Weakly electric fish provide excellent study systems to analyze. The fish has independently evolved electric organs from muscle. However, how the fish could evolve its electric organs remains unclear.
A new study explains how small genetic changes enabled electric fish to evolve electric organs. The finding might also help scientists pinpoint the genetic mutations behind some human diseases.
Evolution took advantage of a quirk of fish genetics to develop electric organs. All fish have two copies of the same gene, creating sodium channels, which are tiny muscle motors.
Electric fish evolved electric organs by turning off one copy of the sodium channel gene in muscles and turning it on in other cells. The tiny motors that normally contract muscles were repurposed to generate electrical signals and bingo! There was the birth of a new organ with incredible potential.
Harold Zakon, professor of neuroscience and integrative biology at The University of Texas at Austin and corresponding author of the study, said, “This is exciting because we can see how a small change in the gene can completely change where it’s expressed.”
Scientists determined that a short section of this sodium channel gene, which is about 20 letters long, controls whether the gene is expressed in any given cell. They confirmed that this control region is either changed or absent in electric fish. That’s why one of the two sodium channel genes in electric fish muscles is switched off. However, the ramifications are much broader than the evolution of electrified fish.
Zakon said, “This control region is in most vertebrates, including humans. So, the next step in terms of human health would be to examine this region in databases of human genes to see how much variation there is in normal people and whether some deletions or mutations in this region could lead to a lowered expression of sodium channels, which might result in disease.”
“The sodium channel gene had to be turned off in muscle before an electric organ could evolve.”
“If they turned on the gene in both muscle and the electric organ, then all the new stuff happening to the sodium channels in the electric organ would also be occurring in the muscle. So, it was important to isolate the expression of the gene to the electric organ, where it could evolve without harming muscle.”
There are two groups of electric fish in the world: South American and African weakly electric fish. Electric fish in Africa had mutations in the control region, while electric fish in South America lost it entirely. Both groups arrived at the same solution for developing an electric organ—losing expression of a sodium channel gene in muscle—though from two different paths.
Gallant said, “If you rewound the tape of life and hit play, would it play back the same way, or find new ways forward? Would evolution work the same way over and over again?. Electric fish, let us try to answer that question because they have repeatedly evolved these incredible traits. We swung for the fences in this paper, trying to understand how these sodium channel genes have been repeatedly lost in electric fish. It was a collaborative effort.”
- Sarah Lapotin et al. Divergent cis-regulatory evolution underlies the convergent loss of sodium channel expression in electric fish. DOI: 10.1126/sciadv.abm2970