Six lineages of electric fish independently evolved their electricity-generating organs with essentially the same genes and development and cellular pathways.
The research, published in the current issue of Science, sheds light on the genetic blueprint used to evolve these complex, novel organs, which the creatures use for defense, predation, navigation, and communication.
“It’s truly exciting to find that complex structures like the electric organ, which evolved completely independently in six groups of fish, seem to share the same genetic toolkit,” says Jason Gallant, a zoologist at Michigan State University and co-lead author of the paper detailing the electric eel genome.
“Biologists are starting to learn, using genomics, that evolution makes similar structures from the same starting materials, even if the organisms aren’t even that closely related.”
Not really an eel but a fish more closely related to the catfish, the electric eel produces a jolting electric field of up to 600 volts, about 100 volts per foot of fish.
Worldwide, there are hundreds of species of electric fish in six broad lineages. Their 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.
“An exciting result of this work is that it pinpoints steps in various cellular pathways that are the most likely to evolve in other animals as well,” says co-lead author Harold Zakon of the University of Texas at Austin.
“For example, the pathways that transmit electrical pulses in the vertebrate heart, including our own heart, derive from muscles. We find that electric organs in fish and these pathways in our hearts share some of the same regulatory genes.”
Like batteries in a flashlight
All muscle and nerve cells have electrical potential. Simple contraction of a muscle will release a small amount of voltage. But between 100 and 200 million years ago, some fish began to amplify that potential by evolving electrocytes from muscle cells, organized in sequence and capable of generating much higher voltages than those used to make muscles work.
“Evolution has removed the ability of muscle cells to contract and changed the distribution of proteins in the cell membrane; now all electrocytes do is push ions across a membrane to create a massive flow of positive charge,” says Lindsay Traeger, University of Wisconsin graduate student and co-author of the study.
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 Michael Sussman, a biochemist at the University of Wisconsin.
The additional current required for the power comes from the fact that an eel body contains many millions of such “batteries” working together and firing their electrical discharge simultaneously.
The National Science Foundation, the W.M. Keck Foundation, and the National Institutes of Health funded the work. Researchers from Rice University also contributed to the paper.