Physical mapping of rDNA genes establishes the karyotype of almond

The localisation of ribosomal RNA genes on chromosomes of almond (Prunus amygdalus, 2n = 16) was studied by fluorescence in situ hybridisation. Simultaneous double-colour hybridisation with both 18S-5.8S-25S and 5S rDNA probes demonstrated that all chromosomes can be identified. In spite of the small size, differences in length between chromosomes that hybridised with the same rDNA probe as well as between chromosomes without hybridisation signal are apparent. Chromosomes were ordered in the karyotype according to their length. The 18S-5.8S-25S rDNA genes were detected in subdistal positions of chromosomes 2, 3, and 8. Sites located on chromosomes 2 and 3 carry a higher number of repeats than the site of chromosome 8. The 5S rDNA genes were found proximally located on chromosomes 5 and 7, the signal on chromosome 5 showing higher intensity than the signal on chromosome 7. Chromosomes 1, 4, and 6 show no hybridisation signal.