Publication by researchers from the Faculty of Biology UW in Proceedings of the Royal Society B: Biological Sciences

30 10 2025

Researchers from the Faculty of Biology, University of Warsaw – Katarzyna Tratkiewicz and Dr. Ludwik Gąsiorowski – have described a remarkable developmental error occurring in the microscopic flatworm Stenostomum brevipharyngium. As a result of a spontaneous mistake during asexual reproduction (paratomy), an additional, ectopic head forms instead of a new tail, leading to the emergence of double-headed individuals. Surprisingly, such animals are capable of reversing their body axis polarity by 180 degrees and can continue reproducing asexually.

Paratomy is an unusual mode of asexual reproduction in which a new individual is formed within the parental body – first establishing a new head and tail, before the two individuals separate. In most animals, the antero-posterior axis is determined once, during early embryogenesis. However, in organisms that reproduce somatically, such as flatworms of the genus Stenostomum, this body axis must be re-established repeatedly in the context of the fully formed adult body.

The team from the Comparative Invertebrate Zoology Group, Institute of Evolutionary Biology, Faculty of Biology, University of Warsaw, observed the spontaneous formation of individuals with two heads – exhibiting ectopic head structures in place of the posterior pole. As demonstrated by the authors, this double-head phenotype is not heritable: worms arising from the sectioning or fission of such double-headed individuals develop normally, giving rise to healthy populations.

Moreover, a body fragment containing an ectopic head is capable of regenerating a tail at its previously anterior pole, effectively reversing the polarity of the entire body axis by 180 degrees. This phenomenon is extremely rare among bilaterians and reveals extraordinary developmental plasticity and regenerative capacity in flatworms.

The full study has been published in Proceedings of the Royal Society B: Biological Sciences “Spontaneous ectopic head formation enables reversal of the body axis polarity in microscopic flatworms”