Abstract
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.
Highlights
Eukaryotes require a functional centromere for faithful transmission of genetic material during cell division
Our results indicate that both reactivation and de novo centromere formation could occur on fragments derived from the progenitor possessing an inactive centromere
The results showed that mini B1104 has the same breakpoints as 9-B inactivated centromere-1 (9-Bic-1) in the centromere and chromosome arm 9S with its B portion broken at the centromere region adjacent to the centromeric knob, which suggest that mini B1104 was a 9-Bic-1 derivative (Fig. 3, Supplemental Fig. 2)
Summary
Eukaryotes require a functional centromere for faithful transmission of genetic material during cell division. Centromeres typically contain highly repetitive, species-specific DNA sequences (for review: Hartley and O’Neill 2019). Many studies have shown that the DNA sequences found in canonical centromeres are neither required, nor sufficient, to establish kinetochore formation (for review: Burrack and Berman 2012). The DNA sequence of maize (Zea mays) centromeres is composed of a 156-bp centromere-specific satellite repeat CentC and Centromeric Retrotransposon of Maize (CRM) family members (Jiang et al 1996; Ananiev et al 1998; Wolfgruber et al 2009). Supernumerary maize B chromosome centromeres have similar DNA components, but CentC and CRM elements are joined by arrays of a B-specific repeat (ZmBs) in the centromeres of the maize B chromosome, which enables easy identification of the B centromere in cytological spreads (Lamb et al 2005; Blavet et al 2021)
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