THE MOLECULAR CYTOGENETICS OF VIGNA-UNGUICULATA (L) WALP - THE PHYSICAL ORGANIZATION AND CHARACTERIZATION OF 18S-5.8S-25S RIBOSOMAL-RNA GENES, 5S RIBOSOMAL-RNA GENES, TELOMERE-LIKE SEQUENCES, AND A FAMILY OF CENTROMERIC REPETITIVE DNA-SEQUENCES

A knowledge of genome organization is important for understanding how genomes function and evolve, and provide information likely to be usef ul in plant breeding programmes involving hybridization and genetic ma nipulation. Molecular techniques, including in situ hybridization, mol ecular cloning and DNA sequencing, are proving valuable tools to inves tigate the structure, organization, and diversity of chromosomes in ag ricultural crops. Heterologous labelled 18s-5.8s-25s (pTa71) and 5s rD NAs (pTa794) were used for in situ hybridization on Vigna unguiculata (L.) Walp, chromosomes. Hybridization with 18 s-5.8 s-25s rRNA gene pr obes occurred at the same chromosomal sites which were positive to the CMA fluorochrome. Silver staining of nucleolar-organizing regions ind icated that all the rDNA sites detected using the 18 s-5.8 s-25s rRNA gene probe possessed active genes. Degenerate telomeric repeats gave h ybridization signals at the telomeres of most chromosomes and no inter calary sites were detected at metaphase; the sequences appear to have no preferential distribution in interphase nuclei. A repetitive DraI f amily from V. unguiculata was cloned (pVuKB1) and characterized. The D raI repeat is 488 nucleotides long, AT rich (74%), and hybridized on a ll chromosomes in the centromeric areas. The presence of this sequence family was investigated by Southern hybridization in different Vigna species and other Leguminoseae. It was only detected in V. unguiculata , and hence represents a species-specific DNA sequence.