6533b7d3fe1ef96bd1260855

RESEARCH PRODUCT

Onset of DNA synthesis in experimentally activated ascidian eggs

subject

description

DNA synthesis was studied autoradiographically in unfertilized ascidian eggs (Ascidia malaca and Ciona intestinalis) that had been artificially activated by a K+-free external medium or a Ca-ionophore. Naked eggs of A. malaca were incubated in K+-free seawater that contained [3H]-thymidine for 30 min, and naked eggs of C. intestinalis were incubated in seawater supplemented with a Ca-ionophore and [3H]-thymidine for 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 [3H]-thymidine was detected in female nuclei of treated eggs which had not ejected their polar bodies. In eggs treated with the Ca-ionophore, a known activating agent, the rate of incorporation of [3H]-thymidine was higher and nuclear duplication was also observed. Comparison of activation under the two different sets of artificial conditions indicated that, in K+-free medium, the early calcium-dependent events, such as deformation of egg shape, extrusion of the polar bodies, and ooplasmic segregation, do not occur, while DNA synthesis is initiated. The results are discussed on the basis of the hypothesis that, in Ascidians, egg-activation events are only partly regulated by an intracellular increase in levels of calcium ions. DNA synthesis appears to be an independent process that is initiated by changes in the egg plasma membrane. © 1994 Wiley-Liss, Inc.