As It Happens

Why fruit flies produce giant sperm 20 times their body length

A scientist is trying to find out why male fruit flies are producing giant sperm that grow much longer than the bodies of the insects themselves.
(Wikimedia Commons)

You may think of fruit flies as diminutive, weak, boring creatures. But you know what they say: don't judge a creature's sperm by its tiny, short-lived cover.

Some people say that. Well, only scientists, actually. But they've been saying it a lot lately because it turns out that male fruit flies are packing an impressive punch, sperm-wise. Until now, trying to explain this evolutionary arms race causing fruit flies to produce bigger and bigger sperm has been difficult.

A laboratory assistant sorts out fruit flies. (Herwig Prammer/Reuters)

"In the most extreme case, the sperm are about 58 millimetres long or roughly 20 times the total body length of the males producing them," Scott Pitnick tells As It Happens host Carol Off.

Pitnick is an evolutionary biologist at Syracuse University who recently published a study on fruit fly sperm in the journal Nature.

"Males have to grow these outrageously large testes and the sperm are all rolled up inside," Pitnick explains. "The testes are about 11 per cent of the male's total body mass."

Scott Pitnick, evolutionary biologist at Syracuse University (Syracuse University)

Pitnick uses an unravelling "ball of yarn" analogy to try to describe the flies' elaborate storage system, which he says puts extreme demands on the insects. But, as his study shows, lengthy sperm is key to mating because it occupies the female longer.

"We think about sperm competition as a lottery," Pitnick argues. "It's how it was always traditionally viewed, meaning the more tickets you buy the higher your probability of winning. And in order to produce the most tickets, males produce the smallest sized sperm as possible."

But Pitnick's finding suggest that fruit flies are unique, opting for size and "quality over quantity" when it comes to sperm production traits.

A scanning electron micrograph image of a fruit fly sperm, which when unraveled reaches about 2.3 inches long, 1,100 times bigger than human sperm. (Romano Dallai/Italian National Academy of Entomology/AP)

"In this case, what we found is that the males that have the most competitive edge are those producing a few long sperm," Pitnicks explains.

"We figured out that these long sperm provide an advantage in the competition to occupy these female sperm storage organs."



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