Fly pheromones can say yes and no: attracting chemicals may also repel, limit interspecies mating.

The sweet smell of honey attracts more flies than vinegar's sour odor, but the ultimate fruit fly magnet is eau de nothing.

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Ditching pheromones makes both male and female fruit flies supersexy to males, even of other species, neurogeneticist Joel Levine of the University of Toronto Mississauga and his colleagues report in the October 15 Nature. The discovery suggests pheromones can be "back off" as well as "come-hither" signals, helping animals find the right gender and species to mate with.

It was a mystery how fruit fly species could tell each other apart. Many species look similar, at least to humans. "We geneticists can hardly tell them apart unless we dissect them," says Nicolas Gompel of the Developmental Biology Institute of Marseilles-Luminyin France. Scientists have known that pheromones are important in finding mates, but no one knew how those signals work in combination. Levine and colleagues selectively killed pheromone-producing cells called oenocytes on flies' abdomens, creating scentless flies.

Surprisingly, the lack of a come-hither signal was more of an aphrodisiac for male flies than pheromones were. Normal males were more attracted to both male and female flies lacking pheromones than to normal females. Males from three other Drosophila species also courted scentless D. melanogaster females, which the males wouldn't do in the wild.

The team then added back specific pheromones to scentless flies to see what the chemicals did. Unexpectedly, adding a female pheromone thought to be an aphrodisiac--(7Z,11Z)-heptacosadiene or 7,11 HD--didn't make flies more attractive.

But if the perfume blend contained both 731 HD and cis-vaccenyl acetate--a male pheromone used to warn off other males, abbreviated eVA--the female chemical could "counter the chemical chastity belt imposed by cVA," Gompel and a coauthor write in a commentary appearing in the same issue of Nature.

"Males are only after one thing. They want to mate," Levine says. Even when facing conflicting signals, males "would rather hedge their bets and go for it."

Females are more discriminating, he adds. Given a choice, they chose males that produce pheromones. That could mean that male pheromones put females in the mood.

The researchers found that just one pheromone barred mating between species. Adding 7,11 HD--not produced by other Drosophila species--to scentless D. melanogaster females made other species lose interest in those females.