Proc. Natl Acad. Sci. USA 114, 10888–10893 (2017)

Credit: blickwinkel/Alamy Stock Photo

Planarians (pictured) are intriguing creatures: these little freshwater worms reproduce by ripping themselves in two — literally separating head from tail — and then regenerating the bit that each half lost. But they're also rather shy, and seldom actually commit to the task of reproducing. These quirks have frustrated planarian research for centuries, and the seemingly random location of division along the body axis has come to be thought of as entirely unpredictable. But now, Paul Malinowski and colleagues revealed that pinpointing the fission site is simply a matter of mechanics.

Decapitated planarians are known to divide more frequently, so Malinowski and co-workers exploited this tendency to ramp up their data collection. A mechanical model comprising a thin elastic shell in place of the planarian's head succeeded in reproducing the body dynamics preceding fission. And a statistical analysis revealed that the time between fission events was correlated with the location of division relative to the pharynx, which the worms use to ingest food. Variable stays between divisions translated into variable fission sites, due to the sheer mechanics of rupturing around a large muscle. AK