It definitely takes two when it comes to making baby flies. Scientists have known for years that fly semen can trigger ovulation and make females lay more eggs. But no one knew exactly what those proteins did to the female reproductive tract. A study published in PNAS takes a deep look inside fruit flies to find out.
The research team, led by Mariana F. Wolfner at Cornell University, flash-froze female Drosphilia melanogaster in liquid nitrogen during and at set times after mating. They took high-resolution microCT scans of those animals, getting 3D snapshots of how the female reproductive tract changed shape after they had sex. By comparing those results to images they collected from females they mated to mutant males lacking specific seminal proteins, they determined which shape changes were caused by the chemicals inside semen.
They discovered that seminal proteins make eggs move through female reproductive structures. Soon after Drosphilia mating ends, these proteins make the uterus expand enough to fit an egg inside, then contract snugly around the egg to hold it in place for fertilization. Another protein, called ovulin, uncoils loops in the oviducts to let eggs pass through. Seminal proteins also make eggs mature, swelling the ovaries until the female’s abdomen is full of eggs .
How does semen have such a widespread effect on a female’s reproductive system? Wolfner’s team found one possible reason: the tip of the male’s aedeagus — his version of a penis — punctures the female’s vagina while they copulate. You can see this happening in the image to the left: the aedeagus, outlined by dots, pokes through the muscular wall of the vagina (M) at the arrow, immediately behind his ejaculate (E). In flies, blood isn’t cooped up in vessels; instead, it surrounds and bathes all the tissues in the body. A leak in the vagina that frees a little semen can let those proteins travel all over the body.
Images from Mattei et al. 2015