Spontaneous ectopic head formation enables reversal of the body axis polarity in microscopic flatworms

In most of the animals, the antero-posterior axis is specified during early embryogenesis. However, in the organisms that undergo somatic asexual reproduction, constant re-establishment of the body axis occurs during each asexual act in the context of the fully formed adult body. In microscopic flatworms from the genus <i>Stenostomum</i> the new head and tail structures are inserted in the pre-existing body plan during the asexual process known as paratomy. Here, we report a spontaneously occurring developmental error that results in the formation of worms with double heads at opposite ends of their bodies, lacking posterior pole identity. In the set of experiments, we show that the double-head phenotype is not heritable on the organismal level. Worms originating from the sectioning or fission of the double-head animals give rise to the healthy populations that do not display the erroneous asexual development. We also demonstrate that the piece of the worm with ectopic head can survive, regenerate the tail on its previously anterior pole and resume asexual reproduction. Effectively, such regeneration allows stable reversal of the body axis polarity without impairment of the survival or reproductive abilities of the animal, an exceptionally rare phenomenon among bilaterians.