Don't underestimate the manliness of the humble fruit fly: He may be small, but his sperm is not.
In fact, the sperm of the fruit fly Drosophila bifurca can stretch up to nearly 6 centimeters in length. That's several times the length of the male fruit fly himself, and about 1,000 times the length of a human sperm cell.
If this massive sperm length seems unusual, that's because it is. The D. bifurca's spaghetti-like zygote takes a lot of energy to produce, and therefore he can only produce a few of them. That means he can't implement the more common male reproductive strategy of quantity over quality.
Still, evolutionary theory suggests that the fruit fly's gigantic sperm is no accident; it must have come about for some reason. But what? The phenomenonon is perplexing enough to have garnered its own name. Scientists call it "the big-sperm paradox."
Scott Pitnick, an evolutionary biologist at Syracuse University, has been studying this conundrum for 15 years. In a paper published Wednesday in Nature, he and his colleagues offer some new insights on what might be driving the fruit fly to produce just a few large sperm.
As Pitnick sees it, D. bifurca's long sperm is a type of male ornamentation, not so different from a peacocks' dazzling feathers, an elk's enormous antlers, or a lizard's flashy dewlap. All of these features exist primarily to help a male suitor out-compete his rivals and snag a female mate. This ensures that his genes will be passed on to the next generation.
In the case of the fruit fly, however, the ornamentation is not something you can see -- the female fruit fly can't size up a male's sperm before deciding whether to mate with him. But it's still an ornament, Pitnick said -- it's just that the competition is happening inside the female's reproductive organs.
"It's an idea that is hard to get your head around because we're so used to thinking of female preference as being sensory-driven," he said. "But it still meets the criteria because a long sperm doesn't fertilize an egg better; it's just about competition."
Just as the male fruit fly has evolved very long sperm, the female fruit fly has evolved a similarly long sperm storage tubule in her reproductive tract. Because she mates with several males in a short period of time, sexual selection continues inside her body.
And here is where sperm length shows its true advantage: The longer sperm a male fruit fly can produce, the more likely his sperm will fertilize an egg.
But Pitnick's investigations were not done yet.
"Saying a male fruit fly has evolved a long sperm because females have evolved a long reproductive tract is not a satisfying answer," he said. "What is driving the evolution of the female reproductive tract?"
In the new paper, Pitnick and his colleagues from the University of Zurich, George Washington University and the National University of Singapore suggest a two-part answer.
The first part has to do with genetics. The research team found that the genes influencing the length of a male's sperm and the length of a female's sperm storage tubule work in tandem. They can see this because when one changes, the other is able to adapt.
"That could mean that the exact same genes are influencing both traits, although we can't say that for sure right now," Pitnick said.
The authors also discovered that there is an advantage for female fruit flies in having their eggs fertilized by extra-long sperm. It turns out that big sperm are correlated with health, and size, in male fruit flies.
Therefore, when a female's reproductive organ favors the longest sperm, she is picking the best mate, even if she doesn't know it.
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