Physical mapping of 5s and 18s-25s rDNA and repetitive DNA sequences

Abstract: An accurate physical map of the location of the 5 s and the 18s-5.8s-25s rRNA genes and a repetitive DNA sequence has been produced on Aegilops umbellulata Zhuk., (2n = 2x = 14) chromosomes by in situ hybridization. Chromosome morphology together with the hybridization pattern of pSc 119.2, a DNA sequence from rye, allowed identification and discrimination of different chromosomes; pSc119.2 hybridizes with all Ae. umbellulata chromosomes at the telomeres, except for the short arm of chromosome 6U, and shows intercalary sites on the long arms of chromosomes 6U and 7U. The 5 s and 18s-25s rDNA have been mapped physically only on the short arms of chromosomes 1U and 5U. On chromosome 1U the order of the genes is 5 s rDNA subterminal and 18s-25s rDNA more proximal, while on chromosome 5U the position of the genes is reversed. The relative order of the genes, together with the hybridization pattern of the pSc 119.2, is useful in identifying whole chromosomes or chromosome segments from Ae. umbellulata in recombinant or addition lines with wheat. The data help link the physical organization of chromosomes to the genetic map. Other members of the Triticeae vary in the presence and order of the 5 s and 18s-25s rDNA sequences on groups 1 and 5, indicating multiple and complex evolutionary rearrangements of the chromosome arms