Size mat­ters to tiny bugs

Sci­en­tists think they know why fruit flies have gi­ant sperm

SundayXtra - - LIFE / SCIENCE - By Rachel Feltman

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 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 a few choice an­i­mals buck this 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 coiledup sperm more than five cen­time­tres long, which is 20 times big­ger than their own body size. That’s 1,000 times longer 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 36.5 me­tres long.

In the­ory, this shouldn’t work. Sperm gen­er­ally gets the ge­netic job done by pro­vid­ing so many op­tions, 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, such as the fruit fly, longer, scarcer sperm seems to give males a mat­ing ad­van­tage.

Scott Pit­nick, 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.

“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 the public’s vis­ual im­age of a “typ­i­cal sperm cell,” a tad­pole-like thing, is flawed — sperm evolve and adapt so quickly there 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 Na­ture, Pit­nick and his fel­low re­searchers come up with a frame­work for un­der­stand­ing the evo­lu­tion­ary drive be­hind these per­plex­ing sperm cells.

The team noted 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 duelling 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 said, 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 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 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 aren’t sure how well this ex­pla­na­tion for fruit fly sperm’s weird­ness will trans­fer over to other bigspermed an­i­mals, so the evo­lu­tion­ary mys­tery of su­per­sized sperm isn’t 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.


When un­rav­elled, the fruit fly’s sperm is more than five cen­time­tres in length.


A scan­ning elec­tron mi­cro­graph im­age of a fruit fly sperm.

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