EFFECTS OF SILVER IONS (AG-OBSOLETA() ON CONTRACTILE RING FUNCTION AND MICROTUBULE DYNAMICS DURING FIRST CLEAVAGE IN ILYANASSA)

The terminal phase of cell division involves tight constriction of the cleavage furrow contractile ring, stabilization/elongation of the int ercellular bridge, and final separation of the daughter cells. At firs t cleavage, the fertilized eggs of the mollusk, Ilyanassa obsoleta, fo rm two contractile rings at right angles to each other in the same cyt oplasm that constrict to tight necks and partition the egg into a tref oil shape. The cleavage furrow contractile ring (CF) normally constric ts around many midbody microtubules (MTs) and results in cleavage; the polar lobe constriction contractile ring (PLC) normally constricts ar ound very few MTs and subsequently relaxes without cleavage. In the pr esence of Ag+ ions, the PLC 1) begins MT-dependent rapid constriction sooner than controls, 2) encircles more MTs than control egg PLCs, 3) elongates much more than control PLCs, and 4) remains tightly constric ted and effectively cleaves the polar lobe from the egg. If Ag+-incuba ted eggs are returned to normal seawater at trefoil, tubulin fluoresce nce disappears from the PLC neck and the neck relaxes. If nocodazole, a drug that depolymerizes MTs, is added to Ag+-incubated eggs during e arly PLC constriction, the PLC is not stabilized and eventually relaxe s. However, if nocodazole is added to Ag+-incubated eggs at trefoil, t ubulin fluorescence disappears from the PLC neck but the neck remains constricted. These results suggest that Ag+ accelerates and gradually stabilizes the PLC constriction by a mechanism that is initially MT-de pendent, but that progressively becomes MT-independent. (C) 1994 Wiley -Liss, Inc.