Unstable transmission of rice chromosomes without functional centromeric repeats in asexual propagation

During sexual propagation of primary trisomic 8, chromosome 8 breaks in some rice plants, resulting in a telotrisomic (2n+.8S) line. In this study, we observed that the extra short arm of chromosome 8 (.8S) can easily be lost in the telotrisomic, and we determined by fluorescence in-situ hybridization (FISH) analysis that the centromeric region of the extra .8S did not contain the rice centromeric satellite repeat (CentO) and centromere-specific retrotransposon (CRR); however, the extra .8S contained part of the CentO and CRR sequences in the initially preserved telotrisomic line. We confirmed by real-time quantitative PCR (RQ-PCR) analysis that the original functional centromere of the extra .8S was lost. Using both FISH and RQ-PCR, the breakage point of the extra .8S was found within the BAC clone a0070J19 sequence containing the first part of the short arm near the centromere region of chromosome 8 but without any CentO or CRR sequences. However, part of the DNA sequence within the a0070J19 BAC clone played a role in the new functional centromere, contributing to the morphological variations by asexually propagated plants of rice telotrisomics in the field. We conclude that CENH3, a key element in the eukaryotic kinetochore, may not always bind properly with the new functional centromere, resulting in loss of the extra .8S during mitosis and the chromosome numbers returning to diploid levels in subsequent generations.