Published online 18 March 2010 | Nature | doi:10.1038/news.2010.135

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Insect sperm fight one another with brute force and chemical weapons.

Reproductive tract of female fruitfly mated to males with sperm containing green or red fluorescent protein. Science/AAAS

When the sperm of different male insects meet inside a female, they use everything from wrestling to chemical warfare to try and fertilize as large a share of her eggs as possible, according to two studies published this week. The studies also show that females don't just let the battle take its course, but manipulate it to their own ends.

A US team has genetically engineered fruitflies to produce sperm that fluoresce in different colours. The researchers use the technique to watch the sperm of different males as they jostled for position inside a female, giving a first look at sperm competition in action1.

And researchers in Denmark and Australia have shown that the seminal fluid of some ants and bees aids a male's own sperm and attacks his rivals. But queen ants, which need huge sperm reserves for the long years of egg-laying ahead, suppress this competition2. Both studies are published in Science.

For the fruitfly Drosophila melanogaster the most recent male to mate with a female fertilizes most of her eggs — 80% — and his predecessors lose out.

But the mechanism by which the last male got this advantage wasn't known. "The female reproductive tract has been a black box," says Scott Pitnick of Syracuse University in New York.

Speedy sperm

Pitnick and his colleagues have shed light on the mystery by using transgenic male flies with sperm heads illuminated by green or red fluorescent protein. The researchers mated female flies with one green-spermed male and one red, and tracked the sperms' fate by freezing females at different intervals after mating and dissecting them, or examining the reproductive tract in real time under a microscope (see videos of fluorescent sperm in female reproductive tracts here and here).

They saw a complex series of events during and after mating, in which both sexes have a say.

Males transfer about 1,400 sperm per mating. The female stores about 500 of these in three storage organs — blind-ended tubes that branch off from the reproductive tract.

“The female reproductive tract has been a black box.”



As soon as the second mating begins, before any sperm are transferred, the female releases some of the first male's sperm from storage. This might be triggered by physical stimulation, or by chemicals in the second male's seminal fluid.

Then, the second male's sperm arrive, and seem to flush their competitors out through vigorous motion. This ousting of other sperm might be why males transfer about three times more sperm cells than females have room to store.

Several hours after mating, females bring the contest to a close by ejecting any sperm that hasn't made it to the storage organs.

The sight of sperm in motion has wowed researchers in the field. "No one knew how dynamic and fast the ejaculates are inside a female," says Tommaso Pizzari, a biologist at the University of Oxford, UK. "It's like watching schools of fish chase each other."

Long struggle

Among female insects, the champions of storing and manipulating sperm are queen ants and bees.

In the largest and most complex insect societies, such as honeybees, and leafcutter and army ants, queens mate with up to 20 males in a few hours. Males die after mating, but their sperm can live on for years.

A leafcutter ant queen, for example, takes around 100 million–400 million sperm on board. Over the next decade or two, she will use them to fertilize some 50 million–150 million eggs. Honeybee queens have shorter lives, but might still produce more than a million offspring.

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Sperm don't simply wait their turn inside. "The queen's body is an arena where sperm are allowed to fight it out for a while," says Jacobus Boomsma, a population biologist at the University of Copenhagen.

To see what weapons males used in this fight, Boomsma and his colleagues put sperm from male bees and ants on a microscope slide with either saline, a male's own seminal fluid, or that of another male.

Sperm survived longer in their own male's seminal fluid, but died more quickly in another male's. The fluid seems to contain chemicals that nurture a male's own sperm and attack the sperm of other males. Studies of honeybees suggest that it is proteins that attack alien cells, in the same way the immune system does inside the body.

Uneasy truce

For species in which queens mate just once, such as bumblebees, sperm of different males never meet, so there is no competition. The seminal fluid of these species does not harm rival sperm, the researchers found.

Bumblebee queens mate just once so the sperm of these species d not compete. B. Baer

"Self-recognition is more pronounced in species with higher levels of sperm competition. It's exactly what you'd expect," says Pizzari.

By letting sperm fight it out, females let the fittest males father their offspring. But were this competition to continue, it might threaten the long-term supplies of sperm — particularly in leafcutter ants, where queens pay out just a few sperm to fertilize each egg. "Every sperm they store is precious," says Boomsma.

To see how these queens police sperm competition, Boomsma's team added fluid from the leafcutter queen's sperm storage organ to the mix. This, they say, cancelled out the negative effect of another male's seminal fluid.

It is thought that queens benefit from multiple matings by producing more genetically diverse offspring, and so healthier colonies. If this is so, it might also be in a male's interest not to do too much damage to his rival's sperm, notes Tracey Chapman, an evolutionary biologist at the University of East Anglia in Norwich, UK. "Once they're in the storage organs, everybody had better settle down," she says.