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Calyculin-A Induces Cleavage in a Random Plane in Unfertilized Sea Urchin Eggs

¹ Structural Physiology Research Group, RIKEN Spring8 Center, Harima Institute, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan
² Marine Biological Laboratory, 7 MBL Street, Woods Hole, Massachusetts 02543-1015
³ Institute for Biomolecular Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan

^* To whom correspondence should be addressed. E-mail: issei.mabuchi{at}gakushuin.ac.jp

Calyculin-A (CLA), a protein phosphatase inhibitor, has been known to induce cleavage resembling normal furrowing in unfertilized sea urchin eggs. In CLA-treated eggs, actin filaments and myosin assemble to form a contractile ring-like structure in the egg cortex; however, this occurs in the absence of a mitotic spindle or asters. Here, we investigated the relationship between the plane of CLA-induced cleavage and the intrinsic animal-vegetal polar axis in sea urchin eggs. The animal-vegetal axis was established using black ink to visualize the jelly canal located at the animal pole in the jelly coat surrounding the egg. We measured the acute angle between the jelly canal axis and the cleavage plane for both fertilized eggs and CLA-treated unfertilized eggs. Although the acute angle lay within 10 degrees for most of the fertilized eggs, it varied widely for CLA-treated unfertilized eggs. Measurements of the diameter of blastomeres revealed that cleavage of fertilized eggs took place in the mid-plane of the egg, but that CLA-induced divisions were unequal. These results suggest that neither the orientation nor the location of the CLA-induced cleavage furrow is related to the animal-vegetal polar axis of the egg, even though the furrowing mechanism itself is not dissimilar to that in fertilized eggs.

Abbreviations: CLA, Calyculin-A • CR, contractile ring