The DNA sequence of medaka chromosome LG22.

We report the genomic DNA sequence of a single chromosome (linkage group 22; LG22) of the small teleost fish medaka (Oryzias latipes) as a first whole chromosome sequence from a non-mammalian vertebrate. The order and orientation of 633 protein-coding genes were deduced from 18,803,338 bp of DNA sequence, providing the opportunity to analyze chromosome evolution of vertebrate genomes by direct comparison with the human genome. The average number of genes in the "conserved gene cluster" (CGC), a strict definition of "synteny" at the sequence basis, between medaka and human was 1.6. These and other data suggest that approximately 38.8% of pair-wise gene relationships would have been broken from their common ancestor in the human and medaka lineages and further imply that approx 20,000 (15,520-23,280) breaks would have occurred from the entire genome of the common ancestor. These breaks were generated mainly by intra-chromosomal shufflings at a specific era in the vertebrate lineage. These precise comparative genomics allowed us to identify the pieces of ancient chromosomes of the common vertebrate ancestor and estimate chromosomal evolution in the vertebrate lineage.

A first generation physical map of the medaka genome in BACs essential for positional cloning and clone-by-clone based genomic sequencing.

In order to realize the full potential of the medaka as a model system for developmental biology and genetics, characterized genomic resources need to be established, culminating in the sequence of the medaka genome. To facilitate the map-based cloning of genes underlying induced mutations and to provide templates for clone-based genomic sequencing, we have created a first-generation physical map of the medaka genome in bacterial artificial chromosome (BAC) clones. In particular, we exploited the synteny to the closely related genome of the pufferfish, Takifugu rubripes, by marker content mapping. As a first step, we clustered 103,144 public medaka EST sequences to obtain a set of 21,121 non-redundant sequence entities. Avoiding oversampling of gene-dense regions, 11,254 of EST clusters were successfully matched against the draft sequence of the fugu genome, and 2363 genes were selected for the BAC map project. We designed 35mer oligonucleotide probes from the selected genes and hybridized them against 64,500 BAC clones of strains Cab and Hd-rR, representing 14-fold coverage of the medaka genome. Our data set is further supplemented with 437 results generated from PCR-amplified inserts of medaka cDNA clones and BAC end-fragment markers. Our current, edited, first generation medaka BAC map consists of 902 map segments that cover about 74% of the medaka genome. The map contains 2721 markers. Of these, 2534 are from expressed sequences, equivalent to a non-redundant set of 2328 loci. The 934 markers (724 different) are anchored to the medaka genetic map. Thus, genetic map assignments provide immediate access to underlying clones and contigs, simplifying molecular access to candidate gene regions and their characterization.