Certain fruit flies of the genus Drosophila make the world's longest sperm. One species that biologist Daniel Lachaise studies makes sperm that are two-thirds of an inch long--more than 300 times longer than human sperm, 600 times longer than hippopotamus sperm (which are the world's shortest), and 6 times longer than the fly itself. The fly is not ripped apart by its own stupendous sex cells, however, because the sperm are hair-thin and all balled up. Still, making such gametes is a considerable effort for a small insect. In humans, says Lachaise, an equivalent energy investment would mean that males would make sperm 40 feet long.
It sounds like a colossal waste, and in fact many species of Drosophila take the more conventional approach of making lots of tiny sperm. But according to Lachaise and his colleagues at the National Center for Scientific Research in Paris and the University of Orsay, making giant sperm can pay off. A fly that does so makes far fewer sperm, but a much higher proportion succeed in fertilizing an egg. Moreover, the giant sperm may enable male flies to deliver resources directly to their offspring-- something that is usually thought to be the exclusive province of the female and her egg.
In all species of Drosophila, the male deposits sperm in the uterus, and the female then draws them into an adjoining storage chamber. There the hapless gametes huddle--often with rivals deposited by other males--waiting for the female to release her eggs. As an egg moves past the opening of the storage chamber, it draws in the lucky sperm whose gene- carrying head is protruding from the opening. One mystery in all this is the function of the sperm's tail: with the female doing all the work, Drosophila sperm have no need to swim.
The sperm that Lachaise and his colleagues study, from the species D. hydei and D. littoralis, are far too long for swimming in any case; they are tightly coiled when the male deposits them and are uncoiled only when the female draws them, like so much spaghetti, into her hugely elongated, hoselike storage chamber. Yet the giant sperm clearly get the job done. The French researchers tallied how many sperm a male fly deposited in one mating and then how many eggs were fertilized. Although each male typically deposited no more than 250 sperm, those sperm were astonishingly successful. In D. hydei, nearly 80 percent of them fertilized an egg; in D. littoralis, nearly half did. In contrast, a human male ejaculates hundreds of millions of sperm at once, and usually not one of them does its job.
Clearly one reason for the high success rate of an individual fly's giant sperm is that there are so few of them. But that doesn't explain why such behemoths evolved in the first place. Being larger must have given the first largish sperm (the giants didn't evolve overnight) an advantage over rivals deposited by other male flies; perhaps it simply made it harder for them to be shoved aside in whatever quiet struggle goes on in the female's sperm-storage chamber. But Lachaise thinks a giant sperm's tail is far longer than it need be for that purpose alone. It's not used for propulsion, he says. So the only possible function is to introduce something into the egg.
While in many animal species only the gene-carrying head of a sperm cell penetrates the egg, a fruit fly's giant sperm gets pulled into the egg tail and all. By tagging the tail with antibodies, Lachaise and his colleagues showed that it was still visible inside the embryo well after the fertilized egg had divided into layers of different cells. That suggests the tail is providing a resource of some kind--energy, perhaps--to the developing embryo.
Anything at all would be more help than most fathers get to give their offspring. The only way most males, flies or otherwise, can aid the embryo is by providing food to the female. That strategy can backfire, though: a male may feed a mate that ends up being fertilized by a rival male. Giant sperm, says Lachaise, may offer an end run around this dilemma. We've long thought that evolution could produce only smaller and smaller sperm in larger and larger amounts, but this is not true, he says. Evolution is like water running on a flat surface--it goes where it can.