The Impact of Recombination on dN/dS within Recently Emerged Bacterial Clones

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Abstract
The development of next-generation sequencing platforms is set to reveal an unprecedented level of detail on short-term molecular evolutionary processes in bacteria. Here we re-analyse genome-wide single nucleotide polymorphism (SNP) datasets for recently emerged clones of methicillin resistant Staphylococcus aureus (MRSA) and Clostridium difficile. We note a highly significant enrichment of synonymous SNPs in those genes which have been affected by recombination, i.e. those genes on mobile elements designated “non-core” (in the case of S. aureus), or those core genes which have been affected by homologous replacements (S. aureus and C. difficile). This observation suggests that the previously documented decrease in dN/dS over time in bacteria applies not only to genomes of differing levels of divergence overall, but also to horizontally acquired genes of differing levels of divergence within a single genome. We also consider the role of increased drift acting on recently emerged, highly specialised clones, and the impact of recombination on selection at linked sites. This work has implications for a wide range of genomic analyses. As bacteria diversify, many of the nucleotide changes that emerge will render the cell slightly less competitive, and these mutations will tend to be removed by natural selection. However, this purging process does not happen instantaneously, and this delay allows deleterious mutations to survive in the population long enough to be sampled. Genomes at the very initial stages of diversification therefore exhibit a relatively high proportion of slightly deleterious mutations and, as most of these are non-synonymous mutations, this is manifest as a high dN/dS ratio. However, the effective population size will also impact on this ratio, as will selection operating on neighbouring SNPs. Here we examine the distribution of synonymous and non-synonymous SNPs within recently emerged clones of two important nosocomial pathogens, methicillin resistant Staphylococcus aureus (MRSA) and Clostridium difficile. In both species, we note a much higher proportion of synonymous changes in those single nucleotide polymorphisms (SNPs) likely to have emerged through recombination compared to de novo mutations. We argue that this effect is explained by the very recent emergence of the mutational SNPs combined with a reduction in the efficiency of selection due to niche specialisation.