Quantitative chromosome maps and rDNA localization in the T subgenome of Nicotiana tabacum L. and its putative progenitors

Using DAPI-stained prometaphase chromosomes, quantitative idiograms were co nstructed for the T subgenome of Nicotiana tabacum (2n = 4x = 48, SSTT) and two putative candidates for its T subgenome progenitor, Nicotiana otophora and Nicotiana tomentosiformis (both have 2n = 24, TT). The large chromosom es of the three karyotypes could be identified from the distributional patt ern of the DAPI signal. Fluorescence in situ hybridization (FISH) with 5S r DNA gave not only good cytogenetical landmarks for identification of small chromosomes of the karyotypes but also phylogenetical information. in all t hree idiograms, 5S rDNA was localized in the proximal region of the long ar m of a small submetacentric pair, but an additional 5S rDNA locus was detec ted terminally on the short arm of a small metacentric pair in N. otophora. The 18S rDNA locus detected here corresponded to satellite regions in all three karyotypes. Two satellited pairs in N. otophora and one satellited pa ir in N. tomentosiformis had single large subterminal DAPI blocks and two i nterstitial DAPI bands on their long arms, respectively. For the T subgenom e component of N. tabacum, the single intense DAPI band was depicted on the center of the long arm of a satellited pair in the idiogram, although two interstitial bands were often detected on the long arm of the satellited pa ir in some spreads. Therefore, it was suggested that the T component of N. tabacum was more similar to that of N. tomentosiformis than N. otophora, es pecially in respect of the number and location of rDNA and the distribution al patterns of DAPI signals.