A radiation hybrid map of the zebrafish genome.

Recent large-scale mutagenesis screens have made the zebrafish the first vertebrate organism to allow a forward genetic approach to the discovery of developmental control genes1, 2, 3. Mutations can be cloned positionally, or placed on a simple sequence length polymorphism (SSLP) map4, 5, 6 to match them with mapped candidate genes and expressed sequence tags7, 8 (ESTs). To facilitate the mapping of candidate genes and to increase the density of markers available for positional cloning, we have created a radiation hybrid (RH) map of the zebrafish genome. This technique is based on somatic cell hybrid lines produced by fusion of lethally irradiated cells of the species of interest with a rodent cell line. Random fragments of the donor chromosomes are integrated into recipient chromosomes or retained as separate minichromosomes9, 10. The radiation-induced breakpoints can be used for mapping in a manner analogous to genetic mapping, but at higher resolution and without a need for polymorphism. Genome-wide maps exist for the human, based on three RH panels of different resolutions11, 12, 13, as well as for the dog14, rat15 and mouse16, 17. For our map of the zebrafish genome, we used an existing RH panel18, 19 and 1,451 sequence tagged site (STS) markers, including SSLPs, cloned candidate genes and ESTs. Of these, 1,275 (87.9%) have significant linkage to at least one other marker. The fraction of ESTs with significant linkage, which can be used as an estimate of map coverage, is 81.9%. We found the average marker retention frequency to be 18.4%. One cR3000 is equivalent to 61 kb, resulting in a potential resolution of approximately 350 kb.