The classic definition of a biological species is the ability to breed within its group, and the inability to breed outside it. For instance, breeding a horse and a donkey may result in a live mule offspring, but mules are nearly always sterile due to genomic incompatibility between the two species. The vast majority of the time, mating across species is merely unsuccessful in producing offspring. However, when researchers at the University of Maryland and the University of Toronto mated Caenorhabditis worms of different species, they found that the lifespan of the female worms and their number of progeny were drastically reduced compared with females that mated with the same species. In addition, females that survived cross-species mating were often sterile, even if they subsequently mated with their own species.
When the researchers observed the sterile and dying female worms under a microscope using a fluorescent stain to visualize sperm in live worms, they discovered that the foreign sperm had broken through the sphincter of the worm’s uterus and invaded the ovaries. There, the sperm prematurely fertilized the eggs, which were then unable to develop into viable offspring. The sperm eventually destroyed the ovaries, resulting in sterility. The sperm then traveled farther throughout the worm’s body, resulting in tissue damage and death.
“Our findings were quite surprising because females typically just select sperm from males of their own species during fertilization, an action that does not lead to long-term consequences because there is no gene flow between the species,” said Asher Cutter, associate professor of ecology and evolutionary biology at the University of Toronto.
The researchers believe the “killer sperm” may be the result of a divergence in the evolution of worm species’ sexual organs—in particular, the ability of sperm to physically compete with one another. When a female worm mates with multiple males, the sperm jostle each other, competing for access to the eggs. Female worms’ bodies must be able to withstand this competition to survive and produce offspring. The researchers hypothesize that the aggressiveness of the sperm and the ability of the uterus to tolerate the sperm are the same within a single species, but not across multiple species. Thus, a female from a species with less active sperm may not be able to tolerate the aggressive sperm from a different species.