Сперма плодовых мушек движется как скоординированный трафик, чтобы избежать узлов

These insects probably have the largest sperm-to-body-size ratio. A male fruit fly’s sperm is nearly 40 times larger than that of a human. To make it work, the fruit fly performs a feat that should end in a knot. It stores thousands of sperm nearly as long as its own body inside two sacs only about one-tenth as long as each cell. Pack ordinary threads that tightly and they snarl. A new study in Nature Physics suggests that the sperm avoid tangling by constantly moving against one another. Instead of swimming freely through liquid like they do in humans, they pack into orderly layers, with neighboring cells often traveling in opposite directions. As their long tails beat, they push against nearby sperm and slide past them. That shared motion keeps the cells stretched out and the entire mass loose enough to flow rather than collapse into a knot. In other words, a fruit fly’s sperm resembles something closer to coordinated traffic. “It’s not just one heroic swimmer getting to the egg; it’s a collective behavior,” Jasmin Imran Alsous, the study’s first author, told The New York Times. A Sperm Highway Inside a Storage Sac The researchers studied Drosophila melanogaster, the humble laboratory fruit fly. Its sperm average about 1.8 millimeters long, roughly 36 times the length of human sperm. The majority of each cell consists of a thin, beating tail. But male flies store the cells in seminal vesicles measuring roughly 200 micrometers across. The team genetically labeled sperm heads and tails in different fluorescent colors, then used confocal microscopy and three-dimensional electron microscopy to reconstruct their arrangement. The cells filled an estimated 40 to 60 percent of the storage organ, but they did not form a random wad. They lay in highly aligned bands that bent and folded through the confined space. The sperm mass churned slowly, at roughly 2.4 micrometers per second, while individual sperm moved through it at about 15.6 micrometers per second. Among more than 1,100 neighboring pairs, 79 percent traveled in opposite directions. “It’s like a 1,000-lane highway where all the cars are moving in opposite directions,” Michael Shelley, the study’s senior author, told The New York Times. The sperm’s dependence on one another became clear when the researchers removed them from the crowded organ. Alone in liquid, each cell still sent rippling waves down its tail, but it mostly wriggled in place. Inside the tightly packed vesicle, those same waves pressed against neighboring sperm moving in the opposite direction, giving the cells something to push against and allowing them to slide forward. To confirm the pattern, the team briefly darkened small sections of fluorescently labeled tails. The marked bands soon separated and traveled in opposite directions, while computer simulations of flexible, beating filaments produced the same collective motion. From Evolutionary Mystery to Living Material Biologists already had an explanation for why fruit flies evolved extravagant sperm. A 2002 Science study found that fertilization success depends partly on the fit between sperm length and the female’s sperm-storage anatomy. Research published in Nature in 2016 argued that competition after mating can drive a feedback loop favoring longer sperm and longer female storage organs, despite the cost of producing fewer cells. The new work shifts the focus from evolution to mechanics: Once giant sperm exist, how do they remain usable? The researchers describe the packed cells as “active matter,” a material whose components generate their own motion. Each sperm behaves somewhat like a polymer sliding through an imaginary tube formed by its neighbors. Physicists call that snake-like movement reptation. Here, however, internal tail waves power the motion rather than random molecular jostling. The team found similar counterflow inside the female’s long, narrow seminal receptacle, where sperm can remain for about two weeks before fertilization. That suggests the behavior does more than solve a male storage problem. It may keep sperm organized throughout the reproductive journey. “This is a stunningly important contribution to reproductive biology because it finally tackles the functional biology of sperm within the complex social and spatial environments of the male and female reproductive tracts — no easy task,” Scott Pitnick, a Syracuse University biologist who was not involved with the study, told Physics Magazine.