Cytogenetics and Comparative Analysis of Phaseolus Species

The genus Phaseolus includes approximately 75 species, most of which have 2n = 22 small meta- or submetacentric chromosomes. This considerable karyotypic stability has been further reinforced by comparative cytogenetic mapping of single-copy sequences using Bacterial Artificial Chromosomes (BACs) and the Fluorescent in situ Hybridization (FISH) technique. These physical maps have revealed complete macrosynteny among the investigated species and only a few breaks in collinearity due to chromosomal inversions, except for Phaseolus leptostachyus, with 2n = 20 and several rearrangements. The variation in the repetitive fraction of the genome is much greater, however, as expected. Considerable variation has been found in the number of 35S rDNA sites in the representatives of the Vulgaris group, and repetitive pericentromeric and subtelomeric sequences vary among Phaseolus species. This repetitive genome fraction has also been investigated in Phaseolus vulgaris through an epigenetic approach, revealing a higher degree of complexity in the heterochromatin than previously thought. The available molecular tools for the common bean have permitted a systematic analysis of the synteny, not only between the common bean and other cultivated and wild species of the genus, but also at the intergeneric level, contributing to a more detailed understanding of the cytogenetics of the group.