Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins (2000) / Chapter Skim
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5 The Evolution of RNA Viruses: A Population Genetics View
5 The Evolution of RNA Viruses: A Population Genetics View
Pages 61-82
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From page 61... ...
It does not consider or make simplistic assumptions on the transformation laws within and between genotypic and phenotypic spaces. However, such laws are minimized in the case of RNA viruses because the phenotypic space maps onto the genotypic space in a much more linear way than on higher DNAbased organisms.
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From page 62... ...
RNA VIRUSES MEET THE POPULATION GENETICS THEORY OF EVOLUTION: THEORETICAL BACKGROUND According to population genetics, evolution is the change in the genetic properties of populations. Changes considered to be evolutionarily
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From page 63... ...
The closest environmental component is formed by cytoplasmic components of the infected cell, but intercellular spaces within tissues, tissues within individual hosts, and the ecological environment where host species are living are other components that modulate the adaptive response of RNA viruses. The theoretical framework to understand the dynamics of haploid organisms can be found in a series of classic papers that appeared many years ago (Moran, 1957; Robson, 1957; Muller, 1964; Drobn~k and Dlouha, 1966; Felsenstein, 1971; Karlin and McGregor, 1971; Cook and Nassar, 1972; Gillespie, 1973; Gladstein, 1973; Trajstman, 1973; Cannings, 1974, 1975; Haigh, 1978; Emigh, 1979a, b; Strobeck, 1979~.
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From page 64... ...
, constitute the main body of population genetics to account for the dynamics of RNA viral populations. EXPERIMENTAL VIRUS MODEL AND FITNESS ASSAYS Vesicular stomatitis virus (VSV)
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From page 65... ...
Chao (1990) provided the first experimental evidence for the action of Muller's ratchet in RNA viruses.
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From page 66... ...
TABLE 1. Percentage of fitness decline, with respect to the corresponding initial viral clone, in experiments with different RNA viruses subjected to a different number and continuous bottleneck transfers No.
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From page 68... ...
The effect of deleterious mutations on long-term survival of viral population should be greater if we consider large numbers of mutations of small average effect compared with few changes of large effect because, in the latter case, selection should be more efficient in purifying deleterious load. The population size necessary to stop Muller's ratchet not only depended on the demographic experimental regimes of the viral populations, but also on the genetic composition and the fitness of the initial populations.
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From page 69... ...
ADAPTATION: COMPETITION IN CONSTANT, CHANGING, AND SUBDIVIDED ENVIRONMENTS Until now, we have shown experimental evidence for the accumulation of many deleterious mutations during the evolution of RNA viral populations, especially those subjected to strong bottlenecking, a com
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From page 70... ...
But from time to time, the appearance of mutations with beneficial effect on fitness cannot be ruled out. This, and the next two sections, will provide experimental evidence for the presence of such mutations and its effect on the long-term evolution of RNA viruses.
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From page 71... ...
It catches some relevant TABLE 5. Estimated parameters for the hyperbolic model of fitness, with standard errors, in four independent experiments of fitness recovery of MARM F clone during 40 large and consecutive population expansions Replicate In WO a b R2 W I -9.3 + 0.7 9.3 + 0.8 0.4 + 0.2 0.95 1.1 + 0.0 II -7.9 + 0.6 7.9 + 0.7 0.3 + 0.1 0.95 1.2 + 0.1 III -9.0 + 0.6 9.3 + 0.8 0.8 + 0.3 0.99 1.2 + 0.2 IV -10.0 + 0.3 10.2 + 0.4 1.2 + 0.2 1.00 1.0 + 0.1 Last column corresponds to fitness estimates at passage 40.
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From page 72... ...
The model has two main properties: mean fitness evolves rapidly during the first passages and asymptotically approaches a maximum value. The reason for the deceleration in the rate of adaptation has to do with the availability of beneficial mutations and the magnitude associated with each available mutation.
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From page 73... ...
CLONAL INTERFERENCE IMPOSES A LIMIT ON THE RATE OF VIRUS ADAPTATION Viral populations adapt through the appearance and fixation of beneficial mutations. In large asexual populations such as of VSV, where mutation rate and population size are the key state variables, beneficial mutations may arise frequently enough that two or more are coexisting.
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From page 74... ...
(A) Influence of effective population size on the fitness effect of beneficial mutations that are fixed.
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From page 75... ...
. However, the low numbers of mutations detected in this study forced us to consider two key points related with the estimation of genetic variability of RNA viral populations and its relationship with fitness change.
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From page 77... ...
It is expected that the fitness recovery of an expanding population of a highly debilitated clone should be characterized at the molecular level by repeated nonsynonymous substitution fixation, a pattern that has not been observed in the regions studied. If we consider that the estimated number of beneficial mutations in VSV expanding populations is low, then it might be possible that the two regions selected have not been the appropriate ones.
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From page 78... ...
formally contemplates the action of random drift and eventual fixation of some neutral mutants. Most of the evidence giving support to the quasispecies theory comes from RNA viruses because they show high mutation rates and reach very high population numbers in a short time.
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From page 79... ...
In summary, objections and explanatory power of both theories can be exchanged. Once stated the formal equivalence between quasispecies theory and group selection models, the major problem to be solved is to gain experimental evidence in favor of a supra individual unit of selection.
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From page 80... ...
(1993) Genetic bottlenecks and population passages cause profound fitness differences in RNA viruses.
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From page 81... ...
(1991) Quantitation of relative fitness and great adaptability of clonal populations of RNA viruses.
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From page 82... ...
(1995) Extreme fitness differences in mammalian and insect hosts after continuous replication of vesicular stomatitis virus in sanfly cells.
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