Developmental Genetics in the Zebrafish

The zebrafish owes its elevation from common pet shop aquarium fish to one of the few model systems for the study of vertebrate development largely to the "Oregon School." George Streisinger, at the University of Oregon at Eugene, first settled on the zebrafish for his pioneering genetic studies. The medaka may have some advantages over the zebrafish for certain procedures and is still used for developmental studies; for example, it has been used recently for the analysis of mesoderm formation (10).

Streisinger established a number of methods for studying zebrafish, including the generation of isogenic homozygous diploid lines and the screening of haploid embryos for developmentally interesting mutations (11). The haploid zebrafish embryo develops for several days and eventually dies; a screening program based on haploid embryos has the advantage that recessive mutations present in the female can be revealed in a single generation. This method has been used effectively by Kimmel's laboratory at the University of Oregon to reveal several mutants either induced or present in the genetic background of the zebrafish stock held in Eugene. Such mutants include spadetail (12), cyclops (13), and no-tail (14); lines that have subsequently been extensively studied, the last two being important for studies of patterning of the axial mid-line of the embryonic axis. Mapping of candidate genes has allowed a number of mutants to be identified. These include spadetail (15), cyclops (16), and other important regulatory genes, such as fleating head (17) and FgF8 (18).

Since the first attempts to establish zebrafish for developmental genetics the single most important step has been the selection of this system for a number of major screens using chemical mutagenesis and analysis of sib-crosses in the F2 generation to reveal mutations originally induced into male germ cells by chemical mutagenesis using ethyl nitrosourea (ENU) (19). The strategy, methodology, and results from these screens have recently been published by these two laboratories, and are the subject of Chapter 30. Together with the continuing screening in Oregon and elsewhere, these two large-scale screens are now at the stage where mutations are being characterized genetically and morphologically. The screens have revealed many mutants of developmental interest affecting the major organ systems and structures of the embryo (Development, vol. 123, 1996).

Considerable progress has been made in generating a genetic recombination map for genes relative to the chromosomes. Methods based on polymerase chain reaction (PCR) (so-called RAPD method) have been used to identifiy polymorphic short sequences which, in conjunction with generation of hap-loids, have been applied by John Postlethwait's laboratory (20-22) to generate

Fig. 1. The reticulospinal complex of the zebrafish. (A) The cells of the complex indicating rhombomeres (rl-7) and the principal identified cells (based on ref. 25). (B) Back-filled reticulospinal neurons in a normal embryo. The Mauthner cell is arrowed, v indicates the vestibulospinal neurons, and nmlf indicates the midbrain nucleus of the longitudinal vesiculus. (See color plate 6 appearing after p. 368.)

Fig. 1. The reticulospinal complex of the zebrafish. (A) The cells of the complex indicating rhombomeres (rl-7) and the principal identified cells (based on ref. 25). (B) Back-filled reticulospinal neurons in a normal embryo. The Mauthner cell is arrowed, v indicates the vestibulospinal neurons, and nmlf indicates the midbrain nucleus of the longitudinal vesiculus. (See color plate 6 appearing after p. 368.)

a recombination map (21,22). Genes newly isolated by mutation and genes cloned by homology to those in other species are now being added to the map. It is clear the map will have many markers within a short time and will be a valuable resource for molecular identification of novel mutations either by mapping of cloned genes to the same site as an identified mutation (15-17) or by positional cloning (23).

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