ABSTRACT
In the late 1970s and 1980s, the cloning and sequencing of DNA segments of the rye genome were initiated in several countries around the world. This activity focused on repetitive DNA sequences because they were more accessible for establishing the technologies required for using these sequences as molecular markers in genetic mapping and characterizing breeding lines. Early studies in Australia showed the value of the repetitive DNA sequences for characterizing chromosome arms through visualizing their distribution by in situ hybridization to root tip squashes (Appels et al. 1978). By excluding the chromosomes that contain C+/N+ heterochromatin (Schlegel and Gill 1984) from the correlation analysis, the correlation between chromosome size and DNA content per chromosome is r = +0.87. Only 10%–20% of the genome can be assigned, biochemically, to the major part of the genome, which belongs to the repeated sequence category. The kinetics of genome organization has revealed that repeated sequences are generally interspersed among unrepeated sequences. The discovery of a very rapidly reannealing class of DNA constitutes 4%–10% of the genome, and although it is believed to be composed largely of sequences capable of renaturation, this class contains long tandem arrays of simple, repeated sequences (Appels 1982). Ranjekar et al. (1974) were the rst to demonstrate several buoyant-density components in a fraction of DNA renaturing with a density of 0-0.1 (10%–12% of the genome). However, the predominant component is a well-dened species at 1.7 g/cc in a CsCl gradient. Smith and Flavell (1977) considered this class of DNA to consist mainly of palindromic sequences, which are distributed in clusters throughout at least 30% of the genome. DNA, with a mean fragment length of 500 bp, was fractionated to allow recovery of very rapidly renaturing fraction (Cot 0-0.2). This DNA was shown to contain several families of highly repeated sequence DNA. Two of them were puried which resulted in a fraction renaturing to a density of 1.7 g/cc comprising 0.1% of the total genome. The second fraction, the polypyrimidine tract of DNA, also comprised 0.1% of the genome. Other hybridization studies between cereals have shown that 22%–24% of the DNA is rye-specic repeated sequence (Rimpau et al. 1978).
