R-048. Estimation of Bounds of Gene Flow by Homologous Recombination among Coastal Vibrio Isolates

D. Gevers, S. P. Preheim, Y. Boucher, M. F. Polz;
Massachusetts Inst. of Technology, Cambridge, MA.

The frequency with which bacterial genomes can incorporate divergent sequences is an important element of genome and ultimately population differentiation. In particular, laboratory experiments have suggested that decreasing frequency of homologous recombination with increasing genetic distance establishes effective boundaries to gene flow akin to eukaryotic species. However, the constraints on homologous recombination within natural populations, where there are confounding factors like ecological structure, have yet to be described. We analyzed the phylogenetic structure of coastal vibrios isolated from bacterioplankton. Multi-locus sequencing of hundreds of isolates revealed that named species are differentiated into phylogenetic clusters; however, some species were further differentiated into subclusters. To explore to what extent these subclusters are evolutionarily stable units, it is necessary to estimate frequency of recombination within and between clusters. The results show that different genes have different frequencies and that none of the clusters are genetically isolated. Nonetheless, we observe that the rate of homologous recombination is higher among closely related strains, suggesting an increasing barrier to gene flow as clusters diverge. These results are discussed in light of current theories of genetic differentiation and speciation among prokaryotes.