It was as if this ion channel were a dial that could twist one neuron type into the other. But what was actually different about this protein in the snake’s body and rattle?
At first, the researchers thought that rattle motor neurons must have extra KV72/3 potassium channels. If the rattle neurons had more channels, the scientists figured, then they could discharge ions more quickly, bringing the voltage back down to prepare the channels to quickly fire again.
To find out, Bothe and Chagnaud extracted and sequenced RNA from both types of rattlesnake motor neurons and sent the data to Jason Gallant, an evolutionary biologist at Michigan State University, so he could compare the expression of the KV72/3 channel gene between the two tissues. The gene for KV72/3 channels is the same in every cell of the animal’s body — but if the rattle neurons had more KV72/3 channels, the researchers would expect to see higher gene expression in that tissue.
Alas, their simple explanation was not proved out. “There really is no difference in the level of gene expression in these potassium channels, which was disappointing,” Gallant said. “But I think it opens up a more realistic view of biology.”
Variations in the gene’s expression would have provided a simple, open-and-shut way to explain how the evolutionary screws on rattlesnake motor neurons are adjusted. But biology offers other possibilities. Chagnaud and Bothe speculated that after the channel proteins are constructed from the genetic blueprint, they could be modified into slightly different forms that manage ions differently. More research will be needed to pin down the details — to find the control that adjusts the control.
For his part, Katz didn’t consider the result disappointing at all. “So they didn’t see a [change in] gene expression. That was the answer they expected,” he said. “But the fact is that that’s a cool result.”
For many decades, researchers have assumed that motor circuits “exist as they will be used,” Katz said — meaning that initiating a behavior like walking or swimming is simply a matter of turning on the right circuit. In this view, evolving a new behavior would require an entirely new circuit layout. But in studies of organisms as diverse as crustaceans, sea slugs and now possibly snakes, researchers are finding that interactions with neuromodulators and other chemicals can modulate the activity that a circuit evokes, leading the same networks of cells to produce markedly different behaviors.
The new study, Katz said, hints that playing with this plasticity could be a way that new movement behaviors evolve. Perhaps the difference between rattle and body behavior has something to do with subtle differences in their cells’ chemical environments, not the structure or expression of the ion channel itself.
“For a lot of evolutionary modifications, your primary goal is to not break the animal, right?” Bagnall said. “Anything that you can do that tunes traits without becoming an on/off switch is a powerful means of driving change without being deeply deleterious.”
Turning and Tuning
This new study shows that it’s possible to tune motor neurons for wildly different behaviors by tweaking a single protein. But motor neurons are just one piece of the movement puzzle. They’re the last link in a chain that begins with circuits in the central nervous system known as central pattern generators, which generate the rhythmic patterns involved in walking or swimming. Those upstream circuits are better understood in other organisms, like zebra fish. In rattlesnakes, puzzling them out would be a next logical step.
“The number-one missing link,” Katz said, “is how do you create the frequency for the rattle? Where does that come from?”
Chagnaud is eager to find out if a similar Stellschraube tunes motor neurons in another species feared for its bite. Like rattlesnakes, piranhas execute two rhythmic movements with radically different frequencies: swimming, with a frequency of up to six cycles per second, and vibrating their swim bladders at frequencies of up to 140 cycles per second to make noises that sound like barks, yips and drumbeats. However, unlike rattlesnakes, piranhas use the same section of their spine to control both movement types.
“I’m curious to know, will it be KV72/3? We have no idea,” Chagnaud said. “Did evolution find the same solution to the same problem?”
He has his doubts. Although he’s hopeful about finding a similar mechanism, the surprising — and at times frustrating — discovery in rattlesnakes “was an eye-opener,” he said. Evolution is not a human designer with a goal in mind. Its methods are mysterious, and its toolbox is vast. “And you have very different screws that you can turn.”