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Genetic screens in zebrafish (Danio rerio) have identified mutations that define the roles of hundreds of essential vertebrate genes. Genetic maps can link mutant phenotype with gene sequence by providing candidate genes for mutations and polymorphic genetic markers useful in positional cloning projects. Here we report a zebrafish genetic map comprising 4073 polymorphic markers, with more than twice the number of coding sequences localized in previously reported zebrafish genetic maps. We use this map in comparative studies to identify numerous regions of synteny conserved among the genomes of zebrafish, Tetraodon, and human. In addition, we use our map to analyze gene duplication in the zebrafish and Tetraodon genomes. Current evidence suggests that a whole-genome duplication occurred in the teleost lineage after it split from the tetrapod lineage, and that only a subset of the duplicates have been retained in modern teleost genomes. It has been proposed that differential retention of duplicate genes may have facilitated the isolation of nascent species formed during the vast radiation of teleosts. We find that different duplicated genes have been retained in zebrafish and Tetraodon, although similar numbers of duplicates remain in both genomes. Finally, we use comparative mapping data to address the proposal that the common ancestor of vertebrates had a genome consisting of 12 chromosomes. In a three-way comparison between the genomes of zebrafish, Tetraodon, and human, our analysis delineates the gene content for 11 of these 12 proposed ancestral chromosomes.

Large-scale genetic screens in zebrafish have identified thousands of mutations in hundreds of essential genes. The genetic mapping of these mutations is necessary to link DNA sequences to the gene functions defined by mutant phenotypes. Here, we report two advances that will accelerate the mapping of zebrafish mutations: (1) The construction of a first generation single nucleotide polymorphism (SNP) map of the zebrafish genome comprising 2035 SNPs and 178 small insertions/deletions, and (2) the development of a method for mapping mutations in which hundreds of SNPs can be scored in parallel with an oligonucleotide microarray. We have demonstrated the utility of the microarray technique in crosses with haploid and diploid embryos by mapping two known mutations to their previously identified locations. We have also used this approach to localize four previously unmapped mutations. We expect that mapping with SNPs and oligonucleotide microarrays will accelerate the molecular analysis of zebrafish mutations.

To help understand mechanisms of vertebrate genome evolution, we have compared zebrafish and tetrapod gene maps. It has been suggested that translocations are fixed more frequently than inversions in mammals. Gene maps showed that blocks of conserved syntenies between zebrafish and humans were large, but gene orders were frequently inverted and transposed. This shows that intrachromosomal rearrangements have been fixed more frequently than translocations. Duplicated chromosome segments suggest that a genome duplication occurred in ray-fin phylogeny, and comparative studies suggest that this event happened deep in the ancestry of teleost fish. Consideration of duplicate chromosome segments shows that at least 20% of duplicated gene pairs may be retained from this event. Despite genome duplication, zebrafish and humans have about the same number of chromosomes, and zebrafish chromosomes are mosaically orthologous to several human chromosomes. Is this because of an excess of chromosome fissions in the human lineage or an excess of chromosome fusions in the zebrafish lineage? Comparative analysis suggests that an excess of chromosome fissions in the tetrapod lineage may account for chromosome numbers and provides histories for several human chromosomes.

Zebrafish mutations define the functions of hundreds of essential genes in the vertebrate genome. To accelerate the molecular analysis of zebrafish mutations and to facilitate comparisons among the genomes of zebrafish and other vertebrates, we used a homozygous diploid meiotic mapping panel to localize polymorphisms in 691 previously unmapped genes and expressed sequence tags (ESTs). Together with earlier efforts, this work raises the total number of markers scored in the mapping panel to 2119, including 1503 genes and ESTs and 616 previously characterized simple-sequence length polymorphisms. Sequence analysis of zebrafish genes mapped in this study and in prior work identified putative human orthologs for 804 zebrafish genes and ESTs. Map comparisons revealed 139 new conserved syntenies, in which two or more genes are on the same chromosome in zebrafish and human. Although some conserved syntenies are quite large, there were changes in gene order within conserved groups, apparently reflecting the relatively frequent occurrence of inversions and other intrachromosomal rearrangements since the divergence of teleost and tetrapod ancestors. Comparative mapping also shows that there is not a one-to-one correspondence between zebrafish and human chromosomes. Mapping of duplicate gene pairs identified segments of 20 linkage groups that may have arisen during a genome duplication that occurred early in the evolution of teleosts after the divergence of teleost and mammalian ancestors. This comparative map will accelerate the molecular analysis of zebrafish mutations and enhance the understanding of the evolution of the vertebrate genome.

Genetic screens in zebrafish (Danio rerio) have isolated mutations in hundreds of genes essential for vertebrate development, physiology, and behavior. We have constructed a genetic linkage map that will facilitate the identification of candidate genes for these mutations and allow comparisons among the genomes of zebrafish and other vertebrates. On this map, we have localized 771 zebrafish genes and expressed sequence tags (ESTs) by scoring single-stranded conformational polymorphisms (SSCPs) in a meiotic mapping panel. Of these sequences, 642 represent previously unmapped genes and ESTs. The mapping panel was comprised of 42 homozygous diploid individuals produced by heat shock treatment of haploid embryos at the one-cell stage (HS diploids). This "doubled haploid" strategy combines the advantages of mapping in haploid and standard diploid systems, because heat shock diploid individuals have only one allele at each locus and can survive to adulthood, enabling a relatively large quantity of genomic DNA to be prepared from each individual in the mapping panel. To integrate this map with others, we also scored 593 previously mapped simple-sequence length polymorphisms (SSLPs) in the mapping panel. This map will accelerate the molecular analysis of zebrafish mutations and facilitate comparative analysis of vertebrate genomes.

Genetic screens in zebrafish (Danio rerio) have isolated mutations in hundreds of genes with essential functions. To facilitate the identification of candidate genes for these mutations, we have genetically mapped 104 genes and expressed sequence tags by scoring single-strand conformational polymorphisms in a panel of haploid siblings. To integrate this map with existing genetic maps, we also scored 275 previously mapped genes, microsatellites, and sequence-tagged sites in the same haploid panel. Systematic phylogenetic analysis defined likely mammalian orthologs of mapped zebrafish genes, and comparison of map positions in zebrafish and mammals identified significant conservation of synteny. This comparative analysis also identified pairs of zebrafish genes that appear to be orthologous to single mammalian genes, suggesting that these genes arose in a genome duplication that occurred in the teleost lineage after the divergence of fish and mammal ancestors. This comparative map analysis will be useful in predicting the locations of zebrafish genes from mammalian gene maps and in understanding the evolution of the vertebrate genome.