Academic journal article Genetics

Genetic Load in Sexual and Asexual Diploids: Segregation, Dominance and Genetic Drift

Academic journal article Genetics

Genetic Load in Sexual and Asexual Diploids: Segregation, Dominance and Genetic Drift

Article excerpt

ABSTRACT

In diploid organisms, sexual reproduction rearranges allelic combinations between loci (recombination) as well as within loci (segregation). Several studies have analyzed the effect of segregation on the genetic load due to recurrent deleterious mutations, but considered infinite populations, thus neglecting the effects of genetic drift. Here, we use single-locus models to explore the combined effects of segregation, selection, and drift. We find that, for partly recessive deleterious alleles, segregation affects both the deterministic component of the change in allele frequencies and the stochastic component due to drift. As a result, we find that the mutation load may be far greater in asexuals than in sexuals in finite and/or subdivided populations. In finite populations, this effect arises primarily because, in the absence of segregation, heterozygotes may reach high frequencies due to drift, while homozygotes are still efficiently selected against; this is not possible with segregation, as matings between heterozygotes constantly produce new homozygotes. If deleterious alleles are partly, but not fully recessive, this causes an excess load in asexuals at intermediate population sizes. In subdivided populations without extinction, drift mostly occurs locally, which reduces the efficiency of selection in both sexuals and asexuals, but does not lead to global fixation. Yet, local drift is stronger in asexuals than in sexuals, leading to a higher mutation load in asexuals. In metapopulations with turnover, global drift becomes again important, leading to similar results as in finite, unstructured populations. Overall, the mutation load that arises through the absence of segregation in asexuals may greatly exceed previous predictions that ignored genetic drift.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

MOST eukaryotes engage in sexual reproduction despite potentially high costs, such as the famous twofold cost of sex (MAYNARD SMITH 1978; BARTON and CHARLESWORTH 1998). Genetically, the key components of sexual reproduction are recombination and, in diploid organisms, segregation. Both are absent under pure asexual reproduction. Recombination and segregation rearrange the genotypic composition of offspring from sexual matings, by bringing together novel allelic combinations at a locus (segregation) or at a set of different loci (recombination). Hence, these processesmay affect the distribution of fitness values within populations and may therefore generate indirect selective pressure for sexual reproduction (BARTON and CHARLESWORTH 1998; OTTO and LENORMAND 2002; OTTO 2003; AGRAWAL 2006; DE VISSER and ELENA 2007).

One possible advantage of recombination and segregation is that they allow sexual populations to reduce their genetic load through an improved efficiency of selection against deleterious alleles (KIMURA and MARUYAMA 1966; CROW 1970; CROW and KIMURA 1970). This requires the existence of negative disequilibria such as when bene- ficial and deleterious alleles (within or between loci) occur more often in the same individual than expected by chance. Recombination and segregation bring together favorable alleles within the same individuals (and unfavorable alleles in others) and hence improve the efficiency of natural selection. Selection against recurrent deleterious mutation can create negative disequilibria between loci ("negative linkage disequilibrium") if deleterious alleles at different loci interact synergistically (KONDRASHOV 1982; CHARLESWORTH 1990). Equivalently, selection can create negative disequilibria within loci ("heterozygote excess") if deleterious alleles are fully or partially recessive. This is because (with partially recessive deleterious alleles) the fitness of heterozygotes is higher than the average fitness of the homozygotes, and hence heterozygote excess develops during selection. Once a heterozygote excess is established, sexual reproduction leads to improved selection and therefore to reduced genetic load, because segregation eliminates the heterozygote excess, resulting in an increased variance in fitness (CHASNOV 2000). …

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