Abbreviation of the second meiotic division by precocious fertilization in fish oocytes

Abstract

To clarify the mechanisms of fertilization in fish, we have established an experimental system using Oryzias latipes oocytes. When females that had spawned every day at 2628°C were transferred to 10°C about 5 hr before the initiation of germinal vesicle breakdown (GVBD), GVBD was arrested for at least 20 hr at 10°C. When the females were returned to 27°C, the oocytes underwent GVBD about 2 hr later and meiosis proceeded to metaphase of the second meiotic division in a normal time course. Using this experimental system, isolated oocytes at various stages were inseminated immediately after follicular layers were mechanically removed. Oocytes at the GVBD stage were penetrated by spermatozoa but not activated. The rate of sperm penetration increased progressively beginning 1 hr after GVBD, while the rate of activation increased more slowly. Pronucleus formation and karyogamy were observed in oocytes inseminated 2 hr or more after GVBD. When a spermatozoon penetrated an immature oocyte during the first meiotic division but global exocytosis of the cortical alveoli failed to occur, the sperm nucleus transformed to metaphase chromosomes. On the other hand, when exocytosis of cortical alveoli occurred in the entire oocyte cortex, meiotic chromosomes and the sperm nucleus in metaphase I or anaphase I oocytes decondensed after extrusion of the first polar body and transformed to female and male pronuclei, respectively. These pronuclei underwent karyogamy without the oocyte undergoing the second meiotic division. These results reveal that in the medaka oocytes competent to undergo propagative exocytosis skip the second meiotic division upon activation by sperm penetration before completion of the first meiotic division.