When it comes to fertilizing an egg, it's usually every sperm for itself. But in species where a female mates with several males in quick succession, sperm can hook up with each other--sometimes by the hundreds--to outswim competitors. Researchers have now demonstrated that, in mice, these cooperative sperm recognize and prefer their closest relatives to make a group push for the egg.
Linking up with other sperm to swim faster makes sense, but it's not without risk. It can set off a chemical reaction that makes an individual sperm infertile. So it's only worth the trouble if sperm cooperate with other sperm from the same male. Researchers have assumed that the sperm just link up with their closest neighbors, which are likely to be related because they entered the reproductive tract at the same moment. But do sperm have a better way of figuring out who is who?
Heidi Fisher, a postdoctoral fellow at Harvard University, compared sperm of highly promiscuous deer mice (Peromyscus maniculatus) with that of monogamous oldfield mice (P. polionotus). She fluorescently labeled the sperm so that she could tell which father they came from. Facing no competition from other males, the sperm of oldfield mice linked up indiscriminately when in a dish with mixed sperm. The deer mice sperm, however, were choosy about whom they associated with: Even sperm from two brothers kept to themselves, only linking up with those from the same male. "Whatever the recognition factor is, it is extremely variable, like a fingerprint," says Fisher, who reports her findings with Harvard evolutionary biologist Hopi Hoekstra online today in Nature.
The fingerprint probably has a genetic basis, says Harry Moore, a biologist at the University of Sheffield in the United Kingdom who helped pioneer the study of sperm cooperation. Fisher says the next step is to identify the genes responsible.
David Quellar, who studies cooperation in slime molds at Rice University in Houston, Texas, thinks Fisher needs to first test more mice to know for sure if the two species of mice differ in sperm behavior. But "if it holds up, it's a really nice result because it extends kin cooperation and kin recognition to the realm of sperm interactions," he says. "If microbes can recognize and help kin, why not sperm?"