Is calcium ionophore a universal activator for unfertilised eggs?

Abstract

WE have previously exposed sea urchin eggs to micromolar amounts of the divalent transporting ionophore A23187, and observed every parameter of normal fertilisation. The cortical reaction with elevation of the fertilisation membrane, the plasma membrane conductance changes, the respiratory burst, and increases in protein and DNA synthesis were all initiated in the usual fashion¹. These activations of Lytechinus pictus and Strongylocentrotus purpuratus eggs were independent of the ionic composition of the external solutions¹. A23187 seemed to act by releasing intracellular Ca²⁺, for eggs preloaded with ⁴⁵Ca showed a twentyfold increase in ⁴⁵Ca-efflux when activated by ionophore or fertilisation¹. Measurements of free and bound calcium and magnesium in homogenates of unfertilised eggs showed that most of the Mg²⁺ was already available in soluble form, whereas the Ca²⁺ was sequestered but available for release¹. After we found that 50 µM A23187 could activate eggs of the mollusc Acmaea insessa and the protochordate Ciona intestinalis to undergo several abnormal cleavages, we wondered whether ionophore activation might be a general phenomenon. To our knowledge artificial parthenogenesis has not been obtained with Acmaea and Ciona eggs by other methods. We have now found that calcium ionophore activation is common to several species widely separated phylogenetically.