CHARACTERIZATION OF THE NICOTIANA-TABACUM-L GENOME BY MOLECULAR CYTOGENETICS

Nicotiana tabacum (2n = 48) is a natural amphidiploid with component g enomes S and T. We used non-radioactive in situ hybridization to provi de physical chromosome markers for N. tabacum, and to determine the ex tant species most similar to the S and T genomes. Chromosomes of the S genome hybridized strongly to biotinylated total DNA from N. sylvestr is, and showed the same physical localization of a tandemly repeated D NA sequence, HRS 60.1, confirming the close relationship between the S genome and N. sylvestris. Results of dot blot and in situ hybridizati ons of N. tabacum DNA to biotinylated total genomic DNA from N. toment osiformis and N. otophora suggested that the T genome may derive from an introgressive hybrid between these two species. Moreover, a compari son of nucleolus-organizing chromosomes revealed that the nucleolus or ganizer region (NOR) most strongly expressed in N. tabacum had a very similar counterpart in N. otophora. Three different N. tabacum genotyp es each had up to 9 homozygous translocations between chromosomes of t he S and T genomes. Such translocations, which were either unilateral or reciprocal, demonstrate that intergenomic transfer of DNA has occur red in the amphidiploid, possibly accounting for some results of previ ous genetic and molecular analyses. Molecular cytogenetics of N. tabac um has identified new chromosome markers, providing a basis for physic al gene mapping and showing that the amphidiploid genome has diverged structurally from its ancestral components.