Heterochrony of axis segmentation underlies extreme morphogenesis in the Japanese eel

Heterochrony is a major mode of vertebrate body plan evolution, yet its molecular and cellular basis remains poorly characterized. Here, we show that axis segmentation in Anguilla japonica —a species that forms 120 vertebrae—is driven by the temporal extension of somitogenesis. This is achieved through the prolonged maintenance of axial progenitors in the tail beyond the hatching stage, coupled to an extreme segment scaling regime that operates under the constraint of minimal axis growth. We identify delayed Hox13 activation and sustained Oct4 expression as molecular signatures of the prolonged segmentation program. Furthermore, we describe two spatially distinct axial progenitor pools that expand stemness in the tail. These findings reveal how the modulation of stemness in time and space drives extreme morphological evolution in vertebrates.