Kinetochore microtubules and chromosome movement during prometaphase in Drosophila melanogaster spermatocytes studied in life and with the electron microscope

Dynamics and control of sister kinetochore behavior during the meiotic divisions in Drosophila spermatocytes

Sister kinetochores are connected to the same spindle pole during meiosis I and to opposite poles during meiosis II. The molecular mechanisms controlling the distinct behavior of sister kinetochores during the two meiotic divisions are poorly understood. To study kinetochore behavior during meiosis, we have optimized time lapse imaging with Drosophila spermatocytes, enabling kinetochore tracking with high temporal and spatial resolution through both meiotic divisions. The correct bipolar orientation of chromosomes within the spindle proceeds rapidly during both divisions. Stable bi-orientation of the last chromosome is achieved within ten minutes after the onset of kinetochore-microtubule interactions. Our analyses of mnm and tef mutants, where univalents instead of bivalents are present during meiosis I, indicate that the high efficiency of normal bi-orientation depends on pronounced stabilization of kinetochore attachments to spindle microtubules by the mechanical tension generated by spindle forces upon bi-orientation. Except for occasional brief separation episodes, sister kinetochores are so closely associated that they cannot be resolved individually by light microscopy during meiosis I, interkinesis and at the start of meiosis II. Permanent evident separation of sister kinetochores during M II depends on spindle forces resulting from bi-orientation. In mnm and tef mutants, sister kinetochore separation can be observed already during meiosis I in bi-oriented univalents. Interestingly, however, this sister kinetochore separation is delayed until the metaphase to anaphase transition and depends on the Fzy/Cdc20 activator of the anaphase-promoting complex/cyclosome. We propose that univalent bi-orientation in mnm and tef mutants exposes a release of sister kinetochore conjunction that occurs also during normal meiosis I in preparation for bi-orientation of dyads during meiosis II.

For production of oocytes and sperm, cells have to complete meiosis which includes two successive divisions. These divisions convert diploid cells with a maternal and a paternal copy of each chromosome into haploid cells with only one copy of each chromosome. Chromosome copy reduction requires regulation of sister kinetochore behavior during the meiotic divisions. Kinetochores are protein networks assembled at the start of divisions within the centromeric region of chromosomes. They provide attachment sites for spindle microtubules which in turn exert poleward pulling forces. During pre-meiotic S phase, each chromosome is duplicated into two closely associated sister chromatids. At the start of the first meiotic division, both sister chromatids together assemble only one functional kinetochore, permitting subsequent separation of paired homologous chromosomes to opposite spindle poles. In contrast, at the onset of the second meiotic division, each sister chromatid organizes its own kinetochore followed by separation of sister chromatids to opposite spindle poles. To analyze when and how sister kinetochores are individualized, we have improved time lapse imaging with Drosophila spermatocytes. Our analyses in normal and genetically altered spermatocytes suggest that the release of sister kinetochore conjunction occurs during the first meiotic division after activation of the anaphase promoting complex/cyclosome.

Using Photobleaching to Measure Spindle Microtubule Dynamics in Primary Cultures of Dividing Drosophila Meiotic Spermatocytes

In dividing animal cells, a microtubule (MT)-based bipolar spindle governs chromosome movement. Current models propose that the spindle facilitates and/or generates translocating forces by regionally depolymerizing the kinetochore fibers (k-fibers) that bind each chromosome. It is unclear how conserved these sites and the resultant chromosome-moving mechanisms are between different dividing cell types because of the technical challenges of quantitatively studying MTs in many specimens. In particular, our knowledge of MT kinetics during the sperm-producing male meiotic divisions remains in its infancy. In this study, I use an easy-to-implement photobleaching-based assay for measuring spindle MT dynamics in primary cultures of meiotic spermatocytes isolated from the fruit fly Drosophila melanogaster. By use of standard scanning confocal microscopy features, fiducial marks were photobleached on fluorescent protein (FP)-tagged MTs. These were followed by time-lapse imaging during different division stages, and their displacement rates were calculated using public domain software. I find that k-fibers continually shorten at their poles during metaphase and anaphase A through the process of MT flux. Anaphase chromosome movement is complemented by Pac-Man, the shortening of the k-fiber at its chromosomal interface. Thus, Drosophila spermatocytes share the sites of spindle dynamism and mechanisms of chromosome movement with mitotic cells. The data reveal the applicability of the photobleaching assay for measuring MT dynamics in primary cultures. This approach can be readily applied to other systems.

Adaptive changes in the kinetochore architecture facilitate proper spindle assembly

Mitotic spindle formation relies on the stochastic capture of microtubules at kinetochores. Kinetochore architecture affects the efficiency and fidelity of this process with large kinetochores expected to accelerate assembly at the expense of accuracy, and smaller kinetochores to suppress errors at the expense of efficiency. We demonstrate that upon mitotic entry, kinetochores in cultured human cells form large crescents that subsequently compact into discrete structures on opposite sides of the centromere. This compaction occurs only after the formation of end-on microtubule attachments. Live-cell microscopy reveals that centromere rotation mediated by lateral kinetochore-microtubule interactions precedes formation of end-on attachments and kinetochore compaction. Computational analyses of kinetochore expansion-compaction in the context of lateral interactions correctly predict experimentally-observed spindle assembly times with reasonable error rates. The computational model suggests that larger kinetochores reduce both errors and assembly times, which can explain the robustness of spindle assembly and the functional significance of enlarged kinetochores.

Differing requirements for Augmin in male meiotic and mitotic spindle formation in Drosophila

Animal cells divide using a microtubule-based, bipolar spindle. Both somatic, mitotic cells and sperm-producing male meiotic spermatocytes use centrosome-dependent and acentrosomal spindle-forming mechanisms. Here, we characterize the largely undefined, centrosome-independent spindle formation pathway used during male meiosis. Our live and fixed cell analyses of Drosophila spermatocytes reveal that acentrosomal microtubules are nucleated at kinetochores and in the vicinity of chromatin and that together these assemble into functional spindles. Mutational studies indicate that γ-tubulin and its extra-centrosomal targeting complex, Augmin, are vital for this process. In addition, Augmin facilitates efficient spindle assembly in the presence of centrosomes. In contrast to the pronounced recruitment of Augmin on spindles in other cell types, the complex is absent from those of spermatocytes but does accumulate on kinetochores. Polo kinase facilitates this kinetochore recruitment while inhibiting Augmin's spindle association, and this in turn dictates γ-tubulin distribution and spindle density. Polo's negative regulation of Augmin in male meiosis contrasts with its requirement in loading Augmin along mitotic spindles in somatic Drosophila cells. Together our data identify a novel mechanism of acentrosomal spindle formation in spermatocytes and reveal its divergence from that used in mitotic cells.

Ex vivo culture of Drosophila pupal testis and single male germ-line cysts: dissection, imaging, and pharmacological treatment

During spermatogenesis in mammals and in Drosophila melanogaster, male germ cells develop in a series of essential developmental processes. This includes differentiation from a stem cell population, mitotic amplification, and meiosis. In addition, post-meiotic germ cells undergo a dramatic morphological reshaping process as well as a global epigenetic reconfiguration of the germ line chromatin-the histone-to-protamine switch. Studying the role of a protein in post-meiotic spermatogenesis using mutagenesis or other genetic tools is often impeded by essential embryonic, pre-meiotic, or meiotic functions of the protein under investigation. The post-meiotic phenotype of a mutant of such a protein could be obscured through an earlier developmental block, or the interpretation of the phenotype could be complicated. The model organism Drosophila melanogaster offers a bypass to this problem: intact testes and even cysts of germ cells dissected from early pupae are able to develop ex vivo in culture medium. Making use of such cultures allows microscopic imaging of living germ cells in testes and of germ-line cysts. Importantly, the cultivated testes and germ cells also become accessible to pharmacological inhibitors, thereby permitting manipulation of enzymatic functions during spermatogenesis, including post-meiotic stages. The protocol presented describes how to dissect and cultivate pupal testes and germ-line cysts. Information on the development of pupal testes and culture conditions are provided alongside microscope imaging data of live testes and germ-line cysts in culture. We also describe a pharmacological assay to study post-meiotic spermatogenesis, exemplified by an assay targeting the histone-to-protamine switch using the histone acetyltransferase inhibitor anacardic acid. In principle, this cultivation method could be adapted to address many other research questions in pre- and post-meiotic spermatogenesis.

Disruption of redox homeostasis leads to oxidative DNA damage in spermatocytes of Wolbachia-infected Drosophila simulans

Molecular interactions between symbiotic bacteria and their animal hosts are, as yet, poorly understood. The most widespread bacterial endosymbiont, Wolbachia pipientis, occurs in high density in testes of infected Drosophila simulans and causes cytoplasmic incompatibility (CI), a form of male-derived zygotic lethality. Wolbachia grow and divide within host vacuoles that generate reactive oxygen species (ROS), which in turn stimulate the up-regulation of antioxidant enzymes. These enzymes appear to protect the host from ROS-mediated damage, as there is no obvious fitness cost to Drosophila carrying Wolbachia infections. We have now determined that DNA from Wolbachia-infected mosquito Aedes albopictus (Aa23) cells shows a higher amount of the base 8-oxo-deoxyguanosine, a marker of oxidative DNA damage, than DNA from uninfected cells, and that Wolbachia infection in D. simulans is associated with an increase in DNA strand breaks in meiotic spermatocytes. Feeding exogenous antioxidants to male and female D. simulans dramatically increased Wolbachia numbers with no obvious effects on host fitness. These results suggest that ROS-induced DNA damage in sperm nuclei may contribute to the modification characteristic of CI expression in Wolbachia-infected males and that Wolbachia density is sensitive to redox balance in these flies.

Phosphorylation of human Sgo1 by NEK2A is essential for chromosome congression in mitosis

Chromosome segregation in mitosis is orchestrated by the interaction of the kinetochore with spindle microtubules. Our recent study shows that NEK2A interacts with MAD1 at the kinetochore and possibly functions as a novel integrator of spindle checkpoint signaling. However, it is unclear how NEK2A regulates kinetochore-microtubule attachment in mitosis. Here we show that NEK2A phosphorylates human Sgo1 and such phosphorylation is essential for faithful chromosome congression in mitosis. NEK2A binds directly to HsSgo1 in vitro and co-distributes with HsSgo1 to the kinetochore of mitotic cells. Our in vitro phosphorylation experiment demonstrated that HsSgo1 is a substrate of NEK2A and the phosphorylation sites were mapped to Ser(14) and Ser(507) as judged by the incorporation of (32)P. Although such phosphorylation is not required for assembly of HsSgo1 to the kinetochore, expression of non-phosphorylatable mutant HsSgo1 perturbed chromosome congression and resulted in a dramatic increase in microtubule attachment errors, including syntelic and monotelic attachments. These findings reveal a key role for the NEK2A-mediated phosphorylation of HsSgo1 in orchestrating dynamic kinetochore-microtubule interaction. We propose that NEK2A-mediated phosphorylation of human Sgo1 provides a link between centromeric cohesion and spindle microtubule attachment at the kinetochores.

Evolutionary conservation of lampbrush-like loops in drosophilids

Background

Loopin-1 is an abundant, male germ line specific protein of Drosophila melanogaster. The polyclonal antibody T53-F1 specifically recognizes Loopin-1 and enables its visualization on the Y-chromosome lampbrush-like loop named kl-3 during primary spermatocyte development, as well as on sperm tails. In order to test lampbrush-like loop evolutionary conservation, extensive phase-contrast microscopy and immunostaining with T53-F1 antibody was performed in other drosophilids scattered along their genealogical tree.

Results

In the male germ line of all species tested there are cells showing giant nuclei and intranuclear structures similar to those of Drosophila melanogaster primary spermatocytes. Moreover, the antibody T53-F1 recognizes intranuclear structures in primary spermatocytes of all drosophilids analyzed. Interestingly, the extent and conformation of the staining pattern is species-specific. In addition, the intense staining of sperm tails in all species suggests that the terminal localization of Loopin-1 and its orthologues is conserved. A comparison of these cytological data and the data coming from the literature about sperm length, amount of sperm tail entering the egg during fertilization, shape and extent of both loops and primary spermatocyte nuclei, seems to exclude direct relationships among these parameters.

Conclusion

Taken together, the data reported strongly suggest that lampbrush-like loops are a conserved feature of primary spermatocyte nuclei in many, if not all, drosophilids. Moreover, the conserved pattern of the T53-F1 immunostaining indicates that a Loopin-1-like protein is present in all the species analyzed, whose localization on lampbrush-like loops and sperm tails during spermatogenesis is evolutionary conserved.

Contribution of noncentrosomal microtubules to spindle assembly in Drosophila spermatocytes

Previous data suggested that anastral spindles, morphologically similar to those found in oocytes, can assemble in a centrosome-independent manner in cells that contain centrosomes. It is assumed that the microtubules that build these acentrosomal spindles originate over the chromatin. However, the actual processes of centrosome-independent microtubule nucleation, polymerisation, and sorting have not been documented in centrosome-containing cells. We have identified two experimental conditions in which centrosomes are kept close to the plasma membrane, away from the nuclear region, throughout meiosis I in Drosophila spermatocytes. Time-lapse confocal microscopy of these cells labelled with fluorescent chimeras reveals centrosome-independent microtubule nucleation, growth, and sorting into a bipolar spindle array over the nuclear region, away from the asters. The onset of noncentrosomal microtubule nucleation is significantly delayed with respect to nuclear envelope breakdown and coincides with the end of chromosome condensation. It takes place in foci that are close to the membranes that ensheath the nuclear region, not over the condensed chromosomes. Metaphase plates are formed in these spindles, and, in a fraction of them, some degree of polewards chromosome segregation takes place. In these cells that contain both membrane-bound asters and an anastral spindle, the orientation of the cytokinesis furrow correlates with the position of the asters and is independent of the orientation of the spindle. We conclude that the fenestrated nuclear envelope may significantly contribute to the normal process of spindle assembly in Drosophila spermatocytes. We also conclude that the anastral spindles that we have observed are not likely to provide a robust back-up able to ensure successful cell division. We propose that these anastral microtubule arrays could be a constitutive component of wild-type spindles, normally masked by the abundance of centrosome-derived microtubules and revealed when asters are kept away. These observations are consistent with a model in which centrosomal and noncentrosomal microtubules contribute to the assembly and are required for the robustness of the cell division spindle in cells that contain centrosomes.

Time-lapse confocal microscopy reveals a potential role for noncentrosomal microtubules nucleated near the nuclear envelope in spindle assembly in Drosophila spermatocytes

Chromosome-microtubule interactions during mitosis

Spindle microtubules interact with mitotic chromosomes, binding to their kinetochores to generate forces that are important for accurate chromosome segregation. Motor enzymes localized both at kinetochores and spindle poles help to form the biologically significant attachments between spindle fibers and their cargo, but microtubule-associated proteins without motor activity contribute to these junctions in important ways. This review examines the molecules necessary for chromosome-microtubule interaction in a range of well-studied organisms, using biological diversity to identify the factors that are essential for organized chromosome movement. We conclude that microtubule dynamics and the proteins that control them are likely to be more important for mitosis than the current enthusiasm for motor enzymes would suggest.

Drosophila dd4 mutants reveal that gammaTuRC is required to maintain juxtaposed half spindles in spermatocytes

The weak spindle integrity checkpoint in Drosophila spermatocytes has revealed a novel function of the gamma-tubulin ring complex (gammaTuRC) in maintaining spindle bipolarity throughout meiosis. Bipolar and bi-astral spindles could form in Drosophila mutants for dd4, the gene encoding the 91 kDa subunit of gammaTuRC. However, these spindles collapsed around metaphase and began to elongate as if attempting anaphase B. The microtubules of the collapsing spindle folded back on themselves, their putative plus ends forming the focused apexes of biconical figures. Cells with such spindles were unable to undergo cytokinesis. A second type of spindle, monopolar hemi-spindles, also formed as a result of either spindle collapse at an earlier stage or failure of centrosome separation. Multiple centrosome-like bodies at the foci of hemi-spindles nucleated robust asters of microtubules in the absence of detectable gamma-tubulin. Time-lapse imaging revealed these to be intermediates that developed into cones, structures that also had putative plus ends of microtubules focused at their tips. Unlike biconical figures, however, cones seemed to contain a central spindle-like structure at their apexes and undergo cytokinesis. We conclude that spermatocytes do not need astral microtubules nucleated by opposite poles to intersect in order to form a central spindle and a cleavage furrow.

Synchronous nuclear-envelope breakdown and anaphase onset in plant multinucleate cells

Multinucleate plant cells with genetically balanced nuclei can be generated by inhibiting cytokinesis in sequential telophases. These cells can be used to relate the effect of changes in the distribution of nuclei in the cytoplasm to the control of the timing of cell cycle transitions. Which mitotic cell cycle events are sensitive to differences in the amount of cytoplasm surrounding each chromosomal complement has not been determined. To address this, we maximized the cell size by transiently inhibiting replication, while cell growth was not affected. The nuclei of 93% of the elongated cells reached prophase asynchronously compared to 46% of normal-sized multinucleate cells. The asynchronous prophases of normal-sized cells became synchronous at the time of nuclear-envelope breakdown, and the ensuing metaphase plate formation and anaphase onset and progression occurred synchronously. The elongated multinucleate cells were also very efficient in synchronizing the prophases at nuclear-envelope breakdown, in the prophase-to-prometaphase transition. However, 2.4% of these cells broke down the nuclear envelope asynchronously, though they became synchronous at the metaphase-to-anaphase transition. The kinetochore-microtubular cycle, responsible for coordinating the metaphase-to-anaphase transition and for the rate of sister segregation to opposite spindle poles during anaphase, remained strictly controlled and synchronous in the different mitoses of a single cell, independently of differences in the amount of cytoplasm surrounding each mitosis or its ploidy. Moreover, the degree of chromosome condensation varied considerably within the different mitotic spindles, being higher in the mitoses with the largest surrounding cytoplasm.

The rate of poleward chromosome motion is attenuated in Drosophila zw10 and rod mutants

Here we show that the rate of poleward chromosome motion in zw10-null mutants is greatly attenuated throughout the division process, and that chromosome disjunction at anaphase is highly asynchronous. Our results show that ZW10 protein, together with Rod, is involved in production and/or regulation of the force responsible for poleward chromosome motion.

Dynein is a transient kinetochore component whose binding is regulated by microtubule attachment, not tension

Cytoplasmic dynein is the only known kinetochore protein capable of driving chromosome movement toward spindle poles. In grasshopper spermatocytes, dynein immunofluorescence staining is bright at prometaphase kinetochores and dimmer at metaphase kinetochores. We have determined that these differences in staining intensity reflect differences in amounts of dynein associated with the kinetochore. Metaphase kinetochores regain bright dynein staining if they are detached from spindle microtubules by micromanipulation and kept detached for 10 min. We show that this increase in dynein staining is not caused by the retraction or unmasking of dynein upon detachment. Thus, dynein genuinely is a transient component of spermatocyte kinetochores. We further show that microtubule attachment, not tension, regulates dynein localization at kinetochores. Dynein binding is extremely sensitive to the presence of microtubules: fewer than half the normal number of kinetochore microtubules leads to the loss of most kinetochoric dynein. As a result, the bulk of the dynein leaves the kinetochore very early in mitosis, soon after the kinetochores begin to attach to microtubules. The possible functions of this dynein fraction are therefore limited to the initial attachment and movement of chromosomes and/or to a role in the mitotic checkpoint.

Probing spindle assembly mechanisms with monastrol, a small molecule inhibitor of the mitotic kinesin, Eg5

Monastrol, a cell-permeable small molecule inhibitor of the mitotic kinesin, Eg5, arrests cells in mitosis with monoastral spindles. Here, we use monastrol to probe mitotic mechanisms. We find that monastrol does not inhibit progression through S and G2 phases of the cell cycle or centrosome duplication. The mitotic arrest due to monastrol is also rapidly reversible. Chromosomes in monastrol-treated cells frequently have both sister kinetochores attached to microtubules extending to the center of the monoaster (syntelic orientation). Mitotic arrest-deficient protein 2 (Mad2) localizes to a subset of kinetochores, suggesting the activation of the spindle assembly checkpoint in these cells. Mad2 localizes to some kinetochores that have attached microtubules in monastrol-treated cells, indicating that kinetochore microtubule attachment alone may not satisfy the spindle assembly checkpoint. Monastrol also inhibits bipolar spindle formation in Xenopus egg extracts. However, it does not prevent the targeting of Eg5 to the monoastral spindles that form. Imaging bipolar spindles disassembling in the presence of monastrol allowed direct observations of outward directed forces in the spindle, orthogonal to the pole-to-pole axis. Monastrol is thus a useful tool to study mitotic processes, detection and correction of chromosome malorientation, and contributions of Eg5 to spindle assembly and maintenance.

A phospholipid kinase regulates actin organization and intercellular bridge formation during germline cytokinesis

The endgame of cytokinesis can follow one of two pathways depending on developmental context: resolution into separate cells or formation of a stable intercellular bridge. Here we show that the four wheel drive (fwd) gene of Drosophila melanogaster is required for intercellular bridge formation during cytokinesis in male meiosis. In fwd mutant males, contractile rings form and constrict in dividing spermatocytes, but cleavage furrows are unstable and daughter cells fuse together, producing multinucleate spermatids. fwd is shown to encode a phosphatidylinositol 4-kinase (PI 4-kinase), a member of a family of proteins that perform the first step in the synthesis of the key regulatory membrane phospholipid PIP2. Wild-type activity of the fwd PI 4-kinase is required for tyrosine phosphorylation in the cleavage furrow and for normal organization of actin filaments in the constricting contractile ring. Our results suggest a critical role for PI 4-kinases and phosphatidylinositol derivatives during the final stages of cytokinesis.

Visualizing the spindle checkpoint in Drosophila spermatocytes

The spindle assembly checkpoint detects defects in spindle structure or in the alignment of the chromosomes on the metaphase plate and delays the onset of anaphase until defects are corrected. Thus far, the evidence regarding the presence of a spindle checkpoint during meiosis in male Drosophila has been indirect and contradictory. On the one hand, chromosomes without pairing partners do not prevent meiosis progression. On the other hand, some conserved components of the spindle checkpoint machinery are expressed in these cells and behave as their homologue proteins do in systems with an active spindle checkpoint. To establish whether the spindle checkpoint is active in Drosophila spermatocytes we have followed meiosis progression by time-lapse microscopy under conditions where the checkpoint is likely to be activated. We have found that the presence of a relatively high number of misaligned chromosomes or a severe disruption of the meiotic spindle results in a significant delay in the time of entry into anaphase. These observations provide the first direct evidence substantiating the activity of a meiotic spindle checkpoint in male Drosophila.

Attachment of kinetochores to spindle microtubules during meiosis I of Lilium microsporocytes

Kinetochores and microtubules were visualized simultaneously during spindle formation at the first meiotic division in microsporocytes of Lilium longiflorum (2n = 24) under a confocal laser-scanning microscope, after immunofluorescence staining with centromere-recognizing antiserum and tubulin-specific antibody. During early prometaphase I, each kinetochore of bivalent chromosomes appeared to be an amorphous flat structure upon its initial attachment to microtubules. It became compact and spherical with the development of the spindle. From late prometaphase I, when the bipolar spindle was nearly complete, each kinetochore resembled a double disk that was suggestive of a pair of sister kinetochores and the homologous kinetochores were oriented towards opposite poles. Thus, the bipolar spindle at metaphase I included 12 bivalent chromosomes with a total of four kinetochores each. At anaphase I, the sister kinetochores moved to the same spindle pole as a paired unit. In microsporocytes arrested at prometaphase I by colchicine treatment, the sister kinetochores also came to be distinguishable. These results suggest that the change of kinetochore structure during meiosis I may be under chromosomal control but be somewhat associated with its attachment to spindle microtubules.

Chromosomal strategies for adaptation to univalency

The orientation and segregation behaviour of different types of univalents, namely sex chromosomes, B chromosomes and autosomal univalents, were analysed in living spermatocytes of eight evolutionarily distant grasshopper species. The meiotic behaviour of each univalent was characterized in terms of velocity of prometaphase movements, frequency of reorientations, types of final orientation at metaphase I and modes of segregation at anaphase I. All these features were found to vary between different univalents. Certain combinations of these traits, defining a 'chromosomal strategy', appear commonly together in certain chromosome types, indicating that they are the result of selection acting on the chromosomes to increase transmission effectiveness. The sex univalents show in general a strategy in which all the features favouring an eventual equational segregation at anaphase I tend to be minimized. There is much more variation in behaviour among B chromosomes than among X chromosomes, which is a reflection of their heterogeneous nature. Induced autosomal univalents are studied in Locusta migratoria. They show a very irregular behaviour, indicating their lack of adaptation to univalency.

Male sterility and meiotic drive associated with sex chromosome rearrangements in Drosophila. Role of X-Y pairing

In Drosophila melanogaster, deletions of the pericentromeric X heterochromatin cause X-Y nondisjunction, reduced male fertility and distorted sperm recovery ratios (meiotic drive) in combination with a normal Y chromosome and interact with Y-autosome translocations (T(Y;A)) to cause complete male sterility. The pericentromeric heterochromatin has been shown to contain the male-specific X-Y meiotic pairing sites, which consist mostly of a 240-bp repeated sequence in the intergenic spacers (IGS) of the rDNA repeats. The experiments in this paper address the relationship between X-Y pairing failure and the meiotic drive and sterility effects of Xh deletions. X-linked insertions either of complete rDNA repeats or of rDNA fragments that contain the IGS were found to suppress X-Y nondisjunction and meiotic drive in Xh-/Y males, and to restore fertility to Xh-/T(Y;A) males for eight of nine tested Y-autosome translocations. rDNA fragments devoid of IGS repeats proved incapable of suppressing either meiotic drive or chromosomal sterility. These results indicate that the various spermatogenic disruptions associated with X heterochromatic deletions are all consequences of X-Y pairing failure. We interpret these findings in terms of a novel model in which misalignment of chromosomes triggers a checkpoint that acts by disabling the spermatids that derive from affected spermatocytes.

Bipolar spindle attachments affect redistributions of ZW10, a Drosophila centromere/kinetochore component required for accurate chromosome segregation

Previous efforts have shown that mutations in the Drosophila ZW10 gene cause massive chromosome missegregation during mitotic divisions in several tissues. Here we demonstrate that mutations in ZW10 also disrupt chromosome behavior in male meiosis I and meiosis II, indicating that ZW10 function is common to both equational and reductional divisions. Divisions are apparently normal before anaphase onset, but ZW10 mutants exhibit lagging chromosomes and irregular chromosome segregation at anaphase. Chromosome missegregation during meiosis I of these mutants is not caused by precocious separation of sister chromatids, but rather the nondisjunction of homologs. ZW10 is first visible during prometaphase, where it localizes to the kinetochores of the bivalent chromosomes (during meiosis I) or to the sister kinetochores of dyads (during meiosis II). During metaphase of both divisions, ZW10 appears to move from the kinetochores and to spread toward the poles along what appear to be kinetochore microtubules. Redistributions of ZW10 at metaphase require bipolar attachments of individual chromosomes or paired bivalents to the spindle. At the onset of anaphase I or anaphase II, ZW10 rapidly relocalizes to the kinetochore regions of the separating chromosomes. In other mutant backgrounds in which chromosomes lag during anaphase, the presence or absence of ZW10 at a particular kinetochore predicts whether or not the chromosome moves appropriately to the spindle poles. We propose that ZW10 acts as part of, or immediately downstream of, a tension-sensing mechanism that regulates chromosome separation or movement at anaphase onset.

Mutations in twinstar, a Drosophila gene encoding a cofilin/ADF homologue, result in defects in centrosome migration and cytokinesis

We describe the phenotypic and molecular characterization of twinstar (tsr), an essential gene in Drosophila melanogaster. Two P-element induced alleles of tsr (tsr1 and tsr2) result in late larval or pupal lethality. Cytological examination of actively dividing tissues in these mutants reveals defects in cytokinesis in both mitotic (larval neuroblast) and meiotic (larval testis) cells. In addition, mutant spermatocytes show defects in aster migration and separation during prophase/prometaphase of both meiotic divisions. We have cloned the gene affected by these mutations and shown that it codes for a 17-kD protein in the cofilin/ADF family of small actin severing proteins. A cDNA for this gene has previously been described by Edwards et al. (1994). Northern analysis shows that the tsr gene is expressed throughout development, and that the tsr1 and tsr2 alleles are hypomorphs that accumulate decreased levels of tsr mRNA. These findings prompted us to examine actin behavior during male meiosis to visualize the effects of decreased twinstar protein activity on actin dynamics in vivo. Strikingly, both mutants exhibit abnormal accumulations of F-actin. Large actin aggregates are seen in association with centrosomes in mature primary spermatocytes. Later, during ana/telophase of both meiotic divisions, aberrantly large and misshaped structures appear at the site of contractile ring formation and fail to disassemble at the end of telophase, in contrast with wild-type. We discuss these results in terms of possible roles of the actin-based cytoskeleton in centrosome movement and in cytokinesis.

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.

A comparative study of orientation at behavior of univalent in living grasshopper spermatocytes

Orientational movements and modes of segregation at anaphase I were analyzed in three different types of univalents in living spermatocytes of the grasshopper species Eyprepocnemis plorans, namely the sex univalent, three types of accessory chromosomes and spontaneous and induced autosomal univalents. When two or more univalents were present in the same spindle, their dynamics were directly compared. Chromosomes may show variable velocity and number of reorientations: the X and the most common B types (B1 and B2) are slow and rarely reorient, a more geographically restricted B (B5) is faster and reorients more often, and autosomal univalents are the fastest and show the highest frequency of reorientations. Nonetheless, the X and the accessories are rigorously reductional at anaphase I whereas autosomal univalents often fail to migrate or divide equationally. This indicates that orientational and segregational behavior are controlled mainly by chromosomal rather than cellular characteristics and that chromosomes may display a great variety of strategies to achieve regular segregation.

Chromatin and microtubule organization during premeiotic, meiotic and early postmeiotic stages of Drosophila melanogaster spermatogenesis

Larval and pupal testes of Drosophila melanogaster were fixed with a methanol/acetone fixation procedure that results in good preservation of cell morphology; fixed cells viewed by phase-contrast optics exhibit most of the structural details that can be seen in live material. Fixed testis preparations were treated with anti-tubulin antibodies and Hoechst 33258 to selectively stain microtubules and DNA. The combined analysis of cell morphology, chromatin and microtubule organization allowed a fine cytological dissection of gonial cell multiplication, spermatocyte development, meiosis and the early stages of spermatid differentiation. We placed special emphasis on the spermatocyte growth phase and the meiotic divisions, providing a description of these processes that is much more detailed than those previously reported. In addition, by means of bromo-deoxyuridine incorporation experiments, we were able to demonstrate that premeiotic DNA synthesis occurs very early during spermatocyte growth.

Meiosis in Drosophila males. I. The question of separate conjunctive mechanisms for the XY and autosomal bivalents

The conjunctive mechanism of the XY bivalent is believed to differ from that of the autosomal bivalents in the achiasmate Drosophila melanogaster male. It has been proposed that hypothetical cohesive elements, termed collochores, hold the X and Y chromosomes together at or near their nucleolar organizing regions (NORs) and that collochores are not exhibited by autosomal bivalents. In electron micrographs, unique fibrillar material is observed between the X and Y chromosomes at the synaptic site. Recently, the 240 bp nontranscribed spacer associated with rRNA genes at the NOR has been implicated as the essential DNA sequence for XY pairing. To test whether this DNA sequence is always associated with XY pairing and to determine its relationship to the unique fibrillar material, we studied the XY bivalent in Drosophila simulans. The D. simulans Y chromosome has few, if any, rRNA genes, but does have a large block (3,000 kb or 12,500 copies) of the nontranscribed spacer repeat located at the distal end of its long arm. This is in contrast to the D. melanogaster Y, which has the repeat located among rRNA genes on its short arm. Using light and electron microscopy, we show that the X does indeed pair with the distal end of the long arm of the D. simulans Y. However, no fibrillar material is evident in serial thin sections of the D. simulans XY bivalent, suggesting that this material (in D. melanogaster) may be remnants of the NOR rather than a morphological manifestation of the hypothetical collochores.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromosome mal-orientation and reorientation during mitosis

We argue that mal-orientation of mitotic chromosomes is not as rare as once believed. However, unlike bivalents during meiosis I, the reorientation of a mal-oriented mitotic chromosome has yet to be observed. This appears to be due, in part, to the difficulty in differentiating mal-oriented chromosomes from mono-oriented ones which are common during spindle formation in living mitotic cells. We assume that mitotic cells possess mechanisms for correcting chromosome mal-orientations that are similar to those operating during meiosis. However, unlike meiosis, where reorientation appears to be triggered when tension on a K-fiber is relieved or reduced, other factors related to the close proximity of sister kinetochores may also induce reorientation in mal-oriented mitotic chromosomes. We favor a model in which the reorientation of a mitotic kinetochore depends on, and is initiated by, the kinetochore capturing MTs from the pole to which it is reorienting.

Genetic scrambling as a defence against meiotic drive

Genetic recombination has important consequences, including the familiar rules of Mendelian genetics. Here we present a new argument for the evolutionary function of recombination based on the hypothesis that meiotic drive systems continually arise to threaten the fairness of meiosis. These drive systems act at the expense of the fitness of the organism as a whole for the benefit of the genes involved. We show that genes increasing crossing over are favoured, in the process of breaking up drive systems and reducing the fitness loss to organisms.

Differential mechanisms governing segregation of a univalent in oocytes and spermatocytes of Drosophila melanogaster

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.

Tension, microtubule rearrangements, and the proper distribution of chromosomes in mitosis

The basis for stable versus unstable kinetochore orientation was investigated by a correlated living-cell/ultrastructural study of grasshopper spermatocytes. Mal-oriented bivalents having both kinetochores oriented to one spindle pole were induced by micromanipulation. Such mal-orientations are stable while the bivalent is subject to tension applied by micromanipulation but unstable after tension is released. Unstable bivalents always reorient with movement of one kinetochore toward the opposite pole. Microtubules associated with stably oriented bivalents, whether they are mal-oriented or in normal bipolar orientation, are arranged in orderly parallel bundles running from each kinetochore toward the pole. Similar orderly kinetochore microtubule arrangements characterized mal-oriented bivalents fixed just after release of tension. A significantly different microtubule arrangement is found only some time after tension release, when kinetochore movement is evident. The microtubules of a reorienting kinetochore always include a small number of microtubules running toward the pole toward which the kinetochore was moving at the time of fixation. All other microtubules associated with such a moving kinetochore appear to have lost their anchorage to the original pole and to be dragged passively as the kinetochore proceeds to the other pole. Thus, the stable anchorage of kinetochore microtubules to the spindle is associated with tension force and unstable anchorage with the absence of tension. The effect of tension is readily explained if force production and anchorage are both produced by mitotic motors, which link microtubules to the spindle as they generate tension forces.

The effects of benzodiazepines upon the fidelity of mitotic cell division in cultured Chinese hamster cells

4 benzodiazepine sedatives, namely diazepam, medazepam, midazolam and bromazepam were investigated for their effects upon the fidelity of cell division in both low passage number and immortalised Chinese hamster cell lines. The study revealed substantial differences in the effect of these structurally related drugs upon mitosis, which may reflect different mechanisms of action of the drugs in cultured cells. Diazepam and medazepam exposure of immortal and low passage number cells resulted in the formation of monopolar mitotic spindles and subsequent metaphase arrest. The production of these spindles may be explained by the inhibition or centriole separation . In contrast, midazolam and bromazepam failed to produce observable changes in spindle structure. All 4 benzodiazepines produced significant toxicity in low passage number cells whereas, immortalised cells were more resistant to their toxic effects. They all induced metaphase chromosome dislocations in immortalised cells, whereas only diazepam and medazepam produced such effects in the low passage number cell line. In general, immortal cells appeared to be less sensitive to the toxic effects of benzodiazepines than the low passage number cells.

Micromanipulated bivalents can trigger mini-spindle formation in Drosophila melanogaster spermatocyte cytoplasm

Single (individual) bivalents in cultured Drosophila melanogaster primary spermatocytes were detached from the spindle with a micromanipulation needle and placed in the cytoplasm. Such bivalents are prevented from rejoining the spindle by a natural membrane barrier that surrounds the spindle, but they quickly orient as if on a spindle of their own and the half-bivalents separate in anaphase. Serial section electron microscopy shows that a mini-spindle forms around the cytoplasmic bivalent, i.e., the microtubule density in the vicinity of the bivalent is much greater than in other cytoplasmic regions. This microtubule population cannot be accounted for solely by kinetochore nucleation and/or capture of microtubules. Furthermore, the mini-spindles frequently form at odd angles to the main spindle, so that at least one pole has no relationship to the poles of the main spindle. We conclude that a bivalent, or factors that become associated with the bivalent as a result of the manipulation, can either stabilize microtubules or promote their assembly. The bivalent activates latent microtubule organizing centers, or alternatively, polar organizing material has been passively transported from the main spindle to the cytoplasm by the micromanipulation procedure.

Malorientation and abnormal segregation of chromosomes during recovery from colcemid and nocodazole

Reversal of meiotic arrest in crane-fly spermatocytes by U. V. irradiation of Colcemid-arrested cells or by rinsing Nocodazole-arrested cells in fresh buffer results in the induction of chromosome malorientation. Malorientations observed among Colcemid-recovering and Nocodazole-recovering spermatocytes at frequencies higher than normally observed in untreated cells included associations of sister kinetochores of half-bivalents with both spindle poles (amphitely), in contrast with associations of sisters with only one pole (syntely) as is usually found during the first meiotic division. In several cases, prior to anaphase onset, maloriented bivalents appeared unusually tilted with respect to the spindle axis, and during anaphase they gave rise to laggard half-bivalents that did not segregate during anaphase along with half-bivalents having proper syntelic orientation. The results parallel previous findings obtained during cold recovery, and the properties of the drugs used here suggest that their action on microtubules, although reversible, induces malorientation during recovery from meiotic arrest.

Bivalent orientation and behavior in crane-fly spermatocytes recovering from cold exposure

At metaphase in crane-fly primary spermatocytes, the two sister kinetochores at the centromere of each homologue in a bivalent normally are adjacent and face the same pole; one homologue has all its kinetochore microtubules (kMTs) extending toward one pole and its partner has all its kMTs extending toward the opposite pole. In contrast, during recovery from exposure to 2 degrees C, one or both homologues in many metaphase bivalents had bipolar malorientations: all kMTs of one kinetochore extended toward one pole and some or all those of its sister extended toward the other. Metaphase sister kinetochores that had most of their kMTs extending toward the same pole were adjacent, and those with most extending toward opposite poles were separated from each other. Distances between homologous centromeres were similar to those in properly oriented bivalents. Maloriented bivalents were tilted relative to the spindle axis, and analysis of living cells showed that tilted configurations were rare during prometaphase in untreated cells but frequently arose in cold-recovering cells as initial configurations, then persisted through metaphase. This was in contrast to unipolar configurations of bivalents (configurations suggesting orientation of both homologous centromeres toward the same pole), which always reoriented shortly after the configuration arose. We conclude that in cold-recovering cells, bipolar malorientations are more stable than unipolar malorientations, and the orientation process is affected such that bipolar malorientations arise in bivalents upon initial interaction with the spindle and persist through metaphase.