Karyotype variability of sour orange (Citrus aurantium L.) and the origin of its heteromorphic karyotypes

Sour orange (Citrus aurantium L.) is a very well-known citrus product. As sweet orange [Citrus sinensis (L.) Osbeck], it originated from the crossing between mandarin (Citrus reticulata Blanco) and pummelo [Citrus maxima (Burm.) Merril]. In order to investigate the origin and variability of its karyotype, eight cultivars of C. aurantium and one cultivar of Citrus natsudaidai Hayata, closely related to C. aurantium, were characterized with the fluorochromes CMA (chromomycin A3) and DAPI (4′-6-diamidino-2-phenylindole) and FISH (fluorescent in situ hybridization) with 5S and 35S rDNA, as well as BAC (bacterial artificial chromosomes) probes. The sour orange accessions analyzed presented three distinct karyotype formulae, but six of eight accessions had the formula 1A + 1B + 1C + 8D + 7F. Citrus natsudaidai presented the formula 2A + 1C + 6D + 9F. The accessions ‘Smooth Flat Seville’, ‘Standard’, and ‘Thornless’ presented three sites of 35S rDNA in total, in A or B chromosomes, as well as in one D. The accessions ‘Smooth Flat Seville’ and ‘Thornless’ presented always two sites of 5S rDNA, in one F and one D chromosomes. The presence of chromosome types A/35S and F/5S reinforces C. aurantium as a first-generation hybrid derived from C. maxima, while a D/5S-35S chromosome corroborates C. reticulata as a second parent. However, several chromosome changes occurred after the hybridization event, which can be partly explained by the reduction of heterochromatic blocks and partly by the occurrence of introgressions in some accessions.