Besides stunning predators and killing prey, electric fish also use their shocking organ to navigate and communicate in murky waters at nighttime. Some of them can discharge up to 600 volts -- a jolt several times more powerful than a current from the standard household outlet. According to a new study, electric organs evolved six times, and fish from all six of those lineages used the same genes and developmental pathways to make the organ.
Just as flight arose independently in birds, bats, and bugs, organs capable of generating an electric field have shown up repeatedly in totally unrelated fish: from electric eels (Electrophorus electricus) to electric catfish (Malapterurus electricus), from streams in Gabon, West-Central Africa, to flooded forests in the Amazon. Yet, scientists couldn’t explain the genetic mechanisms leading to this apparent case of convergent evolution.
"These fish have converted a muscle to an electric organ," Michael Sussman from University of Wisconsin-Madison says in a news release. The organ has only evolved in fish because water is needed as a conductor. A 6-foot (1.8-meter) eel essentially has a built-in 5.5-foot (1.6-meter) cattle prod, he adds. “Since all of the visceral organs are near the face, the remaining 90 percent of the fish is almost all electric organ."
To better understand the evolution of genes expressed in electric organs, Sussman and colleagues assembled the genome of the electric eel and sequenced RNA products from electric organs and skeletal muscle of three other fish lineages that had independently evolved the organ. Each of them evolved on a distinct path for millions of years, but despite separate trajectories, the lineages shared similar patterns of gene expression.
All muscle cells have electrical potential. Any simple muscle contraction releases a small amount of voltage, and at least 100 million years ago, some fish started amplifying that potential. Over time, in each electric fish lineage, the expression of genes related to directing electrical current in muscles increased; meanwhile, the expression of genes involved in converting electrical stimuli into muscle contractions (activities not needed in electric organs) decreased. Muscle cells became larger cells capable of generating higher voltages, called electrocytes.
"Evolution has removed the ability of muscle cells to contract and changed the distribution of proteins in the cell membrane,” UW-Madison’s Lindsay Traeger explains. “Now all electrocytes do is push ions across a membrane to create a massive flow of positive charge.” The alignment and polarity of the electrocytes work like batteries stacked in a flashlight, Sussman adds. Millions of such “batteries” work together in the fish’s body, firing their electrical discharge simultaneously.
"Complex structures like the electric organ, which evolved completely independently in six groups of fish, seem to share the same genetic toolkit," Jason Gallant of Michigan State University says in an MSU release. "Evolution makes similar structures from the same starting materials, even if the organisms aren't even that closely related."
The findings were published in Science this week.