When fully grown, electric eels (Electrophorus electricus) are 2 meters long and weigh 20 kg. They live in the swamps and streams of the Amazon and Orinoco basins, where they stun prey and deter predators by delivering millisecond electric pulses of up to 600 volts and up to 1 ampere.
Of the 60 or so species of electric ray, the largest is the Atlantic torpedo (Torpedo nobiliana), which can reach up to 1.8 meters in length and 40 kg in mass. As its name suggests, the Atlantic torpedo is found along the coasts of the Atlantic Ocean. Its electric organs can deliver pulses potent enough to knock a human diver unconscious.
Despite its name, the electric eel (Electrophorus electricus) is not an eel, but a knifefish. CREDIT: Süddeutschen Zeitung
As fish go, E. electricus and T. nobiliana look nothing like each other. One inhabits freshwater; the other, seawater. To Charles Darwin, the fact they and other species of electric fish share the ability to generate electric fields was, if not a challenge to evolution, then at least an observation that his theory had to accommodate.
In On the Origin of Species Darwin argued that because of their diversity, the electric fishes could not have a recent common ancestor. Most fishes lack the ability to generate electric fields, a fact that militates against their having a distant common ancestor. That's because the inevitably large number of intervening branches that would connect the common ancestor to today's fish would yield an implausibly large number of species that had lost their electric ability.
Having rejected the common ancestor explanations, Darwin reached a different conclusion:
I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings, which owe but little of their structure in common to inheritance from the same ancestor.
The process that Darwin identified has become known as convergent evolution. In a sense, its existence is not surprising. Faced with similar environment challenges, different species can acquire similar traits. Bats and dolphins, for example, have both evolved the ability to locate prey through echolocation.
Genomic basis of convergent evolution
In a paper published this week in Science, Michael Sussman of the University of Wisconsin and his collaborators describe their investigation of the genomic basis of convergent evolution. The focus of their study was on the electric organs of E. electricus and other electric fish.
The team's starting point was the long-held assumption that electrocytes (as the cells that generate the electric fields are known) evolved from muscle cells, with which they share some properties. Like muscle cells, electrocytes are covered in transmembrane ion channels and are connected to neurons.
But electrocytes lack the molecular structures that endow muscle cells with their ability to contract. And because potential difference depends on charge separation, electrocytes are large—larger than muscle cells.
Myosin is a motor protein essential for contraction. A muscle cell makes it when the cell's protein factories, the ribosomes, receive a stretch of messenger RNA (mRNA) that encodes the myosin gene. If electrocytes are not descended from muscle cells, none of the mRNA molecules present in electrocytes would carry the myosin gene.
But if electrocytes are descended from muscle cells, the various regulatory processes that control which mRNAs are transcribed might still produce some myosin-encoding mRNA, albeit in small quantities.
To find out, Sussman and his collaborators extracted mRNA from the electric organs of five species of electric fish. They also extracted mRNA from the fishes' kidney, brain, skeletal muscle, and heart. Taken together, the mRNAs represented around 20 000 protein-encoding genes.
Thanks to sequencing technology and the statistical tools of bioinformatics, the team could identify genes and measure the frequency with which the researchers showed up in the variously sourced mRNA. In particular, they could determine which genes are more common (up-regulated is the biological term) in electric organs than in skeletal muscle and which genes are less common (down-regulated).
The team's findings paint a remarkably consistent picture. The most strongly up-regulated genes in electric organs turned out to be ones associated with electricity. For example, a gene called scn4aa that encodes a sodium ion channel is strongly up-regulated in electric organs and strongly down-regulated in muscle. The converse is also applies. The myosin gene smyd1a is strongly down-regulated in electric organs and strongly up-regulated in muscle.
In general, evolution entails the mutation of proteins. In the case of electric organs, the mutated proteins are evidently transcription factors and other proteins that regulate the expression of genes.
Although the paper that Sussman and his colleagues wrote is about convergent evolution, I struggled to discern anything in it that marked convergent evolution as being fundamentally different from ordinary, nonconvergent evolution. Indeed, the evolution of electric organs from skeletal muscle could serve as a textbook example of how evolution plays out at the molecular level. And as Darwin's analogy of the two inventors seems to imply, convergent evolution could simply be a matter of coincidence.