Griffin D K, Robertson L B W, Tempest H G, Skinner B M
University of Kent, Department of Biosciences, Canterbury, UK.
Cytogenet Genome Res. 2007;117(1-4):64-77. doi: 10.1159/000103166.
Birds are characterised by feathers, flight, a small genome and a very distinctive karyotype. Despite the large numbers of chromosomes, the diploid count of 2n approximately 80 has remained remarkably constant with 63% of birds where 2n = 74-86, 24% with 2n = 66-74 and extremes of 2n = 40 and 2n = 142. Of these, the most studied is the chicken (2n = 78), and molecular cytogenetic probes generated from this species have been used to further understand the evolution of the avian genome. The ancestral karyotype is, it appears, very similar to that of the chicken, with chicken chromosomes 1, 2, 3, 4q, 5, 6, 7, 8, 9, 4p and Z representing the ancestral avian chromosomes 1-10 + Z; chromosome 4 being the most ancient. Avian evolution occurred primarily in three stages: the divergence of the group represented by extant ratites (emu, ostrich etc.) from the rest; divergence of the Galloanserae (chicken, turkey, duck, goose etc.)--the most studied group; and divergence of the 'land' and 'water' higher birds. Other than sex chromosome differentiation in the first divergence there are no specific changes associated with any of these evolutionary milestones although certain families and orders have undergone multiple fusions (and some fissions), which has reduced their chromosome number; the Falconiformes are the best described. Most changes, overall, seem to involve chromosomes 1, 2, 4, 10 and Z where the Z changes are intrachromosomal; there are also some recurring (convergent) events. Of these, the most puzzling involves chromosomes 4 and 10, which appear to have undergone multiple fissions and/or fusions throughout evolution - three possible hypotheses are presented to explain the findings. We conclude by speculating as to the reasons for the strange behaviour of these chromosomes as well as the role of telomeres and nuclear organisation in avian evolution.
鸟类的特征是有羽毛、能飞行、基因组小且核型非常独特。尽管鸟类的染色体数量众多，但二倍体数2n约为80一直保持得相当稳定，63%的鸟类2n = 74 - 86，24%的鸟类2n = 66 - 74，极端情况是2n = 40和2n = 142。其中，研究最多的是鸡（2n = 78），从该物种产生的分子细胞遗传学探针已被用于进一步了解鸟类基因组的进化。看来，祖先核型与鸡的核型非常相似，鸡的染色体1、2、3、4q、5、6、7、8、9、4p和Z代表祖先鸟类染色体1 - 10 + Z；染色体4是最古老的。鸟类进化主要发生在三个阶段：现存平胸鸟类（鸸鹋、鸵鸟等）所代表的类群与其他类群的分化；鸡形目和雁形目（鸡、火鸡、鸭、鹅等）——研究最多的类群的分化；以及“陆地”和“水生”高等鸟类的分化。除了第一次分化中的性染色体分化外，这些进化里程碑中没有任何一个与特定变化相关，尽管某些科和目经历了多次融合（以及一些分裂），这减少了它们的染色体数量；隼形目是描述得最清楚的。总体而言，大多数变化似乎涉及染色体1、2、4、10和Z，其中Z染色体的变化是染色体内的；也有一些反复出现（趋同）的事件。其中，最令人困惑的涉及染色体4和10，它们在整个进化过程中似乎经历了多次分裂和/或融合——提出了三种可能的假说来解释这些发现。我们通过推测这些染色体奇怪行为的原因以及端粒和核组织在鸟类进化中的作用来结束本文。
