ABSTRACT
Tetraodon nigroviridis is a freshwater puffer fish with the smallest known vertebrate genome. Here, we report a draft genome sequence with long-range linkage and substantial anchoring to the 21 Tetraodon chromosomes. Genome analysis provides a greatly improved fish gene catalogue, including identifying key genes previously thought to be absent in fish. Comparison with other vertebrates and a urochordate indicates that fish proteins have diverged markedly faster than their mammalian homologues. Comparison with the human genome suggests approximately 900 previously unannotated human genes. Analysis of the Tetraodon and human genomes shows that whole-genome duplication occurred in the teleost fish lineage, subsequent to its divergence from mammals. The analysis also makes it possible to infer the basic structure of the ancestral bony vertebrate genome, which was composed of 12 chromosomes, and to reconstruct much of the evolutionary history of ancient and recent chromosome rearrangements leading to the modern human karyotype.
Subject(s)
Chromosomes/genetics , Fishes/genetics , Gene Duplication , Genome , Vertebrates/genetics , Animals , Base Composition , Chromosomes, Human/genetics , Conserved Sequence/genetics , Evolution, Molecular , Genes/genetics , Humans , Karyotyping , Mammals/genetics , Models, Genetic , Molecular Sequence Data , Physical Chromosome Mapping , Proteome , Sequence Analysis, DNA , Synteny/genetics , Urochordata/geneticsABSTRACT
Tetraodon nigroviridis is among the smallest known vertebrate genomes and as such represents an interesting model for studying genome architecture and evolution. Previous studies have shown that Tetraodon contains several types of tandem and dispersed repeats, but that their overall contribution is >10% of the genome. Using genomic library hybridization, fluorescent in situ hybridization, and whole genome shotgun and directed sequencing, we have investigated the global and local organization of repeat sequences in Tetraodon. We show that both tandem and dispersed repeat elements are compartmentalized in specific regions that correspond to the short arms of small subtelocentric chromosomes. The concentration of repeats in these heterochromatic regions is in sharp contrast to their paucity in euchromatin. In addition, we have identified a number of pseudogenes that have arisen through either duplication of genes or the retro-transcription of mRNAs. These pseudogenes are amplified to high numbers, some with more than 200 copies, and remain almost exclusively located in the same heterochromatic regions as transposable elements. The sequencing of one such heterochromatic region reveals a complex pattern of duplications and inversions, reminiscent of active and frequent rearrangements that can result in the truncation and hence inactivation of transposable elements. This tight compartmentalization of repeats and pseudogenes is absent in large vertebrate genomes such as mammals and is reminiscent of genomes that remain compact during evolution such as Drosophila and Arabidopsis.
