New study reveals evolutionary pathways of brain complexity and behavior in Hymenoptera insects

A group of researchers has proposed a novel model for the evolution of higher brain functions and behaviors in the Hymenoptera order of insects. The study focused on comparing the Kenyon cells, a type of neuronal cell, in the mushroom bodies of “primitive” sawflies and sophisticated honey bees. The mushroom bodies are involved in learning, memory, and sensory integration in the insect brain.

The research team discovered that three distinct specialized subtypes of Kenyon cells in honey bee brains appear to have evolved from a single multifunctional subtype in their ancestor. This finding suggests that studying these insects could provide valuable insights into the evolution of higher brain functions and behaviors, including those observed in humans.

Comparatively, mammalian brains are large and complex, making it challenging to identify the co-development of specific behaviors, neural changes, and genetic factors over time. In contrast, insect brains are smaller and simpler, making them useful models for studying brain function and behavior.

Professor Takeo Kubo, co-author of the study and affiliated with the University of Tokyo, explains that in a previous study in 2017, they found that the complexity of Kenyon cell subtypes increases with behavioral diversification in Hymenoptera. In other words, insects with more Kenyon cell subtypes tend to have more complex brains and exhibit a wider range of behaviors. However, the evolutionary origins of these different subtypes remained unknown, motivating the current research.

The team selected two representative Hymenoptera species with different behaviors: the solitary turnip sawfly, which possesses a single subtype of Kenyon cell, and the social honey bee, which has three subtypes. By examining the gene expression profiles of these subtypes using transcriptome analysis, the researchers aimed to uncover potential evolutionary pathways between the two species and speculate about their functions.

In conclusion, this research sheds light on the evolution of brain functions and behaviors in insects, particularly in the Hymenoptera order. By studying these relatively simpler insect brains, scientists hope to gain a better understanding of the evolution of higher brain functions and behaviors in both insects and more complex animals, including humans.

This image illustrates how the diverse and complex Kenyon cell (KC) subtypes of the honey bee evolved from the ancestorlike single KC subtype, represented by the turnip sawfly. Credit: Takayoshi Kuwabara

Assistant Professor Hiroki Kohno, a co-author from the Graduate School of Science, expressed surprise at the findings, stating that each of the three subtypes of Kenyon cells in honey bees showed comparable similarity to the single subtype in sawflies. Initially, the researchers had assumed that additional subtypes had been added one by one, based on their analysis of several genes. However, it appears that the subtypes evolved through functional segregation and specialization from a multifunctional ancestral type.

As the number of Kenyon cell subtypes increased, each subtype inherited distinct properties from the ancestral type, which were then modified in different ways to result in their diverse present-day functions.

To provide a behavioral example of how ancestral Kenyon cell functions are present in both sawflies and honey bees, the researchers trained sawflies to perform a common honey bee behavior test involving the association of an odor stimulus with a reward. Although initially challenging, the sawflies were eventually able to engage in the memory task.

The researchers further manipulated a gene called CaMKII in sawfly larvae, a gene associated with long-term memory formation in honey bees. When the manipulated larvae developed into adults, their long-term memory was impaired, indicating that CaMKII plays a similar role in both sawflies and honey bees. While CaMKII was expressed throughout the single Kenyon cell subtype in sawflies, it was preferentially expressed in only one subtype in honey bees. This suggests that the role of CaMKII in long-term memory was passed down to the specific subtype in honey bees.

Despite the differences in size and complexity between insect and mammalian brains, there are shared functional and architectural characteristics in the nervous systems. This is why the model proposed in the study, elucidating the evolution and diversification of Kenyon cell subtypes, has the potential to enhance our understanding of the evolution of human behavior. The team is now interested in investigating Kenyon cell types acquired in parallel with social behaviors, such as the honey bee’s “waggle dance.”

Takayoshi Kuwabara, the lead author and a doctoral student from the Graduate School of Science, expressed the team’s desire to explore whether the model presented in the study is applicable to the evolution of other behaviors. The neural basis for controlling social behavior, be it in insects, animals, or humans, remains largely unknown. Therefore, this study is considered pioneering in the field, addressing some of these mysteries.

Source: University of Tokyo

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