Rodionov A V
Genetika. 1996 May;32(5):597-608.
Karyotypes of lower vertebrates mainly consist of microchromosomes. In higher vertebrates, microchromosomes are present in each class of the most primitive orders. Birds have more microchromosomes in their karyotype than other vertebrates. Accumulation of microchromosomes in the avian karyotype probably occurred after separation of birds from reptilians in Triassic, but prior to radiation of ancestors of the modern orders (late Cretaceous-early Jurassic). In this review, the structural, molecular, and functional organization of avian macro- and microchromosomes and their participation in genetic processes are discussed. The average size of an avian microchromosome is about 12.4 Mb, which is ten times less than the size of an average macrochromosome. In contrast to macrochromosomes, medium and small avian chromosomes lack the highest level of chromosomal organization: their chromonemes do not have spiral coiling. Microchromosomal euchromatin largely consists of GC-rich R regions. More than half of the mapped avian genes are located on microchromosomes. Crossing-over frequency in microchromosomes is approximately threefold higher than in macrochromosomes. This may be caused by high GC content and recombination hot spots, which are present on each microchromosome. High recombination frequency in microchromosomes increases the probability of their correct meiotic segregation.
低等脊椎动物的核型主要由微染色体组成。在高等脊椎动物中，微染色体存在于最原始目级别的每个类别中。鸟类核型中的微染色体比其他脊椎动物更多。鸟类核型中微染色体的积累可能发生在三叠纪鸟类与爬行动物分离之后，但在现代目祖先辐射（晚白垩世 - 早侏罗世）之前。在这篇综述中，讨论了鸟类大染色体和微染色体的结构、分子和功能组织以及它们在遗传过程中的参与情况。鸟类微染色体的平均大小约为12.4 Mb，比平均大染色体的大小小十倍。与大染色体不同，鸟类的中、小染色体缺乏最高水平的染色体组织：它们的染色线没有螺旋盘绕。微染色体常染色质主要由富含GC的R区域组成。超过一半的已定位鸟类基因位于微染色体上。微染色体中的交叉频率比大染色体高约三倍。这可能是由每个微染色体上存在的高GC含量和重组热点引起的。微染色体中的高重组频率增加了它们正确减数分裂分离的概率。
