Abstract:The impact of epistasis on the evolution of multilocus traits depends on recombination. While sexually-reproducing eukaryotes recombine so frequently that epistasis between polymorphisms is not considered to play a large role in short-term adaptation, many bacteria also recombine, some to the degree that their populations are described as 'panmictic' or 'freely recombining'. However, whether this recombination is sufficient to limit the ability of selection to act on epistatic contributions to fitness is unknown. We quantify homologous recombination in five bacterial pathogens and use these parameter estimates in a multilocus model of bacterial evolution with additive and epistatic effects. We find that even for highly recombining species (e.g. Streptococcus pneumoniae or Helicobacter pylori), selection on weak interactions between distant mutations is nearly as efficient as for an asexual species, likely because homologous recombination typically transfers only short segments. However, for strong epistasis, bacterial recombination accelerates selection, with the dynamics dependent on the amount of recombination and the number of loci. Epistasis may thus play an important role in both the short-and long-term adaptive evolution of bacteria and, unlike in eukaryotes, is not limited to strong effect sizes, closely linked loci, or other conditions that limit the impact of recombination.Author Summary: Like sexual eukaryotes, many bacterial species recombine so extensively that mutations are only weakly correlated from being transferred to many genetic backgrounds. Recombination in these species breaks mutation combinations and may limit the ability of selection to act on epistatic interactions between mutations. However, even for bacteria that have been historically described as "fully sexual", we show selection may still act on very weak epistatic effects, such that epistasis may frequently comprise an important genetic component of adaptive phenotypes in this kingdom of life despite large variation in recombination rates among species. Since the rate of adaptation for epistatic multilocus traits depends on the number of loci and the recombination rate, bacteria may differ dramatically in their adaptive solutions to selective pressures.
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