Hundreds of millions of years before the evolution of animals with segmented bodies, jointed skeletons, or appendages, soft-bodied invertebrates like sea slugs dominated the seas. A new study finds parallels between the brain architecture that drives locomotion in sea slugs and that of more complex segmented creatures with jointed skeletons and appendages.
The study, published in the Journal of Neuroscience, suggests that instead of evolving an entirely new set of neural circuits to govern the movement of segmented body parts, insects, crustaceans, and even vertebrates such as mammals adapted a network of neurons, a module, than guided locomotion and posture in much simpler organisms.
“Sea slugs may still have that module, a small network of neurons called ‘cluster A,’ with 23 neurons identified so far,” said University of Illinois Urbana-Champaign professor of molecular and integrative physiology Rhanor Gillette. , who led the new investigation. .
“The question we address in this study is whether the similarities we see between sea slugs and more complex creatures evolved independently or whether those with segmented body parts and appendages may have inherited their underlying neural circuitry from a common bilateral ancestor. soft-bodied.” he said.
To answer that question, Gillette and his colleagues, former graduate students Colin Lee and Jeffrey Brown, videotaped the movements of sea slugs and combined that data with recorded responses to stimulation of specific nerves and neurons in the brain. of sea slugs.
“The predatory sea slug we studied, Pleurobranchea californicait uses the cilia on its foot to crawl, paddling through the secreted mucus,” Gillette said. – all powered by group A.”
Previous studies from the Gillette laboratory showed that Pleurobranchea it performs cost-benefit calculations each time it encounters another creature in the wild. If he is very hungry, the neurons that control his attack and feeding behavior are in a heightened state of arousal and he will chase almost anything that smells like food. In other circumstances, it will do nothing or even actively avoid the stimulus.
“This is a good idea if you don’t need the food and can avoid other cannibals. Pleurobranchea attracted to him,” Gillette said. “All of these behaviors involve how the A group coordinates with action choices.”
In mammals, a special module in the rhombencephalon called the reticular system translates specific instructions for action choices from higher brain regions for posture and locomotion, Gillette said. This region then sends the motor commands to the spinal cord for final transmission to the muscles.
“In particular, the reticular system relies on critical serotonin-producing neurons to control body movements in posture and locomotion,” he said. “In the new study, we found that similar serotonin-producing neurons in group A sea slugs drive behaviors such as chasing, avoidance and escape.
“In their relative simplicity, sea slugs resemble in many respects the expected simpler ancestor of today’s complex animals,” Gillette said. “All of the major modules of the action choice circuitry, which translate that choice into motor commands and the generation of motor patterns found in complex animal nervous systems are also identifiable in the simplest soft-bodied sea slugs.”
The study offers the first evidence that the circuits that drive locomotion in animals with complex bodies and behaviors “have close functional analogies in the simplest gastropod molluscs and may share a common inheritance,” Gillette said.
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