Ogawa A, Murata K, Mizuno S
Laboratory of Molecular Biology, Department of Applied Biological Chemistry, Faculty of Agriculture, Tohoku University, 1-1 Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981 Japan.
Proc Natl Acad Sci U S A. 1998 Apr 14;95(8):4415-8. doi: 10.1073/pnas.95.8.4415.
Perhaps the most striking fact about early Cenozoic avian history some 70 million years ago was the rapid radiation of large, flightless, ground-living birds. It has been suggested that, for a time, there was active competition between these large terrestrial birds and the early mammals. Probably reflecting the above noted early start of Ratitae of the infraclass Eoaves, the presumptive sex chromosomes of their present day survivors, such as the emu and the ostrich, largely remained homomorphic. The signs of genetic differentiation between their still-homomorphic Z and W chromosomes were tested by using two marker genes (Z-linked ZOV3 and the gene for the iron-responsive element-binding protein) and one marker sequence of a part of a presumptive pseudogene (W-linked EE0.6 of the chicken). Their homologues, maintaining 71-92% identities to the chicken counterparts, were found in both the emu (Dromaius novaehollandiae) and the ostrich (Struthio camelus). Their locations were visualized on chromosome preparations by fluorescence in situ hybridization. In the case of the emu, these three marker sequences were localized on both members of the fifth pair of a female, thus revealing no sign yet of genetic differentiation between the Z and the W. The finding was the same with regard to both members of the fourth pair of male ostriches. In the female ostrich, however, the sequence of the gene for the iron-responsive element-binding protein was missing from one of the pairs, thus revealing the differentiation by a small deletion of the W from the Z.
大约7000万年前的新生代早期鸟类历史中，最引人注目的事实可能是大型、不会飞的陆栖鸟类的迅速辐射。有人提出，在一段时间内，这些大型陆生鸟类与早期哺乳动物之间存在着激烈的竞争。可能反映了上述低等鸟类平胸总目早期的起源，其现存幸存者，如鸸鹋和鸵鸟的假定性染色体在很大程度上仍保持同态。通过使用两个标记基因（Z连锁的ZOV3和铁反应元件结合蛋白的基因）以及假定假基因一部分的一个标记序列（鸡的W连锁EE0.6），测试了它们仍然同态的Z和W染色体之间的遗传分化迹象。在鸸鹋（Dromaius novaehollandiae）和鸵鸟（Struthio camelus）中都发现了与鸡对应物保持71-92%同一性的它们的同源物。通过荧光原位杂交在染色体标本上观察它们的位置。就鸸鹋而言，这三个标记序列定位在雌性第五对染色体的两条成员上，因此尚未显示出Z和W之间的遗传分化迹象。雄性鸵鸟第四对染色体的两条成员也是如此。然而，在雌性鸵鸟中，铁反应元件结合蛋白的基因序列在其中一对中缺失，从而通过W相对于Z的小缺失揭示了分化。
