A team in Japan reports that switching on one gene in a specific set of brain cells made one fruit fly species perform the courtship ritual of another.
The two species involved, split from a common ancestor roughly 30 to 35 million years ago, so copying behavior across that divide is no small feat.
Changing behavior by gene transfer
“When we activated the fru gene in insulin-producing neurons of singing flies to produce FruM proteins, the cells grew long neural projections,” said Ryoya Tanaka, co-lead author and lecturer at Nagoya University’s Graduate School of Science.
The project was led by Tanaka alongside colleagues at Japan’s National Institute of Information and Communications Technology (NICT), in partnership with Nagoya University.
The manipulated males of Drosophila melanogaster stopped relying only on song and instead offered a nuptial gift, a regurgitated droplet, (vomit) the hallmark move of Drosophila subobscura.
The team boosted a sex-specific isoform of the fruitless gene, FruM, inside insulin-producing neurons that sit in the brain and normally regulate metabolism and growth signals.
Those neurons sprouted new connections to the brain’s courtship control center, forming a circuit that drives gift offering rather than song, according to the paper and editor’s summary.
The authors also pinpointed a small cluster of 16 to 18 such neurons and tied their output to mouthpart motor pathways that enable regurgitation and transfer to the female’s proboscis.
This shifts views of evolution
“Our findings indicate that the evolution of novel behaviors does not necessarily require the emergence of new neurons,” noted Yusuke Hara, co-lead author at NICT.
Small genetic changes that rewire existing cells can yield new behaviors, which is a tighter, more economical route than inventing whole networks from scratch.
That idea is consistent with work showing modular, plug-and-play features in fly courtship circuits that can accept different sensory inputs or outputs without rebuilding the entire system.
Fruitless gene in courtship behavior
The fruitless program sets up the male courtship network during development, and experiments show that FruM is required in a critical window to produce normal male-female courtship.
About 2,000 FruM-positive neurons dot the male nervous system, and subsets control steps such as song, pursuit, and copulation timing.
Earlier work comparing species already hinted that fru-labeled circuitry is wired differently in subobscura versus melanogaster, supporting the notion that conserved genes can drive distinct behaviors in related animals.
Insulin neurons guide courtship behavior
In flies, a compact set of brain endocrine cells secretes insulin-like peptides, and this cluster sits in a neuroendocrine hub called the pars intercerebralis.
Those cells normally track nutritional status and help regulate growth, stress resistance, and lifespan, so recruiting them into a courtship circuit links internal state to mating tactics in a direct way.
The new study shows that giving these cells FruM builds a bridge from internal metabolic nodes to the courtship command center, adding a behavioral output that one species has never used before.
What gift giving does for flies
Drosophila subobscura males offer a regurgitated droplet to females during courtship, and experiments show that this gift influences female choice and reproductive output.
The gift is tiny, but in a resource-limited world it carries information about the male’s condition and foraging success, and it can provide nutrients that matter to a female’s egg production.
Embedding that behavior in the primary courtship sequence ties mating success to an ecologically meaningful performance.
How the circuit likely works
The study’s mapping implicates neurite growth from insulin cells and connections onto proboscis-innervating motoneurons, which can trigger regurgitation and mouth-to-mouth transfer seen in subobscura courtship.
That path bypasses the wing-vibration pathway that dominates in melanogaster song, so the act of giving replaces the act of singing in a way the nervous system can execute without training.
By locating the few neurons that matter and showing how they wire in, the team gives neurogenetics a clear, testable circuit to manipulate in either species.
Limits and next questions
“We’ve shown how we can trace complex behaviors like nuptial gift-giving back to their genetic roots to understand how evolution creates entirely new strategies that help species survive and reproduce,” said Daisuke Yamamoto, senior author at NICT.
This result does not mean complex social behaviors in other animals are controlled by a single switch, or that they can be changed safely outside the lab.
It does show that a few preexisting neurons, given the right transcriptional program, can be redirected toward a new behavioral output.
Genes, behavior, and future science
The work strengthens the case that behavior evolves by redeploying shared parts, not just by adding parts, which helps explain how related species end up with strikingly different courtship rituals over time.
It also provides a tidy experimental blueprint for tracing a behavior to its molecular driver, the specific cell type, and the synapses that make the action happen.
That level of precision is rare for any innate behavior, so the field now has a scaffold for testing how internal state, environment, and sex-specific genes integrate inside the brain.
The study is published in Science.
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