Molecular cytogenetics of Musa species, cultivars and hybrids: Location of 18S-5.8S-25S and 5S rDNA and telomere-like sequences

The physical sites of 18S-5.8S-25S and 5S rRNA genes and telomeric sequences in the Musa L. genome were localized by fluorescent in situ hybridization on mitotic chromosomes of selected lines. A single major intercalary site of the 18S-5.8S-25S rDNA was observed on the short arm of the nucleolar organizing chromosome in each genome. AA and BB genome diploids had a single pair of sites, triploids had three sites while a tetraploid hybrid had four sites. The probe is useful for quick determination of ploidy, even using interphase nuclei from slowly growing tissue culture material. Variation in the intensity of signals was observed among heterogeneous Musa lines indicating variation in the number of copies of the 18S-5.8S-25S rRNA genes. Eight subterminal sites of 5S rDNA were observed in Calcutta 4 (AA) while Butohan 2 (BB) had six sites; some were weaker in both genotypes. Triploid lines showed six to nine major sites of 5S rDNA of widely varying intensity and near the limit of detection. The diploid hybrids had five to nine sites of 5S rDNA while the tetraploid hybrid had 11 sites. The telomeric sequence was detected as pairs of dots at the ends of all the chromosomes analysed but no intercalary sequences were seen. The molecular cytogenetic studies of Musa using repetitive and single copy DNA probes should yield insight into the genome and its evolution and provide data for Musa breeders, as well as generating genetic markers in Musa.