Cloning and characterisation of polymorphic heterochromatic segments of Brachycome dichromosomatica

After selective enrichment and differential hybridisation of C(o)t-1 DNA fr actions of plants with and without polymorphic heterochromatic segments, a repetitive sequence (called Bds1) specific to the polymorphic chromosome se gments of Brachycome dichromosomatica (Brachyscome dichromosomatica) was is olated. A single repeat unit of Bds 1 is 92 bp long and is organised in tan dem arrays at three different polymorphic segment sites on the chromosomes of cytodeme A2. Although all three sites showed extensive polymorphism betw een plants, the karyotypes of all analysed mitotic root cells were stable w ithin a single plant. Electron microscopy revealed heavily condensed chroma tin structures at the most obvious polymorphic site. The mechanisms that ge nerate and maintain the observed chromosome structure polymorphisms are dis cussed.