CALCIUM INFLUX, FERTILIZATION POTENTIAL AND EGG ACTIVATION IN FUCUS-SERRATUS

Fertilisation in the marine alga Fucus serratus is accompanied by incr eased influx of Ca2+ from the external medium. The onset of this incre ase, monitored with the Mn2+ fluorescence quench technique, correspond ed with the depolarisation phase of the fertilisation potential. Exter nal Ca2+ was necessary for the onset of the fertilisation potential an d the early activation events, including cell wall exocytosis. Removal of Ca2+ from, or addition of Sr2+ to, the external medium during the fertilisation potential reduced the magnitude of the depolarisation an d prolonged its duration. While fertilisation potentials could not be elicited in the presence of 0.1 mM Ca2+, addition of Ba2+ in the prese nce of 0.1 mM Ca2+ allowed normal fertilisation potential and egg acti vation. Microinjection of ryanodine or cyclic guanosine 5'-monophospha te (cGMP) did not induce cytoplasmic Ca2+ elevation or egg activation. Inositol 1,4,5-triphosphate [Ins(1,4,5)P-3] produced a transient elev ation of cytoplasmic Ca2+, monitored using ratio photometry, but did n ot cause cell wall exocytosis except at the site of microinjection. Th e results demonstrate an essential role for Ca2+ influx during Fucus e gg activation. The relative importance of influx and intracellular Ca2 + release in Fucus egg activation is discussed.