Horizontal gene transfer and the role of restriction-modification systems in bacterial population dynamics

Abstract

Horizontal gene transfer (HGT) mediates non-vertical exchange of genetic elements thereby obfuscating the phylogenetic signal associated with vertically inherited mutations. Bacterial species exposed to significant HGT deviate from the clonal paradigm of a-sexual reproduction, towards pan-mictic admixtures. Intermediate population structures were also observed in which, despite high HGT rates, well-defined lineages coexist with a pan-mictic background. Different “forces” have been proposed to account for the containment of the HGT pan-mixing effect, including selection, fitness-related expansions and micro-epidemic evolution. Restriction-modification systems (RMSs) modulate the length of horizontally transferred DNA by selective cleavage of genetic material with heterologous methylation patterns. In a pan-genomic analysis of the Neisseria meningitidis bacterial species, sets of RMSs associated to specific lineages were shown to generate a differential barrier to DNA exchange, consistent with the inferred phylogeny.These data suggest that HGT, instead of being a “force” opposed to the emergence, persistence and global dissemination of consistent lineages, when modulated by RMSs can be the very cause of the intermediate population structures observed for the majority of pathogenic bacteria.