Fruit fly sperm is enormous
Popular Science...
They may have tiny bodies, but fruit flies produce giant sperm. Adult male Drosophila melanogaster fruit flies are about two millimeters long (roughly the size of a sesame seed) and their sperm can be just as long, with the sperm tails taking up most of that space.
In order to keep the thousands of sperm cells inside their bodies untangled, the insects use movement in an ingenious way. The sperm cells are organized into densely aligned groups that move by pushing off one another to keep themselves more taut. The more taut, the less likely it is that the tails will get tangled. The findings are detailed in a study published today in the journal Nature Physics.
Fruit fly sperm are about 40 times larger than human sperm, and thousands of sperm cells are stored inside the tightly confined spaces of both the male and female reproductive systems. One of those organs is the seminal vesicle, where males store the sperm before mating.
The seminal vesicle is only about 200 micrometers in size. That’s really small compared to the sperm cells that are two millimeters long. In theory, this cell-to-storage size ratio should be similar to putting a pair of wired earphones inside of a pants pocket. The earphones often come out as a tangled mess.
“Now imagine putting thousands [of earbuds] in your pocket. The sperm are of course different from passive wires: the sperm are active, generating bending waves along their long tail,” Jasmin Imran Alsous, a study co-author and computational biologist at the Center for Computational Biology, tells Popular Science. “So how do thousands of active sperm organize and move within the tight confines of the reproductive system?”
To see how male fruit flies keep so many lengthy sperm organized, the team dyed the sperm’s heads and tails with a fluorescent substance and examined them with a powerful 3D electron microscope. They saw that the sperm cells are densely packed, but instead of staying still, they generate collective movements. These movements, called flows, can span the entire seminal vesicle for hours at a time.
According to Imran Alsous, individual sperm move smoothly past each other, similarly to cars moving along a highway in opposite directions.
“Each sperm is attached to a long tail, that tail is moving just as quickly as the heads, but the tails collectively move more slowly together, in this flowy slow churn. It would be as if the highways now began to fold and bend while the cars within them continued to dart in opposite directions,” she explains.
The team believes that these forces help to keep the sperm cells’ long tails from getting tangled in both male and female reproductive organs. The contact-based interactions help them move by pushing off of each other, which keeps the cells more taut, instead of a mushy mess.
Another example of these tight-packed systems in biology is how six feet of DNA is stored inside of a cell nucleus or how humans’ large and small intestines are about 30-feet-long when combined. This set-up allows living things to store more than their bodies should allow. For sperm cells, this increases the chance reproduction will be successful and the species will survive.
“Our work suggests that sperm can move in a way that’s very different from the textbook picture of a lone sperm swimming through fluid,” says Imram Alsous. “Individually, fruit fly sperm exhibit very little directed motion, but together they can move through interactions with their neighbors.”
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