Male or Female? In Flies, Some Cells Can't Tell

An experienced fruit fly researcher can tell at a glance whether the fly she is observing is male or female. Distinct pigmentation patterns on the flies' bodies, a type of bristle found only on the legs of males, and differences in the genitalia are all dead giveaways. But most of the fly's body parts look identical in males and females, and until now, scientists had no idea whether "maleness" or "femaleness" extended to all of the insect's cells and tissues.

Many cell types in the fruit fly do not have a male or female sexual identity because they do not express the sex determination gene doublesex. The red dots in the left image show the position of all cells in the fruit fly antenna. The red cells in the right image are a subset of specialized neuronal cells in the antenna that sense odors, while the green cells are those cells that express the doublesex gene. Notably, none of the odor-sensing cells express doublesex. Thus many of the odor-sensing cells do not have the potential to express a sexual identity.
Illustration: Bruce Baker
In a new study investigating just how pervasive a fruit fly's sexual identity actually is, researchers at the Janelia Farm Research Campus of the Howard Hughes Medical Institute find that most cells in flies' bodies are identical, regardless of whether they are in a male or a female. This is because the influence of sex-determining genes is restricted to specific tissues, says Bruce Baker, a Janelia Farm group leader who conducted the research with colleagues Carmen Robinett, Alex Vaughn, and Jon Michael Knapp.

"There's a lot more to a fly's life than just reproducing. There's no reason to believe males and females shouldn't have the same wings, for instance, or the same eyes." says Bruce Baker.

In flies, as in most animals, only a few tissues exhibit overt differences between the sexes. But, Baker notes, "we tend to think of maleness and femaleness not as something that applies to parts of the body, but to the whole individual."

Baker says scientists often assume that even tissues and organs that are not discernibly different between the sexes possess genetic regulators that are sexually specific. Although those tissues might look the same, they could actually differ in subtle ways between males and females. His lab's new work, however, demonstrates that only a subset of cells is likely to know whether they are male or female.

By tracking the activity of a master gene called doublesex, which determines sexual characteristics, Robinett and her colleagues found that male and female fruit flies are really a mixture of cells that are sex-specific and cells that are sexless.

The new work, published May 4, 2010, in the journal PLoS Biology, shows that many of the fly's cells never turn on the doublesex gene. So while, some cells "know" their sex and take on sex-appropriate characteristics during development, those that do not express doublesex remain identical in males and females.

Because doublesex or analogous genes are present in myriad organisms, including humans, this theme may apply to more than fruit flies. "It may be broadly that males and females are made up of a mosaic of cells that know their sex and cells that don't," Baker says. "Given what we know about the enormous commonality between all organisms from lowly fruit fly to a person, it's not unreasonable to think this is going to be something very extensive in the animal kingdom."

Baker has been studying the genetics of fruit fly sex for 30 years. By the early 1980s, his group had discovered a cascade of genes that determines how tissues and organs develop in a sex-specific manner. In this bureaucratic chain of command, each gene tells the next one how and when to act. The doublesex gene is the last gene in the cascade, Baker discovered. In flies, females express one form of doublesex, and males another. Baker and his colleagues have come to view the gene as a master switch that tells female cells to create female sexual characteristics and male cells to make male ones. Until now, fly geneticists clung to the notion that the male version of doublesex was switched on in every cell of male flies, while the female version was present in every cell in female flies.

Baker says that paradigm has survived despite recent evidence that has begun to chip away at it. In recent years, other labs reported that doublesex is turned on only in small areas in the fly embryo, and only in specific parts of the brain. These red flags were "pushed aside," says Baker, because the prevailing theory was so entrenched.

Prompted by those other studies, Robinett and her colleagues employed a genetic engineering technique to quickly and easily watch for doublesex expression in the cells of developing flies. The technique attaches a reporter gene to doublesex that makes cells expressing the gene glow green. Tracking doublesex then becomes as easy as watching for green spots in fly embryos, larvae, and adults.

With this approach, Baker's group found that during fly development, doublesex switches on and off at different times and in different tissues. As expected, they saw doublesex protein in the genitalia and in other body parts that display differences between males and females. But in body parts where there are not overt differences between males and females, they found that doublesex was expressed only in certain cells and not expressed in many other types of cells. "It's a simple observational study with profound implications," Baker says.

"We saw that doublesex was expressed in a highly regulated pattern," says Baker. "It is turned on during certain times of development but not during other times. And it is only on in some tissues and not in other tissues."

While sexual reproduction obviously requires different equipment for males and females, Baker says "there's a lot more to a fly's life than just reproducing. There's no reason to believe males and females shouldn't have the same wings, for instance, or the same eyes. The evolutionary argument for the mosaic pattern we see is that for some bodily functions, there's one best way to do it for a species."

Baker adds that he expects the publication to prompt other researchers to check for similar doublesex patterns in other model organisms such as roundworms, zebrafish, and mice. If similar findings emerge, Baker says it could shake up the entire notion of sex differences in the animal kingdom.

And if the finding applies to humans - a big "if" at this point - he says the implications could be even more significant. "If it applies to people, it's certainly going to change our sociological view of what we think of as maleness and femaleness."

Source: Howard Hughes Medical Institute