Recent results from a University of Maine research team with expertise in fish biology, genetics and protein chemistry are shedding light on the evolution of life in the world’s coldest ocean. The findings are beginning to answer questions about how Antarctic icefish have evolved to thrive in an extreme environment despite genetic mutations which would probably doom them elsewhere.
The work has generated new information about the interplay of life and the environment and has attracted the attention of medical researchers who need to understand how cells respond to stress and disease. The project has also led to a separate study of the breakdown of proteins leading to fish meat spoilage, a topic of considerable importance to the seafood processing industry.
Collaborators include Bruce Sidell, a physiologist and director of the School of Marine Sciences, and Michael Vayda and Robert Cashon of the Department of Biochemistry, Molecular Biology and Microbiology. Students participating in the research include Kristin O’Brien, Theresa Grove and Deena Small. Two UMaine graduates working on the project as research technicians are Lori Costello and Tom Moylan.
Early years The team focuses on icefish, one of six families of fishes in the Antarctic. This group has evolved in relative isolation over the past 25 to 40 million years and today dominates the southern ocean around Antarctica.
“The early years of our work were centered around questions of energy metabolism,” says Sidell. “We wanted to get at the underlying mechanisms that permit normal cell function at very cold temperatures. Cold temperatures constrain a lot of biological processes. We were interested in understanding how these fish thrive at a body temperature of around zero degrees Centigrade for their entire life history.” Sidell and his graduate students have looked at topics such as how quickly oxygen moves through tissues and how muscle cells change when they are exposed to cold temperatures for long periods. Their work is supported by grants from the National Science Foundation’s Antarctic Program. The extended trips south earned them the Antarctic Service Medal which is awarded for more than 30 hours of work below 60 degrees south latitude. “It looks like adaptation to cold temperatures turns on a genetic program of some sort. It’s very similar to the change in gene expression that happens in endurance exercise training or in some disease states,” Sidell explains. “Chances are, when you see something like that happening, there's a signal here. It may well be that the signal is exactly the same in these fishes adapting to cold temperatures, as it is in endurance training and diabetic myopathy. Nobody knows what it is. And you may be able to get at the answer directly and cleanly using this model.”
In the early 1990s, they shifted gears to focus on a related mystery. It turned out that, as icefish had evolved throughout the millennia, some species had lost the ability to make two chemicals which are vital to most animals. The compounds are hemoglobin, the oxygen-carrying pack horse in red blood cells, and myoglobin, which is found in muscles. Something about icefish allows them to get along without hemoglobin. It was thought until the mid-1980s that the same was true across the board for myoglobin.
Myoglobin is found in high concentrations in the heart and other hard working muscles. It gives beef its red color. It plays a crucial role as a storehouse of oxygen which muscles need in times of stress. While scientists have known about myoglobin for many years, they have yet to completely understand how it works.
It turns out that icefish are a particularly useful model for studying the role of myoglobin in the heart. “Icefish are absolutely unique among all vertebrate animals in their cardio-vascular physiology,” says Sidell. “If you look at the blood of these animals, it’s sort of a cloudy opalescent gray color. There are no other vertebrate species in the world which as adults do not express hemoglobin or red blood cells. They also have some fairly draconian changes in their cardio-vascular system that appear to compensate for this lack of oxygen binding ability. They have very large hearts, large blood volumes and very high cardiac output.”
Since icefish species were first described in the scientific literature in the early 1950s, the evidence suggested that all 15 of them lacked myoglobin. In the mid-1980s, scientists reported finding the chemical in two species. That report was viewed with skepticism, and Sidell was among the skeptics. His opinion changed during a research trip to the Antarctic in early 1990s.
“We had collected an icefish species that we don’t see very often, and we had decided to harvest this particular animal for some tissue experiments. We opened it up, and because it has no hemoglobin or red blood cells, everything is really pale. And there, sitting in the chest cavity of the animal, was a very rose colored heart ventricle. “I got pretty excited. I managed to get the physician at the Palmer Station (in Antarctica) whipped up enough about it to bring in a surgical light and take some pictures.” Over the next few months, Sidell and his colleagues performed a series of tests which confirmed that indeed, myoglobin was present. That finding raised a variety of important questions. Did other icefish species have it? Is it necessary for those species, or is it akin to the human appendix, an unnecessary evolutionary artifact? If indeed myoglobin is important, how have species without myoglobin adapted to its loss? The questions are not fully answered, and indeed Vayda and Sidell have an unresolved bet over the evolutionary artifact question. In fact, says Vayda, the answer may be more complicated than they thought. Experiments have shown that icefish hearts containing myoglobin are stronger than hearts without it.
However, in the environment of the deep cold southern ocean, mutations leading to the loss of myoglobin are not lethal. Icefish without myoglobin get along quite well, thank you.
The team has confirmed that during the evolution of icefish species, genetic mutations leading to the loss of myoglobin have occurred independently at four different times. Moreover, the mutations are not identical. In other words, there is no single genetic smoking gun which eliminated myoglobin from some icefish species. Cashon has confirmed that fish myoglobin is structurally different from mammalian myoglobin. The fish variety is more flexible and appears to bind and release its store of oxygen more rapidly than human myoglobin.
The team has published numerous articles about its findings, and more than half of the scientists who have requested reprints of those articles are medical researchers. For example, a University of Rochester Medical School scientist recently contacted Sidell about the myoglobin work which may be helpful in research on cardiac physiology and oxygen starved tissues. “If you look at most of the work being done now on artificial blood substitutes, it’s based on myoglobin. There’s more to be understood about how myoglobin structures can be changed to result in different functional characteristics,” says Sidell.
The above story is based on materials provided by University Of Maine. Note: Materials may be edited for content and length.
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