The Mutational Meltdown in Asexual Populations

Abstract

Loss of fitness due to the accumulation of deleterious mutations appears to be inevitable in small, obligately asexual populations, as these are incapable of reconstituting highly fit genotypes by recombination or back mutation. The cumulative buildup of such mutations is expected to lead to an eventual reduction in population size, and this facilitates the chance accumulation of future mutations. This synergistic interaction between population size reduction and mutation accumulation leads to an extinction process known as the mutational meltdown, and provides a powerful explanation for the rarity of obligate asexuality. We give an overview of the theory of the mutational meltdown, showing how the process depends on the demographic properties of a population, the properties of mutations, and the relationship between fitness and number of mutations incurred.

How populations persist when asexuality requires sex: the spatial dynamics of coping with sperm parasites

“…Asexual lineages are therefore typically fairly short-lived (e.g. Muller 1964;Lynch & Gabriel 1990;Lynch et al 1993;Crow 1994;Dunbrack et al 1995; but see ancient asexuals: Judson & Normark (1996)). Indeed, much of the literature on the evolution of sex attempts to weigh the short-term advantage of the twofold benefit of asexual reproduction with the long-term benefits of sex, as well as asking whether sex has significant benefits that act already over shorter time-scales (e.g.…”

Accumulation of Deleterious Mutations in Small Abiotic Populations of RNA

“…Another issue of great interest is the relationship between mutational load and population size. It has been predicted that these two factors can act synergistically, in that as the load increases, the population size should decrease, leading to a higher probability of fixing new deleterious mutations (Lynch and Gabriel 1990;Gabriel et al 1993;Lynch et al 1993). Eventually a threshold is crossed, and the population spirals into extinction via a ''mutational meltdown,'' as can be seen in ciliated protozoans and fibroblast cultures, for example (Smith and Pereira-Smith 1977;Tagaki and Yoshida 1980).…”

“…Eventually a threshold is crossed, and the population spirals into extinction via a ''mutational meltdown,'' as can be seen in ciliated protozoans and fibroblast cultures, for example (Smith and Pereira-Smith 1977;Tagaki and Yoshida 1980). Mutation-accumulation (MA) experiments have been used effectively for .40 years to address questions related to the buildup of deleterious mutations in populations such as those of Arabidopsis, Caenorhabditis elegans, Daphnia, Drosophila, Escherichia coli, Saccharomyces cerevisiae, and others (reviewed in Mukai 1964;Kibota and Lynch 1996;Lynch and Walsh 1998;Schultz et al 1999;Pfrender and Lynch 2000;Zeyl et al 2001;Estes et al 2004). A species lineage is propagated in a very controlled environment over a large number of generations and is typically forced through a bottleneck in size each generation to exacerbate the effects of random genetic drift.…”

Molecular tracking new migrations of an old pathogen : the re-emergence of potato late blight

“…The speed of the ratchet dé-pends on the rate of mutation and the population size; it proceeds fastest with high mutation rates and small populations. Populations can reach "mutational meltdown" after only 10 3 asexual générations (Lynch et al 1993). The US-1 clonal lineage may hâve experienced near this number of asexual générations by the 1980s and 1990s Goodwin 1997), so its fitness could hâve been much lower than when it originally escaped from Mexico.…”