The art and design of genetic screens: zebrafish

The zebrafish offers a forward genetic system in which to explore vertebrate biological processes. Zebrafish were used to carry out the first large-scale genetic screens in vertebrates, which identified more than 2,000 mutations involved in embryonic development. Haploid and homozygous diploid screens expose recessive alleles in a generation, eliminating the need to work up an entire generation of fish. Screens can use various mutagens, which offer different advantages in mutation rate or the ability to clone the gene. Fluorescent reporter genes are used to visualize biological processes in zebrafish and have been used successfully in genetic screens to identify mutants (for example, digestive and retinal axon mutants). Specific zebrafish behaviours are amenable to genetic screens, including vision response, locomotion and addiction behaviours. Gene function can be explored in zebrafish by identifying mutations that enhance or suppress a phenotype, by isolating new and conditional alleles, as well as by using gene knockdown and gain-of-function technologies. Small molecules can penetrate the chorion of zebrafish embryos and induce embryonic phenotypes within a controlled time during development. Screen design requires a balance between selecting parameters that will best identify mutations of interest with those that are achievable in individual laboratories.