For Female Fruit Flies, Mr. Right Has The Biggest Sperm

May 25, 2016
Originally published on May 26, 2016 6:03 pm

You can't help but notice that Scott Pitnick has a big tattoo. It's a sperm with a long tail that winds down his right arm.

People sometimes stare. "And when I tell them what it is, they either are very interested or they pivot on their heel and walk away," says Pitnick, an evolutionary biologist at Syracuse University. "All eye contact ceases."

Some people just don't like talking about sperm. But not him. He's spent his career trying to unravel the mystery of giant sperm.

The sperm are made by some fruit fly species, and they can be more than 2 inches long — or 20 times the length of the fly's body. The massive sperm get rolled up like a ball of yarn, and they're so costly to make that males produce very few of them.

"I mean, every decent scientist I know has this dogma-busting streak," says Pitnick. "And so you always have your eyes out for something that doesn't make any sense, that doesn't jive with prevailing theory."

And these bizarre sperm defied easy explanation. After all, males in most species typically produce tons of sperm. An adult man, for example, makes millions of new sperm each day. It's generally the females that produce relatively few eggs.

"So for me, discovering species where males have this very female-like reproductive strategy of producing very few high-investment gametes, or sperm, was a thrilling moment," says Pitnick.

The question was, why do the flies do this?

It's not unusual for males to try to impress females with big body parts. Consider antlers on deer, or elaborate tails on peacocks. Females check these out to decide who is good enough to mate with. "A crappy male in poor condition isn't able to afford to grow really huge antlers and carry them around on his head," Pitnick points out.

But the size of a fly's sperm isn't as obvious or flashy. Nonetheless, Pitnick and his colleagues have results from a study published Wednesday in the journal Nature comparing the evolution of long sperm in fruit flies, or Drosophila, with other male ornaments in the animal kingdom that are used to woo females — everything from deer antlers to stag beetle mandibles to lizard horns.

"Sperm length in Drosophila has been subject to incredibly strong sexual selection, more so than any other ornament that's ever been studied before," Pitnick says.

It turns out that as female flies flit around mating with different males, the females store the sperm inside a special organ before the eggs get fertilized, and the sperm jostle around in there.

"The advantage to having relatively long sperm is that they are better competitors for this limited storage space inside of the female reproductive tract," says Pitnick. Any increase in the size of the female's storage space seems to drive the evolution of longer sperm, and vice versa.

But that's not the only thing that's been working to make sperm longer and longer, he says.

"In the species with long sperm, males transfer very few sperm — a few to dozens of sperm per copulation," he says. "Females have to mate multiple times a day just to get an adequate supply of sperm."

Only the guys with the best genes can make enough long sperm to take advantage of all these mating opportunities. That's because making giant sperm isn't easy — it takes a lot out of the poor guys.

"As sperm get longer, the number of sperm males produce becomes more and more reliant on the health, the condition the male is in," says Pitnick.

That's a plus for females, who only want to make babies with the most hardy, manly flies.

"There's this whole world that we've barely scratched the surface of, of complex interactions between sperm and females," says Pitnick. "You have to understand that it's not just a simple game of numbers where males are competing through a lottery and the female reproductive tract doesn't matter. A lot of people, I think, still subscribe to the idea that females are passive vessels in which this competition plays out, and nothing can be further from the truth."

Biologist and sperm researcher Tim Karr, who is currently a visiting scholar at Kyoto Institute of Technology in Japan, says the new work is impressive and that these huge sperm have long been an enigma.

"The biology is so very different and there has been no good explanation or even attempt to explain it," says Karr. "I think it will spark an awful lot of conversation and hopefully additional research."

It's often the weird outliers that can reveal nature's secrets, says Karr, and this study shows how female choice and male competition can be linked together in surprisingly complex ways.

Copyright 2017 NPR. To see more, visit http://www.npr.org/.

AUDIE CORNISH, HOST:

Now we're going to talk about sperm. Yep, the world's largest sperm are more than 2 inches long, and they're made by male fruit flies. Now scientists think they now understand why these tiny flies produce the biggest sperm on Earth. NPR's Nell Greenfieldboyce reports.

NELL GREENDFIELDBOYCE, BYLINE: Scott Pitnick is all about giant sperm. He even has a huge sperm tattoo that winds its way down his right arm. People sometimes stare.

SCOTT PITNICK: And when I tell them what it is, they either are very interested, or they pivot on their heel and walk away.

GREENDFIELDBOYCE: He's an evolutionary biologist at Syracuse University, and when he finds someone who doesn't walk away, he is only too eager to explain why he has devoted his career to unraveling the mystery of giant sperm.

PITNICK: I mean, every decent scientist I know has this dogma-busting streak, and so you always have your eyes out for something that doesn't make any sense.

GREENDFIELDBOYCE: Here's why giant sperm didn't seem to make sense. Males of other species like humans make millions of tiny sperm a day, and it's the females who produce relatively few eggs. But in these flies, the males make small numbers of long sperm.

PITNICK: So for me, discovering species where males have this very female-like reproductive strategy of producing very few high-investment gametes or sperm was a thrilling moment.

GREENDFIELDBOYCE: Now think about big body parts that males use to impress females like antlers on dear or elaborate tales on peacocks. Pitnick says females check these out to decide who's good enough to mate with.

PITNICK: A crappy male in poor condition isn't able to afford to grow really huge antlers and carry them around on his head.

GREENDFIELDBOYCE: The size of a fly's sperm is not as obvious or flashy. Nonetheless, Pitnick says his research shows that sperm length has been subject to incredibly strong sexual selection.

PITNICK: More so than any other ornament that's ever been studied before.

GREENDFIELDBOYCE: And here's why. As a female flies around, mating with different males, she stores all of the sperm inside a special organ. The sperm stays in there for a while until an egg comes along to be fertilized. Pitnick's work shows that longer sperm seem to elbow out their shorter rivals.

PITNICK: They are better competitors for this limited storage space inside of the female reproductive tract.

GREENDFIELDBOYCE: The size of the female's storage space and the size of the sperm have been co-evolving. When one gets bigger, so does the other. It's tough for the guy flies. It's not easy to make sperm 20 times the length of your own body

PITNICK: As sperm get longer, the number of sperm males produce becomes more and more reliant on the health, the condition the male is in.

GREENDFIELDBOYCE: He's found that in species with long sperm, the females mate much more frequently. Only the most hardy, manly flies can produce enough long sperm to capitalize on this situation. And that pleases the ladies just fine. They want to make babies with the best.

Pitnick and his colleagues lay out this tangled sperm tale in the journal Nature, and it's impressed Tim Karr. He's a biologist who's currently a visiting scholar at Kyoto Institute of Technology in Japan. He's studied giant sperm in the past.

TIM KARR: The biology is so very different. And there has been no good explanation or even attempt to explain it.

GREENDFIELDBOYCE: He likes this explanation.

KARR: I think it will spark an awful lot of conversation and hopefully additional research.

GREENDFIELDBOYCE: Because there's no doubt more to learn from giant sperm. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.