A MOLECULAR ANALYSIS OF THE ORIGIN OF THE CREPIS-CAPILLARIS B-CHROMOSOME

The origin of the B chromosome of Crepis capillaris has been studied b y using in situ hybridization with different DNA probes. Genomic in si tu hybridization (GISH) with DNA from plants with and without Bs as pr obes indicates that the B chromosome has many DNA sequences in common with A chromosomes, showing no region rich in B-specific sequences. Si x additional DNA probes were used to test the possible origin of this B from the standard NOR chromosome (chromosome 3). In the short arm of the NOR chromosome, we detected not only 18 S + 25 S rDNA, but also 5 S rDNA and a specific repetitive sequence from the NOR chromosome (pC cH32); in the heterochromatic bands of the long arm, we found two diff erent repetitive sequences (pCcE9 and pCcD29). In the B chromosome, ho wever, only the 18 S + 25 S rDNA and the telomeric sequences from Arab idopsis thaliana were observed. Our in situ hybridization data with te lomeric repeats indicate that the two telomeres of the B are larger th an those of the A chromosomes, confirming the isochromosomal nature of this B. Hybridizations of 18 S + 25 S rDNA and telomeric repeats to b lots of DNA from plants with and without Bs reveal a high homology bet ween A and B 18 S + 25 S rDNA genes, but some sequence dissimilarities between A and B telomeres. Taken as a whole, these data indicate that the entire B of C capillaris, although possibly having originated fro m the standard genome, did not derive directly from the NOR chromosome .