Nervous systems may have evolved more than once, new research reveals

It crawls!

But how?

That was the question Adriano Senatore faced in his study of Trichoplax adhaerens, an extremely simple aquatic animal that emerged more than 600 million years ago, and has been squirming around the planet ever since.

“Under a microscope, you can only distinguish six different types of cells in a Trichoplax,” says the U of T Mississauga assistant professor of biology. “It doesn’t look like it has muscles, or a nervous system. It doesn’t have synapses, which are the canonical structures for neural communication. But it can crawl around, find food, move in response to light, chemical signals, and gravity.” Somehow, Trichoplax cells communicate with each other in very specific ways, as occurs in nervous systems.

In explaining where the tiny creatures derive their power of locomotion, Senatore’s research pointed at a much larger discovery: nervous systems, an essential part of almost all animal life, may have evolved more than once. 

“If you try to nail down what the nervous system really is, it’s about cells communicating with each other and passing information along,” Senatore says. “But single celled organisms communicate with each other too, and so do non-neural cells. We think that the necessity to communicate between specific cell types was the main driver for nervous system evolution.”

Cellular communication allows even very simple animals to coordinate their cells into unified action — like the bodily contractions and expansions required to move. Signals within and between cells happen at the molecular level, through the action of proteins called “ion channels” that propagate electrical impulses, and proteins that form synapses, which convert these impulses into chemical signals that affect other cells.

Senatore studies organisms on both genetic and multicellular level to understand how these signalling systems evolved, and how they do their job. Recent findings indicate that most genes required for neural communication arose hundreds of millions of years ago. In collaboration with a team from U of T Mississauga, Brock University, and the National Institutes of Health, he and his team studied the evolution of one particularly important gene called RIM-I, which plays a crucial role in structuring synapses in both vertebrates and invertebrates. 

Evolutionary geneticists have proposed that RIM is one of only 25 “essential” animal-specific genes, and is implicated in the story of how animals evolved nervous systems and synapses.

In Trichoplax, Senatore and his team discovered a second RIM gene, dubbed “RIM II,” which also emerged at the onset of animals. Senatore and his co-authors examined the genomes of many other species to look for RIM II. They reported recently in the journal Genome Biology and Evolution that many animals, including humans, lost the RIM II gene somewhere along their evolutionary path. Their nervous systems are based on RIM I.

Trichoplaxes have both RIM genes, and most surprising of all, at least one group of animals, mysterious jellyfish-like creatures known as ctenophores, only have RIM II and not RIM I. That makes them possibly the most divergent animals on the planet. Some researchers now suspect their animalian nervous system evolved independently.

Senatore says these kinds of discoveries, which involve analyzing huge amounts of data, have only become possible with the technological advances of just the past few years. His laboratory includes a node on U of T Mississauga’s new high-powered computing cluster.

“We have gotten huge support from Information and Instructional Technology Services. In fact, one of the authors on the paper is [U of T Mississauga Senior Systems Administrator] Brian Novogradac, who was a central figure in helping us get our analysis and programs up and running on the cluster.”

Processing power and computer-assisted Big-Data analysis do more than make it easier to answer longstanding questions about the evolution and genetics of nervous systems — they also broaden the range of questions that researchers can ask.

Senatore describes how until recently, research in his field has focused for decades on just a few key “model” species — yeasts, rodents, fruit flies, nematodes, zebrafish — that were considered representative of larger evolutionary principles. Now, he says an explosion of computing power allows evolutionary genomicists to explore more genes in more species in more nuanced ways. The field, he says, is entering a new “golden age.”

“There is a gold mine of possibilities. We often look at genes in a very small number of model systems. And if you see the same gene in three models, you say, ‘This is what it does.’ But when you see it somewhere else, something completely unexpected happens.”