Pachytene Chromosomes and Origin of a Tetraploid Species of Phaseolus

A detailed description with reference to the identification of 19 of the 22 pachytene chromosomes of a tetraploid species of Phaseolus from Brazil is reported. The chromosomes are identified on the basis of their relative length, arm ratio, chromomere pattern, nucleolar association and presence or absence of telochromomeres or knobs. These studies also led to the realization of allosyndetic nature of pairing, chromosomal differentiation in the parental sets and nucleolus organizer suppression in the 4n Phaseolus sp. These observations indicate an allotetraploid origin for the species. The somatic chromosomes fall into nine classes with respect to their differences in length, arm ratio and nucleolar association.