Sucessive karyotipic changes revealed for comparative cytogenetic mapping in the Leptostachyus group, Phaseolus (Fabaceae)

Polyploidy and disploidy have been reported as the main events in karyotype evolution of plants. In the genus Phaseolus L., numerically stable (2 n = 22), a small monophyletic group of three species from Mexico and Central America, the Leptostachyus group, presents the dysploid karyotype (2 n = 20) and one larger chromosome pair. Many rearrangements, including translocations, inversions and a Nested Chromosome Fusion (NCF), were observed in P. leptostachyus Benth, revealing a high rate of karyotype evolution. The analysis of the first diverging species of this clade, P. macvaughii Delgado, revealed that the NCF event is a synapomorphy of the Leptostachyus clade, resulting in the descending dysploidy. Thus, this study aimed to understand the karyotype evolution of this group, verifying if P. macvaughii shares with P. leptostachyus the same rearrangements in the homeologs to P. vulgaris chromosomes Pv1, Pv2, Pv4 and Pv8. For this, fluorescence in situ hybridizations (FISH) were performed on P. macvaughii mitotic slides using nine sequences of P. vulgaris cloned into bacterial artificial chromosomes (BACs) and previously mapped on P. leptostachyus chromosomes. Sequences selected for Pv1 hybridized collinearly in P. macvaughii , contrasting to the observed in P. leptostachyus, which showed synteny break. Markers from Pv2 showed loss of synteny in both species, due to a translocation. Part of the long arm of chromosome 2 translocated to the short arm of chromosome 1, altering the arm ratio of these chromosomes in P. macvaughii . The chromosomes Pv 4 and Pv 8 kept their collinearity to P. vulgaris in P. macvaughii, different to the observed in P. leptostachyus . The presence of shared and non-shared rearrangements indicates a progressive accumulation of karyotypic changes in the Leptostachyus group.