Evidence of an American Origin for Symbiosis-Related Genes in Rhizobium lusitanum

Randomly amplified polymorphic DNA (RAPD) analysis was used to investigate the diversity of 179 bean isolates recovered from six field sites in the Arcos de Valdevez region of northwestern Portugal. The isolates were divided into 6 groups based on the fingerprint patterns that were obtained. Representatives for each group were selected for sequence analysis of 4 chromosomal DNA regions. Five of the groups were placed withinRhizobium lusitanum , and the other group was placed withinR. tropici type IIA. Therefore, the collection of Portuguese bean isolates was shown to include the two speciesR. lusitanum andR. tropici . In plant tests, the strains P1-7, P1-1, P1-2, and P1-16 ofR. lusitanum nodulated and formed nitrogen-fixing symbioses both withPhaseolus vulgaris andLeucaena leucocephala . A methyltransferase-encodingnodS gene identical with theR. tropici locus that confers wide host range was detected in the strain P1-7 as well as 24 others identified asR. lusitanum . A methyltransferase-encodingnodS gene also was detected in the remaining isolates ofR. lusitanum , but in this case the locus was that identified with the narrow-host-rangeR. etli . Representatives of isolates with thenodS ofR. etli formed effective nitrogen-fixing symbioses withP. vulgaris and did not nodulateL. leucocephala . From sequence data ofnodS , theR. lusitanum genes for symbiosis were placed within those of eitherR. tropici orR. etli . These results would support the suggestion thatR. lusitanum was the recipient of the genes for symbiosis with beans from bothR. tropici andR. etli .