Ezaz Tariq, Stiglec Rami, Veyrunes Frederic, Marshall Graves Jennifer A
Research School of Biological Sciences, The Australian National University, Canberra, ACT 2601, Australia.
Curr Biol. 2006 Sep 5;16(17):R736-43. doi: 10.1016/j.cub.2006.08.021.
The peculiar cytology and unique evolution of sex chromosomes raise many fundamental questions. Why and how sex chromosomes evolved has been debated over a century since H.J. Muller suggested that sex chromosome pairs evolved ultimately from a pair of autosomes. This theory was adapted to explain variations in the snake ZW chromosome pair and later the mammal XY. S. Ohno pointed out similarities between the mammal X and the bird/reptile Z chromosomes forty years ago, but his speculation that they had a common evolutionary origin, or at least evolved from similar regions of the genome, has been undermined by comparative gene mapping, and it is accepted that mammal XY and reptile ZW systems evolved independently from a common ancestor. Here we review evidence for the alternative theory, that ZW<-->XY transitions occurred during evolution, citing examples from fish and amphibians, and probably reptiles. We discuss new work from comparative genomics and cytogenetics that leads to a reconsideration of Ohno's idea and advance a new hypothesis that the mammal XY system may have arisen directly from an ancient reptile ZW system.
性染色体独特的细胞学特征和独特的进化历程引发了许多基本问题。自H.J. 穆勒提出性染色体对最终由一对常染色体进化而来以来，一个多世纪里，关于性染色体为何以及如何进化一直存在争议。该理论后来被用于解释蛇类ZW染色体对以及哺乳动物XY染色体对的变异情况。四十年前，S. 大野指出哺乳动物的X染色体与鸟类/爬行类的Z染色体存在相似之处，但他关于它们有共同进化起源，或者至少是从基因组的相似区域进化而来的推测，已被比较基因图谱研究削弱，目前人们普遍认为哺乳动物的XY系统和爬行类的ZW系统是从一个共同祖先独立进化而来的。在此，我们回顾支持另一种理论的证据，即ZW<-->XY转变在进化过程中发生，文中列举了鱼类、两栖类以及可能还有爬行类的例子。我们讨论了比较基因组学和细胞遗传学方面的新研究成果，这些成果促使人们重新审视大野的观点，并提出一个新的假说，即哺乳动物的XY系统可能直接起源于古代爬行类的ZW系统。
