An astral simulacrum of the central spindle accounts for normal, spindle-less, and anucleate cytokinesis in echinoderm embryos.
Bottom Line: Although the nature, origin, and variety of such cues have long been obscure, one component is certainly the Rho activator Ect2.Here we describe the behavior and function of Ect2 in echinoderm embryos, showing that Ect2 migrates from spindle midzone to astral microtubules in anaphase and that Ect2 shapes the pattern of Rho activation in incipient furrows.In all these cases, the cell assembles essentially the same cytokinetic signaling ensemble—opposed astral microtubules decorated with Ect2 and Cyk4.
Affiliation: Oregon Institute of Marine Biology, Charleston, OR 97420 Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms EN6 3LD, United Kingdom.Show MeSH
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Mentions: We next tested whether Ect2 and centralspindlin are localized to furrows that form between two asters without a spindle between them (“secondary” furrows). The classic instance is the toroidal cell, which, as described originally by Rappaport (1961), first cleaves to make a binucleate, U-shaped cell; at second mitosis, it cleaves from one to four cells with two furrows crossing spindles and one that passes between two asters. We perforated Ect2- or Cyk4-expressing starfish zygotes or blastomeres from the 2- to 16-cell stage with a 40- to 60-μm glass bead. Ect2 consistently appeared, albeit more faintly than in the primary furrows, in the overlap zone where a secondary furrow ingressed, becoming concentrated to a midbody in all cases in which the secondary furrow completed (Figure 7, A and B, and Supplemental Video S9). Cyk4 always recruited robustly to the secondary furrow (Figure 7C and Supplemental Video S10). Of interest, perforated cells expressing Ect2 or Cyk4 also displayed elevated cortical contractility, and occasionally an ingressing furrow, on the far side of the torus during the first mitosis (Supplemental Videos S9 and S10); although we did not unambiguously detect Ect2 in such cases, Cyk4 consistently decorated distal portions of astral microtubules on the far side of the torus, albeit much less well focused than at second mitosis (Figure 7C and Supplemental Video S10).
Affiliation: Oregon Institute of Marine Biology, Charleston, OR 97420 Cancer Research UK London Research Institute, Clare Hall Laboratories, South Mimms EN6 3LD, United Kingdom.