Accumulation of Deleterious Mutations in Endosymbionts: Muller's Ratchet with Two Levels of Selection

2000 ◽  
Vol 156 (4) ◽  
pp. 425 ◽  
Author(s):  
Rispe ◽  
Moran
Keyword(s):  
Levels Of Selection ◽  
Deleterious Mutations ◽  
Muller's Ratchet ◽  
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 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.

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.

scholarly journals Germline Bottlenecks and the Evolutionary Maintenance of Mitochondrial Genomes

Genetics ◽  
1998 ◽  
Vol 149 (4) ◽  
pp. 2135-2146 ◽  
Author(s):  
Carl T Bergstrom ◽  
Jonathan Pritchard
Keyword(s):  
Equilibrium Distribution ◽  
Genetic Model ◽  
Mutation Rates ◽  
Mitochondrial Genomes ◽  
Uniparental Inheritance ◽  
Small Populations ◽  
Deleterious Mutations ◽  
Muller's Ratchet ◽  
Large Populations ◽  
Muller’S Ratchet

Abstract Several features of the biology of mitochondria suggest that mitochondria might be susceptible to Muller's ratchet and other forms of evolutionary degradation: Mitochondria have predominantly uniparental inheritance, appear to be nonrecombining, and have high mutation rates producing significant deleterious variation. We demonstrate that the persistence of mitochondria may be explained by recent data that point to a severe “bottleneck” in the number of mitochondria passing through the germline in humans and other mammals. We present a population-genetic model in which deleterious mutations arise within individual mitochondria, while selection operates on assemblages of mitochondria at the level of their eukaryotic hosts. We show that a bottleneck increases the efficacy of selection against deleterious mutations by increasing the variance in fitness among eukaryotic hosts. We investigate both the equilibrium distribution of deleterious variation in large populations and the dynamics of Muller's ratchet in small populations. We find that in the absence of the ratchet, a bottleneck leads to improved mitochondrial performance and that, over a longer time scale, a bottleneck acts to slow the progression of the ratchet.

scholarly journals The spatial Muller’s ratchet: surfing of deleterious mutations during range expansion

2019 ◽  
Author(s):  
Félix Foutel-Rodier ◽  
Alison Etheridge
Keyword(s):  
Population Size ◽  
Range Expansion ◽  
Allee Effect ◽  
Large Population ◽  
Small Population ◽  
Deleterious Mutations ◽  
Muller's Ratchet ◽  
Muller’S Ratchet ◽  
Expansion Load ◽  
The Impact

AbstractDuring a range expansion, deleterious mutations can “surf” on the colonisation front. The resultant decrease in fitness is known as expansion load. An Allee effect is known to reduce the loss of genetic diversity of expanding populations, by changing the nature of the expansion from “pulled” to “pushed”. We study the impact of an Allee effect on the formation of an expansion load with a new model, in which individuals have the genetic structure of a Muller’s ratchet. A key feature of Muller’s ratchet is that the population fatally accumulates deleterious mutations due to the stochastic loss of the fittest individuals, an event called a click of the ratchet. We observe fast clicks of the ratchet at the colonization front owing to small population size, followed by a slow fitness recovery due to migration of fit individuals from the bulk of the population, leading to a transient expansion load. For large population size, we are able to derive quantitative features of the expansion wave, such as the wave speed and the frequency of individuals carrying a given number of mutations. Using simulations, we show that the presence of an Allee effect reduces the rate at which clicks occur at the front, and thus reduces the expansion load.

scholarly journals Genome expansion in early eukaryotes drove the transition from lateral gene transfer to meiotic sex

2020 ◽  
Author(s):  
Marco Colnaghi ◽  
Nick Lane ◽  
Andrew Pomiankowski
Keyword(s):  
Gene Transfer ◽  
Genome Size ◽  
Lateral Gene Transfer ◽  
Genetic Material ◽  
Theoretical Models ◽  
Deleterious Mutations ◽  
Strong Constraint ◽  
Muller's Ratchet ◽  
Muller’S Ratchet ◽  
Recombination Length

ABSTRACTProkaryotes generally reproduce clonally but can also acquire new genetic material via lateral gene transfer (LGT). Like sex, LGT can prevent the accumulation of deleterious mutations predicted by Muller’s ratchet for asexual populations. This similarity between sex and LGT raises the question why did eukaryotes abandon LGT in favor of sexual reproduction? Understanding the limitations of LGT provides insight into this evolutionary transition. We model the evolution of a haploid population undergoing LGT at a rate λ and subjected to a mutation rate μ. We take into account recombination length, L, and genome size, g, neglected by previous theoretical models. We confirm that LGT counters Muller’s ratchet by reducing the rate of fixation of deleterious mutations in small genomes. We then demonstrate that this beneficial effect declines rapidly with genome size. Populations with larger genomes are subjected to a faster rate of fixation of deleterious mutations and become more vulnerable to stochastic frequency fluctuations. Muller’s ratchet therefore generates a strong constraint on genome size. Importantly, we show that the degeneration of larger genomes can be resisted by increases in the recombination length, the average number of contiguous genes drawn from the environment for LGT. Large increases in genome size, as in early eukaryotes, are only possible as L reaches the same order of magnitude as g. This requirement for recombination across the whole genome can explain the strong selective pressure towards the evolution of sexual cell fusion and reciprocal recombination during early eukaryotic evolution – the origin of meiotic sex.

scholarly journals The speed of Muller's ratchet with background selection, and the degeneration of Y chromosomes

2001 ◽  
Vol 78 (2) ◽  
pp. 149-161 ◽  
Author(s):  
ISABEL GORDO ◽  
BRIAN CHARLESWORTH
Keyword(s):  
Deleterious Mutation ◽  
Selection Effect ◽  
Background Selection ◽  
Deleterious Mutations ◽  
Effective Population ◽  
Muller's Ratchet ◽  
Analytical Approximations ◽  
Y Chromosomes ◽  
Muller’S Ratchet ◽  
Rate Of Accumulation

The rate of accumulation of deleterious mutations by Muller's ratchet is investigated in large asexual haploid populations, for a range of parameters with potential biological relevance. The rate of this process is studied by considering a very simple model in which mutations can have two types of effect: either strongly deleterious or mildly deleterious. It is shown that the rate of accumulation of mildly deleterious mutations can be greatly increased by the presence of strongly deleterious mutations, and that this can be predicted from the associated reduction in effective population size (the background selection effect). We also examine the rate of the ratchet when there are two classes of mutation of similar but unequal effects on fitness. The accuracy of analytical approximations for the rate of this process is analysed. Its possible role in causing the degeneration of Y and neo-Y chromosomes is discussed in the light of our present knowledge of deleterious mutation rates and selection coefficients.

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