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Genetic load

genetic load, genetic load formula
Genetic load is the difference between the fitness of an average genotype in a population and the fitness of some reference genotype, which may be either the best present in a population, or may be the theoretically optimal genotype The average individual taken from a population with a low genetic load will generally, when grown in the same conditions, have more surviving offspring Genetic load can also be seen as reduced fitness at the population level compared to what the population would have if all individuals had the reference high-fitness genotype High genetic load may put a population in danger of extinction

Contents

  • 1 Fundamentals
  • 2 Causes
    • 21 Deleterious mutation
    • 22 Beneficial mutation
    • 23 Inbreeding
    • 24 Recombination/segregation load
  • 3 Migration load
  • 4 References

Fundamentals

Consider n genotypes A 1 , … , A n _,\dots ,\mathbf _} , which have the fitnesses w 1 , … , w n ,\dots ,w_} and frequencies p 1 , … , p n ,\dots ,p_} , respectively Ignoring frequency-dependent selection, the genetic load L may be calculated as:

L = w max − w ¯ w max -}} \over w_}}

where w max } is either some theoretical optimum, or the maximum fitness observed in the population In calculating the genetic load, w 1 … w n \dots w_} must be actually found in at least a single copy in the population, and w ¯ }} is the average fitness calculated as the mean of all the fitnesses weighted by their corresponding frequencies:

w ¯ = ∑ i = 1 n p i w i }=^w_}}}

where the i t h }} genotype is A i _} and has the fitness and frequency w i } and p i } respectively

One problem with calculating genetic load is that it is difficult to evaluate either the theoretically optimal genotype, or the maximally fit genotype actually present in the population This is not a problem within mathematical models of genetic load, or for empirical studies that compare the relative value of genetic load in one setting to genetic load in another

Causes

Deleterious mutation

Deleterious mutation load is the main contributing factor to genetic load overall Most mutations are deleterious, and occur at a high rate The Haldane-Muller theorem of mutation-selection balance says that the load depends only on the deleterious mutation rate and not on the selection coefficient Specifically, relative to an ideal genotype of fitness 1, the mean population fitness is exp ⁡ − U where U is the total deleterious mutation rate summed over many independent sites The intuition for the lack of dependence on the selection coefficient is that while a mutation with stronger effects does more harm per generation, its harm is felt for fewer generations

A slightly deleterious mutation may not stay in mutation-selection balance but may instead become fixed by genetic drift when its selection coefficient is less than one divided by the effective population size In asexual populations, the stochastic accumulation of mutation load is called Muller's ratchet, and occurs in the absence of beneficial mutations, when after the most-fit genotype has been lost, it cannot be regained by genetic recombination Deterministic accumulation of mutation load occurs in asexuals when the deleterious mutation rate exceeds one per replication Sexually reproducing species are expected to have lower genetic loads This is one hypothesis for the evolutionary advantage of sexual reproduction Purging of deleterious mutations in sexual populations is facilitated by synergistic epistasis among deleterious mutations

High load can lead to a small population size, which in turn increases the accumulation of mutation load, culminating in extinction via mutational meltdown

The accumulation of deleterious mutations in humans has been of concern to many geneticists, including Hermann Joseph Muller, James F Crow, Alexey Kondrashov, W D Hamilton, and Michael Lynch

Beneficial mutation

New beneficial mutations create fitter genotypes than those previously present in the population When load is calculated as the difference between the fittest genotype present and the average, this creates a substitutional load The difference between the theoretical maximum which may not actually be present and the average is known as the "lag load" Motoo Kimura's original argument for the neutral theory of molecular evolution was that if most differences between species were adaptive, this would exceed the speed limit to adaptation set by the substitutional load However, Kimura's argument confused the lag load with the substitutional load, using the former when it is the latter that in fact sets the maximal rate of evolution by natural selection

Inbreeding

Inbreeding increases homozygosity In the short run, an increase in inbreeding increases the probability with which offspring get two copies of a recessive deleterious alleles, lowering fitnesses via inbreeding depression In a species that habitually inbreeds, eg through self-fertilization, recessive deleterious alleles are purged

Recombination/segregation load

Combinations of alleles that have evolved to work well together may not work when recombined with a different suite of coevolved alleles, leading to outbreeding depression Segregation load is the presence of underdominant heterozygotes ie heterozygotes that are less fit than either homozygote Recombination load arises through unfavorable combinations across multiple loci that appear when favorable linkage disequilibria are broken down Recombination load can also arise by combining deleterious alleles subject to synergestic epistasis, ie whose damage in combination is greater than that predicted from considering them in isolation

Migration load

Migration load is the result of nonnative organisms that aren’t adapted to a particular environment coming into that environment If they breed with individuals who are adapted to that environment, their offspring will not be as fit as they would have been if both of their parents had been adapted to that particular environment Migration load can also occur in asexually reproducing species, but in this case, purging of low fitness genotypes is more straightforward

References

  1. ^ Whitlock, Michael C; Bourguet, Denis 2000 "Factors affecting the genetic load in Drosophila: synergistic epistasis and correlations among fitness components" Evolution 54 5: 1654–1660 doi:101554/0014-3820200005420CO;2 PMID 11108592 
  2. ^ Crist, Kathryn Carvey; Farrar, Donald R 1983 "Genetic load and long-distance dispersal in Asplenium platyneuron" Canadian Journal of Botany 61 6: 1809–1814 doi:101139/b83-190 
  3. ^ JF Crow 1958 "Some possibilities for measuring selection intensities in man" Human Biology 30 1: 1–13 PMID 13513111 
  4. ^ Agrawal, Aneil F; Whitlock, Michael C 2012 "Mutation load: the fitness of individuals in populations where deleterious alleles are abundant" Annual Review of Ecology, Evolution, and Systematics 43 1: 115–135 doi:101146/annurev-ecolsys-110411-160257 
  5. ^ Klekowski, EdwardJ 1988 "Genetic load and its causes in long-lived plants" Trees 2 4: 195–203 doi:101007/BF00202374 
  6. ^ Bürger, Reinhard 1998 "Mathematical properties of mutation-selection models" Genetica 102/103: 279–298 doi:101023/a:1017043111100 
  7. ^ Lande, Russell October 1994 "Risk of Population Extinction from Fixation of New Deleterious Mutations" Evolution 48 5: 1460 doi:102307/2410240 
  8. ^ Kondrashov, A S 1988 "Deleterious mutations and the evolution of sexual reproduction" Nature 336 6198: 435–440 Bibcode:1988Natur336435K doi:101038/336435a0 PMID 3057385 
  9. ^ Marriage, Tara N 2009 Mutation, asexual reproduction and genetic load: A study in three parts PhD thesis University of Kansas 
  10. ^ a b Crow, James F 5 August 1997 "The high spontaneous mutation rate: Is it a health risk" Proceedings of the National Academy of Sciences 94 16: 8380–8386 Bibcode:1997PNAS948380C doi:101073/pnas94168380 ISSN 0027-8424 PMC 33757  PMID 9237985 
  11. ^ Lynch, Michael; Conery, John; Burger, Reinhard December 1995 "Mutational Meltdowns in Sexual Populations" Evolution 49 6: 1067 doi:102307/2410432 
  12. ^ Lynch, Michael; Conery, John; Burger, Reinhard 1 January 1995 "Mutation Accumulation and the Extinction of Small Populations" The American Naturalist 146 4: 489–518 doi:101086/285812 JSTOR 2462976 
  13. ^ Muller, H J 1 June 1950 "Our load of mutations" American Journal of Human Genetics 2 2: 111–176 ISSN 0002-9297 PMC 1716299  PMID 14771033 
  14. ^ Kondrashov, Alexey S 21 August 1995 "Contamination of the genome by very slightly deleterious mutations: why have we not died 100 times over" Journal of Theoretical Biology 175 4: 583–594 doi:101006/jtbi19950167 PMID 7475094 
  15. ^ Hamilton, WD Narrow Roads of Gene Land vol 2: Evolution of Sex pp 449–463 
  16. ^ Lynch, M 7 March 2016 "Mutation and Human Exceptionalism: Our Future Genetic Load" Genetics 202 3: 869–875 doi:101534/genetics115180471 PMC 4788123  PMID 26953265 
  17. ^ Smith, J Maynard 1 January 1976 "What Determines the Rate of Evolution" The American Naturalist 110 973: 331–338 doi:101086/283071 JSTOR 2459757 
  18. ^ Kimura, Motoo 1968 "Evolutionary rate at the molecular level" – Scholar search Nature 217 5129: 624–626 Bibcode:1968Natur217624K doi:101038/217624a0 PMID 5637732 
  19. ^ Ewens, Warren J 2003 Mathematical population genetics 2nd ed New York: Springer p 78 ISBN 978-0387201917 
  20. ^ Saccheri, I J; Lloyd, H D; Helyar, S J; Brakefield, P M 2005 "Inbreeding uncovers fundamental differences in the genetic load affecting male and female fertility in a butterfly" Proceedings of the Royal Society B: Biological Sciences 272 1558: 39–46 doi:101098/rspb20042903 PMC 1634945  PMID 15875568 
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  22. ^ Barrett, S C H; Charlesworth, D 1991 "Effects of a change in the level of inbreeding on the genetic load" Nature 352 6335: 522–524 Bibcode:1991Natur352522B doi:101038/352522a0 PMID 1865906 
  23. ^ Haag, C R; Roze, D 2007 "Genetic load in sexual and asexual diploids: segregation, dominance and genetic drift" Genetics 176 3: 1663–1678 doi:101534/genetics107073080 PMC 1931546  PMID 17483409 
  24. ^ King, J 1966 "The gene interaction component of the genetic load" Genetics 53 3: 403–413 PMC 1211027  PMID 5919323 
  25. ^ Bolnick, Daniel I; Nosil, Patrik 2007 "Natural selection in populations subject to a migration load" Evolution 61 9: 2229–2243 doi:101111/j1558-5646200700179x 
  26. ^ Hu, Xin-Sheng; Li, Bailian 2003 "On migration load of seeds and pollen grains in a local population" Heredity 90 2: 162–168 doi:101038/sjhdy6800212 PMID 12634823 
  27. ^ Ilkka Hanski; Oscar E Gaggiotti, eds 2004 Ecology, Genetics, and Evolution of Metapopulations Academic Press ISBN 978-0-12-323448-3 

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    29.10.2014


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