Here鈥檚 a powerful incentive for animals of separate species not to have sex: killer sperm. When two particular kinds of nematode worms mate, the sperm of one sterilises or kills the other. It is the first instance of sperm causing physical harm to another species.
of the University of Maryland in College Park and his colleagues tried to get two species of nematode worm to interbreed. Males of Caenorhabditis nigoni, which reproduces sexually with males and females, mated with , a self-fertilising hermaphrodite.
鈥淏ut it just wasn鈥檛 working,鈥� says Haag. When Haag examined the C. briggsae worms, he saw C. nigoni sperm in the body cavity, even in the worm鈥檚 head. The sperm had broken out of the worm鈥檚 uterus, destroyed the ovaries and then rampaged through the body, causing severe tissue damage.
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鈥淭he two species are very close in evolutionary terms, yet when they mate all hell breaks loose,鈥� says Haag.
He tried mating many different Caenorhabditis worms and found that mating with a male of a different species was always harmful, but more so for hermaphrodites like C. briggsae. That may be because they do not have to deal with sperm jostling each other to reach the eggs, whereas in species that regularly mate, females have to evolve reinforced reproductive tracts to cope with the sperm. 鈥淥ver time the hermaphrodites become very vulnerable to encounters with relatives that still do it the old way,鈥� says Haag.
鈥淚 don鈥檛 know of any other examples in nature where the sperm themselves do this kind of damage,鈥� says of the University of St Andrews, UK.
The find could help explain why some species stay separate, says Haag: if they mate, one of them could die. 鈥淧unishing cross-species mating by sterility or death would be a powerful evolutionary way of maintaining a species barrier.鈥�
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