Physical Mapping of the 5S Ribosomal RNA Gene in Citreae of Aurantioideae Species using Fluorescence in situ Hybridization

The location of the 5S ribosomal RNA gene (rDNA) in species from six genera of the Citreae of Aurantioideae was determined using fluorescence in situ hybridization (FISH). A 5S rDNA probe was labeled with biotin-16-dUTP. The probe was detected using a fluorescein isothiocyanate (FITC)-avidin conjugate with chromosomes counterstained with propidium iodide (PI). When the chromosomes were observed under a G filter, PI-stained chromosomes were classified into the following five types based on the number and position of PI-positive (+) bands; B: one telomeric and one proximal band, C: two telomeric bands, D: one telomeric band, E: without bands and Dst: type D with a satellite chromosome. Two 5S rDNA sites were located in type D chromosomes in Citropsis gabunensis, whereas in Citrus reshni, Fortunella japonica, Clymenia polyandra and Swinglea glutinosa they were found in type E chromosomes. In Poncirus trifoliata, three rDNA sites were located together at the proximal PI (+) band of a type B chromosome, one site was located in the proximal region of a type D chromosome, and two sites were located together at the PI (+) band of a type D chromosome.