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The Use of Model Systems

When you wake up in the morning, you choose what to wear and what to eat. You probably know why you make those choices. Maybe you always pick your favorite color shirt. Or maybe you eat a big breakfast because you have an important test in school. But how do you make these choices? What parts of your brain and what hormones do you use?

We still don't know very much about how the brain works. But every day more research is being done to understand it a little bit better. Because many of the necessary experiments cannot be done on humans, scientists will use animals as model systems.

Animals are used in research all the time. For example, lab rats may be used to test the effectiveness of a new drug. When an animal is used to learn something that also applies to other animals, it is called a model system. Besides being used for testing new drugs, model systems can also be used to understand the mechanisms of protein synthesis, cell division, gene expression, and other invisible but important processes. Researchers will most often use animals that can reproduce quickly and are easily cared for, like fruit flies, yeast, and mice. But why do the discoveries that are made in yeast, of all things, also apply to the biology of humans?

Fruit Fly
Yeast
Lab rat

One of the most fascinating aspects of our understanding of evolution is that every living being that exists and has ever existed is descended from one common ancestor.

This can be hard to believe. But after decades of careful study, practically all evolutionary researchers have all arrived at the same conclusion. They even yield identical "trees of life" like the one below. These phylogenetic trees show which organisms descended from what ancestors and how closely they are related.

Phylogenetic Tree of Life

Image courtesy of Wikimedia Commons

The small black line from which the rest of the “tree” branches represents the common ancestor of all life on earth. Over millions of years the descendants of that ancestor differentiated and evolved into all the different branches of bacteria, eukaryotes, and archaea.

Because we share common ancestors with the animals used in experiments, researchers know that our cells and DNA all work the same way. This means that any new understandings of how other animals’ cells function can also be applied to human cells.

By studying the signals that determine behavior in other animals, we can begin to understand why humans behave the way they do. When someone has a disorder that changes their behavior, it is usually caused by a change in their brain or hormones. If we first understand what is causing the disorder and its symptoms, more research can be done to find a new way of treating it.

For example, the brains of people with schizophrenia work differently than the brains of other people. This can cause them to show irregular behaviors that make their lives more difficult. Because we now know which chemicals in the brain cause this disease, patients can take medications that regulate these chemicals. This helps them control their symptoms and live a more normal life.

This research on context-dependent behavior uses toads as a model system because we aleady know a lot about why they behave the way they do. For example, we know that they bury themselves in the sand for most of the year to keep from drying out. When they behave a certain way, we know why they do it, and we at least know the short term effects of that action. This means that researchers can now focus their attention on how the behaviors evolved and the hormonal and neurological signals for the behavior.

This lab is looking at how certain hormones affect mate-choice behavior in toads. Remember, breeding with another species is like jumping out of a moving car. It never ends well. But when the mother toad is faced with the "cliff" of rapidly dropping water levels, she knows that her tadpoles need to develop very quickly to survive. So she will choose to sacrifice some of their fertility so that they can metamorphose on time.

But how do they do this? How does the toad receive the signal to mate with the other species? What other hormones and neurons are involved in making this decision? We have no idea, so that means it's time for some science!

Hooray!

This unusual behavior has been verified experimentally. Now, scientists are building on Dr. Pfennig's research to further understand the forces behind it. We know that animals translate an external event into a combination of signals and then into a behavior. But what are those signals in the middle?

Nick Garcia’s research focuses on the role of a hormone called leptin in this behavior. This involves looking closely at how leptin levels change throughout the frog’s reproductive cycle, where it binds in the brain, and whether the two species of spadefoot toad respond to leptin in the same way. This means that he spends time observing the toads themselves as well as testing their DNA and proteins.

But to do all of these experiments, you need a good supply of toads! What does the lab do to keep them healthy and ready to go?