Analysis of Rhizobium etli and of its symbiosis with wild Phaseolus vulgaris supports coevolution in centers of host diversification

Abstract

Common beans (<i>Phaseolus vulgaris</i>) comprise three major geographic genetic pools, one in Mexico, Central America, and Colombia, another in the southern Andes, and a third in Ecuador and northern Peru. Species <i>Rhizobium etli</i> is the predominant rhizobia found symbolically associated with beans in the Americas. We have found polymorphism in the common nodulation gene node among <i>R. etli</i> strains from a wide range of geographical origins, which disclosed three <i>nodC</i> types. The different <i>nodC</i> alleles in American strains show varying predominance in their regional distributions in correlation with the centers of bean genetic diversification (BD centers). By cross-inoculating wild common beans from the three BD centers with soils from Mexico, Ecuador, Bolivia, and Northwestern Argentina, the <i>R. etli</i> populations from nodules originated from Mexican soil again showed allele predominance that was opposite to those originated from Bolivian and Argentinean soil, whereas populations from Ecuadorian soil were intermediate. These results also indicated that the preferential nodulation of beans by geographically related <i>R. etli</i> lineages was independent of the nodulating environment. Coinoculation of wild common beans from each of the three BD centers with an equicellular mixture of <i>R. etli</i> strains representative of the Mesoamerican and southern Andean lineages revealed a host-dependent distinct competitiveness: beans from the Mesoamerican genetic pool were almost exclusively nodulated by strains from their host region, whereas nodules of beans from the southern Andes were largely occupied by the geographically cognate <i>R. etli</i> lineages. These results suggest coevolution in the centers of host genetic diversification.