Unusual Courtship: Male Drosophila santomea Engage in Same-Sex Behavior
Unlike most fruit fly species where males exhibit aggression towards one another, male Drosophila santomea, an African fruit fly, engage in same-sex courtship behavior instead of fighting. This behavior was observed by researchers at Caltech's Anderson lab, led by Youcef Ouadah and David Anderson.
Research Findings
The study investigated the biological mechanisms behind this male same-sex sexual behavior (SSB) in D. santomea. It identified that evolutionary changes in pheromonal signaling pathways in both male and female flies are responsible.
Despite strong male-male courtship, D. santomea males can still distinguish between males and females and maintain typical reproductive rates.
Experimental Observations
Initial observations involved isolating male flies for a week and then placing two males of the same species in a chamber with a food substrate, conditions that typically induce aggression in other fruit fly species. D. santomea males, however, displayed courtship behaviors such as raising a wing to a 90-degree angle and vibrating it to produce a species-specific song, which are normally performed by males towards females.*
Pheromonal Analysis
Collaborating with Joseph Parker's lab, pheromone profiling using gas chromatography-mass spectrometry revealed three key pheromonal differences between D. melanogaster and D. santomea. These differences are linked to selection pressure on D. santomea females to maintain reproductive isolation.
Evolutionary Context
D. santomea are endemic to São Tomé and are still undergoing speciation.
They can mate with the closely related D. yakuba, also found on the island, but produce sterile male hybrid offspring. Researchers propose that D. santomea females evolved an aversion to a pheromone produced at high levels by D. yakuba males. Consequently, D. santomea males reduced their production of this same pheromone, which ordinarily prevents courtship among males. The reduction of this pheromone is linked to the emergence of high levels of male SSB.
Broader Implications
These pheromonal changes cause D. santomea males to be courted by males of other fruit fly species, such as D. melanogaster and D. simulans.
While the direct reproductive benefits of same-sex sexual behavior (SSB) for D. santomea are not fully understood, it may allow for the establishment of dominance relationships without injurious fighting.
Similar social and pheromonal evolutionary patterns resulting in spontaneous male-male courtship have been identified in D. persimilis, another distantly related fruit fly species.
Caveats and Future Research
The researchers emphasize that their findings pertain to rigorously measured and quantified behaviors, not sexual identity or intent.
The intent behind male-directed courtship may differ from female-directed courtship, possibly serving as an intimidation tactic. Future research will explore the genetic basis of these pheromonal changes and investigate potential modifications in aggression circuits within D. santomea brains.
The findings were published on March 20 in Current Biology.