The CHH motif in sugar beet satellite DNA: a modulator for cytosine methylation.

Methylation of DNA is important for the epigenetic silencing of repetitive DNA in plant genomes. Knowledge about the cytosine methylation status of satellite DNAs, a major class of repetitive DNA, is scarce. One reason for this is that arrays of tandemly arranged sequences are usually collapsed in next-generation sequencing assemblies. We applied strategies to overcome this limitation and quantified the level of cytosine methylation and its pattern in three satellite families of sugar beet (Beta vulgaris) which differ in their abundance, chromosomal localization and monomer size. We visualized methylation levels along pachytene chromosomes with respect to small satellite loci at maximum resolution using chromosome-wide fluorescent in situ hybridization complemented with immunostaining and super-resolution microscopy. Only reduced methylation of many satellite arrays was obtained. To investigate methylation at the nucleotide level we performed bisulfite sequencing of 1569 satellite sequences. We found that the level of methylation of cytosine strongly depends on the sequence context: cytosines in the CHH motif show lower methylation (44-52%), while CG and CHG motifs are more strongly methylated. This affects the overall methylation of satellite sequences because CHH occurs frequently while CG and CHG are rare or even absent in the satellite arrays investigated. Evidently, CHH is the major target for modulation of the cytosine methylation level of adjacent monomers within individual arrays and contributes to their epigenetic function. This strongly indicates that asymmetric cytosine methylation plays a role in the epigenetic modification of satellite repeats in plant genomes.

Nondisjunction in favor of a chromosome: the mechanism of rye B chromosome drive during pollen mitosis.

B chromosomes (Bs) are supernumerary components of the genome and do not confer any advantages on the organisms that harbor them. The maintenance of Bs in natural populations is possible by their transmission at higher than Mendelian frequencies. Although drive is the key for understanding B chromosomes, the mechanism is largely unknown. We provide direct insights into the cellular mechanism of B chromosome drive in the male gametophyte of rye (Secale cereale). We found that nondisjunction of Bs is accompanied by centromere activity and is likely caused by extended cohesion of the B sister chromatids. The B centromere originated from an A centromere, which accumulated B-specific repeats and rearrangements. Because of unequal spindle formation at the first pollen mitosis, nondisjoined B chromatids preferentially become located toward the generative pole. The failure to resolve pericentromeric cohesion is under the control of the B-specific nondisjunction control region. Hence, a combination of nondisjunction and unequal spindle formation at first pollen mitosis results in the accumulation of Bs in the generative nucleus and therefore ensures their transmission at a higher than expected rate to the next generation.