A Mutational Hotspot and Strong Selection Contribute to the Order of Mutations Selected for during Escherichia coli Adaptation to the Gut.
The relative role of drift versus selection underlying the evolution of bacterial species within the gut microbiota remains poorly understood. The large sizes of bacterial populations in this environment suggest that even adaptive mutations with weak effects, thought to be the most frequently occurring, could substantially contribute to a rapid pace of evolutionary change in the gut. We followed the emergence of intra-species diversity in a commensal Escherichia coli strain that previously acquired an adaptive mutation with strong effect during one week of colonization of the mouse gut. Following this first step, which consisted of inactivating a metabolic operon, one third of the subsequent adaptive mutations were found to have a selective effect as high as the first. Nevertheless, the order of the adaptive steps was strongly affected by a mutational hotspot with an exceptionally high mutation rate of 10-5. The pattern of polymorphism emerging in the populations evolving within different hosts was characterized by periodic selection, which reduced diversity, but also frequency-dependent selection, actively maintaining genetic diversity. Furthermore, the continuous emergence of similar phenotypes due to distinct mutations, known as clonal interference, was pervasive. Evolutionary change within the gut is therefore highly repeatable within and across hosts, with adaptive mutations of selection coefficients as strong as 12% accumulating without strong constraints on genetic background. In vivo competitive assays showed that one of the second steps (focA) exhibited positive epistasis with the first, while another (dcuB) exhibited negative epistasis. The data shows that strong effect adaptive mutations continuously recur in gut commensal bacterial species.
Instituto Gulbenkian de Ciência, Oeiras, Portugal.
Marta
M
Lourenço
Instituto Gulbenkian de Ciência, Oeiras, Portugal.
Ricardo S
http://orcid.org/0000-0002-4834-6997
RS
Ramiro
Instituto Gulbenkian de Ciência, Oeiras, Portugal.
Daniela
D
Güleresi
Instituto Gulbenkian de Ciência, Oeiras, Portugal.
João
J
Barroso-Batista
Instituto Gulbenkian de Ciência, Oeiras, Portugal.
Karina B
http://orcid.org/0000-0001-5210-1434
KB
Xavier
Instituto Gulbenkian de Ciência, Oeiras, Portugal.
Isabel
I
Gordo
Instituto Gulbenkian de Ciência, Oeiras, Portugal.
Ana
http://orcid.org/0000-0002-7276-4195
A
Sousa
D000222:Adaptation, Physiological / Q000235:genetics*; D000483:Alleles; D000818:Animals; D029382:Dicarboxylic Acid Transporters / Q000235:genetics*; D004843:Epistasis, Genetic; D004926:Escherichia coli / Q000235:genetics / Q000472:pathogenicity; D029968:Escherichia coli Proteins / Q000235:genetics*; D019143:Evolution, Molecular; D000069196:Gastrointestinal Microbiome / Q000235:genetics; D014644:Genetic Variation; D026901:Membrane Transport Proteins / Q000235:genetics*; D051379:Mice; D009154:Mutation; D012641:Selection, Genetic
D029382:Dicarboxylic Acid Transporters; D029968:Escherichia coli Proteins; C088103:FocA protein, E coli; D026901:Membrane Transport Proteins; C086256:dcuB protein, E coli