Nagoya University and NICT elucidate the genes and brain neural circuits that determine courtship behavior patterns in Drosophila

A research group led by Lecturer Ryoya Tanaka from the Graduate School of Science, Nagoya University and Senior Researcher Yusuke Hara and Laboratory Head Daisuke Yamamoto from the Advanced ICT Research Institute at the National Institute of Information and Communications Technology (NICT), in collaboration with Tohoku University, Kyorin University, Tokyo University of Science, the National Institute of Genetics, and Nagoya University's Institute of Transformative Bio-Molecules (ITbM), announced their success in transplanting courtship behavior from Drosophila subobscura to D. melanogaster through manipulation of a single gene. The genetically manipulated D. melanogaster males began exhibiting gift-giving courtship behavior that they normally do not perform. This is expected to advance our understanding of biological evolution. The results were published in the journal Science on August 14.

Animal behavioral patterns vary greatly between species, and even closely related species are known to differ in social behaviors, parenting methods, and mating behaviors. These interspecies behavioral differences are thought to result from differences in the way nerve cells connect to each other in the brain (neural connections). However, it has not been well understood which specific neural connections produce which differences.

In this study, the research group focused on courtship behavior in Drosophila. During courtship behavior, D. melanogaster males vibrate their wings to "play love songs." In contrast, males of the same genus, D. subobscura, have the habit of regurgitating food they have swallowed and offering it as a "gift." The two species show completely different behaviors.

Previously, Yamamoto and his colleagues had revealed that the fruitless gene (fru gene) is important for courtship behavior, that this gene has the function of masculinizing nerve cells, and that the neural circuit in which this gene operates (fru circuit) directly controls male courtship behavior. It is known that when this gene is forced to operate, courtship behavior occurs even in the absence of females.

Furthermore, in 2017, Yamamoto and Tanaka jointly revealed that D. subobscura also possesses the fru circuit that controls courtship behavior, that this circuit is common to both species, and that they also share the same number of nerve cells: around 2,000. Therefore, in this study, they verified the possibility that differences in the fru circuits between the two species produce differences in courtship behavior.

When they examined the differences between the neural circuits of the two species in detail, they discovered 18 nerve cells in the pars intercerebra of D. subobscura where the fru gene operates, and observed that neural projections extended extensively. When they investigated this function, they found that these nerve cells were insulin neurons. When insulin neurons were activated during D. subobscura courtship behavior, gift presentation occurred frequently, while inactivation suppressed this behavior. It became clear that insulin neurons play a central role in the gift-giving courtship behavior of D. subobscura.

Since insulin neurons also exist in D. melanogaster, the researchers conducted comparisons between the two species. As a result, they found that D. subobscura expresses the fru gene in insulin neurons, but D. melanogaster does not. They also found that while D. subobscura extends neural projections extensively over wide areas, D. melanogaster has short neural projections.

They additionally revealed that the insulin neurons of D. subobscura extend extensively due to fru gene expression and connect with courtship command neurons.

When they forced the fru gene to be expressed in the insulin neurons of D. melanogaster, the neurons extended and connected with courtship command neurons. Males of D. melanogaster that underwent this manipulation began exhibiting "gift" presentation behavior that they normally do not perform when courting females. Courtship behavior was transplanted between species through expression of the fru gene alone. Since D. melanogaster males with forced fru gene expression exhibited both gift presentation and wing-vibrating courtship behavior, it was also suggested that the two circuits may be controlled independently.

Comments from the research group:

Tanaka: "I have always loved insects and have felt that the differences in behaviors among various insects are directly linked to insect prosperity. As I have continued my research, I have been interested in how these behavioral differences are created by genes. I want to continue investigating the mysterious behavioral patterns of various insects that lead to their prosperity."

Yamamoto: "In evolution, it is thought that large changes are produced through the accumulation of small changes, but evolutionary history also shows macroevolution where extreme features suddenly appear. This behavioral switching corresponds to macroevolution, and I believe we were able to show that this can occur simply through the expression of one gene in 18 cells."

Hara: "I am interested in the hidden potential of the brain, and I feel we have been able to elucidate how very small changes can produce large behavioral differences. I want to continue uncovering the hidden potential that still remains in the brain."

Journal Information
Publication: Science
Title: Cross-species implementation of an innate courtship behavior by manipulation of the sex-determinant gene
DOI: 10.1126/science.adp5831