|Van Berkum, Peter|
Submitted to: International Journal of Systematic and Evolutionary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2000
Publication Date: N/A
Interpretive Summary: Globally, soybean is the most important grain legume. Because soybean is a legume it benefits from biological nitrogen fixation through a symbiosis with soil bacteria of two different genera. In agriculture, the benefit is in the form of enhanced efficiency of crop production. Management of biological nitrogen fixation involves the inoculation of the appropriate bacterial cultures at the time of sowing. These bacterial cultures are available to the farmer as inoculants manufactured in industry. The problem is that many different bacteria are available, but comprehensive investigations for evolutionary differences have not been done. Here we report that the bacterial symbionts vary genetically and we have distinguished them by by tracing their evolution. We report the analysis of a novel section of the bacterial chromosome for determining the evolutionary relationships. We concluded that groupings based on phenotypic similarity need not necessarily be as closely related gentically. Also, we indicate that insufficient evidence may have been obtained to support proposals for three different species. Our results will be useful to scientists who are interested in evolutionary biology, taxonomy and strain identification.
Technical Abstract: From sequence divergence of both the 16S rRNA genes and the Internally Transcribed Spacer (ITS) region we report that variation in phylogenetic placement exists among the 17 different serotype strains of Bradyrhizobium that have been isolated from nodules of soybean. Evolutionary relationships among the bradyrhizobia were more resolved using reconstructions derived from ITS than from 16S rRNA gene sequence divergence. The strain USDA 129 was placed together with USDA 62, USDA 110, USDA 122, and USDA 126 and did not cluster with USDA 123 and USDA 127, with which it shares antigenic determinants. We supported the results from the phylogenetic analysis with data from determinations of genetic diversity with additional strains within each of these serogroups using Amplified Fragment Length Polymorhism (AFLP) analysis. From these results we concluded that 129 serogroup strains and 62, 110, and 122 serogroup strains were more similar than the 129 serogroups strains were to 123 and 127 serogroup strains. The serotype strain of B. japonicum USDA 135 and the type strain for B. liaoningense possessed identical 16S rRNA gene and ITS region sequences. Also, the type strain for B. liaoningense cross- reacted with antisera prepared against somatic antigens of USDA 135. We concluded that B. liaoningense belongs to the 135 serogroup and that this species and the 135 serogroup strains are indistinguishable. Our conclusion conflicts with the proposal of B. liaoningense as a species since the 135 serogroup is considered to be B. japonicum.