Physical mapping of DNA repetitive sequences to mitotic and meiotic chromosomes of Brassica oleracea var. alboglabra by fluorescence in situ hybridization

As part of a programme to integrate the genetic and physical chromosome map s of Brassica oleracea we have carried out fluorescence in situ hybridizati on (FISH) to mitotic and meiotic chromosomes of B. oleracea var. alboglabra , using repeat sequence DNA probes. We have confirmed that there are three 18S-5.8S-25S rDNA sites in the haploid genome and accurately determined the ir locations for the first time: two occur subtelomerically on the short ar ms of the two Satellited acrocentric chromosomes (chromosomes 4 and 7), whe reas the third site is adjacent to the centromere on the short arm of a lar ge submetacentric chromosome (chromosome 2). 5S rDNA sequences are located on the long arm of this same chromosome, with closely adjacent major and mi nor loci. A highly repeated sequence (pBcKB4) colocalizes with the pericent romeric heterochromatin on all chromosomes, but six chromosomes exhibit ver y strong signals whereas the remaining three show only weak signals. The co nstruction of a partial karyotype for B. oleracea var. alboglabra, based on several major cytogenetical landmarks provided by FISH localization of rep etitive DNA sequences, provides a valuable framework for the future physica l mapping of nonrepetitive sequences.