Genetic diversity of rhizobia nodulating lentils (Lens culinaris Medik.)

Abstract

Lentil is not only the oldest legume crop but also the oldest of the crops that have been domesticated in the Fertile Crescent and distributed to other regions during the Bronze Age, making it an ideal model to study the evolution of rhizobia associated with crop legumes. This study investigates lentil-nodulating rhizobia from the region where lentil originated (Turkey and Syria) and from regions to which lentil was introduced later (Germany and Bangladesh). There are few studies on lentil-nodulating rhizobia, and no phylogenetic studies on lentil rhizobia using multi locus sequence analyses. Therefore, rhizobia from lentil nodules were chosen to study 1) the genetic diversity 2) the taxonomic position and 3) the transmissible nature of nodulation genes. I have sequenced four housekeeping genes (16S rRNA, recA, atpD, glnII) and three nodulation genes (nodA, nodC, nodD) and analyzed these using phylogenetic and population genetic approaches to achieve these objectives. To supplement these approaches I have also used DNA fingerprinting and phenotypic characterization. Moreover, the symbiotic performance was assessed by nodulation and cross inoculation tests. I identified four different lineages of rhizobia associated with lentil, of which three are new and endemic to Bangladesh, and one lineage was found in the Mediterranean region and Central Europe. The new lineages from Bangladesh are close to Rhizobium etli and correspond to new species in the genus Rhizobium. The endemic lentil grex pilosae may have played a significant role in the origin of these new lineages in Bangladesh. The single lineage from the Mediterranean and Central Europe belongs to Rhizobium leguminosarum. The association of Rhizobium leguminosarum with lentil at the centre of lentil origin and in countries where lentil was introduced later suggests that Rhizobium leguminosarum is the original symbiont of lentil. Lentil seeds might have played a significant role in the initial dispersal of Rhizobium leguminosarum within the Middle East and on to other countries. Analysis of nodulation genes showed that they are prone to horizontal transfer between different chromosomal lineages and sub-lineages of rhizobia. Nodulation genes showed bias to their geographical origin, evidencing that plasmid-borne characters in bacteria rapidly change according to their adaptation to particular environment. Key words: Rhizobium, Lens culinaris, nodulation, multilocus analysis, fingerprint, phylogeny