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Mobile Genetic Elements Generate Diversity in Response to Environmental Stress

Organisms change over time. In order to meet the demands of environments that inevitably produce new circumstances and challenges, species must be able to change and adapt, with each generation being more-and-more fine-tuned to the particular requirements of its ecosystem. Not only does this make good common sense, it is an observable and demonstrable fact. The scientific term given to this kind of natural change and adaptation of organisms over time is evolution. While this theory is as solid an idea as ‘Earth makes the gravitational field that causes stuff to fall’—there are still specific dynamics and mechanisms that have not been completely delineated, just like there are mechanisms underlying the physics of gravity that are still being investigated (are there gravitons? are Newtonian dynamics modified in certain regimes, producing effects of dark matter? dark gravity? emergent gravity? strong gravity? etc..).

One important aspect that has emerged in the extended synthesis of evolutionary theory is the question of evolvability. To what degree do organisms have control over the processes of change that make them responsive to their environment? Obviously, having some response mechanisms that control rates of evolution—evolvability—would be significantly beneficial to organisms who, when presented with a change in the environment, need to produce rapid novel traits in progeny to meet the new challenges. William Brown, a biophysicist with the Resonance Science Foundation, has expounded novel mechanisms of evolvability that enable organisms a degree of directed adaptation, showing that evolution is not as random or blind of a process as has been presumed. An important dynamic in the process involves what Brown calls altruistic genes, which inlcude genetic elements that are shared between organisms and which transpose within the genome, having the ability to mobilize to different locations and change genetic expression.

The ability to rapidly generate different profiles of genetic expression can allow an organism a degree of diversity and flexibility in generating phenotypes (physical traits) that are better suited to environmental conditions. Additionally, altruistic genes involve gene sharing mechanisms between organisms, one proposal of which involves direct transfer via membrane-bound vesicles – which would explain the origin of viruses. Obviously, such a theory would mean that not all virus-like transfers are necessarily detrimental or pathogenic, a postulation that has seen recent observational support with the gene sharing (horizontal gene transfer) among Archea using membrane-bound exchange. Note that viruses, as well as transposable genes (which are endogenous retroviral elements) are mostly only identified when they cause illnesses—which raises the question of whether viral-mediated genetic exchange pathways that appear neutral or even beneficial would be identified. It took a great deal of work and controversy to show that transposable genetic elements undoubtedly can confer benefit via increasing epigenetic diversity and alternative gene expression.

The picture that begins to emerge is that there are a number of existing mechanisms that allow a degree of directed adaptation in which organisms can, to a certain extent, control their own evolvability. A recent study reported from NASA’s Astrobiology Institute at the University of Illinois at Urbana-Champaign, headed by physicist Nigel Goldenfeld, has shown that transposon activity increases in a population of microbes in response to environmental stress—enabling a mechanism by which the microorganisms can rapidly adapt via increased genetic and phenotypic diversity.

“Our work shows that the environment does affect the rate at which transposons become active, and subsequently jump into the genome and modify it,” Goldenfeld said. “Thus, the implication is that the environment does change the evolution rate. What our work does not answer at this point is whether the transposon activity suppresses genes that are bad in the particular environment of the cell. It just says that the rate of evolution goes up in response to environmental stress.

“This conclusion,” he added, “was already known through other studies, for certain types of mutation, so is not in itself a complete reversal of the current dogma. We hope that future work will try to measure whether or not the genome instabilities that we can measure are adaptive.”

 

Article: A Simple Bacterium Reveals How Stress Drives Evolution

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