W. dos S. Soares Filho

Chromosomal markers distinguish hybrids and non-hybrid accessions of mandarin

Mandarin is the common name of a heterogeneous group of Citrus species with a large range of variation in morphological and molecular characters as well as in number of species. Aiming to identify chromosome markers and to clarify the relationship within this group, the karyotype of 13 mandarin accessions were analyzed using CMA/DAPI staining and in situ hybridization with 5S and 45S rDNA probes. The CMA band pattern together with the position of rDNA sites revealed that mandarins can be separated karyologically into three groups: a) C. sunki and C. reshni; b) the Mediterranean mandarin, C. deliciosa, and the closely related C. tangerina cv. Dancy and C. reticulata cv. Cravo; c) the remaining cultivars, which are cytologically heterozygous and most probably interspecific hybrids. The former two groups are assumed to be pure species together with C. medica and C. grandis. A chromosome marker for mandarin species was identified and the relationship among the pure species and some hybrids is discussed.

Linked 5S and 45S rDNA Sites Are Highly Conserved through the Subfamily Aurantioideae (Rutaceae)

Sites of 5S and 45S rDNA are more commonly located on different chromosomes of most angiosperms. Previous investigations have shown that in the subfamily Aurantioideae these sites may appear closely linked (adjacent sites), as in Poncirustrifoliata, or completely isolated, as in some species of Citrus. In the present work, the distribution of rDNA sites was investigated in representatives of 9 genera of Aurantioideae by FISH and CMA banding, aiming to understand the evolution of adjacent sites in the subfamily. A total of 57 rDNA sites were observed, 40 of them being adjacent to each other. All adjacent sites displayed the 45S rDNA array more terminally located. Assuming that the linked 5S-45S rDNA arrangement was the ancestral condition in Aurantioideae, the isolated rDNA sites observed in Clausena excavata,Bergera koenigii, and Fortunella obovata, as well as the complete linkage loss in Citrus maxima and C. medica indicates that unlinked sites arose independently several times in the evolution of the group. The linkage loss may be due to independent dispersion of one or both rDNA sequence families followed by deletion of the corresponding array in the adjacent site. The possible mechanisms behind these events and their occurrence in other groups are discussed.