title: Nondegenerative Evolution in Ancient Heritable Bacterial Endosymbionts of Fungi.
abstract: Bacterial endosymbionts are critical to the existence of many eukaryotes. Among them, vertically transmitted endobacteria are uniquely typified by reduced genomes and molecular evolution rate acceleration relative to free-living taxa. These patterns are attributable to genetic drift-dominated degenerative processes associated with reproductive dependence on the host. The degenerative evolution scenario is well supported in endobacteria with strict vertical transmission, such as essential mutualists of insects. In contrast, heritable endosymbionts that are nonessential to their hosts and engage occasionally in horizontal transmission are expected to display deviations from the degenerative evolution model. To explore evolution patterns in such nonessential endobacteria, we focused on Candidatus Glomeribacter gigasporarum ancient heritable mutualists of fungi. Using a collection of genomes, we estimated in Glomeribacter mutation rate at 2.03 × 10(-9) substitutions per site per year and effective population size at 1.44 × 10(8) Both fall within the range of values observed in free-living bacteria. To assess the ability of Glomeribacter to purge slightly deleterious mutations, we examined genome-wide dN/dS values and distribution patterns. We found that these dN/dS profiles cluster Glomeribacter with free-living bacteria as well as with other nonessential endosymbionts, while distinguishing it from essential heritable mutualists of insects. Finally, our evolutionary simulations revealed that the molecular evolution rate acceleration in Glomeribacter is caused by limited recombination in a largely clonal population rather than by increased fixation of slightly deleterious mutations. Based on these patterns, we propose that genome evolution in Glomeribacter is nondegenerative and exemplifies a departure from the model of degenerative evolution in heritable endosymbionts.
authors.affiliation: School of Integrative Plant Science, Plant Pathology & Plant-Microbe Biology, Cornell University.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Mondo
authors.affiliation: Department of Life Sciences & Systems Biology, University of Torino, Torino, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Salvioli
authors.affiliation: Department of Life Sciences & Systems Biology, University of Torino, Torino, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Bonfante
authors.affiliation: Division of Plant & Soil Sciences, West Virginia University.
authors.forename: Joseph B
authors.initials: JB
authors.lastname: Morton
authors.affiliation: School of Integrative Plant Science, Plant Pathology & Plant-Microbe Biology, Cornell University tep8@cornell.edu.
authors.forename: Teresa E
authors.initials: TE
authors.lastname: Pawlowska
mesh_terms: D001419:Bacteria / Q000235:genetics* / Q000378:metabolism; D005075:Biological Evolution; D019143:Evolution, Molecular; D040961:Genetic Drift; D016678:Genome; D055137:Glomeromycota / Q000235:genetics* / Q000378:metabolism; D059645:Mutation Rate; D010802:Phylogeny; D012641:Selection, Genetic; D013559:Symbiosis
keywords: Glomeribacter gigasporarum; diversifying selection; effective population size; evolution rate; genetic drift; mutation rate; purifying selection; vertical transmission.