3 min readNovel Methods for Analyzing Neural Circuits for Innate Behaviours in Insects

Ishikawa Prefecture, Japan — Insects show a variety of species-specific innate behaviours (instinctive behaviours). For example, a worker honeybee that has found flower nectar exhibits 8-shape waggle-dances upon returning to its beehive. A male moth that has detected a sex pheromone flies around to look for a female counterpart. There remain a number of questions about how a variety of innate behaviours are generated by functions of neural circuits in the insect brain. 

In order to obtain full pictures of neural circuits and their functions responsible for innate behaviours, it is necessary to reveal neural circuits that are activated when an innate behaviour takes place. A method is also required that can control insect behaviour by manipulating activities of neural circuits in an artificial manner.

The present research group at Kanazawa University has been actively engaged in research on functions of neural circuits, focusing on genes whose expression occurs in a neural activity-dependent manner. Previously the group identified a transcription factor gene Hormone receptor 38 (Hr38) *1) that is expressed in a neural activity-dependent manner in the insect brain and found that this gene is a useful marker for neural activities. 

In the present study, the group used the fruit fly (Drosophila melanogaster), a model insect, to generate a genetically modified strain that precisely reflects the expression pattern of Hr38, establishing a method that can specifically visualize active neurons by labelling them with green fluorescent protein (GFP)*2). Using this method, they revealed a full picture of the male fruit fly’s neural circuits in the brain and ventral nerve cord*3) that were activated when a male fly interacted with a female fly. It is known that the sex-determining gene fruitless and doublesex determines the sex of neural systems in the brain and ventral nerve cord of the fruit fly, and that these genes are responsible for development of male-type and female-type neural circuits.

In the present study, the group applied their method specifically to neural circuits that express fruitless or doublesex. This revealed active neural circuits within the male-type neural circuits during the courtship behaviour. As a result, the group found that a neural cluster aSP2 was active specifically when a male fly interacted with a female fly in addition to the neural circuits already known to be important in regulating the mating behaviour. 

In addition to the visualization of neural circuits activated during a behaviour, it is important to be able to manipulate neural circuit activity in a desired manner to reveal neural circuit functions. Therefore, the group generated a Drosophila strain that can activity-dependently express CsChrismon, a light-activated channel rhodopsin*4), in place of GFP. A male fly of this strain was allowed to experience mating with a female fly; on the following day, after removal of the female fly, the male fly alone was illuminated. The male fly, although in the absence of any female fly, showed abdominal bending typical of copulation behaviour. This indicates that the neural circuits that were activated during the mating the previous day can be reactivated by light one day later. 

Furthermore, the group analyzed the functions of the neural cluster aSP2 on mating behaviour. Detailed analysis of courtship behaviour of the male flies whose aSP2 neural activity was inhibited revealed that male flies approached female flies in a normal manner but showed frequent interruption of courtship; as a result, mating success rate was much reduced. In the courtship of the fruit fly, the persistent dynamic approach of male flies to females, who may be reluctant at first, is important in the females’ acceptance of mating. The present result shows that the neural cluster aSP2 plays an important role in regulation of motivation during courtship behaviour.

The present study has established for the first time methods to visualize neural circuits in an activity-dependent manner in insects and to manipulate their activity. These methods should be applicable to the elucidation of neural circuits and their functions in innate behaviours of insects. Understanding the neural basis of innate behaviours of insects is not only significant in fundamental science but could contribute to applications such as the efficient utilization of beneficial insects like the honeybee and the silkworm, getting rid of noxious insects, prevention of epidemics of diseases like malaria, dengue fever, Zika fever, etc. mediated by the mosquito. This study identified the neural cluster aSP2 as an important neural circuit for motivation of an insect behaviour. It is expected that further research on the mechanism of how aSP2 neural circuit controls motivation will allow elucidation of fundamental regulatory mechanisms of innate behaviours in the insect brain.

Article adapted from a Kanazawa University news release.

Publication: Activity-dependent visualization and control of neural circuits for courtship behaviour in the fly Drosophila melanogaster. Takayanagi-Kiya, S et al. Proceedings of the National Academy of Sciences (March 05, 2019): Click here to view.

fruit fly, innate behaviour, Insects, neural circuits, Neurons

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