Sperm can kill female worms of another species by breaking through the uterus and invading the body cavity. This discovery was made when scientists were trying to get two difference species of worms to mate. To their surprise, not only did they discover that the different species of worms could not successfully mate, but the female worms ended up dying. Their report is the first of such an instance in which sperm were shown to cause physical harm to another species.
The study was reported in the journal PLOS Biology. The lead scientists who made the discovery were Asher Cutter, an Associate Professor in the Department of Ecology and Evolutionary Biology at the University of Toronto, and Eric Haag, an Associate Professor in the Department of Biology at the University of Maryland.
It is well-known that there are various ways that animals can make sure their sperm have the best chances to impregnate a female. Sometimes sperm from another animal can be displaced by the second male that mates with a female, or females may develop vaginal plugs after mating that prevent sperm from a second male to travel to meet the egg. These are known mechanisms, however, for males and females that are of the same species.
In the recently reported study, the researchers were attempting to mate worms of different species. Successful interspecies matings are rare in nature. The definition of species are a group of organisms that are reproductively isolated from other organisms, which means individuals in the species can only mate with each other to produce viable offspring, who can also achieve reproductive success in the species. While genetic engineering can produce inter-species animals, for the most part, these inter-species pairings cannot be produced naturally.
In the study, different species of Caenorhabditis worms, such as C. nigoni and C. briggsae, were mated together. The results showed that the sperm became more aggressive. When observing the transparent worms under a microscope, the researchers observed that the sperm broke through the uterus and entered the ovaries. The sperm fertilized the eggs in the ovaries but then continued to travel further into the female’s body. While the destruction of the uterus and ovary sterilized the female worms, their internal organs were also destroyed. The researchers even observed sperm in a female’s head.
These different species of Caenorhabditis worms look very similar to human eyes but clearly they have evolved to have very different biologies that are hell-bent on keeping the species separated. The discovery that sperm can become this aggressive and turn into killer sperm in another species shows that the battles among species can be very serious.
The authors of the study report stated that the results from their study support a role for sexual selection and gamete interaction contributing to reproductive boundaries among species. They suggested that this type of sexual selection would be predicted by evolutionary theory. Sexual selection is a particular form of natural selection in that it is a matter of mating success rather than overall reproductive success. Certainly, the evolution of killer sperm that invade the body of a female from another species, such as observed in these worms, is a good way to keep different species apart.
By Margaret Lutze