The nearly 8 cm long, helical sperm-storage tubule of a female Drosophila bifurca female is the organ of “female sperm choice” that has driven the evolution of giant sperm in fruit flies. (Scott Pitnick)

Sperm is cheap, eggs are dear. If you've ever sat through the most basic of biology classes, you've probably heard some version of that adage. But here's the thing — it isn't always true. Scientists have spent years trying to figure out why, and a new study suggests a possible explanation for the monstrous sperm cells of the humble fruit fly.

First, a little bit about how eggs and sperm are "supposed" to work: In most species, females invest a lot of energy in a relatively small number of sex cells, while males make as many of the smallest sex cells they can manage. Males compete to mate with a female with courtship dances and bells and whistles, but the sex act itself is more of a lottery than anything else. They produce as many of the smallest sperm they can manage, choosing quantity over quality and allowing the fastest (and hopefully fittest) cells to triumph.

It makes a lot of sense, this strategy — so much sense that it's pretty much standard procedure in animals that reproduce sexually. But every rule exists to be broken, and in recent decades, it's become apparent that a few choice animals buck the trend. Instead of producing huge quantities of tiny sperm, they evolve to produce tiny numbers of (relatively) giant sperm.

One extreme example of this is the fruit fly. Some species of these flies can produce coiled-up sperm more than 2 inches long, which is 20 times the length of their own body. That's a thousand times as long as than human sperm. And if a human male of average height produced sperm of the same size as the fruit fly's (relative to body size), it would stretch a staggering 120 feet long.

In theory, this shouldn't work. Sperm generally gets the genetic job done by providing so many options that some of them must fit. With fewer sperm available, reproductive quality should go down the tubes. After all, if every reproductive encounter was between an egg and a single available sperm, there'd be no such thing as sexual selection. All the odds would be resting on one player's fitness. And yet in some animals, like the fruit fly, longer, scarcer sperm seems to give males a mating advantage.

Scott Pitnick, now a professor of biology at Syracuse University, has been asking why this happens since he was a graduate student years ago. Sperm are much more varied — and more impressive — than most people give them credit for, he told The Washington Post.

#NotAllSperm (giphy)
#NotAllSperm (giphy)

"They’re the only cells in the body cast forth to spend their lives essentially as free living organisms in another environment," he said, adding that the public's visual image of a "typical sperm cell," a tadpole-like thing, is flawed — sperm evolve and adapt so quickly that there truly is no such thing as a "typical" model in the animal kingdom. They come in all sorts of shapes and (occasionally horrifying) sizes, including ones that seemingly defy logic.

In a paper published Wednesday in the journal Nature, Pitnick and his fellow researchers come up with a framework for understanding the evolutionary drive behind these perplexing sperm cells.

A detail of the nearly 8 cm long, helical sperm-storage tubule of a Drosophila bifurca female. (Scott Pitnick)

The team noted that longer sperm seemed to be more beneficial when females had longer storage organs. The large sperm could displace the sperm of the female's other recent mates if it was big enough, but it had to be small enough to actually fit. As female storage organs get longer, sperm evolves to get longer as well. That kind of sexual competition is common in the animal kingdom: In ducks, the increasingly labyrinthian twists of the vagina have led to nightmare-inducing corkscrew penises, and the ever-increasing complexity of the dueling sexual organs shows no sign of stopping.

This answer, Pitnick said, was less than satisfying on its own.

"It's, oh, you know, because females prefer them," he said. "That's a really hollow answer, because it's just a restatement of the question. We already understand male competition, but we don't understand the whimsy of the females."

But in sequencing the genomes of some of these fruit flies, the team found another clue: The genes that make sperm long are closely associated with the genes that make female storage organs long.

"Any genetic change in the female preference for long sperm automatically drags the genes for long sperm along with them and vice versa," he explained, which makes the evolutionary effect of this perplexing preference happen more quickly. Not only are females with a preference for long sperm more likely to "select" it from the available pool, but they're also more likely to produce offspring with long sperm — and when you add in the genes for long sperm that come from the dads they find most appealing, their likelihood of producing any other kind of trait in their offspring becomes slim to none.

That feedback loop doesn't solve what Pitnick calls the "big sperm paradox" — the idea that fewer sperm can't possibly have the same odds for producing strong offspring. But the paper suggests one explanation, based on analysis of the energy invested in various sexually selected traits in the animal kingdom.

He and his colleagues noted that re-mating frequency (a tendency for females to mate with multiple males in a short time frame) went up as storage organs (and sperm) got bigger. Females collect far fewer sperm cells in each mating, but they make up for it by gathering lottery tickets from a large group. This seems to be an adaptation that makes even "dear" sperm "cheap" again by giving a statistical advantage to males who can produce more of the energy-intensive sperm.

"When sperm are cheap, any male can produce large quantities," he said. "But in the case of the fruit fly, only the males in the best condition, males with good genes, can produce more than average. The females are all mating like gangbusters, but only a really high-quality male can capitalize on that."

The authors of the study are not sure how well this explanation for the weirdness of fruit fly sperm will transfer over to other big-spermed animals, so the evolutionary mystery of super-size sperm is not quite solved. But Pitnick thinks fruit flies are a good start. He and his colleagues hope to learn more about the mechanisms that drove fruit flies to evolve in this unique way. They suspect this kind of sexual strategy may sometimes help create new species.

"If they’re rapidly co-evolving, then they’re changing in sync, and if those traits are rapidly changing and some populations are geographically isolated, those separate populations may have ejaculate-female incompatibility, which could be an important first step in the formation of new species," Pitnick said. In other words, animals may start to be functionally incapable of reproduction with one another long before their genes make them so. A physical inability for one population's sperm to fertilize another population's eggs would keep the groups genetically isolated from one another — and that could push them further apart.

"This process very well might be an engine of speciation," Pitnick said.

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