Why do bacterial plasmids carry some genes and not others?
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Updated: Aug 11, 2026

High-Resolution Comparison of Bacterial Conjugation Frequencies
Published on: January 10, 2019
1Smithsonian Tropical Research Institute, Costa Rica.
Abstract:
Previous explanations of why bacterial genes for certain "optional" traits tend to occur on plasmids rather than chromosomes are based on an outdated misunderstanding of natural selection. They also fail to explain why certain characters that are ubiquitous in some bacterial species tend to occur on plasmids. This paper shows that all major classes of traits usually associated with plasmids rather than chromosomes confer adaptations to locally restricted conditions. A new "local adaptation" model of plasmid evolution, based on simultaneous application of modern selection theory at the levels of gene, plasmid, cell, and clone reproduction, shows that genes coding local adaptations will reproduce more successfully when on plasmids than when on chromosomes, due to plasmids' greater horizontal mobility.
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