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

Abstract

As part of a programme to integrate the genetic and physical chromosome maps of Brassica oleracea we have carried out fluorescence in situ hybridization (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 their locations for the first time; two occur subtelomerically on the short arms of the two satellited acrocentric chromosomes (chromosomes 4 and 7), whereas the third site is adjacent to the centromere on the short arm of a large submetacentric chromosome (chromosome 2). 5S rDNA sequences are located on the long arm of this same chromosome, with closely adjacent major and minor loci. A highly repeated sequence (pBcKB4) colocalizes with the pericentromeric heterochromatin on all chromosomes, but six chromosomes exhibit very strong signals whereas the remaining three show only weak signals. The construction of a partial karyotype for B. oleracea var. alboglabra, based on several major cytogenetical landmarks provided by FISH localization of repetitive DNA sequences, provides a valuable framework for the future physical mapping of nonrepetitive sequences.