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In early embryos of the neogastropod Ilyanassa obsoleta , cytoplasmic segregation of a polar lobe is required for establishment of the D macromere, empowering its great-granddaughter macromere 3D to act as a single-celled organizer that induces body patterning along the secondary axis. We present evidence that polar lobe inheritance is not sufficient to specify 3D potential, but rather makes the D macromere lineage responsive to some intercellular signal(s) required for activating the special properties of 3D. Experimental removal of micromeres results in loss of organizer-linked MAPK activation, complete and specific defects of organizer-dependent larval organs, and progressive cell cycle retardation leading to equalization of the normally accelerated division schedule of 3D. Ablation of the second-quartet micromere 2d greatly potentiates the effects of first-quartet micromere ablation. Our findings link 3D establishment in I. obsoleta to the putative ancestral spiralian mechanism in which a signal from micromeres leads to specification of 3D among four initially equivalent macromeres.
Cell ablation experiments on embryos of the snail Ilyanassa reveal that specification of the organizer cell 3D depends on cell-extrinsic cues as well as polar lobe inheritance.