Chromosomal characterization based on repetitive DNA distribution in a tetraploid cytotype of Chrysanthemum zawadskii

Abstract

The aim of this study was to gain insight into the basic genetics of Chrysanthemum zawadskii Herbich ssp. lucidum (NAK.) Y. Lee, a Chrysanthemum subspecies from Ulleung Island, South Korea, by investigating its karyotypic and cytogenetic characteristics, and to contribute to the karyosystematic study of the genus Chrysanthemum (tribe: Anthemideae). Fluorescence in situ hybridization (FISH) was used to characterize the ploidy level, chromosome constitution, and distribution of 5S and 18S ribosomal DNA (rDNA) and Arabidopsis-type telomere repeats (TTTAGGG)n in C. zawadskii ssp. lucidum chromosomes. The somatic metaphase chromosome number of C. zawadskii ssp. lucidum was tetraploid (2n = 4x = 36). Signals from four 5S rDNA loci were detected in the middle of the long arms of some chromosomes, whereas signals from eight 18S rDNA loci were located at the termini of the short arms. The two rDNA markers localized to different chromosomes with no co-localization. The Arabidopsis-type telomeric repeats, however, co-localized with 18S rDNA sites at the ends of both chromosomal arms. This co-localization of telomeric sequences and 18S rDNA has also been observed in other Chrysanthemum species with various levels of ploidy, indicating that it is a general feature of the genus. DAPI (4′,6-diamidino-2-phenylindole) bands incidentally detected after FISH are also illustrated. These preliminary data contribute towards understanding the karyotypic trends and role of polyploidy in the evolution of the genus Chrysanthemum.