Homologous recombination in Agrobacterium: potential implications for the genomic species concept in bacteria.

According to current taxonomical rules, a bona fide bacterial species is a genomic species characterized by the genomic similarity of its members. It has been proposed that the genomic cohesion of such clusters may be related to sexual isolation, which limits gene flow between too divergent bacteria. Homologous recombination is one of the most studied mechanisms responsible for this genetic isolation. Previous studies on several bacterial models showed that recombination frequencies decreased exponentially with increasing DNA sequence divergence. In the present study, we investigated this relationship in the Agrobacterium tumefaciens species complex, which allowed us to focus on sequence divergence in the vicinity of the genetic boundaries of genomic species. We observed that the sensitivity of the recombination frequency to DNA divergence fitted a log-linear function until approximately 10% sequence divergence. The results clearly revealed that there was no sharp drop in recombination frequencies at the point where the sequence divergence distribution showed a "gap" delineating genomic species. The ratio of the recombination frequency in homogamic conditions relative to this frequency in heterogamic conditions, that is, sexual isolation, was found to decrease from 8 between the most distant strains within a species to 9 between the most closely related species, for respective increases from 4.3% to 6.4% mismatches in the marker gene chvA. This means that there was only a 1.13-fold decrease in recombination frequencies for recombination events at both edges of the species border. Hence, from the findings of this investigation, we conclude that--at least in this taxon--sexual isolation based on homologous recombination is likely not high enough to strongly hamper gene flow between species as compared with gene flow between distantly related members of the same species. The 70% relative binding ratio cutoff used to define bacterial species is likely correlated to only minor declines in homologous recombination frequencies. Consequently, the sequence diversity, as a mechanistic factor for the efficiency of recombination (as assayed in the laboratory), appears to play little role in the genetic cohesion of bacterial species, and thus, the genomic species definition for prokaryotes is definitively not reconcilable with the biological species concept for eukaryotes.