Along the murky bottom of the Amazon River, there are fish that look like electric eels. The fish use 600-volt electricity to stun or kill someone when they swim by. Some fish species use electricity to navigate through muddy, slow- moving waters and communicate with others of their species through gentle shocks.
It is normal for several species to have the same ability as generating electricity. There are different groups of electric fish in the rivers of South America and Africa. In On the Origin of Species, Charles Darwin mused on both the novelty of their electrical abilities and the strange geographical distribution of them.
The paper published in Science Advances helps to understand the mystery of evolution. Harold Zakon, one of the co-senior authors of the study, said that they were just following up on Darwin. His team in Texas and Michigan State University were able to decipher how a number of strikingly similar electric organs arose in electric fish. There is more than one way to evolve an electric organ.
South American and African fish get their zap from specialized electric organs that extend along much of their body. The organs have modified muscle cells. A burst of current can be produced when the sodium-gate proteins open. It is the simplest signal you could imagine.
In muscle, the electric signals flow between the cells to help them contract for movements, but in the electric organs they are not. The strength of each shock is dependent on how many fire at one time. Electric fish only fire a few at a time, but because electric eels have a lot of electric cells, they can kill small prey.
A key aspect of the evolution of these electric organs was reconstructed by Zakon and his former research technician Sarah LaPotin.
Between 320 million and 400 million years ago, there was a rare genetic accident that duplicated the entire genome of the fish's ancestors. Life-threatening whole-genome duplications are often found in animals. duplications can open up previously undiscovered genetic possibilities because they create redundant copies of everything. A systems biologist at North Carolina State University who was not involved in the study said, "Suddenly, you have the ability to make a whole new pathway, instead of just one new genes."
The more recent ancestors of freshwater electric fish had an extra copy of a gene for an important pump. One copy of the second copy worked in muscle cells.
The second copy of the gene had to be disabled in muscle cells before any electric organ specific adaptation could be made. When Zakon and his colleagues looked at how the electric fish turned off the genes, they were surprised to find that different lines of electric fish did it differently.
The African fish's sodium-pump gene was still functional, but like a lock with no key, it couldn't be activated without helpers. In most of the South American fish, the pump was missing from the muscles because it was missing an essential control element. One of the South American fish's genes was still functioning in muscles. When a different set of genes took over control of the sodium channel in the electric organ, it was turned back on.
In a textbook example of convergent evolution, the various lineages of fish independently hit on the strategy of modifying their muscle tissue to create electrical organs, and they were able to do so by making their sodium pumps work in different tissues. They differed on how to regulate the pumps.
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