Use of population genetic structure to define species limits in the Rhizobiaceae

Symbiotic bacteria of the family Rhizobiaceae are currently defined using a consensus or polyphasic approach, where emphasis in determining species limits is based on the quantification of overall genotypic and phenotypic similarity. In the first part of this review some of the limitations of this approach are examined. In the second part an alternative population-based approach is considered. The primary assumption underlying this approach is that ecological selection is the dominant force constraining genetic diversity in bacterial populations. Practical methods for assessing the range and extent of this diversity are described, along with an example of how such information has been used to provide evidence for two symbiotic nitrogen-fixing species within the genus Sinorhizobium.