Defining the Rhizobium leguminosarum Species Complex

Area of Science:

• Microbiology
• Bacterial Taxonomy
• Genomics

Background:

• The current classification of Rhizobium leguminosarum is too broad, encompassing significant genetic diversity.
• This diversity necessitates a re-evaluation of its species complex status.

Purpose of the Study:

• To analyze the genetic diversity within the Rhizobium leguminosarum species complex (Rlc).
• To delineate distinct genospecies within the Rlc using genomic data.
• To assess the reliability of traditional taxonomic markers versus whole-genome analysis.

Main Methods:

• Phylogenetic analysis using 120 universal (core) genes.
• Calculation of pairwise average nucleotide identity (ANI) for 429 publicly available Rlc genomes.
• Comparison of genomic data with 16S rRNA and housekeeping gene sequences.

Main Results:

• The Rlc comprises 18 distinct genospecies and 7 unique strains, well-separated from other Rhizobium species.
• Genospecies are demarcated by a clear gap in ANI values, typically around 96% ANI.
• Five established species, including R. leguminosarum, are confirmed within these genospecies.
• 16S rRNA diversity is high but insufficient for genospecies delineation; housekeeping genes show limitations in phylogenetic accuracy.

Conclusions:

• The Rhizobium leguminosarum species complex is a collection of distinct genospecies, requiring taxonomic revision.
• Whole-genome sequencing provides a robust method for bacterial diversity characterization.
• Taxonomic decisions for bacteria should leverage comprehensive genomic data, not solely type strains.