Division of labor between ERK and Notch signaling coordinates distinct stem cell populations during jellyfish tentacle regeneration

Regeneration restores lost tissues through diverse cellular strategies that vary across species and tissue contexts. In many systems, regeneration involves blastema formation; however, how blastema formation is coordinated in tissues employing multiple stem/progenitor populations remains unclear. Here, we investigate tentacle regeneration in the hydrozoan jellyfish Cladonema, in which two distinct proliferative populations contribute to regeneration: resident homeostatic stem cells (RHSCs), which supply differentiated cell types, and repair-specific proliferative cells (RSPCs), which are transiently induced upon injury to form the blastema. While extensive cell death occurs shortly after injury, it is largely dispensable for blastema formation. Using pharmacological inhibition and cell proliferation analyses, we identify ERK/MAPK signaling as a key regulator of blastema formation. ERK signaling is rapidly activated at the injury site and selectively promotes proliferation of RSPCs without affecting RHSC proliferation. In contrast, inhibition of Notch signaling disrupts nematocyte differentiation and induces hyperproliferation of RHSCs, while leaving RSPC proliferation unchanged, indicating that Notch signaling governs the balance between differentiation and self-renewal in RHSCs. Together, these findings reveal a division of labor between conserved signaling pathways and support a model in which regeneration is achieved through spatially and functionally compartmentalized control of distinct stem cell populations.