Zebrafish comparative genomics and the origins of vertebrate chromosomes

To help understand mechanisms of vertebrate genome evolution we have compar ed zebrafish and tetrapod gene maps. It has been suggested that translocati ons 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 i ntrachromosomal rearrangements have been fixed more frequently than translo cations. Duplicated chromosome segments suggest that a genome duplication o ccurred in ray-fin phylogeny, and comparative studies suggest that this eve nt happened deep in the ancestry of teleost fish. Consideration of duplicat e chromosome segments shows that at least 20% of duplicated gene pairs may be retained from this event. Despite genome duplication, zebrafish and huma ns 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 chromos ome Fusions in the zebrafish lineage? Comparative analysis suggests that an excess of chromosome fissions in the tetrapod lineage may account for chro mosome numbers and provides histories for several human chromosomes.