Fluorescent fruit flies shed new light
Researchers genetically engineer glow-in-the-dark sperm in fruit flies and reveal a wealth of information about sexual selection.
A lot has changed about the way scientists study sexual selection and reproduction. Some of it has to do with new tools; some of it has to do with new attitudes. There is a lot more going on than just "sperm meets egg."
"It was simply thought of as "this army of sperm competing," so it functioned as a raffle; the more tickets you bought, the more sperm you transferred, the more likely you were to win out in that competition," explains Scott Pitnick, a professor of biology at Syracuse University. "Females were perceived as these passive vessels in which this competition played out — that females didn't play an active role. That's really not the case."
With help from the National Science Foundation (NSF), Pitnick studies reproduction and sexual selection in fruit flies.
"A sea change came in the mid-'90s in earnest when people started paying attention to the female side of things, and sperm-female interactions, and it turns out that's really where all the action is," he continues. "There has been a real male bias in this field, as in most fields of science. And now, in the sperm competition field, there are as many female as male [scientists], and it really has changed the focus considerably and in a very positive way."
For example, modern DNA technology that can confirm paternity opened scientists' eyes to the reality that monogamy is more the exception than the rule in most species.
But knowing there is a lot going on in a female fly's reproductive tract and being able to observe it are two different things.
"It's a difficult field site. And sperm are tough to watch, you can't tell different sperm apart. And so that's really held progress back, in terms of understanding the devil in the details of how post-copulatory sexual selection works," notes Pitnick.
But, Pitnick and his colleague, biology professor John Belote, have found an answer: Glow-in-the-dark sperm.
Belote genetically engineered red and green fluorescent sperm in fruit flies. Observing a female "before" and "after" this new tool is like the difference between watching a grainy, old black-and-white film and a high-definition color video.
"We found a good gene to tag with the fluorescence, which was a gene that would be expressed in the sperm head," says Belote. "Other people had done similar things and labeled sperm tails, and the problem with that is the tails are so long and there are so many sperm in the reproductive tract of the female that there was so much fluorescence, you couldn't really see what was going on."
Pitnick will never forget the "Eureka moment" when he first saw the fluorescent sperm.
"Nothing I knew about sperm from my couple of decades of research prepared me for seeing this, or would have allowed me to predict that sperm would have been behaving this way; that they would have been so dynamically moving throughout the female," says Pitnick.
"It was clear that this was an incredibly exciting moment because we both realized this was just a tool that could just break open a lot of experiments that we could now do," adds Belote.
Biologist Mollie Manier, a postdoctoral researcher at Syracuse University, is using this powerful new tool for some of those experiments. She has a NSF grant to study sperm evolution and sperm competition, research that is now much easier with the color-coded sperm. She has studied snakes and other animals, but loves the challenges of studying the fruit fly, Drosophila melanogaster, and other insects.
"The morphological diversity in insect sperm is off the charts," says Manier. "There are insect sperm that, you look at them and you would have no idea that you are even looking at a sperm cell. They look like more complex, free-living organisms. They're completely bizarre and interesting."
Deciphering the intimate crosstalk and physical interaction between sperm in females is something biologists are only beginning to understand. Their discoveries could someday answer questions about human fertility and might also assist in saving threatened and endangered species.
"In any animal group, people are now just beginning to pay attention to the female side," says Pitnick. "There's a lot going on there, and the architecture, the design, the biochemistry of female reproductive tracts is now a real focus for evolutionary biologists."
"Until we understand those, we don't understand why reproduction is successful in this couple and not successful in that couple. We don't know enough about the moving parts that determine fertilization success. And many, many couples that experience fertility problems get diagnosed with "unexplained infertility," which means we just don't know what the heck is going on," he says.
Scientists around the world are interested in seeing what this new fluorescent light reveals about reproductive and evolutionary biology, as well as sexual selection.