Electrical fish genomes show how evolution repeats itself

Along the murky At the bottom of the Amazon, snakefish called electric eels scour the darkness for unwary frogs or other small prey. When one swims by, the fish releases two 600-volt pulses of electricity to stun or kill it. This high-voltage hunting tactic is distinctive, but a handful of other fish species also use electricity: they create and feel weaker voltages when navigating muddy, slow-moving waters and when communicating with others of their kind through gentle tremors similar to Morse code.

When multiple species share an unusual ability like generating electricity, it’s usually because they’re closely related. But the electric fish in the rivers of South America and Africa comprise six distinct taxonomic groups, and in addition there are three other marine lineages of electric fish. Even Charles Darwin pondered both the novelty of their electrical abilities and the strange taxonomic and geographical distribution of them On the emergence of speciesand wrote: “It is impossible to imagine by what steps these wondrous organs were made” – not just once, but repeatedly.

A recently published paper in scientific advances helps unravel this evolutionary mystery. “We’re really just following Darwin, like most biologists do,” said Harold Zakon, an integrative biologist at the University of Texas, Austin and co-senior author of the study. By stitching together genomic clues, his team in Texas and colleagues at Michigan State University uncovered how a series of strikingly similar electrical organs arose in lineages of electric fish separated by about 120 million years of evolution and 1,600 miles of ocean. It turns out there’s more than one way to design an electric organ, but nature has some favorite tricks to fall back on.

The South American and African fish studied by Zakon’s group get their shock from specialized electrical organs that stretch along much of their bodies. Modified muscle cells, called electrocytes, in the organs create sodium ion gradients. When sodium gate proteins in the membranes of the electrocytes open, it creates a surge of electricity. “It’s pretty much the simplest signal imaginable,” Zakon said.

In muscles, these electrical signals flow through and between cells to help them contract for movement, but in electrical organs the voltage is directed outward. The strength of each shock depends on how many electrocytes are firing at once. Most electrofish fire only a few at a time, but because electric eels contain an unusually high number of electrical cells, they can unleash voltages strong enough to kill small prey.

In the new work, Zakon, his former research technician Sarah LaPotin (now a graduate student at the University of Utah), and his other colleagues reconstructed a key aspect of the evolution of these electrical organs by tracing the fish’s genome history.

It began 320 to 400 million years ago when the ancestor of all fish classified as bony fish survived a rare genetic accident that duplicated its entire genome. Whole genome duplications are often fatal to vertebrates. However, because they create redundant copies of everything in the genome, duplications can also open up previously untapped genetic opportunities. “All of a sudden you have the ability to create a whole new pathway instead of just a new gene,” said Gavin Conant, a systems biologist at North Carolina State University who was not involved in the study.

Harold Zakon, an integrative biologist at the University of Texas, Austin, was one of the leaders of the new study on the evolution of electric fish. “We really just follow Darwin, like most biologists do,” he said.Photo: Lynne McAnelly/Quanta Magazine

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