Published online 20 January 2010 | Nature | doi:10.1038/news.2010.22

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Mouse sperm cells team up with their kin in the race to fertilize eggs.

The sperm of some rodents form trains to speed their passage to the egg. Fisher & Hoekstra

The sperm of a mouse can recognize and team up with sperm from the same male, US biologists have found.

The discovery is further evidence that, far from being simply shells loaded with DNA, sperm cells have evolved sophisticated social behaviours that aid them in the race to fertilize eggs.

The sperm of some rodent species form 'trains'. Up to several dozen cells attach to one another using hooks on their heads, and swim in concert. A sperm train can swim up to 50% faster than a lone cell.

Heidi Fisher and Hopi Hoekstra of Harvard University in Cambridge, Massachusetts, studied the train-making behaviour of sperm cells of the deer mouse (Peromyscus maniculatus) and the oldfield deer mouse (P. polionotus). The mice live in different parts of North America, but they can interbreed successfully in the lab.

Each species' sperm can form trains. Fisher and Hoekstra used a dye to label the sperm of different males with different colours. When the two species' sperm was mixed in a Petri dish, three-quarters of all sperm teamed up with their own kind.

Oh Brother, where art thou?

To test whether the sperms' powers of recognition go beyond the ability to spot their own species, the researchers also mixed sperm from different males of the same species.

The oldfield deer mouse P. polionotus is largely monogamous, so the sperm from two males of this species are unlikely to find themselves in competition in the wild. In this species, mixed sperm leads to mixed sperm trains.

Sperm from different P. polionotus males stick together. Fisher & Hoekstra, Nature

P. maniculatus, by contrast, has a hectic sex life; females have been seen mating with different males within the space of a minute, and the pups in a litter usually have several different fathers. Its sperm stand a good chance of finding themselves racing the sperm of another P. maniculatus male to fertilize an egg.

When the sperm of two P. maniculatus individuals is mixed, the sperm separate out to form trains with sperm from the same male, even if the sperm came from two siblings. "It's quite shocking that the level of discrimination between littermates is the same as that between species," says Fisher. The team's work is published online today in Nature1.

"This study shows very elegantly that sperm have a fantastic ability to discriminate between kin and non-kin," says animal-behaviour expert Tim Birkhead of the University of Sheffield, UK. "When sperm were discovered in the 1600s, they were called 'animalcules' — it was thought they were independent little organisms. In a way, this work fulfils those original observations."

Recognition puzzle

The greater selectiveness of P. maniculatus sperm for who they cooperate with makes sense in the light of the two species' differing sex lives. Only one sperm can fertilize any single egg, so all but one of the cells in the train are, in fact, going nowhere fast.

What's more, many of the cells in the train prematurely trigger the reaction that is used to bore through the egg's wall. This speeds the train on its way, but amounts to suicide for the sperm that triggered the reaction. The more genetically similar the winning sperm is to its train-mates, the more evolutionarily worthwhile their self-sacrifice becomes.

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The ability of sperm to recognize each other may well be common among sperm from promiscuous species, says evolutionary biologist Paula Stockley of the University of Liverpool, UK: "There's no reason not to expect this to be a general effect."

The next step, says Birkhead, is to work out how sperm recognize their brothers. It could be a chemical marker that promiscuous male mice add to the sperms' surface, he suggests, or it could be something that the sperm make themselves, depending on their genetic make-up.

For example, some unicellular yeast and slime moulds carry an extremely variable gene encoding a sticky protein that allows related cells to recognize and attach to one another. Fisher suspects that the deer mice could have something similar.