Embryonic organizer specification in the mud snail Ilyanassa obsoleta depends on intercellular signaling

Abstract

In early embryos of the caenogastropod snail Ilyanassa obsoleta, cytoplasmic segregation of a polar lobe is required for establishment of the D quadrant founder cell, empowering its great-granddaughter macromere 3D to act as a single-celled organizer that induces ectodermal pattern along the embryo's secondary body 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 organizer activation 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.