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Syst Appl Microbiol. 2013 Oct;36(7):490-6. doi: 10.1016/j.syapm.2013.06.004. Epub 2013 Jul 17.

Definition and evolution of a new symbiovar, sv. rigiduloides, among Ensifer meliloti efficiently nodulating Medicago species.

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  • 1Institute of Biological and Environmental Sciences, University of Aberdeen, Aberdeen AB24 3UU, United Kingdom. Electronic address:


Understanding functional diversity is one of the main goals of microbial ecology, and definition of new bacterial ecotypes contributes significantly to this objective. Nitrogen-fixing bacteria provide a good system for investigation of ecotypes/biovars/symbiovars, as they present different specific associations with several host plants. This specific symbiosis is reflected both in the nodulation and fixation efficiency and in genetic characters of the bacteria, and several biovars have already been described in the bacterial species Ensifer meliloti. In the present study, the species affiliation of E. meliloti strains trapped from nodules sampled from Medicago rigiduloïdes roots was analyzed using housekeeping recA genes and DNA-DNA hybridization. The genetic diversity of these isolates was also investigated using several symbiotic markers: nodulation (nodA, nodB, nodC) and nitrogen fixation (nifH) genes, as well as the performance of phenotypic tests of nodulation capacity and nitrogen fixation efficiency. These analyses led to the proposal of a new bacterial symbiovar, E. meliloti sv. rigiduloides, that fixed nitrogen efficiently on M. rigiduloïdes, but not on Medicago truncatula. Using phylogenetic reconstructions, including the different described symbiovars, several hypotheses of lateral gene transfer and gene loss are proposed to explain the emergence of symbiovars within this species. The widespread geographical distribution of this symbiovar around the Mediterranean Basin, in contrast to restriction of M. rigiduloïdes to Eastern European countries, suggests that these isolates might also be associated with other plant species. The description of a new symbiovar within E. meliloti confirms the need for accurate bacterial ecological classification, especially for analysis of bacterial populations.

Copyright © 2013 Elsevier GmbH. All rights reserved.


Biovar; Ecotype; Genetic diversity; Nitrogen fixation; Nodulation genes; Species; Symbiosis

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