One of the most unusual methods of reproduction has just been observed in a flatworm that uses its needle-like penis to stab itself in the head, which leads to egg fertilization.
The method of reproduction in the free-living flatworm, Macrostomum hystrix, exemplifies "selfing." As outlined in a new paper in the Proceedings of the Royal Society B, selfing is when an individual can self-fertilize or self-pollinate.
It "occurs in a broad range of hermaphroditic plants and animals, and is often thought to evolve as a reproductive assurance strategy under ecological conditions that disfavor or prevent (breeding with an unrelated species)," according to lead author Steven Ramm of the University of Basel and his team. Hermaphrodites are people, plants or animals that have both male and female sexual organs or other sexual characteristics.
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The flatworm that Ramm and his colleagues studied is indeed a hermaphrodite -- its blob-resembling body produces both eggs and sperm. In addition to selfing, it can also reproduce with another flatworm. The process in which the penis shoots sperm, hypodermic insemination, sounds more like a vaccination at a doctor's office than mating.
The researchers, however, explained, "Hypodermic insemination occurs when, rather than being directly transferred to the female reproductive tract during mating, sperm are injected following traumatic wounding of the mating partner, usually through the body wall into some extra-genital location."
The wounded worms later heal.
For selfing, the flatworms can inject their "own sperm into their anterior body region, including into their own head," according to the researchers. Observing this process using high magnification, the scientists further noted that the sperm appear to migrate and fertilize an egg located within the flatworm's body. Reproduction then follows.
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Previously it was thought that hermaphrodites could not engage in selfing because their male and female reproductive structures were isolated via "strict separation." Clearly this flatworm has found a way around that supposed limitation.
Selfing isn't ideal, because it doesn't lead to offspring with genetic variation. A more genetically diverse population is better able to withstand different threats.
Nevertheless, it makes reproduction possible under conditions where few, if any, mates are available.
Now the scientists are wondering what other species might engage in hypodermic insemination and selfing. Since at least one type of organism does this, there's a good chance that other hermaphrodites do too.