Genome size, karyotyping and FISH physical mapping of 45S and 5S genes in two cherry rootstocks: Prunus subhirtella and Prunus incisa xserrula.

Physical mapping of rDNA genes was used to karyotype two cherry rootstocks, Prunus subhirtella (PAR) and P. incisa xserrula (PIS), since the identification of homologous pairs is hampered by the high degree of similarity and the small size of the chromosomes. The genome size of PAR and PIS was obtained by flow cytometry. Chromosomal landmarks were identified by FISH of 45S rDNA and 5S rDNAs with DIG or Biotin labelled rDNA probes. Double-colour FISH landmarks were located at the terminal regions of chromosomes, differentiating three chromosome groups. Chromosomes 1, 4, 5 and 8 of both species showed no hybridization signals. The chromosomes 2, 3 and 6 of both species carried signals of 45S rDNA. In PAR, chromosomes 2 and 3 carried the 5S rDNA signals, which in PIS was located on chromosome 7. Secondary constrictions were not visible in PAR chromosomes, while PIS chromosome 2 carries one. The results suggest that the degree of chromosomal differentiation among the two Prunus species is low. This study is the first report of using FISH to determine the number and the physical position of rDNAs in PIS and PAR.