Adaptive mutation

Adaptive mutation is a controversial evolutionary theory. It posits that mutations, or genetic changes, are much less random and more purposeful than traditional evolution. There have been a wide variety of experiments trying to prove (or disprove) the idea of adaptive mutation, at least in microorganisms.

Definition

The most widely accepted theory of evolution states that organisms will diversify based on natural selection where changes caused by mutations improve the organism's chance of survival. Adaptive mutation states that rather than mutations and evolution being random, they are in response to specific stresses. In other words, the mutations that occur are more beneficial and specific to the given stress, instead of random and not a response to anything in particular. It should be noted that the term stress refers to any change in environment, such as temperature, nutrients, population size, etc. Tests with microorganisms have found that for adaptive mutation, more of the mutations observed after a given stress were effective at dealing with the stress than chance alone would suggest is possible.^[1][2] This theory of adaptive mutation was first brought to academic attention in the 1980s by John Cairns.^[3]

Major experiments

Adaptive mutation is a very controversial topic so there have been many experiments to prove or discredit the theory. Three major experiments are the SOS response^[4] and responses to starvation in Escherichia coli,^[5] and testing for revertants of a tryptophan auxotroph of Saccharomyces cerevisiae, or yeast.^[6]

SOS response

This experiment is different from the others in one small way: this experiment is concerned with the pathways leading to adaptive mutation while the others tested the changing environment microorganisms were exposed to.

Simply put, the SOS response in E. coli is a response to DNA damage that must be repaired. The normal cell cycle is put on hold and mutagenesis may begin. This means that mutations will occur to try to fix the damage. This hypermutation, or increased rate of change, response has to have some regulatory process, and some key molecules in this process are RecA, and LexA. These are proteins and act as stoplights for this and other processes. They also appear to be the main contributors to adaptive mutation in E. coli. Changes in presence of one or the other was shown to affect the SOS response, which in turn affected how the cells were able to process lactose, which should not be confused with the lactose starvation experiment. The key point to understand here is that LexA and RecA both were required for adaptive mutation to occur, and without the SOS response adaptive mutation would not be possible.^[7]

Lactose starvation

The E. coli strain FC40 has a high rate of mutation, and so is useful for studies, such as for adaptive mutation. Due to a frameshift mutation, a change in the sequence that causes the DNA to code for something different, FC40 is unable to process lactose. When placed in a lactose-rich medium, it has been found that 20% of the cells mutated from Lac- (could not process lactose) to Lac+, meaning they could now utilize the lactose in their environment. The responses to stress are not in current DNA, but the change is made during DNA replication through recombination and the replication process itself, meaning that the adaptive mutation occurs in the current bacteria and will be inherited by the next generations because the mutation becomes part of the genetic code in the bacteria.^[8] This is particularly obvious in a study by Cairns, which demonstrated that even after moving E. coli back to a medium with minimal levels of lactose, Lac+ mutants continued to be produced as a response to the previous environment.^[9] This would not be possible if adaptive mutation was not at work because natural selection would not favor this mutation in the new environment.

Although there are many genes involved in adaptive mutation, RecG, a protein, was found to have an effect on adaptive mutation. By itself, RecG was found to not necessarily lead to a mutational phenotype. However, it was found to inhibit the appearance of revertants (cells that appeared normally, as opposed to those with the mutations being studied) in wild-type cells. On the other hand, RecG mutants were key to the expression of RecA-dependent mutations, which were a major portion of study in the SOS response experiments, such as the ability to utilize lactose.^[10]

Yeast

Dr. von Borstel, in the 1970s, conducted experiments similar to the Lactose Starvation experiment with yeast, specifically Saccharomyces cerevisiae. He tested for tryptophan auxotroph revertants. A tryptophan auxotroph cannot make tryptophan for itself, but wild-type cells can and so a revertant will revert to the normal state of being able to produce tryptophan. He found that when yeast colonies were moved from a tryptophan-rich medium to a minimal one, revertants continued to appear for several days. The degree to which revertants were observed in yeast was not as high as with bacteria. Other scientists have conducted similar experiments, such as Hall who tested histidine revertants, or Steele & Jinks-Robertson who tested lysine. These experiments demonstrate how recombination and DNA replication are necessary for adaptive mutation. However, in lysine-tested cells, recombination continued to occur even without selection for it. Steele and Jinks-Robertson concluded that recombination occurred in all circumstances, adaptive or otherwise, while mutations were present only when they were beneficial and adaptive.^[11]

References