Electric fish, such as the electric eel, use electric organs to accomplish amazing feats, such as sending and receiving signals akin to bird songs, which allows them to identify other electric fish based on their species, gender, and even individuality. New research published in Science Advances provides insight into how electric fish evolved electric organs as a result of small genetic changes. Scientists may also be able to identify the genetic mutations that lead to some human diseases on the basis of this discovery.
In order for fish to develop electric organs, evolution used a quirk of their genetic makeup. A duplicate copy of the same gene that produces sodium channels, which are tiny motors within the muscles, is present in all fish. Electric fish evolved their electric organs by turning off one duplicate of the sodium channel gene in muscles, and turning it on in other tissues. Tiny motors that are normally used to make muscles contract were repurposed to generate electrical signals and voilà! There is now an organ with a host of astonishing capabilities.
In this study, Harold Zakon, professor of neuroscience and integrative biology at The University of Texas at Austin and coauthor of the study, explained: “This is interesting because we are able to see how a small change in the gene can completely alter where it is expressed.”.
Scientists at the University of Texas at Austin and Michigan State University describe how they discovered a small portion of the sodium channel gene – about 20 letters long – that affects expression in any given cell. They confirmed that the control region of electric fish has either been altered or is completely absent. This is the reason for the disabling of one of the two sodium channel genes in the muscles of electric fish. It has implications that extend beyond the evolutionary development of electric fish.
“Humans possess the same control region as most vertebrates,” Zakon explained. Considering human health, the next step might be to examine this region in genetic databases to determine whether there is a great deal of variation in normal individuals and whether some deletions or mutations in this region are indicative of a decrease in sodium channel expression, which may result in disease.”
A research technician in Zakon’s lab at the time of the study was the first author of the study. As of currently, she is a doctoral candidate at the University of Utah and is a graduate student in visual psychology. Additionally to Zakon and Eberhart, the study’s other senior authors include Jason Gallant, an associate professor of integrative biology at Michigan State University, and Johann Eberhart, professor of molecular biosciences at the University of Texas at Austin.
According to Zakon, an electric organ cannot evolve without the sodium channel gene turned off in muscle.
“If the scientists turned on the gene in the muscle and in the electric organ simultaneously, then together all the exciting stuff that was happening to the sodium channels in the electric organ would also happen in the muscle,” Zakon explained. The reason for isolating the expression of the gene from the muscle is that it can evolve without harming the muscle in a controlled situation.
There are two main groups of electric fish found in the world – one is indigenous to the tropics of Africa, the other to South America. It did not take much for the researchers to notice that the electric fish in Africa possessed mutations in the control region, but the electric fish in South America had completely lost the control region. Although the two research groups approached development of an electric organ from completely different directions, both found the same solution — in which a sodium channel gene was not expressed in muscle.
Would evolution be the same way over and over again if you rewound the tape of life and hit play? Could you have the same experience over and over again?” said Gallant, who breeds the South American electric fish that were used in the study. The researchers concluded that these electrical fish provided us with an excellent opportunity to answer this question due to their repeated evolution of these remarkable characteristics. We swung for the fences in this paper, trying to understand how sodium channel genes are repeatedly lost in electric fish.
It is one of the next questions that researchers hope to answer concerning the evolution of the control region that enables sodium channels to fire in the electric organ.
The National Science Foundation and the National Institutes of Health provided funding for this study.