The chromosomal distribution of repetitive DNA sequences in Chrysanthemum boreale revealed a characterization in its genome

The sequencing of Chrysanthemum boreale genome is now in progress. However, the repetitive DNA sequences that dominates greatly in the genome have not yet been elucidated in Chrysanthemum. Therefore, we performed fluorescence in situ hybridization (FISH) by using the mitotic metaphase chromosome of C. boreale with well-identified repeats and C₀t DNA as probes. The most conserved repeats, rDNAs, as well as telomeric repeats were first observed. Three pairs of 45S rDNA sites were located at the termini of the short arms of chromosomes 3, 7, and 8, while one pair of 5S rDNA signals was observed in the middle of the long arm of chromosome 4. Arabidopsis -type telomeric repeats colocalizing with 45S rDNA sites, were detected at the ends of both chromosome arms. Next, on the basis of DNA reassociation kinetics, three types of C₀t DNA that exhibited different degrees of repetitive nature were fractionated and used as FISH probes to map the repetitive sequences. Signals from all C₀t DNAs were successfully observed, but their coverage on the chromosomes was different for C₀t-1, C₀t-10, and C₀t-100. C₀t-1 FISH signals showed intensity in the telomeric region and were dispersed on both chromosome arms, except for distal regions. C₀t-10 signals were observed almost in all parts of the chromosome, with greater intensity around pericentromeric regions. Bright C₀t-100 signals were observed throughout the chromosome excluding some distal parts. C₀t FISH signals covered the rDNAs and telomeric repeats, thus signifying their repetitive attributes. The relative proportions of repetitive DNA sequences in C. boreale genome is discussed.