The Drosophila mei-S332 gene promotes sister-chromatid cohesion in meiosis following kinetochore differentiation

The Drosophila genes grauzone and cortex are necessary for proper female meiosis

In Drosophila, normal female meiosis arrests at metaphase I. After meiotic arrest is released by egg activation, the two meiotic divisions are rapidly completed, even in unfertilized eggs. Since little is known about the regulation of the meiotic cell cycle after the meiotic arrest, we screened for mutants that arrest in meiosis. Here we describe the phenotype of eggs laid by sterile mothers mutant for either grauzone or cortex. These eggs arrest in metaphase of meiosis II, and although they can enter into an aberrant anaphase II, they never exit meiosis. Prolonged sister-chromatid cohesion is not the cause of this arrest, since a premature release of sister cohesion does not rescue the meiotic arrest of cortex eggs. Aberrant chromosome segregation at meiosis I was the earliest observable defect, suggesting that grauzone and cortex are first required immediately after egg activation. The cortical microtubules are also defective, remaining in a pre-activated state in activated mutant eggs. The mutations had no observable effect on either male meiosis or mitosis. We believe these genes will provide insight into the developmental regulation of meiosis in a genetically tractable organism.

Holding chromatids together to ensure they go their separate ways

Association between sister chromatids is essential for their attachment and segregation to opposite poles of the spindle in mitosis and meiosis II. Sister-chromatid cohesion is also likely to be involved in linking homologous chromosomes together in meiosis I. Cytological observations provide evidence that attachment between sister chromatids is different in meiosis and mitosis and suggest that cohesion between the chromatid arms may differ mechanistically from that at the centromere. The physical nature of cohesion is addressed, and proteins that are candidates for holding sister chromatids together are discussed. Dissolution of sister-chromatid cohesion must be regulated precisely, and potential mechanisms to release cohesion are presented.

Identification of ORD, a Drosophila protein essential for sister chromatid cohesion

Attachment between the sister chromatids is required for proper chromosome segregation in meiosis and mitosis, but its molecular basis is not understood. Mutations in the Drosophila ord gene result in premature sister chromatid separation in meiosis, indicating that the product of this gene is necessary for sister chromatid cohesion. We isolated the ord gene and found that it encodes a novel 55 kDa protein. Some of the ord mutations exhibit unusual complementation properties, termed negative complementation, in which particular alleles poison the activity of another allele. Negative complementation predicts that protein-protein interactions are critical for ORD function. The position and nature of these unusual ord mutations demonstrate that the C-terminal half of ORD is essential for sister chromatid cohesion and suggest that it mediates protein binding.

Separation of sister chromatids in mitosis requires the Drosophila pimples product, a protein degraded after the metaphase/anaphase transition

Mutations in the Drosophila genes pimples and three rows result in a defect of sister chromatid separation during mitosis. As a consequence, cytokinesis is also defective. However, cell cycle progression including the mitotic degradation of cyclins A and B is not blocked by the failure of sister chromatid separation, and as a result, metaphase chromosomes with twice the normal number of chromosome arms still connected in the centromeric region are observed in the following mitosis, pimples encodes a novel protein that is rapidly degraded in mitosis. Our observations suggest that Pimples and Three rows act during mitosis to release the cohesion between sister centromeres.

Meiosis in Drosophila: seeing is believing

Recently many exciting advances have been achieved in our understanding of Drosophila meiosis due to combined cytological and genetic approaches. New techniques have permitted the characterization of chromosome position and spindle formation in female meiosis I. The proteins encoded by the nod and ncd genes, two genes known to be needed for the proper partitioning of chromosomes lacking exchange events, have been identified and found to be kinesin-like motors. The effects of mutations in these genes on the spindle and chromosomes, together with the localization of the proteins, have yielded a model for the mechanism of female meiosis I. In male meiosis I, the chromosomal regions responsible for homolog pairing have been resolved to the level of specific DNA sequences. This provides a foundation for elucidating the molecular basis of meiotic pairing. The cytological techniques available in Drosophila also have permitted inroads into the regulation of sister-chromatid segregation. The products of two genes (mei-S332 and ord) essential for sister-chromatid cohesion have been identified recently. Additional advances in understanding Drosophila meiosis are the delineation of a functional centromere by using minichromosome derivatives and the identification of several regulatory genes for the meiotic cell cycle.

Mei-S332, a Drosophila protein required for sister-chromatid cohesion, can localize to meiotic centromere regions

Mutations in the Drosophila mei-S332 gene cause premature separation of the sister chromatids in late anaphase of meiosis I. Therefore, the mei-S332 protein was postulated to hold the centromere regions of sister chromatids together until anaphase II. The mei-S332 gene encodes a novel 44 kDa protein. Mutations in mei-S332 that differentially affect function in males or females map to distinct domains of the protein. A fusion of mei-S332 to the green fluorescent protein (GFP) is fully functional and localizes specifically to the centromere region of meiotic chromosomes. When sister chromatids separate at anaphase II, mei-S332-GFP disappears from the chromosomes, suggesting that the destruction or release of this protein is required for sister-chromatid separation.

Double or nothing: a Drosophila mutation affecting meiotic chromosome segregation in both females and males

We describe a Drosophila mutation, Double or nothing (Dub), that causes meiotic nondisjunction in a conditional, dominant manner. Previously isolated mutations in Drosophila specifically affect meiosis either in females or males, with the exception of the mei-S332 and ord genes which are required for proper sister-chromatid cohesion. Dub is unusual in that it causes aberrant chromosome segregation almost exclusively in meiosis I in both sexes. In Dub mutant females both nonexchange and exchange chromosomes undergo nondisjunction, but the effect of Dub on nonexchange chromosomes is more pronounced. Dub reduces recombination levels slightly. Multiple nondisjoined chromosomes frequently cosegregate to the same pole. Dub results in nondisjunction of all chromosomes in meiosis I of males, although the levels are lower than in females. When homozygous, Dub is a conditional lethal allele and exhibits phenotypes consistent with cell death.

The yeast med1 mutant undergoes both meiotic homolog nondisjunction and precocious separation of sister chromatids

A mutant at the yeast MED1 locus was isolated in a screen for sporulation-proficient, meiotic-lethal mutants. Synaptonemal complex formation in the med1 mutant is apparently normal and med1 strains undergo meiotic crossing over at approximately 50% of the wild-type level. The med1 mutant undergoes homolog nondisjunction at meiosis I, presumably as a consequence of the decrease in crossing over. In addition, the mutant undergoes precocious separation of sister chromatids, resulting in chromosome missegregation at both meiotic divisions. We suggest that the med1 mutation perturbs chromosome structure, leading to a reduction in recombination and a defect in sister chromatid cohesion.

Regulatory mechanisms in meiosis

Meiosis can be viewed both as a process of cell differentiation and as a modification of the mitotic cell cycle. Here we describe recent progress in defining a variety of regulatory mechanisms that govern the meiotic divisions. Studies in the yeast Saccharomyces cerevisiae and in higher organisms have led to complementary insights into these controls.

P element transposition in Drosophila melanogaster: an analysis of sister-chromatid pairs and the formation of intragenic secondary insertions during meiosis

The gap-repair model proposes that P elements move via a conservative, "cut-and-paste" mechanism followed by double-strand gap repair, using either the sister chromatid or homolog as the repair template. We have tested this model by examining meiotic perturbations of an X-linked ry+ transposon during the meiotic cycle of males, employing the mei-S332 mutation, which induces high frequency equational nondisjunction. This system permits the capture of both sister-X chromatids in a single patroclinous daughter. In the presence of P-transposase, transpositions within the immediate proximity of the original site are quite frequent. These are readily detectable among the patroclinous daughters, thereby allowing the combined analysis of the transposed element, the donor site and the putative sister-strand template. Molecular analysis of 22 meiotic transposition events provide results that support the gap-repair model of P element transposition. Prior to this investigation, it was not known whether transposition events were exclusively or predominantly premeiotic. The results of our genetic analysis revealed that P elements mobilize at relatively high frequencies during meiosis. We estimated that approximately 4% of the dysgenic male gametes have transposon perturbations of meiotic origin; the proportion of gametes containing lesions of premeiotic origin was estimated at 32%.

Sister-chromatid misbehavior in Drosophila ord mutants

In Drosophila males and females mutant for the ord gene, sister chromatids prematurely disjoin in meiosis. We have isolated five new alleles of ord and analyzed them both as homozygotes and in trans to deficiencies for the locus, and we show that ord function is necessary early in meiosis of both sexes. Strong ord alleles result in chromosome nondisjunction in meiosis I that appears to be the consequence of precocious separation of the sister chromatids followed by their random segregation. Cytological analysis in males confirmed that precocious disjunction of the sister chromatids occurs in prometaphase I. This is in contrast to Drosophila mei-S332 mutants, in which precocious sister-chromatid separation also occurs, but not until late in anaphase I. All three of the new female fertile ord alleles reduce recombination, suggesting they affect homolog association as well as sister-chromatid cohesion. In addition to the effect of ord mutations on meiosis, we find that in ord2 mutants chromosome segregation is aberrant in the mitotic divisions that produce the spermatocytes. The strongest ord alleles, ord2 and ord5, appear to cause defects in germline divisions in the female. These alleles are female sterile and produce egg chambers with altered nurse cell number, size, and nuclear morphology. In contrast to the effects of ord mutations on germline mitosis, all of the alleles are fully viable even when in trans to a deficiency, and thus exhibit no essential role in somatic mitosis. The ord gene product may prevent premature sister-chromatid separation by promoting cohesion of the sister chromatids in a structural or regulatory manner.

The Drosophila l(1)zw10 gene product, required for accurate mitotic chromosome segregation, is redistributed at anaphase onset

Mutations in the gene l(1)zw10 disrupt the accuracy of chromosome segregation in a variety of cell types during the course of Drosophila development. Cytological analysis of mutant larval brain neuroblasts shows very high levels of aneuploid cells. Many anaphase figures are aberrant, the most frequent abnormality being the presence of lagging chromosomes that remain in the vicinity of the metaphase plate when the other chromosomes have migrated toward the spindle poles. Finally, the centromeric connection between sister chromatids in mutant neuroblasts treated with colchicine often appears to be broken, in contrast with similarly treated control neuroblasts. The 85-kD protein encoded by the l(1)zw10 locus displays a dynamic pattern of localization in the course of the embryonic cell cycle. It is excluded from the nuclei during interphase, but migrates into the nuclear zone during prometaphase. At metaphase, the zw10 antigen is found in a novel filamentous structure that may be specifically associated with kinetochore microtubules. Upon anaphase onset, there is an extremely rapid redistribution of the zw10 protein to a location at or near the kinetochores of the separating chromosomes.