The genus Phaseolus L. has been subject of extensive cytogenetic studies due to its global economic importance. It is considered karyotypically stable, with most of its ca. 75 species having 2n = 22 chromosomes, and only three species (Phaseolus leptostachyus, Phaseolus macvaughii, and Phaseolus micranthus), which form the Leptostachyus clade, having 2n = 20. To test whether a simple chromosomal fusion was the cause of this descending dysploidy, mitotic chromosomes of P. leptostachyus (2n = 20) were comparatively mapped by fluorescent in situ hybridization (FISH) using bacterial artificial chromosomes (BACs) and ribosomal DNA (rDNA) probes. Our results corroborated the conservation of the 5S and 45S rDNA sites on ancestral chromosomes 10 and 6, respectively. The reduction from x = 11 to x = 10 was the result of the insertion of chromosome 10 into the centromeric region of chromosome 11, supporting a nested chromosome fusion (NCF) as the main cause of this dysploidy. Additionally, the terminal region of the long arm of chromosome 6 was translocated to this larger chromosome. Surprisingly, the NCF was accompanied by several additional translocations and inversions previously unknown for the genus, suggesting that the dysploidy may have been associated to a burst of genome reorganization in this otherwise stable, diploid plant genus.
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We thank M. Wetzel and L. Gonçalves Pereira Neto (Embrapa Recursos Genéticos e Biotecnologia) for providing the seeds, P. Gepts (University of California) for the BAC clones, and V. Geffroy (Université Paris-Sud) for the bacteriophage SJ19.12. A.F. was supported by a grant from Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco (FACEPE), Brazil. A.P.-H. and the project were supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil.
Conflict of interest
All the authors declare that they have no conflict of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
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