Evolutionary Change Is Just That, Change, and Change Often Is a Mixed Bag and More Complex Than Anticipated

There is an unfortunate tendency to conceive of evolutionary change as causing an adaption to a selection factor that marks an improvement in addressing the environment in which the selection factor exists. Were it only so simple. Change may be an improvement, but sometimes it brings unfortunate baggage along with it.

An example. A variation in a single gene enables a strain of fruit flies to miss 12 hours of sleep without building up a sleep deficit. The flies, nicknamed "rovers" for their active behavior, can also learn and remember things after a sleepless night. Flies with the "rover" version of the gene make more of a protein called protein kinase G ("PKG"). Rovers also move around more in search of food than flies with the "sitter" version of the gene, which produces lower levels of PKG. With the advantage of being able to learn and remember in the face of sleep disruption, rovers ought to have completely taken over the fruit fly population. Instead, sitters make up about 30 percent of the fruit fly population in the orchards where the two types of fruit fly were discovered. Why?

It turns out that rovers are much more sensitive to starvation. When starved, the rovers had impaired memory. Sitters, in contrast, actually had an improved memory following a 12-hour period without food. Additionally, rovers generally died after 41 hours of starvation; in contrast, sitters could go for days. Researchers have speculated that the reason that rovers did not dominate completely was that food is often a sometime thing in the wild; it would be the worst possible time to have memory impacted.

It is thus important to view change in a setting, and determine what else is affected by a change that, on its face, might appear beneficial. When examining the implications of a genetic change, it is best to not focus merely on one aspect of behavior.

The study can be found at http://www.pnas.org/content/109/7/2613.

Another example of things not being what they appear to be is related to prions. The general view is that prions are dangerous (think Creutzfeldt-Jakob disease in people and scrapie in sheep). Maybe.

In yeast, prions cause a wide variety of new characteristics that are not wired into DNA, but can still be passed on to daughter cells. This is not such a foreign concept as would appear at first blush. Note the comment on transgenerational epigenetic effects from pollutants, described in the post of March 17, 2012.

Researchers speculate that the changes might act like prototypes that cells can try out before incorporating them into nucleic acid. If that proves to be the case, it may open the door to understanding a whole new way by which evolution may occur. We shall see.

In prion states, proteins change shape and cause other proteins to change shape too. These misshapen proteins come together to form organized clumps, or amyloids. These clumps stop the individual proteins from functioning properly. Though these protein clumps had been identified in many types of yeast grown in artificial lab conditions, it was unclear whether prions in yeast played a biologically important and nonharmful role in the wild. Researchers thus screened over 700 strains of yeast. Many of these strains were collected from natural sources, such as soil, insects and human patients. It was found that one-third of the yeast contained clumps of misfolded proteins.

The researchers focused on yeast that contained clumps of Sup35, a protein involved in assuring that a cell's proteins are cut to the right length. Some types of yeast with Sup35 clumps were able to adapt under stressful conditions, such as in environments with high acidity or that contain DNA-damaging drugs. Certain adaptive advantages also appeared in yeast containing clumps of Mot3, a protein that mediates the transcription of cell wall-building genes (to translate: The process by which genetic information is copied from DNA to RNA, resulting in a specific protein formation). Overall, about 40 percent of changes brought on by the protein clumps appeared to boost yeast growth under stressful conditions, a useful adaptation.

Needless to say, the researchers are not sure (do not know) how the change might be incorporated into yeast DNA, but it seems like an interesting thesis. In any case, it is interesting how traits are passed along, not always by the mechanisms that we were oh so certain were THE mechanisms, at least last week.

The report on the yeast study can be found at http://www.nature.com/nature/journal/v482/n7385/abs/nature10875.html.

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