Karyological Studies in the Genus Phaseolus, Linn

Abstract

Karyotypes of somatic chromosomes of 15 species of Phaseolus, L. have been investigated. All the species have been found to have 11 and 22 as haploid and diploid chromosome numbers (except allotetraploid Phaseolus species 2n=44). Primary base number for the genus is eleven. The somatic chromosomes are small, their size range from 0.85μ to 3.0μ. The longest being 3.0μ in P. vulgaris and shortest 0.85μ is in P. ricciardianus and P. semierectus. TF % have been calculated for each species. On the basis of TF% karyotypes are classified as symmetrical 2A or 3A types. Three species showed asymmetrical karyotypes. Phaseolus ricciardianus have asymmetrical 2B type of karyotype.
The karyotypic differences in the species have been brought about by diminution in chromatin length with advancing evolution and pericentric inversions resulting in the shift of position of the centromere and alteration in the size of chromosomes.
Phaseolus vulgaris and P. aureus with large somatic chromosomes and longest total chromatin length have been considered primitive especially in climbing and erect habit series. Phaseolus bracteatus and P. ricciardianus with short chromosomes and shortest total chromatin length are considered as advanced. Thus, it is concluded that P. vulgaris which is under cultivation in Mexico for the past 7000 year is primitive and has given rise to other species including the erect ones.