Divergent evolution of the Wnt signaling system in flatworms

Regenerative capacity varies widely across flatworms (Platyhelminthes). Whereas catenulids, microstomids and planarians can regenerate a complete head de novo, other flatworms cannot. This striking diversity raises a longstanding evolutionary question: does whole-body regeneration represent an ancestral trait that was subsequently lost in multiple lineages, or did it evolve convergently? Addressing this question requires comparative analyses of the molecular mechanisms underlying regeneration across phylogenetically diverse flatworms. Here, we focus on Wnt signaling, a deeply conserved regulator of antero-posterior (A-P) patterning and a central determinant of head-versus-tail identity during planarian regeneration, to establish a mechanistic framework for such comparisons. Although Wnt signaling has been studied extensively in planarians and parasitic neodermatans, its evolution and deployment in other flatworm clades remain poorly characterized. To address this gap, we characterized the complement of Wnt signaling components in two early-diverging flatworm clades, Catenulida and Macrostomorpha, with particular emphasis on expression and function in the catenulid Stenostomum brevipharyngium. Phylogenetic analyses reveal the ancient loss of six Wnt families and one secreted Frizzled-related protein (sFRP) family in the last common ancestor of flatworms, followed by additional lineage-specific gene losses and expansions. Moreover, several Wnt pathway components display markedly divergent expression patterns between catenulids and other flatworms, while functional analyses indicate corresponding differences in their regenerative deployment. Together, our findings reveal a dynamic evolutionary history of the flatworm Wnt signaling toolkit and establish a comparative framework for testing whether the molecular circuitry underlying head regeneration is ancestrally conserved or has evolved independently in distinct flatworm lineages.