Abstract Body axis formation was a pivotal innovation in animal evolution, providing the spatial framework necessary for organized multicellularity. While the genetic basis of axis specification is well established in bilaterians, how axial organization emerged in early-diverging metazoans remains unresolved. Here, we address this question in Trichoplax adhaerens, a placozoan representing one of the simplest extant animal body plans. We show that Wnt signaling, a conserved regulator of axial patterning, exhibits polarized expression enriched in the peripheral region of this morphologically simple organism. Functional perturbations demonstrate that Wnt activity promotes peripheral cell proliferation and maintains central–peripheral tissue balance. Transcriptomic profiling further reveals distinct molecular identities along this axis, resembling the oral–aboral polarity of cnidarians. Together, our findings uncover a Wnt-dependent axial system in placozoans and support the view that core components of metazoan body axis patterning were already established in early animal evolution.
Di et al. (Tue,) studied this question.