APPEARANCE OF PRONUCLEUS-INDUCING ACTIVITY DURING HORMONALLY INDUCED MEIOTIC MATURATION IN TOAD OOCYTES*

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Abstract
Detergent-pretreated spermatozoa of the toad, B. b. japonicus, transform into pronuclei when injected into progesterone-matured oocytes at 18 h post-hormone treatment (PHT). These sperm do not show any change when injected into the oocytes at the same age from which the germinal vesicle (GV) was removed before the progesterone treatment. To determine when and how the pronucleus-inducing activity (PIA) develops in hormonally induced maturation process, enucleated oocytes were injected with GV and sperm at various stages after the hormone treatment and electrically stimulated at 18 h PHT. Sperm pronuclei are induced only in those oocytes receiving GV before 14 h PHT. The 1 h pulse-treatment of maturing oocytes with cycloheximide between 8-18 h PHT and the injection of sperm at 18 h PHT revealed that PIA does not occur in the oocytes treated with the inhibitor during 10-14 h PHT. Injection of .alpha.-amanitin into maturing oocytes had no effect in this respect. Determination of DNA synthetic activity in vitro of the oocyte extracts from various maturation stages showed that the net increase of the activity occurs before the formation of PIA. The activity of the cycloheximide-treated oocyte extracts utilizing native DNA did not correlate with the sensitivity of oocytes to the inhibitor with respect to PIA in situ. PIA develops, in association with the GV materials, by way of translational events at 10-14 h PHT, being quiescent during later maturation stages, and commences to function as an activation response of oocytes at 18 h PHT.