Cosmid and BAC contig maps have been constructed across two Fugu genomic regions containing the orthologs of human genes mapping to human chromosome 20q. Contig gene contents have been assessed by sample sequencing and comparative database analyses. Contigs are centered around two Fugu topoisomerase1 (top1) genes that were initially identified by sequence similarity to human TOP1 (20q12). Two other genes (SNAI1 and KRML) mapping to human chromosome 20 are also duplicated in Fugu. The two contigs have been mapped to separate Fugu chromosomes. Our data indicate that these linkage groups result from the duplication of an ancestral chromosome segment containing at least 40 genes that now map to the long arm of human chromosome 20. Although there is considerable conservation of synteny, gene orders are not well conserved between Fugu and human, with only very short sections of two to three adjacent genes being maintained in both organisms. Comparative analyses have allowed this duplication event to be dated before the separation of Fugu and zebrafish. Our data (which are best explained by regional duplication, followed by substantial gene loss) support the hypothesis that there have been a large number of gene and regional duplications (and corresponding gene loss) in the fish lineage, possibly resulting from a single whole genome duplication event
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