By: Eliana Zhang
Imagine if you were assigned the task of designing and implementing changes to a crucial mechanical part of an airplane -- while the plane is still flying! That sounds nearly impossible without disaster, right? Your situation would be similar to a puzzle faced by early evolutionary biologists. How could mutations redesign the crucial mechanisms of a living organism while still keeping the organism alive? Although mutations are the main way new traits and features are introduced to an organism during evolution, most mutations cause irreparable damage, sinking our airplane. If a trait is important, how could it possibly be altered while still allowing an organism -- as well as its offspring -- to survive?
Geneticists have found one way mutations can accumulate without obstructing pre-existing cellular function: gene duplication! Errors during cell division as well as efforts to repair DNA breakage can result in an extra (and possibly unused) copy of a gene. These accidents can provide raw material for mutations to accumulate with, facilitating natural selection. This can be seen in plants, where many examples of gene duplication occur. For example, the wild mustard plant species has gone through at least two duplications of all of its chromosomes at some point in its evolutionary history, not to mention its duplications of several individual genes at other times.
One interesting example of gene duplication involves sweet-tasting proteins. Scientists have studied thousands of proteins so far, and most have no noticeable flavor. Around half a dozen, however, have an intensely sweet taste. These rare, sweet proteins can be found in plants from several different continents: “curculin” is present in the fruit of a Malaysian herb; “malabin” can be found in a traditional Chinese herb; “thaumatin” is in the fruit of a West-African, Chinese herb; and “brazzein” is from a wild fruit in Cameroon. Each of these proteins only taste sweet to humans and certain monkeys. They likely provided an advantage to the plants; sweeter fruits would be more alluring to animals who could taste them, and thus their seeds would be distributed more, increasing the proportion of passed-down genes that make sweet proteins in the population. While it is clear why the plants containing these proteins are successful, there is the question of how these proteins came to be.
No one knows exactly how they originated, but it was most likely gene duplication. The proteins are found on different continents in unrelated plants, and they look nothing alike to one another, despite their resemblance to other proteins normally found in healthy plants. For example, brazzein and mabinlin resemble other, known “proteinase inhibitors” -- proteins that can help prevent further damage when a plant is harmed. Most sweet-tasting proteins share a similar story: they resemble other plant proteins with ordinary functions and gene sequences that are either missing or mutated. It seems like pre-existing genes were recycled to become genes for sweet proteins!
At some point in the past of many species that have mutated over time, gene duplication errors resulted in an extra gene that was free to mutate because there was still a valid copy of the gene to support original functions. As time passed, the extra copy acquired mutations. In some plants, those mutations happened to provide sweetness. Plants with that mutation were more appealing to animals, won the game of natural selection, and the rest was history. Duplication of a gene followed by a mutation can lead to the rise of beneficial traits!
What did you learn?
How can mutations arise without impacting the original function?
Complex traits resulting from mutations can come from gene duplication. Duplication of a gene can make a copy that is free to mutate, and a copy that codes for its original function. As a result, if a mutation occurs, the other copy can still fulfill the original task and the new protein can function differently than the original one.
How can sweet proteins benefit plants, and why do they continue to exist?
Sweet fruits would be eaten more often. Since seeds (one way plants can reproduce) are contained within the fruits, getting animals to eat more fruits is beneficial for the productivity of plant reproduction. As a result, there would be more plants with those genes in the next generation, and the proportion of sweet protein genes in the population would grow over time.