DISTRIBUTION AND EVOLUTION OF A GLUCOSEPHOSPHATE ISOMERASE DUPLICATION IN THE LEGUMINOSAE

In the common bean, Phaseolus vulgaris, two loci encode cytosolic glucosephosphate isomerase (GPI) subunits, whereas in the garden pea, Pisum sativum, only one locus is expressed. As a working model, we proposed that this change in isozyme number was produced by a gene-duplication event in the lineage leading to Phaseolus after divergence from that leading to Pisum. This model was tested by analyzing the GPI phenotypes in 119 legume genera, representing all three subfamilies and 23 of the 30 tribes of the Papilionoideae. The duplication was detected in 13 of the 20 papilionoid tribes surveyed, including several members of the putatively primitive tribe Sophoreae. Thus, the duplication appears to be an ancient event, a finding incompatible with the initial hypothesis. Instead, gene silencing is postulated to account for the absence of the duplicated phenotype in many tribes, including such advanced groups as Vicieae, Trifolieae, and Cicereae. Furthermore, silencing has occurred numerous times at lower taxonomic levels, including the subtribe Phaseolinae (Phaseoleae), a monophyletic group in which ten genera were found to have duplicated phenotypes and only one (Strophostyles) appeared to have an unduplicated phenotype. Analysis of GPI phenotypes also revealed numerous cases of partial silencing of duplicate loci as well as nearly equal expression of both loci in many, taxonomically widely scattered species. If our revised hypothesis is correct, this latter result implies that most of the subtribes had formed before significant divergence between the GPI isozymes occurred and, thus, that the radiation of the Papilionoideae was rapid relative to the rate of gene silencing.