Molecular cytogenetic studies on the evolution of genome structure in reptiles and birds.

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To clarify the process of karyotypic evolution and the origins of sex chromosomes and microchromosomes in birds and reptiles, we constructed chromosome maps of Chinese soft-shelled turtle, Japanese four-striped rat snake and Siam crocodile with 79, 105 and 85 functional genes, respectively. The comparative mapping between chicken and three reptile species suggests the followings : 1) the karyotypes of the common ancestor of the Archsauromorh were shared between birds and turtles, 2) chromosome rearrangements by centromeric fusion occurred at a level of whole genome in the lineage of crocodiles after centric fission of chromosomes 1 and 2 of the common ancestor, 3) the intermediate karyotypes between the common ancestor and the crocodiles might have been conserved in the lineage of caimans, and 4) in the Lepidosauria high frequencies of chromosome rearrangements occurred independently from the Archosauromorph.We cloned reptile homologues of six chicken Z-linked genes, ACO1, ATP5A1, CHD1, DMRT1, GHR and RPS6, from the turtle and the snake with ZZ/ZW-type sex chromosomes and the crocodile with undifferentiated sex chromosomes, and mapped them to the chromosomes of the three species. All the genes were localized to the long arm of the turtle chromosome 6, the short arm of the snake chromosome 2 and the short arm of the crocodile chromosome 3. These results indicate that the genetic linkage on chicken Z chromosome has been highly conserved in reptiles, and that the causative genes of sex determination may be different between birds and reptiles.