Chromosome homologies between Citrus and Poncirus—the comparative cytogenetic map of mandarin (Citrus reticulata)

Mandarins are among the main citrus species and constitute a morphologically diverse and taxonomically controversial group. Cytogenetic studies were limited to the determination of its karyotype formula and the localization of ribosomal DNA (rDNA) sites. The cytogenetic map of Poncirus trifoliata, established by fluorescent in situ hybridization (FISH) using single-copy bacterial artificial chromosomes (BACs) together with the localization of rDNA sequences and heterochromatin (CMA/DAPI banding pattern), represented an important milestone for the cytogenetic studies of the citrus species. In this work, the cytogenetic map of Citrus reticulata cv. Cravo, one of the pure species of the genus, was built using BACs from P. trifoliata, including the citrus tristeza virus resistance locus (Ctv), to detect homeologies and structural karyotypic changes between these species. Except for chromosomes 2 and 3, heterochromatin distribution and position of BACs were maintained in comparison to P. trifoliata, indicating a high synteny between both karyotypes and supporting a close phylogenetic relationship between these species. A break of colinearity was observed between BAC clones and the CMA+ bands (including rDNA loci) from chromosomes 2 and 3, which could be mainly explained by amplification and reduction of the satellite DNA sequences that constitute the heterochromatic bands of these chromosome pairs.