Cytogenetic Map of Pummelo and Chromosome Evolution of True Citrus Species and the Hybrid Sweet Orange

Pummelo (Citrus maxima) is considered as one of the true citrus species. Together with mandarin (C. reticulata), it gave rise to the hybrid sweet orange (C. sinensis) and other important citrus crops. Although these species have 2n = 18, each has a unique heterochromatin distribution. The aims of this study were to identify chromosome homoeologies between pummelo and other true citrus species, to investigate the karyotypic changes involved in the chromosomal evolution between true citrus and to shed light into the origin of sweet orange hybrid karyotype. Mitotic metaphase chromosomes of pummelo and sweet orange were double stained with the fluorochromes CMA/DAPI (Chromomycin A3/4’-6-diamidino-2-phenylindole), and identified by FISH (Fluorescence in Situ Hybridization) with chromosome-specific BAC (Bacterial Artificial Chromosome) markers. The results were compared to previously established cytogenetic maps of mandarin, C. medica and Poncirus trifoliata. Only chromosomes 1, 4 and 8 were maintained unaltered among species, with chromosomes 2 and 3 being among the least conserved in heterochromatin distribution. BACs were conserved in position among homoeologs and the markers mapped to chromosomes 2 and 3 indicated that sweet orange karyotype largely conserved one chromosome from pummelo and one from mandarin. Despite conserved synteny, expansion and contraction of heterochromatic blocks accounted for the differences between karyotypes, even between the hybrid sweet orange and pummelo.