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

AGUILAR, O.M.; RIVA, O.; PELTZER, E.

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA

NATL ACAD SCIENCES

Año: 2004 vol. 101 p. 13548 - 13553

0027-8424

Common beans (Phaseolus vulgaris) comprise three major geographicgenetic pools, one in Mexico, Central America, and Colombia,another in the southern Andes, and a third in Ecuador andnorthern Peru. Species Rhizobium etli is the predominant rhizobiafound symbiotically associated with beans in the Americas. Wehave found polymorphism in the common nodulation gene nodCamong R. etli strains from a wide range of geographical origins,which disclosed three nodC types. The different nodC alleles inAmerican strains show varying predominance in their regionaldistributions in correlation with the centers of bean genetic diversification(BD centers). By cross-inoculating wild common beansfrom the three BD centers with soils from Mexico, Ecuador, Bolivia,and Northwestern Argentina, the R. etli populations from nodulesoriginated from Mexican soil again showed allele predominancethat was opposite to those originated from Bolivian and Argentineansoil, whereas populations from Ecuadorian soil were intermediate.These results also indicated that the preferential nodulationof beans by geographically related R. etli lineages wasindependent of the nodulating environment. Coinoculation of wildcommon beans from each of the three BD centers with an equicellularmixture of R. etli strains representative of the Mesoamericanand southern Andean lineages revealed a host-dependentdistinct competitiveness: beans from the Mesoamerican geneticpool were almost exclusively nodulated by strains from their hostregion, whereas nodules of beans from the southern Andes werelargely occupied by the geographically cognate R. etli lineages.These results suggest