Imai H T, Wada M Y, Hirai H, Matsuda Y, Tsuchiya K
National Institute of Genetics, Mishima, Shizuoka-ken 411-8540, Japan.
J Theor Biol. 1999 May 21;198(2):239-57. doi: 10.1006/jtbi.1999.0913.
The nature of the chiasma as a cytological parameter for analysing cross-over was reexamined quantitatively by an improved chiasma graph method. It was reconfirmed in Mus platythrix (n =13) that interstitial chiasmata at diakinesis are distributed randomly and almost uniformly along bivalents except for the centromere and telomere regions. The size of these chiasma blank regions was consistently 0.8% of the total length of haploid autosomes in all chromosomes. There was a minimum value of chiasma interference distance between two adjacent chiasmata, which was constantly 1.8% in all chromosomes. The chiasma frequency at diakinesis was 20.1+/-2. 0 by the conventional method including terminal chiasmata. However, the primed in situ labeling technique revealed that terminal chiasmata were mostly telomere-telomere associations. From these data and also from recent molecular data we concluded that the terminal chiasma is cytologically functional for ensuring the normal disjunction of bivalents at anaphase I, but genetically non-functional for shuffling genes. The chiasma frequency excluding terminal chiasmata was 14.6+/-1.8. Reexamination of the chiasma frequency of 106 animal species revealed that the chiasma frequency increased linearly in proportion to the haploid chromosome number in spite of remarkable difference in their genome size. The increase in chiasma frequency would be evolution-adaptive, because gene shuffling is expected to be accelerated in species with high chromosome numbers.
通过一种改进的交叉点图谱法，对交叉点作为分析交换的细胞学参数的性质进行了定量重新研究。在平颅鼠（n = 13）中再次证实，减数分裂终变期的中间交叉点除着丝粒和端粒区域外，沿二价体随机且几乎均匀分布。所有染色体中，这些交叉点空白区域的大小始终为单倍体常染色体总长度的0.8%。相邻两个交叉点之间的交叉干扰距离有一个最小值，在所有染色体中均恒定为1.8%。采用包括末端交叉点的传统方法，减数分裂终变期的交叉频率为20.1±2.0。然而，引物原位标记技术表明，末端交叉点大多是端粒-端粒关联。根据这些数据以及最近的分子数据，我们得出结论，末端交叉点在细胞学上对确保后期I二价体的正常分离起作用，但在基因改组方面没有遗传功能。不包括末端交叉点的交叉频率为14.6±1.8。对106种动物物种的交叉频率重新研究发现，尽管它们的基因组大小存在显著差异，但交叉频率与单倍体染色体数成线性增加。交叉频率的增加可能是进化适应性的，因为预计在染色体数高的物种中基因改组会加速。
