Here’s how fruit flies’ giant sperm squeeze into tight spaces
Constant motion and mutual pushing prevent the cells from knotting in storage sacs
Male fruit flies’ giant sperm can pack tightly into small spaces without tangling. Inside the seminal vesicles, sperm (heads shown in pink, tails in cyan) organize into an orderly arrangement, traveling in opposing directions in aligned paths.
J.I. Alsous et al/bioRxiv.org 2025
Giant sperm may be the ultimate packers.
Inside male fruit flies, thousands of sperm — each about as long as the fruit fly itself — cram into sacs roughly the size of a very fine pen tip. And somehow this jiggling mass of gigantic sperm stays snarl-free.
“How does a fly manage to do that?” asks Jasmin Imran Alsous, a quantitative biologist at the Flatiron Institute in New York City. Think about how hard it is to keep a pocketful of earbud wires tidy, she says. And with flies, the puzzle is further complicated by the movement of the sperm themselves. The question isn’t just how to store long, thin wires in a container, she says. It’s how to do so while the wires are moving.
Alsous and her colleagues discovered that fly sperm pack so well by aligning into an orderly formation, forming a living liquid crystal, the researchers report July 23 at bioRxiv.org. The sperm line up and move in opposing directions, forming a crystalline-like material that can flow like a fluid, says Michael Shelley, an applied mathematician also at the Flatiron Institute. The sperm and their ever-whipping tails avoid tangling by constantly pushing off each other. This detangling strategy lets the sperm bundle densely inside the fly’s seminal vesicles, the sacs that store mature sperm before ejaculation.
The researchers used 3-D microscopy to visualize the sperm-packed sacs of fruit flies (Drosophila melanogaster) and recorded the living swimmers’ writhing movements. Even when bunched together, the sperm appear to travel in aligned paths that can wend like the whorls of a fingerprint.
The team developed a mathematical description of the sperm’s movement and simulated it on a computer. Experiments on living sperm in the lab confirmed the team’s theory: As the giant sperm slide by one another, the strands push against their neighbors, forcing the sperm to stretch out like cooked spaghetti noodles. “We think that’s actually what keeps them from becoming entangled,” Shelley says.
Such organization is crucial for fruit fly reproduction, Alsous says, because it keeps sperm from getting knotty. Sperm can’t journey into a female’s body if they’re matted together in a lump. The findings, she says, help reveal “how these flies maintain their fertility in the face of such seeming complexity.”
