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The interspecific sugarcane hybrid ''KPS 01-01-25'' is one of Thailand's most successful cultivars, but its genetics and genomic constitution are greatly complicated due to the highly polyploid nature of this crop. Here we analyzed the crop's karyotype, studied chromosome pairing at meiosis I and performed fluorescent in situ hybridization (FISH) of rDNA and rice BACs as probes. Mitotic cell complements in early tapetal cells showed 110 well-spread chromosomes. Pollen mother cells at pachytene clearly demonstrated fully paired configurations of the long chromosomes, in which few pairing partner exchanges and loops could be discerned. In diakinesis cells we observed 74.1 % bivalents, 18.6 % univalents, 2.9 % trivalents and 4.4 % quadrivalents. During this stage secondary associations were abundant, but these threadlike structures disappear after metaphase I. FISH with 5S rDNA produced up to ten fluorescent spots, of which 2-5 are often very small, whereas the 45S rDNA probe showed foci on 8-9 chromosomes. Based on our results and that of related studies we estimate that this sugarcane is composed of 11-12 genomes. We also selected rice BACs containing chromosome 5 genes controlling economically important genes. Eight BACs with relatively strong dot-blot hybridization on genomic sugarcane DNA were selected for FISH on sugarcane chromosome slides. These crossspecies chromosome BAC FISH revealed weak signals, and most of the foci were distributed over parts of the chromosomes. Based on these experiments we concluded that cross-species FISH in sugarcane can better be carried out with sorghum BACs as probes.Keywords Cross-species chromosome painting Á Meiosis Á rDNA FISH Á Saccharum Á Sugarcane

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