Ovulation triggers activation of Drosophila oocytes

Drosophila melanogaster mature oocytes in ovaries are arrested at metaphase I of meiosis. Eggs that have reached the uterus have released this arrest. It was not known where in the female reproductive tract egg activation occ urs and what triggers it. We investigated when and where the egg is activat ed in Drosophila in vivo and at what meiotic stage the egg is fertilized. W e found that changes in the egg's envelope's permeability, one feature of a ctivation, initiate during ovulation, even while most of the egg is still w ithin the ovary. The egg becomes impermeable as it proceeds down the oviduc ts; the process is complete by the time the egg is in the uterus. Cross-lin king of vitelline membrane protein sV23 also increases progressively as the egg moves through the oviducts and the uterus. Activation also triggers me iosis to resume before the egg reaches the uterus, such that the earliest e ggs that reach the uterus are in anaphase I. We discuss models for Drosophi la egg activation in vivo. (C) 2001 Academic Press.