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Applied and Environmental Microbiology, July 2009, p. 4727-4735, Vol. 75, No. 14
0099-2240/09/$08.00+0     doi:10.1128/AEM.00667-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

In Situ Phylogenetic Structure and Diversity of Wild Bradyrhizobium Communities{triangledown} ,{dagger}

J. L. Sachs,* S. W. Kembel,{ddagger} A. H. Lau,§ and E. L. Simms

Department of Integrative Biology, University of California, Berkeley, California 94720

Received 21 March 2009/ Accepted 26 May 2009

Bacteria often infect their hosts from environmental sources, but little is known about how environmental and host-infecting populations are related. Here, phylogenetic clustering and diversity were investigated in a natural community of rhizobial bacteria from the genus Bradyrhizobium. These bacteria live in the soil and also form beneficial root nodule symbioses with legumes, including those in the genus Lotus. Two hundred eighty pure cultures of Bradyrhizobium bacteria were isolated and genotyped from wild hosts, including Lotus angustissimus, Lotus heermannii, Lotus micranthus, and Lotus strigosus. Bacteria were cultured directly from symbiotic nodules and from two microenvironments on the soil-root interface: root tips and mature (old) root surfaces. Bayesian phylogenies of Bradyrhizobium isolates were reconstructed using the internal transcribed spacer (ITS), and the structure of phylogenetic relatedness among bacteria was examined by host species and microenvironment. Inoculation assays were performed to confirm the nodulation status of a subset of isolates. Most recovered rhizobial genotypes were unique and found only in root surface communities, where little bacterial population genetic structure was detected among hosts. Conversely, most nodule isolates could be classified into several related, hyper-abundant genotypes that were phylogenetically clustered within host species. This pattern suggests that host infection provides ample rewards to symbiotic bacteria but that host specificity can strongly structure only a small subset of the rhizobial community.

* Corresponding author. Present address: Department of Biology, University of California, Riverside, CA 92521. Phone: (951) 827-6357. Fax: (951) 827-4286. E-mail: joels{at}ucr.edu

{triangledown} Published ahead of print on 29 May 2009.

{dagger} Supplemental material for this article may be found at http://aem.asm.org/.

{ddagger} Present address: Center for Ecology and Evolutionary Biology, University of Oregon, Eugene, OR 97403.

§ Present address: Keck School of Medicine, University of Southern California, Los Angeles, CA 90089.

Applied and Environmental Microbiology, July 2009, p. 4727-4735, Vol. 75, No. 14
0099-2240/09/$08.00+0     doi:10.1128/AEM.00667-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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