A new study showed female Caenorhabditis worms that mate with other species tend to have a shorter lifespan and are often sterile.
University of Maryland researchers observed the dying worms under a microscope and used a fluorescent stain to keep tabs on the sperm within their bodies. The findings were published in PLOS Biology. They found the sperm from other species had broken through the female worm's uterus and invaded the ovaries; this prematurely fertilized the offspring preventing their development and leaving the mother sterile. If the sperm traveled elsewhere in the body it could result in 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 are wondering if this type of process can be observed in other types of creatures. When the worm mates with multiple males the sperm must compete for fertilization; the level of aggression of the sperm and the ability to withstand it is most likely contained within each species. The researchers observed that three species of hermaphrodite worms (which produce their own sperm and fertilize their own eggs) were most vulnerable to sterility and death when they mated with males of other species.
"We found that hermaphrodites can sense, and try to avoid, males of species that can harm them," said Eric Haag, associate professor of biology at UMD.
This "lethal mating" helps prevent species mixing, but in very rare cases viable offspring can be produced when worms of two species mate.
"Punishing cross-species mating by sterility or death would be a powerful evolutionary way to maintain a species barrier," Haag said.
