You probably haven’t spent much time thinking about the uterus of the fruit fly, Drosophila melanogaster. But neither have most scientists, even though Drosophila is one of the most studied laboratory animals. Now, a team of biologists at the University of California, Davis, has taken the first deep look into Drosophila’s uterus and found some surprises that could have implications not only for understanding insect reproduction and, potentially, pest control. , but also to understand fertility in humans.
The work is published on October 25 in Proceedings of the National Academy of Sciences.
Drosophila has been a favorite subject of geneticists and developmental biologists for more than a century.
“Drosophila is a very productive system in many ways,” said Rachel Thayer, a postdoctoral researcher working with distinguished professor David Begun in the Department of Evolution and Ecology at UC Davis. Extensive catalogs of cell types and genes exist for most fly organ systems. But the female reproductive organs (the uterus, female glands and sperm storage organs) have been largely excluded.
Both humans and insects have internal fertilization, so the female reproductive tract has to deal with foreign material, from sperm to sexually transmitted viruses. Insects, like many other female animals, including birds and reptiles, have organs that can store viable sperm for long periods.
“We wanted to identify all the cell types and their gene usage patterns for these important organs,” Thayer said.
Thayer and Begun, with co-authors Elizabeth Polston and Jixiang Xu, dissected the reproductive tracts of about 150 flies. They were able to separate cell nuclei into individual droplets and label each cell’s RNA with a kind of barcode. By sequencing the RNA, they were able to identify a gene expression profile of individual cells and classify them into types.
“We can identify the types of cells that express certain genes and where they come from,” Thayer said.
Until now, no type of cell from the fly’s uterus had been identified using genetic markers. The new study reveals more than 20 different cell types in the uterus and associated organs.
“The most exciting thing for me is finding cell types that we hadn’t previously predicted would exist,” Thayer said. “This is an anatomy that was previously invisible.”
Support sperm storage
The study found that about 40% of the “seminal fluid protein” genes, previously identified as produced only in male flies, are also expressed in the female fly, especially in sperm storage organs. They may be key to maintaining viable sperm for long periods.
Some proteins in the seminal fluid are thought to manipulate the female fly in ways that benefit the male, for example by delaying the female from mating again. These sexual conflicts have been the subject of considerable studies, mostly theoretical.
“There is controversy about what role these sexual conflicts actually play, because sexual reproduction still needs to be collaborative,” Thayer said. The discovery that many of these proteins are produced by both male and female flies means that scientists will have to adjust their thinking about these ideas, Thayer said.
“It doesn’t completely rule out the possibility of molecular sexual conflict, but it does limit how it could occur,” he said.
While reproduction is obviously very different in humans and insects, the fruit fly may be a model for understanding animal reproduction at a fundamental level. For example, seminal fluid proteins could lead to new ways of growing and storing human sperm without freezing it, which would benefit fertility treatments.
Insects, the most numerous species on Earth, pollinate and provide other benefits in addition to destroying crops and transmitting diseases. A better understanding of insect reproduction could lead to new control strategies.
Thayer is currently working with Drosophila isolates from around the world to better understand how flies evolve in response to environmental stresses, such as climate change and pesticides.
The work was supported by grants from the National Institutes of Health.