ONSET OF DNA-SYNTHESIS IN EXPERIMENTALLY ACTIVATED ASCIDIAN EGGS

DNA synthesis was studied autoradiographically in unfertilized ascidia n eggs (Ascidia malaca and Ciona intestinalis) that had been artificia lly activated by a K+-free external medium or a Ca-ionophore. Naked eg gs of A. malaca were incubated in K+-free seawater that contained [H-3 ]-thymidine for 30 min, and naked eggs of C. intestinalis were incubat ed in seawater supplemented with a Ca-ionophore and [H-3]-thymidine fo r 15 min. The observations revealed limited activation in eggs treated with K+-free seawater. Activation consisted of surface modifications and the onset of DNA synthesis. Incorporation of [H-3]-thymidine was d etected in female nuclei of treated eggs which had not ejected their p olar bodies. In eggs treated with the Ca-ionophore, a known activating agent, the rate of incorporation of [H-3]-thymidine was higher and nu clear duplication was also observed. Comparison of activation under th e two different sets of artificial conditions indicated that, in K+-fr ee medium, the early calcium-dependent events, such as deformation of egg shape, extrusion of the polar bodies, and ooplasmic segregation, d o not occur, while DNA synthesis is initiated. The results are discuss ed on the basis of the hypothesis that, in Ascidians, egg-activation e vents are only partly regulated by an intracellular increase in levels of calcium ions. DNA synthesis appears to be an independent process t hat is initiated by changes in the egg plasma membrane. (C) 1994 Wiley -Liss, Inc.