The fertilization potential and associated membrane potential oscillations during the resumption of meiosis in the egg of the ascidian Phallusia mammillata

Abstract

The fertilization potential in Phallusia mammillata consisted of an initial rapid depolarization. This initial spermtriggered depolarization was followed by a phase of membrane depolarization which was of either long or short duration, depending on the eggs. When of long duration, the phase of membrane depolarization was divided into two periods: the first one began with a plateau (Em = +20.2 ± 1.1 mV; duration = 1.7 ± 0.14 min) which was followed by a series of membrane potential oscillations ( n = 3.1 ± 0.25) lasting 2.4 ± 0.2 min. The second period also began as a plateau (Em = ∼0 mV; duration = 3.40 ± 0.20 min) which was followed by a series of oscillations ( n = 11.5 ± 0.5) lasting 11.8 ± 0.6 min, followed by a membrane repolarization. The second series of oscillations often continued rising from the resting potential value. In the eggs displaying a short duration of membrane depolarization, the second period of depolarization was shortened (lasting only 3.5 ± 0.5 min) since it lacked the second plateau. In addition it displayed a smaller number of oscillations ( n = 4.7 ± 0.6). As a consequence of this shortening, the membrane repolarized sooner. After repolarization, the membrane displayed several potential oscillations that started from the repolarization level. Regardless of the length of the depolarized plateau phases, the total number of membrane oscillations and the time period during which they occurred were constant. Eggs displaying a long depolarization phase had 15.9 ± 0.6 oscillations in a 19.5 ± 0.6 min interval, while eggs having a short depolarization phase had 16.0 ± 0.8 oscillations in a 18.1 ± 0.3 min interval. The time period during which the potential oscillations occurred corresponded remarkably well with the time of the meiotic divisions: the formation of the first polar body was detected about 80 sec after the end of the first series of oscillations; the second polar body was extruded about 85 sec after the last membrane oscillation occurred.