title: GC-Content evolution in bacterial genomes: the biased gene conversion hypothesis expands.
abstract: The characterization of functional elements in genomes relies on the identification of the footprints of natural selection. In this quest, taking into account neutral evolutionary processes such as mutation and genetic drift is crucial because these forces can generate patterns that may obscure or mimic signatures of selection. In mammals, and probably in many eukaryotes, another such confounding factor called GC-Biased Gene Conversion (gBGC) has been documented. This mechanism generates patterns identical to what is expected under selection for higher GC-content, specifically in highly recombining genomic regions. Recent results have suggested that a mysterious selective force favouring higher GC-content exists in Bacteria but the possibility that it could be gBGC has been excluded. Here, we show that gBGC is probably at work in most if not all bacterial species. First we find a consistent positive relationship between the GC-content of a gene and evidence of intra-genic recombination throughout a broad spectrum of bacterial clades. Second, we show that the evolutionary force responsible for this pattern is acting independently from selection on codon usage, and could potentially interfere with selection in favor of optimal AU-ending codons. A comparison with data from human populations shows that the intensity of gBGC in Bacteria is comparable to what has been reported in mammals. We propose that gBGC is not restricted to sexual Eukaryotes but also widespread among Bacteria and could therefore be an ancestral feature of cellular organisms. We argue that if gBGC occurs in bacteria, it can account for previously unexplained observations, such as the apparent non-equilibrium of base substitution patterns and the heterogeneity of gene composition within bacterial genomes. Because gBGC produces patterns similar to positive selection, it is essential to take this process into account when studying the evolutionary forces at work in bacterial genomes.
authors: Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France; CNRS, UMR 5558, Laboratoire de Biométrie et Biologie Evolutive, Villeurbanne, France; CNRS, UMR 5557, Ecologie Microbienne, Villeurbanne, France; INRA, USC 1364, Ecologie Microbienne, Villeurbanne, France; Ecole Normale Supérieure de Lyon, Lyon, France.
authors: Florent
authors: F
authors: Lassalle
authors: Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France; CNRS, UMR 5558, Laboratoire de Biométrie et Biologie Evolutive, Villeurbanne, France.
authors: Séverine
authors: S
authors: Périan
authors: Aarhus University, Bioinformatics Research Center, Århus Denmark1 Université de Lyon, Lyon, France.
authors: Thomas
authors: T
authors: Bataillon
authors: Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France; CNRS, UMR 5557, Ecologie Microbienne, Villeurbanne, France; INRA, USC 1364, Ecologie Microbienne, Villeurbanne, France.
authors: Xavier
authors: X
authors: Nesme
authors: Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France; CNRS, UMR 5558, Laboratoire de Biométrie et Biologie Evolutive, Villeurbanne, France.
authors: Laurent
authors: L
authors: Duret
authors: Université de Lyon, Lyon, France; Université Lyon 1, Villeurbanne, France; CNRS, UMR 5558, Laboratoire de Biométrie et Biologie Evolutive, Villeurbanne, France.
authors: Vincent
authors: V
authors: Daubin
mesh_terms: D001482:Base Composition / Q000235:genetics*; D030541:Databases, Genetic; D019143:Evolution, Molecular; D005785:Gene Conversion / Q000235:genetics*; D016680:Genome, Bacterial; D006801:Humans; D011994:Recombinant Proteins / Q000235:genetics; D012641:Selection, Genetic / Q000235:genetics*
chemical_list: D011994:Recombinant Proteins