The chromosomes of Citrus and Poncirus species and hybrids: identification of characteristic chromosomes and physical mapping of rDNA loci using in situ hybridization and fluorochrome banding.

In situ hybridization of 18S-5.8S-25S rDNA probes labeled with biotin or rhodamine and 5S rDNA probes labeled with digoxigenin was used to locate rDNA sites on root-tip metaphase chromosomes of Citrus sinensis L. (2n = 2x = 18), Poncirus trifoliata L. Raf. (2n = 2x = 18), and Citrus x Poncirus hybrids (2n = 2x = 18). Counterstaining with the fluorochromes chromomycin A3 and DAPI uniquely identified many but not all chromosomes. C. sinensis had five 18S-25S rDNA sites, P. trifoliata had seven, and three different Citrus x Poncirus hybrids had five or six sites. Four 5S rDNA sites were detected, mostly linked to 18S-25S rDNA sites. Overall we observed high levels of chromosomal heterozygosity in all accessions examined.