Discharging 600 volts of electricity, the eel species confused the father of the theory of evolution

At the murky bottom of the Amazon River, electric eels (also known as electric eels) are groping for their unsuspecting prey. When it detects the ill-fated prey swimming by, the eel releases 600 volts of electricity, shocks or even takes the prey's life.

This natural high-voltage hunting strategy is different, but it turns out that many different fish species use electricity in the same way: Throughout the mud or still water they swim through, they generate and sense electric fields when communicating with other species by light currents, similar to Morse code.

When animals share the same anomalous ability as the ability to discharge electricity, often they are closely related biologically. However, the electric fish in South America and Africa live in 6 branches, 3 different families.

Even the father of evolution, Charles Darwin, when talking about these animals' innate ability to shock, became perplexed by the unusual geographical distribution and biological branching. In the book "On the Origin of Species" he once said: "It is impossible to imagine how these miraculous organs come into being." 

In fact, not just once in history, but repeatedly.

"It's impossible to imagine how these amazing organs come to be."

The Origin of Species - Charles Darwin

A new study published in Science Advances may help solve this evolutionary mystery. "We're just adding to what Darwin thinks, as other biologists usually do," said Harold Zakon, an interdisciplinary biologist at the University of Texas, USA, and co-author of the study. ".

By comparing genes, the research team of Harold Zakon and colleagues at Michigan State University, USA discovered how a series of organs and organs that electric fish have evolved over about 120 million years of evolution , along more than 2500km of sea. The result: There is more than one way to build a bioelectrical organ. 

But it turns out that mother nature is also biased in some ways.

How are electric gliders capable of discharge?

The fish from South America and Africa that Harold Zakon's team studied have developed specialized electrical organs along their bodies. Muscle cells are modified to produce sodium ions; Those cells are called Electrocytes. When the protein molecule responsible for opening/closing the sodium door at the cell membrane is open, an electrical impulse is generated.

In fact, even within an organism's muscles, there is often an electrical current, which flows through and between cells, allowing the organism to contract or relax, creating movement; but with the power generator, this current will be transmitted to the environment. The strength of the electric shock also depends on how many electrocytes the organism uses per discharge.

In most cases, electric fish only use a few at a time, but with electric eels, because they carry so many electrical cells, the current they generate is of course very strong, enough for a single fish. small prey immediately lost their lives.

Location of electrogenic organs in the electric eel Electrophorus electricus.

In a new paper by researcher Harold Zakon, his team simulated an important aspect of the evolution of these bioelectrical organs. His team did this by tracing the genetic history of the species.

Specifically, somewhere between 320 million and 400 million years ago, when the ancestors of the true bony fish (Teleost) were able to miraculously survive a problem with the genome code: The entire genetic code suddenly doubled. Normally, such a genetic change would kill vertebrates, but duplicating such genes gives hidden features a chance to manifest.

Commenting on this, systems biologist Gavin Conant at North Carolina State University, said: "Naturally you can create a whole new neurotransmitter, instead of just creating a gene [character] new". 

The sodium pump is a protein, which plays a role in transporting Na + and K + ions across the cell membrane, controlling the concentration of Na + and K + in the cytoplasm and extracellular fluid.

The concentrations of Na+ and K+ play a very important role in maintaining the ability of the cell to generate electricity.

With the near [chronologically, not biologically] ancestral species of today's freshwater electric fish, gene duplication means they have a copy of an important part of the fish. Weight: Sodium pump. Accordingly, a sodium pump continues to function in muscle cells, the new pump has been modified to give the electrocyte new properties.

Most importantly, before any electrically related organs can function, the copy genome must have no effect on muscle cells; otherwise the presence of this organ would disrupt the organism's ability to move. When Harold Zakon's team investigated how electric fish disable this gene, they were overwhelmed by the results: Different varieties of electric fish behave differently.

For most species in South America, the sodium pump does not exist in the muscle.

In the muscle tissue of the African electric fish, the genome of the sodium pump was completely normal, not faulty, but it didn't work - like a lock without a key. For most species in South America, the sodium pump does not exist in the muscle - the sodium pump is often inactivated by lack of a control element. In an anomalous case also in South America, the sodium pump gene was still active, but it was turned off when the fish was still young; As an adult, another set of genes takes over this power-generating organ.

Convergent evolution, also known as concurrent evolution, is when one or more ancestral species develop similar traits. Common examples are the wings of birds, bats, and flies.

The opposite of convergent evolution is divergent evolution, also known as divergent evolution, i.e., new phylogenetic evolution may retain a function but differ in morphology from the ancestor. Examples are domestic dogs.

In typical cases of convergent evolution, different species of fish will "select" ways of modifying muscle tissue to form electrical organs. They can even make the sodium pump work in certain tissues, but the way they do so is different.

Molecular biologist Johann Eberhart at the University of Texas, USA, explains that often, when studying a case of convergent evolution, scientists find that biological characteristics, it turns out, often have a same formation mechanism.

But in the case of electric eels, "it's quite different. It's interesting."

Systems biologist Gavin Conant says the new finding is in some ways "similar to what we've seen" in his team's own study. Gavin Conant's team found that some genes responsible for signaling between the nervous system and muscles in teleost fish were lost and had previously been duplicated, but some fishes electricity can be kept. Without the genes that can control the electrical organs, the electric eels would not be able to discharge their characteristic strong electric currents.

Researcher Harold Zakon and colleagues are also interested in the possible importance of the control region they found in the sodium pump genome, as it appears to be able to precisely control individual tissues. In fact, this control region is also present in the sodium pump of humans and other vertebrates. Most likely, alterations affecting sodium pump activity in human cells may have caused or were partly responsible for pathologies, such as hypertonia (Myotonia).

Harold Zakon's new research reflects only a few representative species of convergent and convergent evolution that some electric fish have. Several other South American fish species can also generate mild shock currents by altering neurons or by altering muscle tissue. Some saltwater electric fish have a very interesting way of development, for example the astrological fish (Stargazer) can generate electrical impulses to attack prey by modifying the muscles in the eye area.

Astrological fish can generate electrical pulses from the eye area.

However, for researcher Harold Zakon, convergent evolution is a good hint to solve a conundrum of biology: Will evolution continue to evolve as it did in reverse?

Researcher Harold Zakon says a new discovery is "extremely exciting", but whether that's the only way to grow the organ remains unclear. Organs that develop in similar and different ways in electric fish have given us more insight into how surprising biology can be.