Dispensable, supernumerary (B) chromosomes are found in diverse eukaryotic species. The origin and genetic consequences of B chromosomes have been the subjects of speculation for more than a century. Until now, there has been no molecular evidence that B chromosome DNA is transcribed and there is no unequivocal evidence as to their origin. B chromosomes are considered to be genetically inert although they appear to cause a variety of phenotypic effects. We report that members of one of two ribosomal RNA gene families that are confined to the B chromosomes of a plant, Crepis capillaris, are transcribed-thus providing the first molecular evidence of gene activity on B chromosomes. Sequence analysis of part of the A and B chromosome rRNA genes, together with comparisons with related species, indicates that the B chromosome rRNA genes originate from the A chromosome.

Abstract. Plant chromosome microdissection techniques together with different isolation and amplification methods of micro isolated DNA are described. Such isolated DNA was used to 'chromosome paint' B chromosomes of the dicot Brachy come dichromosomatica and the monocot Secale cereale. It is demon strated that the specific painting of the described chromosomes was pos sible because of enri chment for chromosome-specific repetiuve sequences, rather than the chromosome specific lowand single-copy sequences which are respon sible for the painting of mammalian chromosomes. The feasibility of 'chromosome painting' of standard chromo somes in plant species with relatively small or large genomes is discu ssed.

Plant chromosome microdissection techniques together with different isolation and amplification methods of microisolated DNA are described. Such isolated DNA was used to 'chromosome paint' B chromosomes of the dicot Brachycome dichromosomatica and the monocot Secale cereale. It is demonstrated that the specific painting of the described chromosomes was possible because of enrichment for chromosome-specific repetitive sequences, rather than the chromosome specific low- and single-copy sequences which are responsible for the painting of mammalian chromosomes. The feasibility of 'chromosome painting' of standard chromosomes in plant species with relatively small or large genomes is discussed.

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