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Mutator Allele

Definition

The definition of a mutator allele is an allele that increases the mutation rate no matter how small or big. If a mutation is beneficial, it is more likely to increase in mutation rate and create a phenomenon called a mutator effect. This allele that causes this mutation to increase in mutation rate is referred to as the mutator allele.

Background

The majority of mutations are deleterious or neutral, meaning they have no beneficial value and as a result are favored against. Even though the majority of mutations are deleterious than beneficial, there are some that are beneficial and carry an adaptive benefit. Mutator allele’s increase the frequency of favorable alleles needed for adaptation in order to increase the probability of fixation of the mutator allele. The strength of mutator alleles does not determine whether or not it will become fixed, the given environment and given population size gives the mutator allele either a higher or lower chance on becoming fixed than another mutation.

Role in Adaptive Evolution

The overall chance of a mutator allele that is beneficial to improve adaptation can be based on the current fitness on a population. If a population has relative high fitness than mutations are more likely to be deleterious or neutral, but if a population has a low fitness these mutations are more prevalent to be beneficial.

Since mutator alleles are chosen to increase mutation rates due to their advantageous effect, there is an implication that there is selection towards change within the population. When selection is high, mutation rates are more likely to increase because the organism is trying to evolve due to its unstable fitness. When selection is low, mutator alleles are favored against because the addition of a mutation may cause counter selection and change the equilibrium and therefore a decrease its fitness. Even if the mutator allele is beneficial, it is unlikely that the organism with a high fitness will consider the mutation.

Genetic Hitch-hiking and Mutator Alleles

The mutator allele acts as a modifier and produces mutations, which if beneficial and recognized by natural selection, will increase in frequency by “hitch-hiking”. Genetic hitchhiking occurs when there is a beneficial allele, i.e. mutation, linked at another locus of another allele. When this benefit is recognized, genetic hitch-hiking is used in order to increase the chances of association between a mutator allele and a beneficial mutation. It has also been found that mutator alleles hitchhike to fixation rather quickly in a small population. So as population size increases, the fixation rate decreases.

Fate of Mutator Alleles in Asexual and Sexual Populations

Mutator alleles occur in both sexual and asexual populations, even though they are more frequently found in asexual than sexual populations.

Sexual Population

The first possible way mutator alleles occur in sexual populations is through transposable elements, like transposons and insertion sequences. Transposable elements are part of a DNA sequence that has the ability to change its position within the genome and as a result create mutations through different base-pair combinations. Once a mutation that is beneficial has been made in the genome, there is a possibility that it will be duplicated and carried on throughout development. Transposable elements act like mutator alleles due to their mutagenic properties which can make them increase in frequency using genetic hitchhiking.

The second possible way mutator alleles occur in a sexual population is through a mutator allele rising from a contingency gene. A contingency gene has an area within a genome that has a higher mutation rate than in other parts of that genome. This area with increased mutation rate scrambles the genetic sequence and results in different gene function or turning a gene on or off. This area acts as a mutator allele due to its mutagenic properties. In a short sequence of DNA in a promoter or coding region, there is possibility that a gene can increase in mutation rate at a specific loci by mispairing, adding, or subtracting a base (A,C,G,T; the DNA’s building blocks) before the replication process takes place. This allows organisms, mainly bacteria, under adaptive pressures or environmental pressures to create a mass amount of different genetic combinations. The increased amount of genetic combinations gives a higher potential that some of these unique combinations will survive over the other combinations. Once a genetic combination has been found that has a higher fitness than the others, it will increase in frequency and be passed on.

Asexual Population

The main reason mutator alleles occur in asexual population is due to the fact that asexual populations don’t go through recombination, the process in which two DNA strands exchange genetic information and create new combinations of alleles. Recombination in sexual populations prevents hitchhiking of mutator alleles, even if beneficial, and creates a decline in mutator alleles actually being picked up by the organism. Since recombination doesn’t occur in asexual populations, there is an increased likelihood that a mutator allele will be picked up, if beneficial.

Another way mutator alleles take place within an asexual populations is due to clonal interference when there is a large amount of linkage disequilibrium (combinations of alleles that are not what they are expected to be). Clonal interference happens when two different beneficial mutations appear in two different individuals. Being asexual, the two individuals cannot use recombination to combine the two mutations therefore they compete with each other until one loses. The individual that one won will reproduce and make identical clones of itself with that beneficial mutation. Since it outcompetes the other individuals in the population, over time the population will have that mutation in each individual.