I remember my visits to the New England Aquarium as a child quite well. The electric eel was a must-see. I watched in awe as this creature swam back and forth angrily in its small tank, sending out jolts that would light up a neon bulb. “600 volts!” the sign read, with a voltmeter to prove it.
While electric eels are perhaps the most famous examples of electric fish, they are not unique in their ability to shock. Electric organs evolved independently in six different orders of fish where the non-contractile muscle-like cells (electrocytes) that make up these organs have very different morphologies. How do these cells turn on the juice?
Michael Sussman, professor of biochemistry and director of the biotechnology center at the University of Wisconsin, Madison and Harold Zakon, professor of neurobiology at the University of Texas, Austin, along with their colleagues, sequenced the genome of the electric eel and compared global gene expression levels in skeletal muscle and electric organs from the eel and the closely related glass knifefish to the electric catfish and elephant fish, which come from different fish orders.
“We expanded our analysis beyond [the electric eel] to ask the question of whether similar genes used in the electric eel were used in the other independently evolved tissues as well,” Sussman explained.
The electric eel has the highest concentration of sodium channels and pumps at the plasma membrane of any known organism, so the researchers expected these to be highly overexpressed in electric organs, “But we were especially interested in looking for the regulatory proteins that allow these transporters to be so very highly overexpressed in this uniquely differentiated tissue,” Sussman said.
Sodium channels were indeed overexpressed in each of the orders, and genes associated with muscle contraction were downregulated, but the team found some surprises too, including overexpression of the Hey1 transcription factor in electric organ tissue from all three divergent fish orders.
“Hey1 inhibits the formation of muscle in muscle precursor cells during development. An interesting thing is that there are also high Hey1 levels in the pacemaker cells of the human heart, which are also non-contractile,” Zakon noted. Another surprising finding was the overexpression of insulin-like growth factor (IGF), which controls the size of cells, in electrocytes, which are known for their large size.
Although the different orders of electric fish are separated by 300 million years of evolution, the power of next-generation sequencing and genomics provides a good picture of convergent evolution, showing that these divergent fish each developed similar, well conserved, molecular toolkits to build cells that pack a punch.
Gallant, JR et al. Nonhuman genetics: Genomic basis for the convergent evolution of electric organs. Science 344(6191):1522-5 June 27, 2014