Ecological genetics of Rhizobium meliloti: Diversity and competitive dominance

Symbiotic effectiveness (nitrogen-fixation ability) is not a measure of inter-strain competitiveness, and Rhizobium strains used as inocula frequently compete poorly with indigenous rhizobia for nodulation of the host legume. Competition between rhizobia delimits the use of Rhizobium inoculum in agriculture. We therefore chose to investigate aspects of the gene pool represented by an indigenous population of R. meliloti selected for maximum diversity, particularly for evidence of competitive dominance. This unadapted population was very heterogeneous in terms of plasmid content, somatic antigens and intrinsic antibiotic resistance (IAR). Little tendency towards competitive dominance (measured in terms of nodule occupancy) was observed. Classical methods (serotype, IAR) of characterising strains did not correlate to define dominance of a strain or a group of strains. The data are consistent with a continuum of symbiotically proficient strains under conditions of maximum diversity.