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

为了弄清鸟类和爬行动物的核型进化过程以及性染色体和微染色体的起源，我们分别构建了含有79个、105个和85个功能基因的中国软壳龟、日本四条纹鼠蛇和暹罗鳄的染色体图谱。鸡与三种爬行动物的对比图显示如下：1)大鳄共同祖先的核型在鸟类和海龟之间是相同的；2)鳄鱼共同祖先的1号和2号染色体发生中心裂变后，在整个基因组水平上发生了着丝体融合的染色体重排；3)鳄鱼共同祖先与大鳄之间的中间核型可能在凯门鳄谱系中被保守。4)鳞翅目的染色体重排频率独立于始祖。从性染色体为ZZ/ zw型的龟、蛇和性染色体未分化的鳄鱼中克隆了6个鸡z连锁基因ACO1、ATP5A1、CHD1、DMRT1、GHR和RPS6的同源基因，并将其定位到这3种动物的染色体上。所有基因均定位于龟6号染色体的长臂、蛇2号染色体的短臂和鳄鱼3号染色体的短臂。这些结果表明，鸡Z染色体上的遗传连锁在爬行动物中高度保守，鸟类和爬行动物性别决定的致病基因可能存在差异。

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.