What triggers a male fly to begin his courtship dance? Is it the sight of a svelte and sinuous female or something else?
You might have heard of pheromones, those airborne chemicals rumored to trigger passionate desire. It turns out that pheromones, more than visual signs, have a powerful effect on Drosophila courtship behavior.
As mentioned above, the Fru gene is made only in special groups of cells, including nerve cells of the brain and sensing organs. One set is in the fly's primitive form of nose.
"Nose" cells that make Fru let the animal smell other flies. Especially smells that identify another fly as an eager-to-mate member of the opposite sex.
Turning off Fru only in those special nose cells of a male fly reduced his courtship behavior. And when the male Fru gene gets turned on in the female's nose, she starts acting more like a male.
Sounds like maybe something the flies are smelling is triggering the courtship dance. To test whether pheromones were to blame, scientists went a step further and created male flies that made female pheromones. These are males that smell female.
They then put these flies together with females that made male Fru. In other words, males that smell like females were put together with females that like the smell of females.
And, as you'd expect, the females started courting the males! Females pursuing males represents a complete reversal of the sex roles for fruit flies.
So for the simple fruit fly, a single gene can change the gender preferred for mating. What about more complicated animals? Do the findings for Fru relate at all to more complex animals?
So far, there is no Fruitless gene known of in humans or other mammals. But there do appear to be genes that are involved in some of the same behaviors. For example, in mice, pheromones help males tell males from females. This is important for knowing if they are encountering a potential mating partner or a mating rival.
An important clue to figuring this out was the TRP2 gene. The TRP2 gene is important for detecting a pheromone and passing that information along. Male mice that lack this gene aren't aggressive towards other males. And they try to mate with both males and females.