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University of Basel

01 July 2015

The Bizarre Mating Habits of Flatworms

Flatworm Macrostomum hystrix
Microscopic image of Macrostomum hystrix showing the anterior eyes (1) in the head, followed by the paired testes (2), paired ovaries (3), developing eggs (4), the female genitalia containing three mature eggs (5), and the male genital region in the tail of the worm (6). (Image: Lukas Schärer)

Failing to find a mating partner is a dent to the reproductive prospects of any animal, but in the flatworm species Macrostomum hystrix it might involve a real headache. Zoologists from the Universities of Basel and Bielefeld have discovered the extraordinary lengths to which this animal is willing to go in order to reproduce – including apparently injecting sperm directly into their own heads. The academic journal Proceedings of the Royal Society B has published their findings.

The absence of a mate usually spells disaster for sexually reproducing animals. However, some simultaneous hermaphrodites – animals who have both male and female sex organs at the same time – have developed an escape route for this scenario: self-fertilization. It is an imperfect solution, as any offspring produced by so-called “selfing” are bound to be inbred, but still better than not reproducing at all.

In previous studies, it had been established that the flatworm species Macrostomum hystrix is capable of switching to just such selfing behavior when isolated from mating partners, a behavior found in many but not all simultaneous hermaphrodites. In their new study, Dr. Lukas Schärer from the University of Basel and his team now show the bizarre, yet remarkable mechanisms Macrostomum hystrix has developed that make this possible.

A shot to the head

The studied flatworms are highly transparent and their insides can therefore be easily observed under the microscope. By doing so, the zoologists discovered that under selfing conditions, when hermaphroditic individuals had to use their own sperm to fertilize their own eggs, the worms had very few sperm in their tail region. This is in stark contrast to worms kept in a group, which contained most sperm in their tails, close to where fertilization actually occurs. Instead, isolated worms had more sperm in their head region.

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