Molecular evolution of Legionella pneumophila dotA gene, the contribution of natural environmental strains.

Given the role of DotA protein in establishing successful infections and the diversity of host cells interacting with Legionella pneumophila in nature, it is possible that this gene product is a target for adaptive evolution. We investigated the influence of L. pneumophila isolates from natural environments with the molecular evolution of this crucial virulence-related gene. The population genetic structure of L. pneumophila was inferred from the partial sequences of rpoB and dotA of 303 worldwide strains. The topology of the two inferred trees was not congruent and in the inferred dotA tree the vast majority of the natural environmental isolates were clustered in a discrete group. The Ka/Ks ratio demonstrated that this group, contrary to all others, has been under strong diversifying selection. The alignment of all DotA sequences allowed the identification of several alleles and the amino acid variations were not randomly distributed. Moreover, from these results we can conclude that dotA from L. pneumophila clinical and man-made environmental strains belong to a sub-set of all genotypes existing in nature. A split graph analysis showed evidence of a network-like organization and several intergenic recombination events were detected within L. pneumophila strains resulting in mosaic genes in which different gene segments exhibited different evolutionary histories. We have determined that the allelic diversity of dotA is predominantly found in L. pneumophila isolates from natural environments, suggesting that niche-specific selection pressures have been operating on this gene. Indeed, the high level of dotA allelic diversity may reflect fitness variation in the persistence of those strains in distinct environmental niches and/or tropism to various protozoan hosts.