Abstract
Bacterial genomes show substantial variations in size. The smallest bacterial genomes are those of endocellular symbionts of eukaryotic hosts, which have undergone massive genome reduction and show patterns that are consistent with the degenerative processes that are predicted to occur in species with small effective population sizes. However, similar genome reduction is found in some free-living marine cyanobacteria that are characterized by extremely large populations. In this Opinion article, we discuss the different hypotheses that have been proposed to account for this reductive genome evolution at both ends of the bacterial population size spectrum.
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Acknowledgements
B.B. thanks the French Research Ministry for Ph.D. funding. This research was funded by the Centre National de la Recherche Scientifique (interdisciplinary programmes PEPS and PEPII) and the French National Research Agency from grant ANR-10-BINF-01-01.
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Glossary
- Codon usage bias
-
The frequency of occurrence of synonymous codons in a coding sequence.
- Effective population size
-
(Ne). The size of an ideal population that has the same level of genetic diversity as a real population; it is used to mathematically model genetic drift.
- Endosymbiotic
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A term used to describe an organism that lives within another organism.
- Error threshold
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A process that limits the amount of information that a genome can store at a given mutation rate.
- Genetic drift
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A change of allele frequencies owing to effects of random sampling from parents to progeny in a finite population.
- Genome streamlining
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A selective process that drives genome reduction by the elimination of the genes that make the lowest contribution to fitness.
- Ka/Ks
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(Also known as dN/dS). The ratio between the rate of non-synonymous substitutions (that is, amino acid replacements) and the rate of synonymous substitutions (that is, changes among the synonymous codons of an amino acid).
- Muller's ratchet
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A process that affects small asexual populations, in which deleterious substitutions tend to accumulate over time.
- Mutators
-
Individuals that have a higher mutation rate than the species-average mutation rate.
- Polymorphism
-
A difference in DNA sequence among individuals of the same species.
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Batut, B., Knibbe, C., Marais, G. et al. Reductive genome evolution at both ends of the bacterial population size spectrum. Nat Rev Microbiol 12, 841–850 (2014). https://doi.org/10.1038/nrmicro3331
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DOI: https://doi.org/10.1038/nrmicro3331
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