Adaptive mutation


Adaptive mutation

Evolutionary theory describes that mutagenesis occurs randomly, regardless of the utility of a genetic mutation to the organism. If it is beneficial or neutral, the organism will survive to reproduce and pass on the mutation. However, molecular biologist John Cairns has proposed that "when populations of single cells are subject to certain forms of strong selection pressure, variants emerge bearing changes in DNA sequence that bring about an appropriate change in phenotype." This suggests that there exists a particular physiological pathway that responds to a specific selective pressure to produce a mutation conferring the correct phenotype that will alleviate this pressure.

Such evidence was first produced by Cairns et al. in 1988. The original experiments involved a strain of E. coli that has a frameshift mutation in the lactose (LacZ) operon, inactivating the proteins needed for utilization of this sugar. The bacteria were then spread on an agar medium in which the only carbon source was lactose. This meant that a cell could grow only if a second mutation occurred in the lactose operon, reversing the effects of the mutation and therefore allowing the enzymes to be synthesized. Mutations with this effect appeared to occur significantly more frequently than expected, and at a rate that was greater than mutations in other parts of the genomes of these E. coli cells.

There is however a serious flaw in this experiment, as Cairns does not distinguish between selection and detection of LacZ revertants. If he is testing to see if the presence of lactose as the selective agent causes mutations that confer the ability to eat lactose, then he should not detect this mutation with lactose present (i.e. looking for cells that grow with lactose as the only carbon source.) He will never know, in this case, if cells have acquired this ability without the presence of lactose - a possibility that his theory cannot reconcile. In fact, the acknowledgment of fundamental limitations on our ability to separate between mutation selection and detection has lead Vasily Ogryzko to suggest that for the proper description of the Cairns' experiments, the formalism of quantum theory would be required, with the phenomenon of adaptive mutations naturally following from such an approach.[1]

Furthermore, when looking for additional mutations only in cells that have already reverted to Lac+, it would certainly be the case that other, unnecessary mutations would be less numerous - especially since most are deleterious. However, Barry Hall has provided evidence that the mutation rate in bacteria under environmental stress increases across the board, most likely indiscriminately. When testing for tryptophan revertants (trp− → trp+, i.e. cells that have regained the ability to make tryptophan), he found that occurrence of auxotrophic mutants increased as well. Tryptophan revertants, which had been exposed to the environmental stress of lacking the amino acid, saw a 1.8% rate of auxotrophy for any other amino acid. When testing for auxotrophy in cells in non-stressed colonies, he found a rate below 0.01%. From these data, Hall hypothesized that cells under stress enter a "hyper-mutable state," where cells increase their general rate of mutation, increasing the overall probability that they will acquire a mutation conferring a phenotype that aids their survival. Hall later determined that his mutants were the result of transposon activity, and concluded that his results were in fact about transposon biology, and not mutagenesis.

Similar results have been observed in other experiments. These experiments suggested that mutations in bacteria are influenced by the selective pressures that the bacteria are placed under.

One possible explanation is that under conditions of stress the global rate of errors in DNA replication and repair mechanisms is increased, and hence the mutation rate is increased.

Another explanation stems from a similarity in cellular mechanisms underlying the acquisition of adaptive mutations in the bacterial stationary phase cells and in the mammalian tumor cells. In both cases the adaptive mutations arise in response to a sustained stress environment and are promoted by high rate of genomic mutations. Cellular processes leading to the mutations are also surprisingly similar between both organisms and include silencing of differentiation, cellular senescence, programmed cell death, and DNA repair on the one hand, and activation of the error-prone replication and transposons on the other. The similarity suggests that the adaptive mutations may be an output of activation in the stressed cell of a special survival strategy for quick adaptation to the stressful environment. This strategy that is also referred as the mutator phenotype is an alternative to other stress-induced strategies, such as senescence and programmed cell death, activated in majority of stressed cells. Continuing stress-induced proliferative and survival signaling may be an important prerequisite for epigenetic reprogramming of some cells to activate the mutator phenotype.

See also

References

  1. ^ Ogryzko, V. V. (1997). [1] [2] [3] [4] Biosystems 43, 83-95

External links


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Mutation — For other uses, see Mutation (disambiguation). Part of the Biology series on Evolution …   Wikipedia

  • Adaptive immune system — The adaptive immune system is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic challenges. Thought to have arisen in the first jawed vertebrates, the adaptive or specific immune system is activated …   Wikipedia

  • Mutation rate — In genetics, the mutation rate is the chance of a mutation occurring in an organism or gene in each generation (or, in the case of multicellular organisms, cell division). The mutation frequency is the number of individuals in a population with a …   Wikipedia

  • Adaptive Landschaft — Sewall [Green] Wright (* 21. Dezember 1889 in Melrose (Massachusetts); † 3. März 1988) war ein amerikanischer Theoretischer Biologe und Genetiker, der zusammen mit Ronald Fisher und John Burdon Sanderson Haldane die Populationsgenetik entwickelte …   Deutsch Wikipedia

  • directed mutation — (adaptive mutation) A mutation that seems to be chosen so the organism can better adapt to its surroundings …   Dictionary of microbiology

  • Lamarckism — (or Lamarckian inheritance) is the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring (also known as heritability of acquired characteristics or soft inheritance). It is named after the French… …   Wikipedia

  • Quantum evolution (alternative) — Quantum evolution is the hypothesis that genetic mutation can be adaptive, or directed through quantum effects.McFadden, Johnjoe (2000). [http://www.surrey.ac.uk/qe/quantumevolution.htm Quantum Evolution .] HarperCollins. ISBN 0 00 255948 X; ISBN …   Wikipedia

  • Fluktuationstest — Das Luria Delbrück Experiment (auch Fluktuationstest) ist ein von Salvador Edward Luria und Max Delbrück erdachtes und durchgeführtes Experiment, das 1943 publiziert wurde und das ein Teil der Arbeiten war, für die Luria und Delbrück 1969… …   Deutsch Wikipedia

  • Luria-Delbrück-Experiment — Das Luria Delbrück Experiment (auch Fluktuationstest) ist ein von Salvador Edward Luria und Max Delbrück erdachtes und durchgeführtes Experiment, das 1943 publiziert wurde und das ein Teil der Arbeiten war, für die Luria und Delbrück 1969… …   Deutsch Wikipedia

  • Maurice Sanford Fox — This article is about a scientist. For the Canadian chess master, see Maurice Fox. Maurice Sanford Fox (born New York, October 11, 1924) is an American geneticist and molecular biologist, and professor Emeritus of Biology at the Massachusetts… …   Wikipedia