Activation of mature oocytes initiates development by releasing the prior arrest of female meiosis, degrading certain maternal mRNAs while initiating the translation of others, and modifying egg coverings. In vertebrates and marine invertebrates, the fertilizing sperm triggers activation events through a rise in free calcium within the egg. In insects, egg activation occurs independently of sperm and is instead triggered by passage of the egg through the female reproductive tract ; it is unknown whether calcium signaling is involved. We report here that mutations in sarah, which encodes an inhibitor of the calcium-dependent phosphatase calcineurin, disrupt several aspects of egg activation in Drosophila. Eggs laid by sarah mutant females arrest in anaphase of meiosis I and fail to fully polyadenylate and translate bicoid mRNA. Furthermore, sarah mutant eggs show elevated cyclin B levels, indicating a failure to inactivate M-phase promoting factor (MPF). Taken together, these results demonstrate that calcium signaling is involved in Drosophila egg activation and suggest a molecular mechanism for the sarah phenotype. We also find the conversion of the sperm nucleus into a functional male pronucleus is compromised in sarah mutant eggs, indicating that the Drosophila egg's competence to support male pronuclear maturation is acquired during activation.
In tricomplex heterozygotes in Drosophila melanogaster three metacentric autosomes (the TRI chromosomes) appear as a trivalent in meiosis while one autosome consisting of two homologous arms attached to the same centromere (a compound) behaves as an obligatory univalent. Cytological analysis of meiosis of tri-complex heterozygotes indicates that in oocytes the univalent compound behaves non-independently in relation to segregation of the trivalent. The compound is distributed preferentially to the same pole as one TRI chromosome. In spermatocytes the compound is distributed at random. In some oocytes the directed segregation is shown to be due to a disjunctional interaction between the compound and one partner of the trivalent at the same time as the other two chromosomes of the trivalent are separating from each other. The basic difference between the segregational mechanisms in the two sexes is discussed with a review of evidence indicating that in males segregation is determined by physical linkage that produces a stable orientation of the homologues at metaphase I. On the other hand, both genetic and cytological evidence indicate that in females a physical linkage (a chiasma) is non-essential for maintenance of co-orientation and stability after the onset of prometaphase. Genetic and cytological evidence support the hypothesis that disjunction is predetermined by non-random arrangement of the centromeric regions of chromosomes in the chromocentre - a suprachromosomal organization characteristic of maturing oocytes.
Cleavage cycles commence and chromosome and centrosome cycles proceed in harmony following fertilization of Drosophila eggs and completion of the meiotic divisions. The sperm-introduced centrioles replicate, separate, and while recruit pericentriolar material centrosomes (CS) form. The CS nucleate asters of microtubules (MT). Spindles form following interaction of some astral MT with kinetochores. In unfertilized eggs, chromosomes do not replicate, and CS and MT asters never form, although their components are present in the egg cytoplasm; unknown mechanisms prevent chromosome replication and CS and MT assembly. In unfertilized Laborc(D) eggs, rudimentary CS assemble spontaneously and instantaneously and nucleate small MT asters. In fertilized Laborc(D) eggs, normal CS form and organize normal asters. However, the CS replicate prior to accomplishment of the first mitosis, and spindles with multiple CS develop. In fertilized Laborc(D) eggs, while the chromosome cycles cease, CS cycles proceed as in wild type. Knowing that Laborc(D) is a dominant-negative mutation and encodes the formation of mutant cytoplasmic dynein heavy chain molecules, we show here that cytoplasmic dynein is involved in prevention of CS assembly in unfertilized eggs and establishing harmony between the chromosome and the CS cycles.
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