PHYSICAL LOCALIZATION OF RIBOSOMAL-RNA GENES BY 2-COLOR FLUORESCENT IN-SITU HYBRIDIZATION AND SEQUENCE-ANALYSIS OF THE 5S RIBOSOMAL-RNA GENE IN PHALARIS-COERULESCENS

The 18S-5.8S-26S rDNA and 5S rDNA loci have been mapped physically by fluorescent in-situ hybridization to the chromosomes of Phalaris coeru lescens. The biotin-labelled heterologous 18S-5.8S-26S rRNA probe (pTa 71) detected one locus, which corresponded to the secondary constricti on (nucleolar organizer) on the long arm of the satellited chromosome II. The homologous 5S rDNA probe (Bam2.12) detected two pairs of 5S rR NA gene clusters which were localized at two different non-satellited chromosomes, one near the telomere on the short arm of the chromosome I, which is the largest chromosome of the complement, and the other ab out 42% out on the long arm of the chromosome III. A BamHI fragment, c ontaining the 5S rRNA gene, has been isolated and characterized. The 5 S rDNA repeat unit is 309 bp in length, consisting of 121 bp highly co nserved coding region and 188 bp variable spacer region. The karyotype of Phalaris coerulescens is characterized by the similar size of chro mosomes within the group 2, group 3, or group 4. This study represents the first step towards the understanding the genome organization of P halaris coerulescens and provides reliable markers for chromosome iden tification in this grass, an important species as a model system for t he study of self-incompatibility in grasses.