Evidence from ITS sequence analysis of 31 and 110 serogroup soybean strains that extant members of the genus Bradyrhizobium are likely the products of reticulate evolutionary events

Authors: Van Berkum, Peter; FUHRMANN, JEFFRY - UNIVERSITY OF DELWARE

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2009
Publication Date: 1/1/2009
Citation: Van Berkum, P.B., Fuhrmann, J.J. 2009. Evidence from ITS sequence analysis of 31 and 110 serogroup soybean strains that extant members of the genus Bradyrhizobium are likely the products of reticulate evolutionary events. Applied and Environmental Microbiology. 75:78-82.

Interpretive Summary: Fertilizers are necessary to produce crops, but the disadvantage is that fossil fuels are used to produce these fertilizers. This increases the cost of fertilizer production, increases food costs, causes widespread pollution, and breaks down the ozone layer which negatively affects the environment and public health. A family of plants know as legumes, including soybean, do not require fertilizer application for growth. The resason is that they form specialized structures on their roots that contain bacteria known as rhizobia. The rhizobia use atmosopheric nitrogen to produce fertilizer in the plant without releasing it into the environment. The choice of the rhizobia to produce an optimum crop is difficult because little is known about their genetic variability and the cause of this variability. In this research, we discovered that rhizobia share genetic information which is a major process causing variability. This information will be valuable to scientists who wish to increase efficieny of soybean production by not using fertilizer.

Technical Abstract: The Internally Transcribed Space (ITS) sequence of several members within each of seventeen soybean bradyrhizobial serogroups was determined to establish whether this region within each genome would serve as a convenient marker for distinguishing serogroup affinity of new isolates. With the exception of serogroups 31 and 110 strains, sequence identity was established within each serogroup. None of the members of serogroup 31 had ITS sequences that were identical with the serotype strain USDA 31, variation ranged from 1 to 24 nucleotides. Sequence identity was established among most strains within serogroup 110. The exceptions were USDA 452 and USDA 456, which had ITS sequences that were identical with the serotype 124 strain, USDA 124. Perhaps this would imply that USDA 452 and USDA 456 are members of rhizobial lineages resulting from genetic exchange and homologous recombination events between the 110 and 124 serogroups. This conclusion would be supported by the construction of a phylogenetic network from the ITS sequence alignment implying that the genomes of extant members of the genus Bradyrhizobium are likely the products of reticulate evolutionary events. The ITS sequences of B. japonicum strains USDA 110 and USDA 124 were more divergent compared with the ITS regions of USDA 110 and bradyrhizobial strains from Lupinus angustifolius, Lotus uliginosus, Crotolaria paulina and Vigna unguiculata. With the exception of B. denitrificans, this would imply that within the genus Bradyrhizobium divergence of individual members is so limited that it would be unjustified to assign discrete species boundaries to any group characterized from phenotypic and molecular data.