We recently discovered that isolated flatworms of the species Macrostomum hystrix are capable of self-fertilization, and now we know how they do it.
In a new study published today in Proceedings of the Royal Society B, I investigated selfing behaviour in M. hystrix together with Lukas Schärer, Aline Schlatter and Maude Poirier at the University of Basel. Because the flatworms are transparent, we can look under the microscope and see where in their body sperm have been received, and thereby compare patterns of sperm transfer under outcrossing and selfing. The results were surprising: whereas outcrossing worms tend to contain most sperm in their tail, close to where fertilization occurs, isolated worms that have begun selfing have very few sperm in this region. Instead, the distribution of sperm is shifted forwards, including observing sperm in their head region, implying a rather strange insemination route: using their needle-like stylet (male copulatory organ), isolated worms apparently self-inject sperm into their own anterior body, from where these sperm then presumably swim through their body to fertilize their own eggs. Such a convoluted route is likely needed because, although all individuals of this species are hermaphrodites, there are no internal connections between their male and female reproductive systems.
What’s fascinating about this behaviour is that it means their needle-like stylet, which is thought to have evolved in the context of sexual conflict and attempting to inject sperm into a mating partner, can be facultatively deployed to inject sperm into themselves when environmental conditions dictate that this is likely to be the only route available for successful reproduction.
Image credit: Maude Poirier & Aline Schlatter. In this microscope image of the tail of the flatworm M. hystrix, the hooked structure towards the rear of the animal is the needle-like copulatory stylet used for injecting sperm into a mating partner…or into themselves.