Department of Agronomy and Range Science, University of California, Davis, California 95616.
Genetics. 1986 Aug;113(4):1037-56. doi: 10.1093/genetics/113.4.1037.
Recombination was investigated within the Nor-B2 locus of wheat chromosome 6B that contains several thousand of the 18S-5.8S-26S rRNA (rDNA) repeated units. Additionally, recombination was assessed for several chromosome regions, in arm 6Bq between the centromere and the B2 locus (awn suppressor) and in arm 6Bp between the centromere and Nor-B2, between Nor-B2 and a distal C-band and between Nor-B2 and Gli-B2 coding for gliadins. The experimental design permitted the distinction between crossing over between homologous chromosomes and exchange between sister chromatids. No homologous crossing over within the Nor-B2 locus was found in a sample of 446 chromosomes, but one exchange with the attributes of unequal sister chromatid exchange was identified. The molecular characteristics of this presumed sister chromatid exchange indicate that the spacer variants present in the Nor-B2 locus are clustered. No homologous recombination was detected within the distal Gli-B2 locus containing repeated genes coding for gliadin seed-storage proteins. Both arms of chromosome 6B showed low crossing-over frequency in the proximal regions. The distance from the centromere to Nor-B2 was only from 0.3 to 2.2 cM although it accounts for about two-thirds of the metaphase chromosome arm, which shows a great distortion of the metaphase map of the arm. The level of homologous recombination within the Nor-B2 locus is lower than in the chromosome region immediately distal to it. Whether it is comparable to that in the chromosome region proximal to it could not be determined. Recombination frequencies of different pairs of chromosome 6B in all but one interval paralleled the frequencies of their metaphase I pairing: Lower pairing at metaphase I was paralleled by lower crossing-over frequency. This relationship indicated that reduced metaphase I pairing between 6B chromosomes from different populations is due to impaired crossing-over and not due to precocious chiasma terminalization.
在包含数千个 18S-5.8S-26S rRNA(rDNA)重复单元的小麦染色体 6B 的 Nor-B2 基因座内研究了重组。此外，还评估了几个染色体区域的重组，包括臂 6Bq 上的着丝粒和 B2 基因座（穗抑制）之间，以及臂 6Bp 上的着丝粒和 Nor-B2 之间、Nor-B2 和编码麦醇溶蛋白的Gli-B2 之间、Nor-B2 和 Gli-B2 之间。实验设计允许区分同源染色体之间的交叉和姐妹染色单体之间的交换。在 446 条染色体的样本中未发现 Nor-B2 基因座内的同源重组，但发现了一个具有不等姐妹染色单体交换特征的交换。这个假定的姐妹染色单体交换的分子特征表明，在 Nor-B2 基因座中存在的间隔变体是聚类的。在包含编码麦醇溶蛋白种子贮藏蛋白的重复基因的远端 Gli-B2 基因座内未检测到同源重组。6B 染色体的两条臂在近端区域显示出低交叉频率。从着丝粒到 Nor-B2 的距离只有 0.3 到 2.2cM，尽管它占中期染色体臂的三分之二左右，但这显示出中期染色体臂的巨大扭曲。Nor-B2 基因座内的同源重组水平低于其紧邻的染色体区域。它是否与紧邻的染色体区域相当尚无法确定。除了一个区间外，不同的 6B 染色体对之间的重组频率与它们的中期 I 配对频率平行：中期 I 配对频率较低与交叉频率较低平行。这种关系表明，来自不同群体的 6B 染色体之间的中期 I 配对减少是由于交叉减少，而不是由于早熟的交叉末端化。
