypothesis. The use of saturation mutagenesis in zebrafish as a means to study vertebrate development has evolved from a convergence of diverse experimental approaches. Saturation mutagenesis of Drosophila has been used with extraordinary success to identify abnormalities in early embryonic patterning. The fact that mutations in mammalian homologs of these genes often have profound developmental consequences demonstrates that many of these basic pathways have been maintained in highly divergent organisms. A similar analysis in vertebrates was proposed as a means to uncover additional loci of developmental importance, especially those affecting formation of internal organs, which are not effectively scored in Drosophila embryonic screens (Nüsslein-Volhard 1994). Zebrafish are particularly well-suited for this purpose, as they generate large numbers of transparent embryos that develop synchronously to a free-swimming hatchling in a period of 3 days (for review, see Driever et al. 1994). Zebrafish have an additional experimental advantage that has been exploited successfully for the purpose of genetic analysis. Based largely on the studies of George Streisinger, a variety of techniques can be used to manipulate the ploidy and parental origin of genes in zebrafish (Streisinger et al. 1981, 1986). For example, gynogenetic haploid embryos can be generated by fertilization of secondary oocytes with UV-irradiated sperm; these embryos develop to hatching, albeit with specific developmental abnormalities. It is also possible to generate both fully homozygous diploid adults, and ‘‘half-tetrad’’ diploid adults, which are partially homozygous (for review, see Postlethwait and Talbot 1997). The uses of these various genetic ‘‘tricks’’ for genetic mapping are outlined in Box 1.