Extremely Adapted!

From minuscule bacteria to roaming elephants, life has taken hold of our planet. Through the process of evolution by natural selection, life on earth has taken various forms and utilizes all kinds of spaces. Even the most extreme environments such as our frozen poles, deep-sea hydrothermal vents, dry deserts, and poisonous waters, are inhabited by life. How animals evolve to survive and thrive in these environments is a curiosity that evolutionary biologists like Ryan Greenway at Kansas State University seek to understand. He and colleagues ask big questions, such as how do organisms adapt to different environments and how can that adaptation lead to new species forming in nature?

Ryan and his colleagues use a combination of experiments in ecology, animal biology, and molecular biology to ask how organisms have evolved to survive in a specific extreme environment: streams with high levels of hydrogen sulfide. Just like cyanide, hydrogen sulfide is toxic to animals because it stops cells from making energy. These highly toxic environments should kill most animals, but scientists have found fish that can survive there. Even though fish can be found in both the sulfide and non-sulfide streams located right next to each other (see picture below), it appears that only some types of fish are able to survive the toxic environment of the sulfide streams. Ryan and his colleagues formulate and test hypotheses that could explain how these fish are able to survive in sulfide streams.

Sulfidic habitats and fish
A. A male Atlantic molly from a regular freshwater stream. B. A male molly from a toxic sulfide stream. C. An area where a clear freshwater stream (left) and cloudy sulfide stream (right) flow together.

One such hypothesis is that the fish living in sulfide streams have adaptations that allow them to live in sulfide that the fish from normal freshwater streams do not possess. Alternatively, the fish from non-sulfide habitats may have the same adaptations and ability to live in sulfide streams. These scientists fly to Mexico where they collect samples and data from naturally occurring sulfide streams to test these and other hypotheses. Ryan records observations about several species of fish including the environments in which they live and data about their physical characteristics. Ryan also conducts experiments, such as putting fish in escape-proof containers and moving them from sulfide streams to non-sulfide habitats, and vice versa, then recording whether the fish live or die. They have found that fish that are moved from non-sulfide streams to sulfide streams cannot survive the unfamiliar environment. This indicates that fish from sulfide streams are in fact uniquely adapted to survive in the toxic waters.

Survival experiments in sulfide stream (top) and freshwater stream (bottom). Fish from different habitat types are placed in the orange escape-proof containers and left in the streams. Every 24 hours, Ryan and colleagues record the number of fish that have lived or died.

After this field ecology work, Ryan is now working to understand the molecular changes that allow fish to survive in sulfide environments. By looking at DNA from fish that can survive in sulfide rich waters and from those that cannot, he is able to identify differences which lead to other types of questions. Once a DNA sequence is known, Ryan can study its protein product more closely. By looking at how a protein from sulfidic fish is different from the same protein in non-sulfidic fish, a finer understanding of what leads to adaptation is possible. For instance, if a protein from a sulfidic fish has changed in such a way that it would no longer function in non-sulfidic fish, then offspring of a sulfidic and a non-sulfidic fish would not be able to survive and reproduce. Changes such as these could lead to the sulfidic fish becoming a distinct species. Seeking a molecular understanding of the origin of species, Ryan and colleagues perform experiments to test this and similar hypotheses.

Using the scientific method, these extremophile researchers have identified morphological and physiological differences between sulfidic and non-sulfidic fish. As previously mentioned, hydrogen sulfide is toxic because it affects an organism’s ability to produce energy. Ryan and colleagues have found that changes in genes and proteins involved with energy production allow fish in sulfide streams to resist the toxicity and survive in these environments. They are now looking at interactions between these specific proteins in sulfidic fish and how those changes may be involved in the evolution of sulfide fish into distinct species.

Rather than retreating from extreme environments, life presses on. The important ongoing work carried out by Ryan and his colleagues will contribute much to our collective understanding of how life changes to thrive.

CaptureThis post was written by Akeem Waite, a fourth-year graduate student. Akeem is a cell biologist working with bakers/brewers yeast to understand how cells control proteins. Yes, this research smells like bread and beer!

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