Do Not Lose Your Head over the Unequal Regeneration Capacity in Prolecithophoran Flatworms

SIMPLE SUMMARY: Some flatworms can regenerate all body parts, including the head, while others cannot. We have studied the regeneration capacity of several prolecithophoran flatworm species to gain insights into the evolution of regeneration in flatworms, particularly the adiaphanidan clade, which includes the well-known planarians. We found that regeneration capacity greatly varies between different species and also between different families of Prolecithophora. No prolecithophoran was found to be able to regenerate the complete head. This suggests that planarians have likely evolved their head regeneration capacity de novo. ABSTRACT: One of the central questions in studying the evolution of regeneration in flatworms remains whether the ancestral flatworm was able to regenerate all body parts, including the head. If so, this ability was subsequently lost in most existent flatworms. The alternative hypothesis is that head regeneration has evolved within flatworms, possibly several times independently. In the well-studied flatworm taxon Tricladida (planarians), most species are able to regenerate a head. Little is known about the regeneration capacity of the closest relatives of Tricladida: Fecampiida and Prolecithophora. Here, we analysed the regeneration capacity of three prolecithophoran families: Pseudostomidae, Plagiostomidae, and Protomonotresidae. The regeneration capacity of prolecithophorans varies considerably between families, which is likely related to the remaining body size of the regenerates. While all studied prolecithophoran species were able to regenerate a tail-shaped posterior end, only some Pseudostomidae could regenerate a part of the pharynx and pharynx pouch. Some Plagiostomidae could regenerate a head including the brain and eyes, provided the roots of the brain were present. The broad spectrum of regeneration capacity in Prolecithophora suggests that head regeneration capacity is not an apomorphy of Adiaphanida.