Rapid fitness losses in mammalian RNA virus clones due to Muller's ratchet.

Abstract

Muller's ratchet is an important concept in population genetics. It predicts that when mutation rates are high and a significant proportion of mutations are deleterious, a kind of irreversible ratchet mechanism will gradually decrease the mean fitness of small populations of asexual organisms. In contrast, sexual recombination may stop or reverse this mutational ratchet by recombinational repair Muller (1) suggested that accumulation of deleterious mutations in asexual (2) organisms occurs by "a kind ofirreversible ratchet mechanism;" in contrast, sexual reproduction allows repair of genetic damage by recombination (2-4). Bell (5) used senescence in protozoa to provide experimental support for Muller's ratchet hypothesis, but some models of Muller's ratchet have been questioned recently (6). Chao (7) recently reported that 40 consecutive plaque-to-plaque (genetic bottleneck) transfers of the tripartite RNA bacteriophage 46 led to a significant decrease in mean fitness. Such clone-to-clone transfers of a virus repeatedly reduce virus population size to one infectious particle and should increase the probability for Muller's ratchet to become manifest if mutation rates are high and sexual recombination is absent. Chao (7) tested fitness of his repeatedly bottlenecked 46 phage clones in paired-growth experiments in which each transferred clone was mixed at a known ratio with a genetically marked parental phage clone. These mixtures were then diluted and plated on lawns of bacterial host cells to form independent plaques during each growth competition transfer. This precluded genetic reassortment of the three genome segments of this tripartite bacteriophage while allowing internally controlled comparisons ofthe relative replication rates of wild-type phage versus repeatedly bottlenecked clones. We have now quantitated the relative fitness of repeatedly bottlenecked clones of vesicular stomatitis virus (VSV) a single-stranded RNA virus of animals, humans, and insect vectors. VSV is the prototype model for negative-stranded animal RNA viruses (which include a large number of medically important human pathogens). Because VSV has only a single, nonsegmented genome, which does not undergo recombination at a detectable level, we are able to quantitate relative fitness directly in mixed-infection competition experiments. We can allow the parental VSV clone and the genetically marked monoclonal antibody (mAb)-resistant (MAR) clones to replicate and compete directly in liquid medium cell cultures (and even in the same cells late during each passage). We previously described quantitative relative fitness assay methods for determining virus fitness during direct replicative competition experiments (8, 9), and these are employed here.RNA viruses generally should be ideal for testing Muller's ratchet theory because most or all of them have extremely high mutation frequencies. RNA viruses apparently lack the mechanisms of proofreading and mismatch repair that are available for high-fidelity DNA replication. Their replic...

An RNA virus can adapt to the multiplicity of infection.

“…It is too early to suggest whether effects of m.o.i., such as those we have described, might be common with other FMDV mutants or with other viruses. However, the results that we have obtained indicate that in competition assays it will be necessary to adjust the initial mixtures to a standard m.o.i., as was done in the fitness determinations with VSV (Holland et al, 1991 ;Duarte et al, 1992 ;Clarke et al, 1993Clarke et al, , 1994, although in this case this precaution was prompted by the need to avoid accumulation of defective-interfering particles. Some effects of frequency-dependent selection in VSV (Elena et al, 1997) could have been influenced by m.o.i.-dependence of relative fitness between two competing viruses.…”

“…Systematic studies on the dynamics of fitness variations with RNA viruses have established that repeated genetic bottlenecks (experimentally realized by serial plaque-to-plaque virus transfers) result in average fitness losses (Chao, 1990 ;Duarte et al, 1992 ;Escarmı! s et al, 1996).…”

Muller's ratchet decreases fitness of a DNA-based microbe.

“…Res. 1,[2][3][4][5][6][7][8][9]. The operation of Muller's ratchet on real populations has been experimentally demonstrated only in RNA viruses.…”

“…(ii) Chao (7) showed that the RNA virus 46, when propagated asexually under intense genetic drift, accumulated deleterious mutations resulting in loss of viability. (iii) Similarly, others (8,9) have shown that vesicular stomatitis virus (VSV) lost fitness when genetic bottlenecks were introduced during virus propagation. Chao et al (10) showed that recombination between mutant 46 lineages could restore viability almost to the wild-type level, demonstrating how sex can slow down the ratchet.…”

Diverse Dengue Type 2 Virus Populations Contain Recombinant and Both Parental Viruses in a Single Mosquito Host

“…This diversification may lead to fitness losses due to the buildup of deleterious mutations or to rapid and unpredictable fitness gains (7,11). Although this genetic variation is ultimately produced by mutation, it also could be shaped by natural selection, most likely from two sources.…”