Fish mapping of rdna and novel tandem repeats in Citrus species

Cytogenetic analysis of Citrus Linnaeus, 1753 species is complicated by the small size and morphological similarity of their chromosomes. In this study, we applied the fluorescence in situ hybridization (FISH) method using rDNA probes, as well as newly developed oligonucleotide probes based on previously uncharacterized tandem repeats identified in citrus genomes. Four tandem repeats (CL1, CL14, CL262, and CL122) were mapped to the chromosomes of five Citrus species: Citrus medica Linnaeus, 1753, Citrus × aurantiifolia (Christmann, 1777) Swingle, 1913, Citrus × limon (Linnaeus, 1753) Osbeck, 1765, Citrus myrtifolia Rafinesque, 1838, and Citrus × latifolia Tanaka, 1951. The repeats exhibited distinct and partially species-specific distribution patterns: CL1 was detected in all species, whereas CL262, CL122, and CL14 exhibited more restricted localization. A comparative analysis of their chromosomal distribution in relation to 45S- and 5S-rDNA loci revealed differences between the conserved regions of ribosomal DNA and the more dynamic tandem repeats. The identified repeats represent useful cytogenetic markers for chromosome identification, karyotyping, and comparative analysis within the genus Citrus.