New research by a team led by UW-Madison biochemistry Professor Michael Sussman shows that electric fish, including the electric eel, evolved their electric organ six times independently over the course of evolutionary history. Sussman's team identified the molecular levers and developmental pathways that all six lineages of electric fish worldwide have in common, resolving a longstanding mystery of what scientists call convergent evolution, a problem Darwin himself pondered. (Credit: Jason Gallant, Michigan State University)
The work establishes the genetic basis for the electric organ, an anatomical feature found only in fish and that evolved independently half a dozen times in environments ranging from the flooded forests of the Amazon to murky marine environments.
"These fish have converted a muscle to an electric organ," explains Sussman, a professor of biochemistry and director of the UW-Madison Biotechnology Center, who first undertook the exploration of the electric organ almost a decade ago. The study published in Science provides evidence to support the idea that the six electric fish lineages, all of which evolved independently, used essentially the same genes and developmental and cellular pathways to make an electric organ, needed for defense, predation, navigation and communication.
"What is amazing is that the electric organ arose independently six times in the course of evolutionary history," says Lindsay Traeger, a UW-Madison graduate student in genetics and a co-lead author of the new report along with Jason Gallant, an assistant professor of zoology at Michigan State University.
Adds Gallant: "The surprising result of our study is that electric fish seem to use the same 'genetic toolbox' to build their electric organ," despite the fact that they evolved independently.
Worldwide, there are hundreds of electric fish in six broad lineages. Their taxonomic diversity is so great that Darwin himself cited electric fishes as critical examples of convergent evolution, where unrelated animals independently evolve similar traits to adapt to a particular environment or ecological niche.
The "in-series alignment" of the electrocytes and unique polarity of each cell allows for the "summation of voltages, much like batteries stacked in series in a flashlight," says Sussman.
The "in-series alignment" of the electrocytes and unique polarity of each cell allows for the "summation of voltages, much like batteries stacked in series in a flashlight," says Sussman.
The additional current required for the power comes from the fact that an eel body contains many millions of such "flashlights" working together and firing their electrical discharge simultaneously.
In addition to sequencing and assembling DNA from the electric eel genome, the team produced protein sequences from the cells of the electric organs and skeletal muscles of three other electric fish lineages using RNA sequencing and analysis. A computationally intense comparative study of the sequences showed that electric organs in fish worldwide used the same genetic tools and cellular and developmental pathways to independently create the electric organ.
"I consider 'exotic' organisms such as the electric fish to be one of nature's wonders and an important 'gift' to humanity," says Sussman. "Our study demonstrates nature's creative powers and its parsimony, using the same genetic and developmental tools to invent an adaptive trait time and again in widely disparate environments. By learning how nature does this, we may be able to manipulate the process with muscle in other organisms and, in the near future, perhaps use the tools of synthetic biology to create electrocytes for generating electrical power in bionic devices within the human body or for uses we have not thought of yet."
