Evolution Update

Evolution Update

Evolution in a Box: How Scientists Evolved a 24hr Pattern in Yeast

Noris Sola February 21, 2015

By taking advantage of ideas in artificial evolution, scientists evolve yeast to have a complex human trait.

Evolutionary biologists typically rely on the idea that differences between modern organisms and their prehistoric counterparts are the result of natural changes that occurred over time. There are many ways to study and observe evolution. Some scientists examine fossils to see how organisms have changed over time. Others look at DNA, focusing on common links between different genes. A group of scientists at Harvard University decided to do things differently and attempt to artificially incite evolution.

To do this, they first asked themselves: how do biological oscillators evolve?

If you’re not a scientist in the field, you have probably never heard of biological oscillators before. Every day, as you wake up and generally go about your day, your body’s rhythms are all part of a cycle. All of these patterns - your heartbeat, sleep patterns, hormones, even what happens when you feel hungry or full- are controlled by tiny machines inside your cells called oscillators.

Most animals also have these tiny machines, but these Harvard scientists asked: can we put something without these patterns onto a schedule like ours? Can we drive an organism to evolve an oscillator when it has none? Can we artificially evolve them?

For these experiment, the scientists chose to work with yeast. Yeast are very well-studied and well-known in science as they have been used by humans for hundreds of years and was one of the first organisms to have their DNA decoded. This makes it ideal for experiments as changes in behavior would easily be noticed. In this case, these yeast-sleuths took a gene that makes yeast glow yellow when exposed to a certain type of light and inserted it into the yeast DNA. This kind of glow is called fluorescence.

Slowly they multiplied yeast populations on a cycle, allowing them to grow and multiply before running them through a machine called a FaCS. What is FaCS? FaCS stands for Fluorescence-activated Cell Sorting, a machine which takes cells and sorts them by whatever parameter you decide. In this case, because they had the yeast glowing, they selected to sort by brightness. After 14 hours, they selected the dimmest and darkest of the yeast and allowed them to continue growing while discarding the rest. After 10 hours they would be run through the machine again. This time, only the brightest yeast would make the cut.

And so the cycle continued, every 24 hours, for hundreds of yeast generations alternating between dimmest and darkest, then boldest and brightest.

You may be wondering: why would this cause them to evolve at all? How does making them glow under a specific light change anything? Why would putting the yeast on this schedule for a few months help them evolve?

Well, it can take a long time for a complex trait to evolve, but nature is impatient. Every day, the world puts pressure on all different animals in all sorts of environments. Is it blistering hot or freezing cold? Has there been a drought or a major flood? Only the animals that survive what nature throws at them get to pass on their genes to the next generation. But its not just their genes that survive! The behavior that led them to survive gets passed on as well.

So yes, this team of scientists 'artificially evolved’ their yeast by getting them to express yellow fluorescence, but they didn’t stop there. They also exerted environmental pressure on their yeast population by selecting only the darkest and brightest to see if their behavior changed. Suddenly, these single-celled organisms whose ancestors had never run on a 24 hour cycle in their evolution was thrust into a cycle where, if they weren’t dark or bright at different times of day, they would perish. So they did what all living things have done for millions of years, they adapted.

Scientists found that just before the selection for the darkest cells, the yeast would separate, moving away from each other so that they were far apart and barely glowing. They kept themselves alone so as to be as dim as possible. Hours later, just before selecting for the brightest cells in the group, the scientists found that the yeast would clump together, forming bright masses of yeast cells, making themselves as bright as possible for selection.

These scientists had done it! They had evolved yeast so that it expressed certain behaviors due to their selection. The gene that made the yeast glow and the one that allows them to clump together weren't originally related. However, after hundreds of generations of cells growing under the selective pressures of the scientist's choosing, the yeast had managed to link the two. When clumped together the yeast were bright; when they were separate they were darker. This allowed them to survive the rigorous selection process, so the yeast began to do this pattern or oscillation-like behavior on their own.

The scientists ran other experiments to see if they had really managed to change the yeast behavior. For example, they stopped running them through the FaCS and let them grow freely to see if they still maintained the separating and clumping behavior twice a day. They did, even without the selective pressure of the FaCS. This is similar to when you spend your day in a dark room, without knowing what time it is outside. You still get hungry at certain times and you still feel tired when its time for bed. The yeast no longer needed to be run through the FaCS in order to run through their schedule.

All of this work helps us understand how we all evolved. Why our behavior and our rhythms are on cycles as well. This experiment was unique in that, instead of tracing differences in genes or pulling apart the genome of yeast to see what they evolved from, the scientists decided to think outside the box. Instead of merely observing, they decided to evolve a complex trait in the yeast themselves. While many people slowly evolve plants and animals in the wild, such as breeding dogs or crossing plants, these scientists took another approach to studying evolution. This 'artificial evolution' of a cell, can provide great insight into how complex traits evolved and how we evolved. Sometimes thinking outside the box can sometimes help us see how the box was built in the first place.

In the following weeks, we will be posting on an exciting new array of experiments that push the envelope exploring how artificial evolution can help us better understand evolution

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