High macro-collinearity between lima bean (Phaseolus lunatus L.) and the common bean (P. vulgaris L.) as revealed by comparative cytogenetic mapping.

Common bean (P. vulgaris) and lima bean (P. lunatus) are the most important crop species from the genus Phaseolus. Both species have the same chromosome number (2n = 22) and previous cytogenetic mapping of BAC clones suggested conserved synteny. Nevertheless, karyotype differences were observed, suggesting structural rearrangements. In this study, comparative cytogenetic maps for chromosomes 3, 4 and 7 were built and the collinearity between the common bean and lima bean chromosomes was investigated. Thirty-two markers (30 BACs and 2 bacteriophages) from P. vulgaris were hybridized in situ on mitotic chromosomes from P. lunatus. Nine BACs revealed a repetitive DNA pattern with pericentromeric distribution and 23 markers showed unique signals. Nine of these markers were mapped on chromosome 3, eight on chromosome 4 and six on chromosome 7. The order and position of all analyzed BACs were similar between the two species, indicating a high level of macro-collinearity. Thus, although few inversions have probably altered centromere position in other chromosomes, the main karyotypic differences were associated with the repetitive DNA fraction.