scholarly journals Muller’s Ratchet in Asexual Populations Doomed to Extinction

2018 ◽  
Author(s):  
Logan Chipkin ◽  
Peter Olofsson ◽  
Ryan C. Daileda ◽  
Ricardo B. R. Azevedo
Keyword(s):  
Process Model ◽  
Population Decline ◽  
Extinction Time ◽  
Deleterious Mutations ◽  
Multitype Branching Process ◽  
Muller's Ratchet ◽  
Constant Size ◽  
Muller’S Ratchet ◽  
Asexual Populations ◽  
Mutational Meltdown

AbstractAsexual populations are expected to accumulate deleterious mutations through a process known as Muller’s Ratchet. Lynch, Gabriel, and colleagues have proposed that the Ratchet eventually results in a vicious cycle of mutation accumulation and population decline that drives populations to extinction. They called this phenomenon mutational meltdown. Here, we analyze the meltdown using a multitype branching process model where, in the presence of mutation, populations are doomed to extinction. We find that extinction occurs more quickly in small populations, experiencing a high deleterious mutation rate, and mutations with more severe deleterious effects. The effects of mutational parameters on extinction time in doomed populations differ from those on the severity of Muller’s Ratchet in populations of constant size. We also 1nd that mutational meltdown, although it does occur in our model, does not determine extinction time. Rather, extinction time is determined by the expected impact of deleterious mutations on fitness.

ADAPTIVE EVOLUTION OF ASEXUAL POPULATIONS UNDER MULLER'S RATCHET

Evolution ◽  
2004 ◽  
Vol 58 (7) ◽  
pp. 1403 ◽  
Author(s):  
Doris Bachtrog ◽  
Isabel Gordo
Keyword(s):  
Adaptive Evolution ◽  
Muller's Ratchet ◽  
Muller’S Ratchet ◽  
Asexual Populations

scholarly journals The advance of Muller's ratchet in a haploid asexual population: approximate solutions based on diffusion theory

1993 ◽  
Vol 61 (3) ◽  
pp. 225-231 ◽  
Author(s):  
Wolfgang Stephan ◽  
Lin Chao ◽  
Joanne Guna Smale
Keyword(s):  
Diffusion Theory ◽  
Large Population ◽  
Approximate Solutions ◽  
Diffusion Approximations ◽  
Asexual Population ◽  
Muller's Ratchet ◽  
Expected Time ◽  
Equilibrium Size ◽  
Muller’S Ratchet ◽  
Asexual Populations

SummaryAsexual populations experiencing random genetic drift can accumulate an increasing number of deleterious mutations, a process called Muller's ratchet. We present here diffusion approximations for the rate at which Muller's ratchet advances in asexual haploid populations. The most important parameter of this process is n0 = N e−U/s, where N is population size, U the genomic mutation rate and s the selection coefficient. In a very large population, n0 is the equilibrium size of the mutation-free class. We examined the case n0 > 1 and developed one approximation for intermediate values of N and s and one for large values of N and s. For intermediate values, the expected time at which the ratchet advances increases linearly with n0. For large values, the time increases in a more or less exponential fashion with n0. In addition to n0, s is also an important determinant of the speed of the ratchet. If N and s are intermediate and n0 is fixed, we find that increasing s accelerates the ratchet. In contrast, for a given n0, but large N and s, increasing s slows the ratchet. Except when s is small, results based on our approximations fit well those from computer simulations.

scholarly journals Mutational Meltdown in Primary Endosymbionts: Selection Limits Muller's Ratchet

PLoS ONE ◽  
2009 ◽  
Vol 4 (3) ◽  
pp. e4969 ◽  
Author(s):  
Julie M. Allen ◽  
Jessica E. Light ◽  
M. Alejandra Perotti ◽  
Henk R. Braig ◽  
David L. Reed
Keyword(s):  
Muller's Ratchet ◽  
Muller’S Ratchet ◽  
Mutational Meltdown

scholarly journals Defying Muller’s Ratchet: Ancient Heritable Endobacteria Escape Extinction through Retention of Recombination and Genome Plasticity

mBio ◽  
2016 ◽  
Vol 7 (3) ◽  
Author(s):  
Mizue Naito ◽  
Teresa E. Pawlowska
Keyword(s):  
Mycorrhizal Fungi ◽  
Arbuscular Mycorrhizal ◽  
Genome Plasticity ◽  
Escape Extinction ◽  
Muller's Ratchet ◽  
Evolutionary Forces ◽  
Slightly Deleterious Mutations ◽  
Muller’S Ratchet ◽  
Asexual Populations ◽  
Host Generation

ABSTRACT   Heritable endobacteria, which are transmitted from one host generation to the next, are subjected to evolutionary forces that are different from those experienced by free-living bacteria. In particular, they suffer consequences of Muller’s ratchet, a mechanism that leads to extinction of small asexual populations due to fixation of slightly deleterious mutations combined with the random loss of the most-fit genotypes, which cannot be recreated without recombination. Mycoplasma-related endobacteria (MRE) are heritable symbionts of fungi from two ancient lineages, Glomeromycota (arbuscular mycorrhizal fungi) and Mucoromycotina . Previous studies revealed that MRE maintain unusually diverse populations inside their hosts and may have been associated with fungi already in the early Paleozoic. Here we show that MRE are vulnerable to genomic degeneration and propose that they defy Muller’s ratchet thanks to retention of recombination and genome plasticity. We suggest that other endobacteria may be capable of raising similar defenses against Muller’s ratchet.

scholarly journals Genetic Hitchhiking and the Evolution of Reduced Genetic Activity of the Y Sex Chromosome

Genetics ◽  
1987 ◽  
Vol 116 (1) ◽  
pp. 161-167
Author(s):  
William R Rice
Keyword(s):  
Y Chromosome ◽  
Alternative Model ◽  
Sex Chromosome ◽  
Deleterious Mutations ◽  
Genetic Hitchhiking ◽  
New Model ◽  
Muller's Ratchet ◽  
Genetic Activity ◽  
Muller’S Ratchet ◽  
Selection For

ABSTRACT A new model for the evolution of reduced genetic activity of the Y sex chromosome is described. The model is based on the process of genetic hitchhiking. It is shown that the Y chromosome can gradually lose its genetic activity due to the fixation of deleterious mutations that are linked with other beneficial genes. Fixation of deleterious Y-linked mutations generates locus-specific selection for dosage tolerance and/or compensation. The hitchhiking effect is most pronounced when operating in combination with an alternative model, Muller's ratchet. It is shown, however, that the genetic hitchhiking mechanism can operate under conditions where Muller's ratchet is ineffective.

scholarly journals Integrating ecology and evolution to study hypothetical dynamics of algal blooms and Muller’s ratchet using Evolvix

2017 ◽  
Author(s):  
Sarah Northey ◽  
Courtney Hove ◽  
Justine Kao ◽  
Jon Ide ◽  
Janel McKinney ◽  
...  
Keyword(s):  
Dna Repair ◽  
Nutrient Availability ◽  
Continuous Time ◽  
Algal Blooms ◽  
Predation Pressure ◽  
Mutation Rates ◽  
Deleterious Mutations ◽  
Muller's Ratchet ◽  
Slightly Deleterious Mutations ◽  
Muller’S Ratchet

Algal blooms have been the subject of considerable research as they occur over various spatial and temporal scales and can produce toxins that disrupt their ecosystem. Algal blooms are often governed by nutrient availability however other limitations exist. Algae are primary producers and therefore subject to predation which can keep populations below levels supported by nutrient availability. If algae as prey mutate to gain the ability to produce toxins deterring predators, they may increase their survival rates and form blooms unless other factors counter their effective increase in growth rate. Where might such mutations come from? Clearly, large populations of algae will repeatedly experience mutations knocking-out DNA repair genes, increasing mutation rates, and with them the chance of acquiring de-novo mutations producing a toxin against predators. We investigate this hypothetical scenario by simulation in the Evolvix modeling language. We modeled a sequence of steps that in principle can allow a typical asexual algal population to escape predation pressure and form a bloom with the help of mutators. We then turn our attention to the unavoidable side effect of generally increased mutation rates, many slightly deleterious mutations. If these accumulate at sufficient speed, their combined impact on fitness might place upper limits on the duration of algal blooms. These steps are required: (1) Random mutations result in the loss of DNA repair mechanisms. (2) Increased mutation rates make it more likely to acquire the ability to produce toxins by altering metabolism. (3) Toxins deter predators providing algae with growth advantages that can mask linked slightly deleterious mutational effects. (4) Reduced predation pressure enables blooms if algae have sufficient nutrients. (5) Lack of recombination results in the accumulation of slightly deleterious mutations as predicted by Muller’s ratchet. (6) If fast enough, deleterious mutation accumulation eventually leads to mutational meltdown of toxic blooming algae. (7) Non-mutator algal populations are not affected due to ongoing predation pressure. Our simulation models integrate ecological continuous-time dynamics of predator-prey systems with the population genetics of a simplified Muller’s ratchet model using Evolvix. Evolvix maps these models to Continuous-Time Markov Chain models that can be simulated deterministically or stochastically depending on the question. The current model is incomplete; we plan to investigate many parameter combinations to produce a more robust model ensemble with stable links to reasonable parameter estimates. However, our model already has several intriguing features that may allow for the eventual development of observation methods for monitoring ecosystem health. Our work also highlights a growing need to simulate integrated models combining ecological processes, multi-level population dynamics, and evolutionary genetics in a single computational run.

scholarly journals Integrating ecology and evolution to study hypothetical dynamics of algal blooms and Muller’s ratchet using Evolvix

2017 ◽  
Author(s):  
Sarah Northey ◽  
Courtney Hove ◽  
Justine Kao ◽  
Jon Ide ◽  
Janel McKinney ◽  
...  
Keyword(s):  
Dna Repair ◽  
Nutrient Availability ◽  
Continuous Time ◽  
Algal Blooms ◽  
Predation Pressure ◽  
Mutation Rates ◽  
Deleterious Mutations ◽  
Muller's Ratchet ◽  
Slightly Deleterious Mutations ◽  
Muller’S Ratchet

Algal blooms have been the subject of considerable research as they occur over various spatial and temporal scales and can produce toxins that disrupt their ecosystem. Algal blooms are often governed by nutrient availability however other limitations exist. Algae are primary producers and therefore subject to predation which can keep populations below levels supported by nutrient availability. If algae as prey mutate to gain the ability to produce toxins deterring predators, they may increase their survival rates and form blooms unless other factors counter their effective increase in growth rate. Where might such mutations come from? Clearly, large populations of algae will repeatedly experience mutations knocking-out DNA repair genes, increasing mutation rates, and with them the chance of acquiring de-novo mutations producing a toxin against predators. We investigate this hypothetical scenario by simulation in the Evolvix modeling language. We modeled a sequence of steps that in principle can allow a typical asexual algal population to escape predation pressure and form a bloom with the help of mutators. We then turn our attention to the unavoidable side effect of generally increased mutation rates, many slightly deleterious mutations. If these accumulate at sufficient speed, their combined impact on fitness might place upper limits on the duration of algal blooms. These steps are required: (1) Random mutations result in the loss of DNA repair mechanisms. (2) Increased mutation rates make it more likely to acquire the ability to produce toxins by altering metabolism. (3) Toxins deter predators providing algae with growth advantages that can mask linked slightly deleterious mutational effects. (4) Reduced predation pressure enables blooms if algae have sufficient nutrients. (5) Lack of recombination results in the accumulation of slightly deleterious mutations as predicted by Muller’s ratchet. (6) If fast enough, deleterious mutation accumulation eventually leads to mutational meltdown of toxic blooming algae. (7) Non-mutator algal populations are not affected due to ongoing predation pressure. Our simulation models integrate ecological continuous-time dynamics of predator-prey systems with the population genetics of a simplified Muller’s ratchet model using Evolvix. Evolvix maps these models to Continuous-Time Markov Chain models that can be simulated deterministically or stochastically depending on the question. The current model is incomplete; we plan to investigate many parameter combinations to produce a more robust model ensemble with stable links to reasonable parameter estimates. However, our model already has several intriguing features that may allow for the eventual development of observation methods for monitoring ecosystem health. Our work also highlights a growing need to simulate integrated models combining ecological processes, multi-level population dynamics, and evolutionary genetics in a single computational run.

scholarly journals Does Muller's ratchet work with selfing?

1978 ◽  
Vol 32 (3) ◽  
pp. 289-293 ◽  
Author(s):  
R. Heller ◽  
J. Maynard Smith
Keyword(s):  
Large Class ◽  
Deleterious Mutations ◽  
Asexual Population ◽  
Muller's Ratchet ◽  
Slightly Deleterious Mutations ◽  
Muller’S Ratchet

SUMMARYThe accumulation of deleterious mutations in a finite diploid selfing population is investigated. It is shown that the conditions for accumulation are very similar to those for the accumulation of mutations in an asexual population by ‘Muller's ratchet’. The ratchet is likely to operate in both types of population if there is a large class of slightly deleterious mutations.

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