Division of Biological Sciences, University of Missouri, Columbia, MO, 65211, USA.
State Key Lab of Plant Cell and Chromosome Engineering, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing, 100101, China.
Chromosome Res. 2021 Dec;29(3-4):313-325. doi: 10.1007/s10577-021-09670-5. Epub 2021 Aug 18.
The B chromosome of maize undergoes nondisjunction at the second pollen mitosis as part of its accumulation mechanism. Previous work identified 9-Bic-1 (9-B inactivated centromere-1), which comprises an epigenetically silenced B chromosome centromere that was translocated to the short arm of chromosome 9(9S). This chromosome is stable in isolation, but when normal B chromosomes are added to the genotype, it will attempt to undergo nondisjunction during the second pollen mitosis and usually fractures the chromosome in 9S. These broken chromosomes allow a test of whether the inactive centromere is reactivated or whether a de novo centromere is formed elsewhere on the chromosome to allow recovery of fragments. Breakpoint determination on the B chromosome and chromosome 9 showed that mini chromosome B1104 has the same breakpoint as 9-Bic-1 in the B centromere region and includes a portion of 9S. CENH3 binding was found on the B centromere region and on 9S, suggesting both centromere reactivation and de novo centromere formation. Another mini chromosome, B496, showed evidence of rearrangement, but it also only showed evidence for a de novo centromere. Other mini chromosome fragments recovered were directly derived from the B chromosome with breakpoints concentrated near the centromeric knob region, which suggests that the B chromosome is broken at a low frequency due to the failure of the sister chromatids to separate at the second pollen mitosis. Our results indicate that both reactivation and de novo centromere formation could occur on fragments derived from the progenitor possessing an inactive centromere.
玉米的 B 染色体在第二次花粉有丝分裂时发生不分离，这是其积累机制的一部分。之前的研究确定了 9-Bic-1（9-B 失活着丝粒-1），它由一个表观遗传沉默的 B 染色体着丝粒组成，该着丝粒易位到 9 号染色体的短臂（9S）上。这条染色体在孤立状态下是稳定的，但当正常的 B 染色体被添加到基因型中时，它会在第二次花粉有丝分裂时尝试发生不分离，通常会使 9S 上的染色体断裂。这些断裂的染色体可以检验失活的着丝粒是否被重新激活，或者是否在染色体的其他部位形成新的着丝粒，以允许片段的恢复。对 B 染色体和 9 号染色体的断点确定表明，迷你染色体 B1104 在 B 染色体着丝粒区域与 9-Bic-1 具有相同的断点，并包含 9S 的一部分。在 B 染色体着丝粒区域和 9S 上都发现了 CENH3 结合，这表明着丝粒的重新激活和新的着丝粒形成。另一个迷你染色体 B496 显示出重排的证据，但它也只显示了新的着丝粒的证据。回收的其他迷你染色体片段直接来源于 B 染色体，断点集中在着丝粒 knob 区域附近，这表明由于姐妹染色单体在第二次花粉有丝分裂时未能分离，B 染色体的断裂频率较低。我们的研究结果表明，在来自具有失活着丝粒的前体的片段中，重新激活和新的着丝粒形成都可能发生。
