ORIGINAL PAPER Andrea Pedrosa-Harand Cı´cero C. Souza de Almeida Magdalena Mosiolek Matthew W. Blair Dieter Schweizer Marcelo Guerra Extensive ribosomal DNA amplification during Andean common bean (Phaseolus vulgaris L.) evolution Received: 22 July 2005 / Accepted: 30 November 2005 / Published online: 6 January 2006 Ó Springer-Verlag 2006 Abstract The extent of 5S and 45S ribosomal DNA (rDNA) variation was investigated in wild and domesti- cated common beans (Phaseolus vulgaris) chosen to rep- resent the known genetic diversity of the species. 5S and 45S rDNA probes were localized on mitotic chromo- somes of 37 accessions by fluorescent in situ hybridization (FISH). The two 5S rDNA loci were largely conserved within the species, whereas a high variation in the number of 45S rDNA loci and changes in position of loci and number of repeats per locus were observed. Domesticated accessions from the Mesoamerican gene pool frequently had three 45S rDNA loci per haploid genome, and rarely four. Domesticated accessions from Andean gene pool, particularly from the race Peru, showed six, seven, eight or nine loci, but seven loci were found in all three races of this gene pool. Between three and eight loci were observed in accessions resulting from crosses between Andean and Mesoamerican genotypes. The presence of two to eight 45S rDNA loci in wild common beans from different geographic locations indicates that the 45S rDNA amplification observed in the Andean lineage took place before domestication. Our data suggest that ectopic recombination between terminal chromosomal regions might be the mechanism responsible for this variation. Common bean (Phaseolus vulgaris, 2n=22) is the world’s most important grain legume for direct food consumption, especially in Latin America and Africa. Although little is known about its genomic organization, the evolution and domestication history of this species have been intensively studied over the last few years. P. vulgaris is thought to have originated in a region encompassing Ecuador and northern Peru (Kami et al. 1995), and dispersed both northwards and southwards establishing the Mesoamerican and Andean gene pools, respectively (reviewed in Gepts 1998). Independent domestication took place in each gene pool (Becerra Velasquez and Gepts 1994). Based on morphological, agroecological and molecular data, the resulting domesticated forms have been grouped into six or seven races (Singh et al. 1991; Beebe et al. 2000). Molecular cytogenetic analysis in common bean has been restricted to a few genotypes. Using a combination of double FISH with 5S and 45S rDNA probes, chro- mosome morphology and heterochromatin distribution, ideograms of two European cultivars were established (Moscone et al. 1999). Later, the chromosomal and ge- netic maps of common bean were integrated by means of direct hybridization of pooled RFLP clones from the University of Florida map (Vallejos et al. 1992) to the mitotic chromosomes of two cultivars of the species (Pedrosa et al. 2003). The cultivars analyzed in these previous studies revealed a high degree of variation in the number and size of the 45S rDNA loci within the species. Based on this small, random sample, it has been suggested that a higher degree of amplification and Communicated by J. S. Heslop-Harrison A. Pedrosa-Harand (&) D. Schweizer Department of Chromosome Biology, University of Vienna, 1030 Vienna, Austria E-mail: andrea.pedrosa.harand@univie.ac.at Tel.: +43-1-427756251 Fax: +43-1-42779562 C. C. S. de Almeida M. Guerra Laboratory of Plant Cytogenetics, Department of Botany, Federal University of Pernambuco, 50670-420 Recife, PE, Brazil M. Mosiolek Department of Plant Cytology and Embryology, Institute of Botany, Jagiellonian University, 31-044 Krakow, Poland M. Mosiolek D. Schweizer GMI—Gregor Mendel Institute of Molecular Plant Biology, 1010 Vienna, Austria M. W. Blair CIAT—International Center for Tropical Agriculture, 6713 Cali, Colombia Theor Appl Genet (2006) 112: 924–933 DOI 10.1007/s00122-005-0196-8