Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins (2000) / Chapter Skim
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7 Bacteria are Different: Observations, Interpretations, Speculations, and Opinions About the Mechanisms of Adaptive Evolution in Prokaryotes
7 Bacteria are Different: Observations, Interpretations, Speculations, and Opinions About the Mechanisms of Adaptive Evolution in Prokaryotes
Pages 99-114
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From page 99... ...
the fundamental role of the horizontal transmission of genes and accessory genetic elements as sources of variation in bacteria. We conclude with speculations about the evolution of accessory elements and their role in the adaptive evolution of bacteria.
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Moreover, a substantial amount of the variation in bacteria is not in their chromosomal genes. Bacteria commonly carry arrays of active and retired accessory genetic elements (plasmids, prophages, transposons, and integrons)
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Bacteria also commonly carry integrons, elements that acquire, accumulate, and control the expression of genes acquired from external sources (Hall, 1997, 1998; Mazel et al., 1998; Row-Magnus and Mazel, 1999~. All of these islands and accessory genetic elements and even some seemingly ordinary chromosomal genes, like those for the resistant forms of the penicillin-binding proteins of Streptococcus pneumonias (Dowson et al., 1989)
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From page 102... ...
On the other hand, at least quantitatively and possibly qualitatively, the chromosomal population genetics of adaptive evolution in bacteria are different from those of sexually reproducing eukaryotes. Adaptive Evolution When There Is Little or No Chromosomal Gene Recombination The primary differences between the chromosomal population genetics of bacteria and those of sexually reproducing eukaryotes arise as a consequence of the low rates of chromosomal gene recombination in bacterial populations.
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From page 103... ...
Sequential evolution Within each bacterial lineage, adaptive evolution will proceed by the sequential accumulation of favorable mutations, rather than by recombinational generation of gene combinations; in this respect, bacterial evolution will be similar to that depicted in the top portion of Muller's famous diagram of evolution in asexual and sexual populations (Crow and Kimura, 1965~. The evolution of a better genotype ABC from its less fit ancestor abc will proceed in stages, one gene at a time, with the order and rate of evolution depending primarily on the fitness of the intermediates (aBc, aBC, etc.~.
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Moreover, if the compensatory mutations become fixed, adaptive valleys may be established and effectively preclude the ascent of even more fit mutations at the deleterious locus itself (Schrag et al., 1997~. Low effective population size Although the total sizes of bacterial populations may be enormous, their genetically effective sizes can be quite low as a consequence of periodic selection, i.e., selective sweeps of better-adapted mutants (Atwood et al., 1951; Koch, 1974; Levin, 1981)
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From page 105... ...
exchange genes, and the intensity of selection for those genes. Adaptive Evolution by the Transmission of Accessory Genetic Elements From one perspective, the accessory genetic elements of bacteria are parasites and symbionts, and their population and evolutionary biology can be and has been treated in that context with little or no reference to their role as sources of variation for their host bacteria.
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From page 106... ...
But can this "niceness" account for the maintenance of accessory genetic elements? If accessory element-borne genes provide a selective advantage to bacteria and the accessory elements themselves are either costly or unstable, why are those genes not sequestered by the host chromosome?
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On the other side, using primarily simulation methods, we demonstrated two seemingly realistic situations under which those genes can be maintained for extended periods on infectiously transmitted accessory elements. The first of these is the continuous entry into that population of lineages that are more fit than existing ones.
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Evolving to Evolve: The Evolution of Infectious Gene Transfer Thus far, we have considered the mechanisms that can maintain infectiously transmitted genetic elements over evolutionary time. But how did these mechanisms and the capacity for acquisition of genes from without evolve in the first place?
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This difference is primarily a consequence of the frequency of homologous gene recombination being low in bacteria and high in sexual eukaryotes and of the phylogenetic range of gene exchange being broad in bacteria and narrow in contemporary eukaryotes. Also contributing to this difference is the prominent role of viruses, plasmids, and other infectiously transmitted accessory genetic elements as bearers and vectors of genes responsible for adaptive evolution and their seemingly negligible role in this capacity in contemporary eukaryotes.
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From page 110... ...
and higher eukaryotes that rarely, if ever, engage in homologous gene recombination (Smith, 1992~. There may also be species of bacteria in which recombination occurs at high rates and species of contemporary eukaryotes in which infectiously transmitted viruses and other accessory elements play a prominent role as sources of variation for adaptive evolution.
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(1986) Niche expansion in bacteria: can infectious gene exchange affect the rate of evolution?
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(1993) The accessory genetic elements of bacteria: existence conditions and (coJevolution.
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(1997~. Adaptation to the fitness cost of antibiotic resistance in Escherichia coli.
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