According to a classic definition of species, two animals belong to the same species if they can make babies that can eventually go on to make their own. Sometimes different species get it on and produce offspring, but hybrids like mules are almost always sterile because of genomic incompatibility. And then there’s the gruesome case of killer nematode sperm, which crawl through the body of females from a different species, destroying ovaries and mangling tissue.
A team led by Asher Cutter from University of Toronto and Eric Haag from University of Maryland mated Caenorhabditis worms from about 10 different species. These include hermaphrodites who produce their own sperm to fertilize their own eggs. The lifespan of females and hermaphrodites mated with males from different species were drastically reduced compared with females mated with the same species. Furthermore, females who survived cross-species mating were often sterile, even when they were later mated with their own species.
The team examined the sterile and dying females and hermaphrodites under a microscope using a fluorescent stain to visualize the sperm. That’s when they discovered that the foreign sperm had broken through the uterus in an attempt to fertilize the eggs. After invading the ovaries, the sperm destroys them, leaving the worm sterile. Then the sperm travels through the body, causing tissue damage and eventually death.
"Punishing cross-species mating by sterility or death would be a powerful evolutionary way to maintain a species barrier," Haag explains in a news release. Sperm and female reproductive tract interactions are a novel reason for failed mating, and similar coordination problems could explain infertility in other animals. The work was published in PLOS Biology this week.
Killer sperm may be the result of divergence in worm sex evolution. If a female mates with more than one male, the sperm physically compete with each other for access to the eggs. For her part, the female must be able to withstand this fierce jostling. “It can turn into kind of an arms race between males and females,” Cutter tells Washington Post. “And it causes this chain reaction of evolutionary change, which can go in really different directions for different species.”
The activeness of sperm -- and the ability of the uterus to tolerate it -- might be specific to species. What may have happened here is that, a female from a species with gentler sperm mated with a species with more aggressive sperm. But without gene flow, there are no long term consequences.
In the most extreme example, when C. nigoni males were bred with C. briggsae hermaphrodites, 95 percent of the maternal worms became sterile. "The two species are very close in evolutionary terms, yet when they mate all hell breaks loose," Haag tells New Scientist. While males mated indiscriminately, the hermaphrodites sensed and even tried to avoid males from harmful species. "Hermaphrodites become very vulnerable to encounters with relatives that still do it the old way," he adds. Though even the most harmful crosses produced some viable hybrids.
Images: Gavin Woodruff (top), Janice Ting (middle)