Evolution is a sneaky beast. The multitudes of selective pressures being placed on all of the pieces of all of the species in the world are enormous. Sometimes it’s easy to see where evolution has done its work and why. Birds have hollow bones and feathers so that they can fly. Lizards in the White Sands region of Arizona changed color from dark to light in order to better fit in to their new surroundings in the span of a few generations.
Sometimes, however, adaptations spring up in unexpected places and for unexplained reasons.
Barry Ganetzky, the Steenbock Professor of Biological Sciences at the University of Wisconsin-Madison, recently published a paper highlighting one of those puzzling changes. He and graduate student Megan Campbell discovered a wide range of diversity in a single neural connection in a fruit fly.
With all of the potential ways that the intricate workings of brains could mutate from selective pressures, the pair did not expect to see much diversity in such a seemingly benign place. After all, so long as the synapse works and it sends a signal, it doesn’t really matter how it gets it done.
But when they looked at neuromuscular junction 4 (NMJ4) where a single motor neuron contacts a specific muscle in the fly body wall to drive its activity, the structure differed wildly, even between several very close subspecies.
The synapse resembles miniature trees decked out with tiny bulbs that are the nerve terminals, called synaptic boutons. Within a species – like those bred in laboratories for experiments – the structures all appeared similar. But once they branched out and examined 21 other species from around the world, there was a surprising amount of diversity. Even stranger, those species more closely related were no more likely to have a similar NMJ4 structure than more distant relatives.
“The results were absolutely flabbergasting — variation far beyond anything we ever anticipated,” Ganetzky says.
The mystery is why. On the surface, it doesn’t appear that it should really matter how the nerve synapse is structured so long as it works. But based on computer simulations, random mutations that don’t affect function aren’t enough to explain the wide variety discovered. There must be something pressuring NMJ4 to evolve differently in separate species.
The genus of fruit flies Drosophila has thousands of species with different behaviors, different food preferences, different environments, different climates, different sizes — with upward of 50 million years of divergence. That’s comparable to the evolutionary separation between mice and humans. With all of those differences, something must be driving the evolution of the synapse.
The question remains unanswered for now, but you can be sure that the researchers will be trying to discover an answer. When they do – and someone someday will – it will reveal much that is unknown about how the nervous system evolves over time.