Molecular diversification of tandemly organized DNA sequences and heterochromatic chromosome regions in some triticeae species

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Abstract
The subtelomeric heterochromatin of rye (Secale cereale) chromosomes makes up 12–18% of the genome and consists largely of a small number of tandemly organized DNA sequence families. The genomic organization, chromosomal locations and the structural organization of monomer units of the major DNA sequences from these regions were investigated and compared in other Triticeae species from the generaSecale, Agropyron, Dasypyrum, Triticum andHordeum. Southern hybridization and polymerase chain reaction analysis established that all studied species preserve the tandem type of sequence organization but the copy number is altered drastically between species. In the pSc200 family, a fraction of the tandem arrays is present with a head-to-head orientation of dimers inS. cereale andS. montanum. Members of the same family are more heterogeneous and present as head-to-head monomers in theDasypyrum species andA. cristatum. In situ hybridization demonstrates different organization of the sequence families in the various species: pSc200 and pSc250 are concentrated in major blocks at the ends of most rye chromosome arms, whereas they are more dispersed and in smaller blocks inDasypyrum andAgropyron indicating that accumulation is not simply due to the sequence itself. In contrast to rye,D. villosum has large blocks of only pSc200 whereasD. breviaristatum shows greater amplification of pSc250. These data indicate that each repetitive family is an independent unit of evolution, and suggest that the twoDasypyrum species are not closely related. The data are discussed in terms of existing evolutionary models for repetitive DNA sequences. The contribution of random events, through molecular drive and selection, to the evolution of heterochromatic regions is considered.