DNA structure of the B chromosome of rye revealed by in situ hybridization using repetitive sequences

To clarify the DNA structure of the B chromosome of rye, Secale cereale L., we carried out in situ hybridization using probes of two highly repetitive sequences (pSc74 and pSc119) of rye A chromosome, of total genomic DNAs from plants with (4 B) or without (0 B) B chromosomes, and probes of fractionated genomic DNA (1.14-kb and 1.28-kb sequences) from a 4 B plant. Clones pSc74 and pSc119 did not hybridize to the B chromosomes except at small interstitial sites. A probe consisting of the total DNA of a 0 B plant hybridized over the entire length of all the A and B chromosomes except for the telomeric C-band region of the long arm of the B chromosome. A probe of total DNA from a 4B plant produced signals only in the terminal C-band region after prehybridization with the 0 B plant DNA. A probe consisting of DraI-digested 1.14-kb fragments from a 4B plant hybridized to the telomeric region of the B chromosome, whereas one of 1.28-kb fragments did so to an interstitial region of the A chromosome. On the basis of the in situ hybridization, and the cytological and molecular biological results, we assume that the B chromosome in rye originated once in the distant past and was then perpetuated in the populations.