Tempo and Mode in Evolution Genetics and Paleontology 50 Years After Simpson (1995) / Chapter Skim
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Explaining Low Levels of DNA Sequence Variation in Regions of the Drosophila Genome with Low Recombination Rates
Explaining Low Levels of DNA Sequence Variation in Regions of the Drosophila Genome with Low Recombination Rates
Pages 275-286
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From page 275... ...
Although the territory is large and much remains to be explored, some descriptive generalizations are now possible. In contrast, the population genetic processes underlying this molecular evolution remain almost entirely obscure.
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From page 276... ...
A Strictly Neutral Hypothesis A very simple though interesting hypothesis to explain the correlation of recombination rates with polymorphism levels is a completely neutral one: Regions of low recombination might have low levels of polymorphism because they have low neutral mutation rates. These low neutral mutation rates in regions of low recombination might result from high average levels of constraint in those parts of the genome or because the spontaneous mutation rates are low there.
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From page 277... ...
Hitchhiking with Favorable Mutations Hypothesis Another hypothesis that has been proposed to explain the pattern of sequence variation is the hitchhiking or "selective sweeps" model (Maynard Smith and Haigh, 1974; Kaplan et al., 1989; Stephan et al., 1992; Wiehe and Stephan, 1993~. Under this model, the low levels of polymorphism in regions of low recombination are due to the hitchhiking effect of selectively advantageous mutants that sweep through the population and, in the process, eliminate variation at tightly linked sites.
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From page 278... ...
between the neutral locus and the site indicated by the arrows is assumed to equal ski. (ski is defined to be the decrement in fitness produced by a single heterozygous deleterious mutation; that is, the relative fitness of an individual heterozygous for a single deleterious mutation is assumed to be 1 sh.)
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From page 279... ...
In contrast, under the background selection model, the mutations that affect fitness are all assumed to be deleterious. In a sufficiently large population, recurrent deleterious mutation and natural selection lead to an equilibrium, with mutation continually adding deleterious alleles and natural selection removing them.
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From page 280... ...
Thus, in Drosophila, He would be the total deleterious mutation rate for loci within 2sh x 100 centimorgans of the neutral locus. If we denote the deleterious mutation rate of the entire diploid genome by UT and the fraction of genome within z centimorgans of a locus by F(z)
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From page 281... ...
Further support for the idea that deleterious mutations are reducing variation in regions of normal recombination is obtained from the observation of transposable element insertion polymorphism from the adh region of Drosophila melanogaster. In a restriction site polymorphism survey of a 15-kb region including the adh locus (Aquadro et al., 1986)
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From page 282... ...
Similarly, the low levels of polymorphism observed on the fourth chromosome of Drosophila melanogaster are not predicted by the background selection model. These are the same conclusions reached by Charlesworth et al., (1993)
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From page 283... ...
Several surveys of polymorphisms in Drosophila have now documented that levels of DNA polymorphism are positively correlated with rates of recombination; i.e., regions with low rates of recombination tend to have low levels of DNA polymorphism within populations of Drosophila. Three hypotheses are reviewed that might account for these observations.
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From page 284... ...
(1986) Molecular population genetics of the alcohol dehydrogenase gene region of Drosophila melanogaster.
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From page 285... ...
(1993) Analysis of a genetic hitchhiking model, and its application to DNA polymorphism data from Drosophila melanogaster.
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