The researchers propose a new model for the evolution of higher brain functions and behaviors in the insect hymenoptera. The team compared Kenyon cells, a type of neuron, in the bodies of mushrooms (the part of an insect’s brain involved in learning, memory and sensory integration) between “primitive” sawflies and complex honeybees. They found that three diverse, specialized subtypes of Kenyon cells in the brains of honey bees appear to have evolved from a single, multifunctional Kenyon cell ancestor. In the future, this research could help us better understand the evolution of some higher brain functions and behaviors.
Are you a “busy as a bee”, a “social butterfly” or a “fly on the wall”? There are many ways we compare our behavior to that of insects, and as it turns out there may be more to it than just fun terms. Studying insects can help us understand not only how their behavior evolved, but also that of highly evolved animals, including us. Mammalian brains are large and complex, so it’s difficult to say which behaviors and neural and genetic changes arose together over time. In comparison, insect brains are much smaller and simpler, making them useful models for study.
“In 2017, we reported that the complexity of Kenyon cell (KC) subtypes in mushroom bodies in insect brains increases with behavioral diversification in Hymenoptera (a large and diverse order of insects),” explained Professor Takeo Kubo from Kenyon University Graduate School of Science. University of Tokyo and co-author of the current study. “In other words, the more KC insect subspecies, the more complex their brain and the behaviors they may exhibit. But we didn’t know how these different subspecies evolved. That was the catalyst for this new study.”
A team from the University of Tokyo and Japan’s National Agricultural and Food Organization (NARO) chose two species of Hymenoptera as representatives of different behaviors: the solitary turnip (which has one KC subspecies) and the evolved social honey bee (which has one KC subspecies). three subspecies KC). Since the sawfly has a more “primitive” brain, it is believed that it has some characteristics ancestral to the brain of the honey bee. To uncover potential evolutionary pathways among them, the researchers used transcriptome analysis to profile gene expression (gene activity) of different KC subtypes and speculate on their functions.
“I was surprised that each of the three KC subspecies in honey bees showed similar similarity to the single KC subspecies in the sawfly,” said Associate Professor Hiroki Kono, co-author from the Graduate School of Science. “Based on our preliminary comparative analysis of several genes, we previously hypothesized that additional KC subtypes were added one after the other. However, they appear to be separated from a multifunctional ancestral species, through functional segregation and specialization.” As the number of KC subspecies increased, each subspecies almost equally inherited some distinct characteristics from KC ancestors. Then it was modified in various ways, diversifying its current functionality.
The researchers wanted a specific behavioral example of how ancestral KC functions exist in both sawflies and honey bees. So, they trained sawflies to engage in a common honey bee behavior test, in which they learned to associate an odor stimulus with a reward. Although challenging at first, the team was eventually able to engage Saw in a memory task. Then the researchers manipulated a gene called CaMKII in sawfly larvaeAnd which in honey bees are associated with long-term memory formation, KC function. As the larvae become adults, their long-term memory is impaired, suggesting that the gene plays a similar role in both sawflies and honey bees. despite of CaMKII It was expressed (i.e. was active) across the entire individual KC subspecies in sawflies, in honey bees it was preferentially expressed in only one KC subspecies. This indicates that the role of CaMKII In long-term memory, it goes by subspecies KC in honey bees.
Despite the differences in size and complexity of the brains of insects and mammals, there are commonalities in terms of the function and basic structure of the nervous system. This is why the model proposed in this study for the evolution and diversification of KC subspecies may help to better understand the evolution of our behaviour. Next, the team is interested in studying acquired KC species in parallel with social behaviors, such as the “waggle dance” of honey bees.
“We would like to clarify whether the model presented here applies to the evolution of other behaviours,” said Takayoshi Kuwabara, PhD student and lead author from the Graduate School of Science. “There are many mysteries about the neural basis that controls social behavior, whether in insects, animals or humans. How it evolved is still largely unknown. I think this study is groundbreaking work in this field.”