Determination of basic chromosome numbers in the genusSaccharumby physical mapping of ribosomal RNA genes

18S-5.6S-25S and 5S ribosomal DNA (rDNA) sites were located by in situ hybridization to the three main species of the Saccharum genus. For each species and each rDNA family, the position and number of sites in the various cytotypes suggested the presence of one locus and basic chromosome numbers of 10 for Saccharum officinarum and Saccharum robustum and 8 forSaccharum spontaneum. The implications of these results for the genetic maps of modern cultivars derived from crosses between the species S. officinarum and S. spontaneum are discussed.Key words: sugarcane, Saccharum, 18S-5.6S-25S rRNA, 5S rRNA, basic chromosome number, in situ hybridization.