scholarly journals MAINTENANCE OF GENETIC VARIABILITY UNDER THE JOINT EFFECT OF MUTATION, SELECTION AND RANDOM DRIFT

Genetics ◽  
1978 ◽  
Vol 90 (2) ◽  
pp. 349-382
Author(s):  
Wen-Hsiung Li
Keyword(s):  
Joint Effect ◽  
Allele Frequencies ◽  
Heterozygote Advantage ◽  
Test Statistic ◽  
Random Drift ◽  
Mutation Pressure ◽  
Rare Alleles ◽  
Overdominant Selection ◽  
The Mean ◽  
Neutral Mutations

ABSTRACT Formulae are developed for the distribution of allele frequencies (the frequency spectrum), the mean number of alleles in a sample, and the mean and variance of heterozygosity under mutation pressure and under either genic or recessive selection. Numerical computations are carried out by using these formulae and Watterson's (1977) formula for the distribution of allele frequencies under overdominant selection. The following properties are observed: (1) The effect of selection on the distribution of allele frequencies is slight when 4Ns ≤ 4, but becomes strong when 4Ns becomes larger than 10, where N denotes the effective size and s the selective difference between alleles. Genic selection and recessive selection tend to force the distribution to be U-shaped, whereas overdominant selection has the opposite tendency. (2) The mean total number of alleles in a sample is much more strongly affected by selection than the mean number of rare alleles in a sample. (3) Even slight heterozygote advantage, as small as 10-5, increases considerably the mean heterozygosity of a population, as compared to the case of neutral mutations. On the other hand, even slight genic or recessive selection causes a great reduction in heterozygosity when population size is large. (4) As a test statistic, the variance of heterozygosity can be used to detect the presence of selection, though it is not efficient when the selection intensity is very weak, say when 4Ns is around 4 or less. A model, which is somewhat similar to Ohta's (1976) model of slightly deleterious mutations, has been proposed to explain the following general patterns of genic variation: (i) There seems to be an upper limit for the observed average heterozygosities. (ii) The distribution of allele frequencies is U-shaped for every species surveyed. (iii) Most of the species surveyed tend to have an excess of rare alleles as compared with that expected under the neutral mutation hypothesis.

scholarly journals GENETIC VARIABILITY MAINTAINED BY MUTATION AND OVERDOMINANT SELECTION IN FINITE POPULATIONS

Genetics ◽  
1981 ◽  
Vol 98 (2) ◽  
pp. 441-459 ◽  
Author(s):  
Takeo Maruyama ◽  
Masatoshi Nei
Keyword(s):  
Mutation Rate ◽  
Allozyme Polymorphism ◽  
Random Genetic Drift ◽  
Effective Population ◽  
Mathematical Properties ◽  
Overdominant Selection ◽  
Mean And Variance ◽  
The Mean ◽  
Neutral Mutations ◽  
The Relationship

ABSTRACT Mathematical properties of the overdominance model with mutation and random genetic drift are studied by using the method of stochastic differential equations (Itô and McKean 1974). It is shown that overdominant selection is very powerful in increasing the mean heterozygosity as compared with neutral mutations, and if 2Ns (N = effective population size; s = selective disadvantage for homozygotes) is larger than 10, a very low mutation rate is sufficient to explain the observed level of allozyme polymorphism. The distribution of heterozygosity for overdominant genes is considerably different from that of neutral mutations, and if the ratio of selection coefficient (s) to mutation rate (ν) is large and the mean heterozygosity (h) is lower than 0.2, single-locus heterozygosity is either approximately 0 or 0.5. If h increases further, however, heterozygosity shows a multiple-peak distribution. Reflecting this type of distribution, the relationship between the mean and variance of heterozygosity is considerably different from that for neutral genes. When s/v is large, the proportion of polymorphic loci increases approximately linearly with mean heterozygosity. The distribution of allele frequencies is also drastically different from that of neutral genes, and generally shows a peak at the intermediate gene frequency. Implications of these results on the maintenance of allozyme polymorphism are discussed.

scholarly journals A two–stage model for Cepaea polymorphism

1998 ◽  
Vol 353 (1375) ◽  
pp. 1577-1593 ◽  
Author(s):  
L. M. Cook
Keyword(s):  
Heterozygote Advantage ◽  
Fluctuating Selection ◽  
Selective Predation ◽  
Random Drift ◽  
Two Stage ◽  
Mutation Pressure ◽  
Simple Simulation ◽  
Human Modification ◽  
History Of

The history of the study of snails in the genus Cepaea is briefly outlined. Cepaea nemoralis and C. hortensis are polymorphic for genetically controlled shell colour and banding, which has been the main interest of the work covered. Random drift, selective predation and climatic selection, both at a macro– and micro–scale, all affect gene frequency. The usual approach to understanding maintenance of the polymorphism, has been to look for centripetal effects on frequency. Possible processes include balance of mutation pressure and drift, heterozygote advantage, relational balance heterosis, frequency–dependent predation, multi–niche selective balance, or some combination of these. Mutational balance is overlaid by more substantial forces. There is some evidence for heterosis. Predation by birds may protect the polymorphism, and act apostatically to favour distinct morphs. Although not substantiated for Cepaea , many studies show that predators behave in the appropriate manner, while shell colour polymorphisms in molluscs occur most commonly in species exposed to visually searching predators. It is not known whether different thermal properties of the shells help to generate equilibria. Migration between colonies is probably greater than originally thought. The present geographical range has been occupied for less than 5000 generations. Climatic and human modification alter snail habitats relatively rapidly, which in turn changes selection pressures. A simple simulation shows that migration coupled with selection which fluctuates but is not centripetal, may retain polymorphism for sufficiently long to account for the patterns we see today. There may therefore be a two–stage basis to the polymorphism, comprising long–term but weak balancing forces coupled with fluctuating selection which does not necessarily balance but results in very slow elimination. Persistence of genetic variants in this way may provide the conditions for evolution of a balanced genome.

scholarly journals INTRACELLULAR POPULATION GENETICS: EVIDENCE FOR RANDOM DRIFT OF MITOCHONDRIAL ALLELE FREQUENCIES IN SACCHAROMYCES CEREVISIAE AND SCHIZOSACCHAROMYCES POMBE

Genetics ◽  
1980 ◽  
Vol 96 (1) ◽  
pp. 237-262
Author(s):  
Kathryn M Thrailkill ◽  
C William Birky ◽  
Gudrun Lückemann ◽  
Klaus Wolf
Keyword(s):  
Saccharomyces Cerevisiae ◽  
Cell Division ◽  
Schizosaccharomyces Pombe ◽  
Molecular Mechanisms ◽  
Recombination Frequency ◽  
Allele Frequencies ◽  
Random Drift ◽  
Frequency Distributions ◽  
The Mean ◽  
Diploid Cells

ABSTRACT We report evidence for random drift of mitochondrial allele frequencies in zygote clones of Saccharomyces cerevisiae and Schizosaccharomyces pombe. Monofactorial and bifactorial crosses were done, using strains resistant or sensitive to erythromycin (alleles ER, ES), oligomycin (OR, OS), or diuron (DR, DS). The frequencies of resistant and sensitive cells (and thus the frequencies of the resistant and sensitive alleles) were determined for each of a number of clones of diploid cells arising from individual zygotes. Allele frequencies were extremely variable among these zygote clones; some clones were "uniparental," with mitochondrial alleles from only one parent present. These observations suggest random drift of the allele frequencies in the population of mitochondrial genes within an individual zygote and its diploid progeny. Drift would cease when all the cells in a clone become homoplasmic, due to segregation of the mitochondrial genomes during vegetative cell divisions. To test this, we delayed cell division (and hence segregation) for varying times by starving zygotes in order to give drift more time to operate. As predicted, delaying cell division resulted in an increase in the variance of allele frequencies among the zygote clones and an increase in the proportion of uniparental zygote clones. The changes in form of the allele frequency distributions resembled those seen during random drift in finite Mendelian populations. In bifactorial crosses, genotypes as well as individual alleles were fixed or lost in some zygote clones. However, the mean recombination frequency for a large number of clones did not increase when cell division was delayed. Several possible molecular mechanisms for intracellular random drift are discussed.

scholarly journals Segregation and the Evolution of Sex Under Overdominant Selection

Genetics ◽  
2003 ◽  
Vol 164 (3) ◽  
pp. 1119-1128 ◽  
Author(s):  
Elie S Dolgin ◽  
Sarah P Otto
Keyword(s):  
Correlated Response ◽  
Heterozygote Advantage ◽  
Genetic Associations ◽  
Asexual Population ◽  
Modifier Locus ◽  
Sexual And Asexual Reproduction ◽  
Overdominant Selection ◽  
Mean Fitness ◽  
Changes Over Time ◽  
Inbred Populations

AbstractThe segregation of alleles disrupts genetic associations at overdominant loci, causing a sexual population to experience a lower mean fitness compared to an asexual population. To investigate whether circumstances promoting increased sex exist within a population with heterozygote advantage, a model is constructed that monitors the frequency of alleles at a modifier locus that changes the relative allocation to sexual and asexual reproduction. The frequency of these modifier alleles changes over time as a correlated response to the dynamics at a fitness locus under overdominant selection. Increased sex can be favored in partially sexual populations that inbreed to some extent. This surprising finding results from the fact that inbred populations have an excess of homozygous individuals, for whom sex is always favorable. The conditions promoting increased levels of sex depend on the selection pressure against the homozygotes, the extent of sex and inbreeding in the population, and the dominance of the invading modifier allele.

The Age of Nonsynonymous and Synonymous Mutations in Animal mtDNA and Implications for the Mildly Deleterious Theory

Genetics ◽  
1999 ◽  
Vol 153 (1) ◽  
pp. 497-506 ◽  
Author(s):  
Rasmus Nielsen ◽  
Daniel M Weinreich
Keyword(s):  
Dna Sequences ◽  
Purifying Selection ◽  
Data Sets ◽  
Deleterious Mutations ◽  
Synonymous Mutations ◽  
Weak Evidence ◽  
Mitochondrial Data ◽  
The Mean ◽  
Excess Number ◽  
Neutral Mutations

Abstract McDonald/Kreitman tests performed on animal mtDNA consistently reveal significant deviations from strict neutrality in the direction of an excess number of polymorphic nonsynonymous sites, which is consistent with purifying selection acting on nonsynonymous sites. We show that under models of recurrent neutral and deleterious mutations, the mean age of segregating neutral mutations is greater than the mean age of segregating selected mutations, even in the absence of recombination. We develop a test of the hypothesis that the mean age of segregating synonymous mutations equals the mean age of segregating nonsynonymous mutations in a sample of DNA sequences. The power of this age-of-mutation test and the power of the McDonald/Kreitman test are explored by computer simulations. We apply the new test to 25 previously published mitochondrial data sets and find weak evidence for selection against nonsynonymous mutations.

scholarly journals Efficacy of SSRIs on cognition of Alzheimer's disease patients treated with cholinesterase inhibitors

2009 ◽  
Vol 22 (1) ◽  
pp. 114-119 ◽  
Author(s):  
Luca Rozzini ◽  
Barbara Vicini Chilovi ◽  
Marta Conti ◽  
Erik Bertoletti ◽  
Marina Zanetti ◽  
...  
Keyword(s):  
Alzheimer’S Disease ◽  
Alzheimer's Disease ◽  
Cholinesterase Inhibitors ◽  
Mmse Score ◽  
Joint Effect ◽  
Negative Effects ◽  
Multidimensional Assessment ◽  
Degree Of Protection ◽  
Depressed Patients ◽  
The Mean

ABSTRACTBackground: This study examines the joint effect on cognition of selective serotonin re-uptake inhibitors (SSRIs) and cholinesterase inhibitors (AChEIs) in depressed patients affected by Alzheimer's disease (AD) living at home.Methods: The study was conducted in two different outpatient neurological clinics. 338 patients with probable AD were treated with ChEis (donepezil, rivastigmine and galantamine) as per the clinician's judgment and were observed for nine months. At study entry, participants underwent a multidimensional assessment evaluating cognitive, functional and psychobehavioral domains. All patients were evaluated at baseline, after one (T1), three (T2) and nine months (T3). Patients were grouped in three different categories (patients not depressed and not treated with SSRIs, patients depressed and treated with SSRIs, and patients depressed but not treated with SSRIs).Results: At baseline 182 were diagnosed as not depressed and not treated with SSRIs, 66 as depressed and treated with SSRIs, and 90 as depressed but not treated with SSRIs. The mean change in MMSE score from baseline to nine months showed that depressed patients not treated worsened in comparison with those not depressed and not treated with SSRIs (mean change −0.8 ± 2.3 vs 0.04 ± 2.9; p = 0.02) and patients depressed and treated with SSRI (mean change −0.8 ± 2.3 vs 0.1 ± 2.5; p = 0.03).Conclusions: In AD patients treated with AChEIs, SSRIs may exert some degree of protection against the negative effects of depression on cognition.

scholarly journals NATURAL SELECTION VS. RANDOM DRIFT: EVIDENCE FROM TEMPORAL VARIATION IN ALLELE FREQUENCIES IN NATURE

Genetics ◽  
1985 ◽  
Vol 111 (3) ◽  
pp. 517-554
Author(s):  
Laurence D Mueller ◽  
Lorraine G Barr ◽  
Francisco J Ayala
Keyword(s):  
Natural Selection ◽  
Strong Evidence ◽  
Allele Frequencies ◽  
Drosophila Pseudoobscura ◽  
Frequency Variation ◽  
Random Drift ◽  
Napa County ◽  
Environmental Variations ◽  
Monthly Sample ◽  
Drosophila Persimilis

ABSTRACT We have obtained monthly samples of two species, Drosophila pseudoobscura and Drosophila persimilis, in a natural population from Napa County, California. In each species, about 300 genes have been assayed by electrophoresis for each of seven enzyme loci in each monthly sample from March 1972 to June 1975. Using statistical methods developed for the purpose, we have examined whether the allele frequencies at different loci vary in a correlated fashion. The methods used do not detect natural selection when it is deterministic (e.g., overdominance or directional selection), but only when alleles at different loci vary simultaneously in response to the same environmental variations. Moreover, only relatively large fitness differences (of the order of 15%) are detectable. We have found strong evidence of correlated allele frequency variation in 13-20% of the cases examined. We interpret this as evidence that natural selection plays a major role in the evolution of protein polymorphisms in nature.

scholarly journals Evaluation of Matured Larval Weight and Larval Duration of New Breeding Lines of Bombyx mori L

2021 ◽  
Vol 8 (10) ◽  
pp. 7-16
Author(s):  
Manjula A. C ◽  
Jenifer Lolita. C ◽  
Shubha Shubha ◽  
Prathibha K.Y ◽  
Keshamma E
Keyword(s):  
Bombyx Mori ◽  
Climatic Conditions ◽  
Sampling Errors ◽  
Random Drift ◽  
Breeding Lines ◽  
Anova Table ◽  
The Mean ◽  
Bombyx Mori L ◽  
Mulberry Silkworm ◽  
Selection Of

We planned to conduct this study with the main aim to develop bivoltine breeds for our tropical climatic conditions by using silkworm breeds with known genetic backgrounds (KA, NB18 and PM) in various hybrid combinations and incorporating them over generations, followed by backcrossing and adequate selection of different generations with the objective of profitability and productivity. The isolated Bivoltin lines (R1 and R2) were reared with their parental races at different times of the year to evaluate their stability in the expression of commercial traits. For the present breeding program, the purebred Bivoltine Kalimpong-A (KA), which spin white oval cocoons, New Bivoltine18 (NB18) white cocoons with rotating dumbbells and Multivoltine Pure Mysore (PM), the yellow pointed cocoons of the mulberry silkworm Bombyx mori L., Selected. One-way and three-way crosses were made using the above three breeds. The first single cross comprised KA females and PM males. The second unique cross comprised NB18 females and PM males. Selection was performed at the egg, larva, pupal, and cocoon stages over the course to determine the desired traits. The offspring of F from the respective crosses were backcrossed with their respective bivoltine males to improve commercial traits. Heterosis in the F1 generations of crosses, including NB18 and PM, was determined by the mean score of the parents (MPV) and the best score of the parents (BPV). A significant test for heterosis was performed using a standard ANOVA table. Based on the results of our study, it was found that the performance of the characters, viz. The weight of mature larvae and the duration of the larvae over generations do not simply increase or decrease regularly, but fluctuate irregularly. The reason for this variation may be due to random genetic drift, sampling errors in estimating generational means, selection pressures, and environmental factors. Therefore, inbreeding variations due to random drift and sampling errors could be reduced by increasing the number sampled and selected.

POPULATION DYNAMICS OF SEX-DETERMINING ALLELES IN HONEY BEES AND SELF-INCOMPATIBILITY ALLELES IN PLANTS

Genetics ◽  
1979 ◽  
Vol 91 (3) ◽  
pp. 609-626 ◽  
Author(s):  
Shozo Yokoyama ◽  
Masatoshi Nei
Keyword(s):  
Population Dynamics ◽  
Population Size ◽  
Honey Bee ◽  
Honey Bees ◽  
Finite Population ◽  
Large Population ◽  
Allele Frequencies ◽  
Self Incompatibility ◽  
Overdominant Selection ◽  
Incompatibility Alleles

ABSTRACT Mathematical theories of the population dynamics of sex-determining alleles in honey bees are developed. It is shown that in an infinitely large population the equilibrium frequency of a sex allele is l/n, where n is the number of alleles in the population, and the asymptotic rate of approach to this equilibrium is 2/(3n) per generation. Formulae for the distribution of allele frequencies and the effective and actual numbers of alleles that can be maintained in a finite population are derived by taking into account the population size and mutation rate. It is shown that the allele frequencies in a finite population may deviate considerably from l/n. Using these results, available data on the number of sex alleles in honey bee populations are discussed. It is also shown that the number of self-incompatibility alleles in plants can be studied in a much simpler way by the method used in this paper. A brief discussion about general overdominant selection is presented.

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