Identification of presumed ancestral DNA sequences of phaseolin in Phaseolus vulgaris

Common bean (Phaseolus vulgaris) consists
oftwo major geographic gene pools, one distributed in Mexico,
Central America, and Colombia and the other in the southern
Andes (southern Peru, Bolivia, and Argentina). Amplification
and sequencing of members of the multigene family coding for
phaseolin, the major seed storage protein of the common bean,
provide evidence for accumulation of tandem direct repeats in
both introns and exons during evolution of the multigene
family in this species. The presumed ancestral phaseolin
sequences, without tandem repeats, were found in recently
discovered but nearly extinct wild common bean populations
of Ecuador and northern Peru that are intermediate between
the two major gene pools of the species based on geographical
and molecular arguments. Our results illustrate the usefulness
of tandem direct repeats in establishing the polarity of
DNA sequence divergence and therefore in proposing phylogenies.