Chromosome segregation during female meiosis in C. elegans: A tale of pushing and pulling

The role of the kinetochore during meiotic chromosome segregation in C. elegans oocytes has been a matter of controversy. Danlasky et al. (2020. J. Cell. Biol. https://doi.org/10.1083/jcb.202005179) show that kinetochore proteins KNL-1 and KNL-3 are required for early stages of anaphase during female meiosis, suggesting a new kinetochore-based model of chromosome segregation.

Meiosis consists of two consecutive chromosome segregation events preceded by a single round of DNA replication. Homologous chromosomes are separated in meiosis I, which is followed by sister chromatid separation in meiosis II to produce haploid gametes. Both of these stages require chromosomes/chromatids to align during metaphase before separating to opposite poles during anaphase. During mitosis, microtubules emanating from centrosomes at opposite poles of the cell bind chromosomes through a multiprotein complex called the kinetochore, allowing chromosomes to be pulled apart (1, 2). This segregation event takes place in two stages: anaphase A, where chromosomes are pulled toward spindle poles due to microtubule depolymerization, and anaphase B, where spindle poles themselves move farther apart, taking the attached chromosomes with them (3, 4). In many organisms, including mammals, oocytes lack centrosomes, and it has been of great interest to clarify the mechanisms used to ensure chromosomes are properly segregated during female meiosis (5, 6). Caenorhabditis elegans has served as a model for studying both mitosis and meiosis, but the mechanisms operating during female meiosis have been a matter of debate and controversy.In 2010, Dumont et al. showed that the kinetochore is required for chromosome alignment and congression during metaphase (7). However, they suggested that chromosome segregation was the result of microtubule polymerization between the segregating chromosomes (Fig. 1), resulting in a pushing force exerted onto chromosomes toward the spindle poles in a largely kinetochore-independent manner (7). This mechanism was also supported by the finding that CLIP-associated protein (CLASP)–dependent microtubule polymerization between the segregating chromosomes is essential for chromosome separation (8). An alternative model suggested that chromosomes are transported through microtubule-free channels toward the spindle poles by the action of dynein (9). Later evidence put in doubt a role for dynein and favored a model in which chromosomes initially separate when the spindle shortens and the poles overlap with chromosomes in an anaphase A–like mechanism. This is then followed by separation of chromosome-bound poles by outward microtubule sliding in an anaphase B–like fashion (10). However, because microtubules emanating from the spindle poles are not required to separate the homologous chromosomes but microtubules between the separating chromosomes are (8), this model is unlikely, at least as an explanation for mid-/late-anaphase movement. Furthermore, although lateral microtubule interactions with chromosomes predominate during metaphase of C. elegans oocyte meiosis, cryo-electron tomography data described end-on attachments between the separating chromosomes as anaphase progresses (11). This led to the suggestion that lateral microtubule interactions with chromosomes are responsible for the initial separation, but microtubule polymerization between the separating chromosomes is required for the later stages of segregation (11). The mechanisms involved in this initial separation have remained obscure. In this issue, Danlasky et al. show that the kinetochore is in fact required for the initial stages of chromosome segregation during female meiosis—an important step forward in our understanding of the mechanisms governing acentrosomal chromosome segregation (12).Open in a separate windowFigure 1.Some of the key findings in Danlasky et al. Kinetochore proteins surround the outer surface of the chromosomes, resulting in a characteristic cup shape. As anaphase progresses, chromosomes come into close contact to the spindle poles (anaphase A). Chromosome stretching is provided by KNL-1, MIS-12 (KNL-3), and NDC-80 (KMN)–dependent forces. Once the spindle starts elongating (anaphase B), central spindle microtubules provide the pushing forces for chromosome segregation. At this stage, kinetochore proteins also occupy the inward face of separating chromosomes. Upon KMN network depletion, bivalents flatten, and chromosome congression and alignment are defective. Anaphase A chromosome movement is almost absent, which leads to error-prone anaphase B.By simultaneously depleting kinetochore proteins KNL-1 and KNL-3 in C. elegans, Danlasky et al. observed the meiotic chromosome congression and alignment defects described in previous studies (7). However, this double-depletion phenotype displayed three key characteristics that suggested a role for kinetochores in chromosome segregation, which are discussed below.The kinetochore is required for bivalent stretching. It was previously shown that the bivalent chromosomes stretch before the initiation of segregation (10). Danlasky et. al found that this stretching of the chromosomes did not occur when KNL-1,3 were depleted, indicating that the kinetochore is required for this process (Fig. 1). Together with the observation that kinetochore proteins appear to extend toward the spindle poles, this finding suggested that pulling forces resulting from the interaction between the kinetochore and spindle microtubules are occurring during metaphase/preanaphase (Fig. 1).The kinetochore is required for anaphase A. In C. elegans female meiosis, anaphase A occurs when homologous chromosomes begin to separate during spindle shortening, and anaphase B when the chromosomes separate alongside the spindle poles (10). Danlasky et al. observed that KNL-1,3 depletion drastically reduced the velocity of anaphase A, as chromosomes only separated when spindle poles began to move apart. This indicated that pulling forces caused by the interaction between the kinetochore and spindle microtubules are also important for the initial separation of homologous chromosomes in anaphase A.The kinetochore is required for proper separation of homologous chromosomes. In KNL-1,3 depletion strains, 60% of bivalents failed to separate before segregation began, resulting in intact bivalents being pulled to the same spindle pole (Fig. 1). This failure of homologous chromosomes to separate was not thought to be a result of KNL-1,3 depletion interfering with the cleavage of cohesin that holds the two homologous chromosomes together because (a) separase and AIR-2^AuroraB, both of which are required for cohesin cleavage, localized normally during metaphase and anaphase, and (b) bivalents separated by metaphase II. This leaves the possibility open that the failure of bivalents to separate was due to the disrupted pulling forces thought to be important in bivalent stretching and anaphase A.Altogether, these data strongly indicate that the kinetochore is required not only for chromosome congression and alignment but also for the early stages of homologue separation. Anaphase B occurred successfully in the absence of KNL-1,3 but was more error prone, likely as a result of the earlier congression and anaphase A defects. While it is clear that chromosome masses do segregate in the absence of the kinetochore, this segregation is highly erroneous as a result of defects during the earlier stages of segregation in anaphase A (Fig. 1).The findings of Danlasky et al. raise testable hypotheses that could significantly enhance our understanding of acentrosomal chromosome segregation. Further investigation of the proposed pulling forces required during metaphase and early anaphase will be of great interest. Additionally, a more detailed analysis of the dynamic localization of separase and Securin, as well as assessing successful cohesin cleavage when KNL-1,3 are depleted, would back up the assertion that the failure of homologous chromosomes to separate was not due to the kinetochore impacting cohesin cleavage. It has previously been shown that the CLASP orthologue CLS-2 in C. elegans localizes to the kinetochore surrounding the bivalent chromosomes during metaphase before relocalizing to the central spindle during anaphase (7, 8, 13). It will be interesting to examine whether this key microtubule-stabilizing protein contributes to anaphase A pulling forces alongside its essential role in microtubule polymerization between chromosomes in anaphase B (8).While the regulation of proper chromosome segregation during acentrosomal meiosis in C. elegans is not yet fully understood, Danlasky et al.’s results represent a significant step forward in this endeavor by showing that the kinetochore is in fact required for the early stages of chromosome segregation.     

Depletion of a Cks homolog in C. elegans embryos uncovers a post-metaphase role in both meiosis and mitosis

In eukaryotic cells, the key regulators of cell-cycle transitions are the cyclin-dependent kinases (CDKs). The best studied CDK is a component of the M-phase promoting factor (MPF), which promotes entry into and progression through meiosis and mitosis. One of the enduring mysteries of the MPF complex has been the role of Cks/Suc1, a highly conserved member of the cell-cycle machinery in eukaryotes [1,2]. Cks has been proposed to be involved in activation of MPF [3], general interactions of MPF with its mitotic substrates [4] and/or inactivation of MPF [5,6]. We identified two Cks homologs in the genome of Caenorhabditis elegans and used RNA-mediated interference (RNAi) to investigate their roles in development. Whereas cks-2(RNAi) embryos display no apparent defects, cks-1(RNAi) embryos display defects in both meiosis and mitosis. Specifically, cks-1(RNAi) embryos fail to exit M phase properly. We propose that CKS-1 has an essential role in the inactivation of MPF during early C. elegans embryogenesis.     

Chromosome cohesion and segregation in mitosis and meiosis

The faithful segregation of the genetic material into daughter cells during cell division is crucial for the production of healthy progeny. Sister chromatid cohesion and separation are fundamental to this process. Progress has been made in our molecular understanding of cohesion and mechanisms for the dissolution of cohesion have been uncovered.     

Supernumerary chromosomes and their behaviour in meiosis of the holocentric Cuscuta babylonica Ghoisy

A supernumerary chromosome and its behaviour during meiosis is described in the parasitic herb Cuscuta babylonica Choisy (2n = 8 + 1B). This species is characterized by holocentric chromosome behaviour, and the occurrence of a supernumerary here is considered noteworthy.     

Separase is required for chromosome segregation during meiosis I in Caenorhabditis elegans.

BACKGROUND: Chromosome segregation during mitosis and meiosis is triggered by dissolution of sister chromatid cohesion, which is mediated by the cohesin complex. Mitotic sister chromatid disjunction requires that cohesion be lost along the entire length of chromosomes, whereas homolog segregation at meiosis I only requires loss of cohesion along chromosome arms. During animal cell mitosis, cohesin is lost in two steps. A nonproteolytic mechanism removes cohesin along chromosome arms during prophase, while the proteolytic cleavage of cohesin's Scc1 subunit by separase removes centromeric cohesin at anaphase. In Saccharomyces cerevisiae and Caenorhabditis elegans, meiotic sister chromatid cohesion is mediated by Rec8, a meiosis-specific variant of cohesin's Scc1 subunit. Homolog segregation in S. cerevisiae is triggered by separase-mediated cleavage of Rec8 along chromosome arms. In principle, chiasmata could be resolved proteolytically by separase or nonproteolytically using a mechanism similar to the mitotic "prophase pathway." RESULTS: Inactivation of separase in C. elegans has little or no effect on homolog alignment on the meiosis I spindle but prevents their timely disjunction. It also interferes with chromatid separation during subsequent embryonic mitotic divisions but does not directly affect cytokinesis. Surprisingly, separase inactivation also causes osmosensitive embryos, possibly due to a defect in the extraembryonic structures, referred to as the "eggshell." CONCLUSIONS: Separase is essential for homologous chromosome disjunction during meiosis I. Proteolytic cleavage, presumably of Rec8, might be a common trigger for the first meiotic division in eukaryotic cells. Cleavage of proteins other than REC-8 might be necessary to render the eggshell impermeable to solutes.     

The many facets of SC function during C. elegans meiosis

Sexually reproducing organisms rely on meiosis for the formation of haploid gametes. This is achieved through two consecutive rounds of cell division (meiosis I and II) after one round of DNA replication. During the meiotic divisions, chromosomes face several challenges to ultimately ensure proper chromosome segregation. Unique events unfold during meiosis I to overcome these challenges. Homologous chromosomes pair, synapse, and recombine. A remarkable feature throughout this process is the formation of an evolutionarily conserved tripartite proteinaceous structure known as the synaptonemal complex (SC). It is comprised of two lateral elements, assembled along each axis of a pair of homologous chromosomes, and a central region consisting of transverse filaments bridging the gap between lateral elements. While the presence of the SC during meiosis has been appreciated now for 50 years (Moses, Biophys Biochem Cytol 2:215–218, 1956; Fawcett, J Biophys Biochem Cytol 2:403–406, 1956), its role(s) remain a matter of intense investigation. This review concentrates on studies performed in Caenorhabditis elegans, a powerful system for investigating meiosis. Studies in this organism are contributing to the unraveling of the various processes leading to the formation of the SC and the various facets of the functions it exerts throughout meiosis.The synaptonemal complex-50 years     

The role of chromosome ends during meiosis in Caenorhabditis elegans

Chromosome ends have been implicated in the meiotic processes of the nematode Caenorhabditis elegans. Cytological observations have shown that chromosome ends attach to the nuclear membrane and adopt kinetochore functions. In this organism, centromeric activity is highly regulated, switching from multiple spindle attachments all along the chromosome during mitotic division to a single attachment during meiosis. C. elegans chromosomes are functionally monocentric during meiosis. Earlier genetic studies demonstrated that the terminal regions of the chromosomes are not equivalent in their meiotic potentials. There are asymmetries in the abilities of the ends to recombine when duplicated or deleted. In addition, mutations in single genes have been identified that mimic the meiotic effects of a terminal truncation of the X chromosome. The recent cloning and characterization of the C. elegans telomeres has provided a starting point for the study of chromosomal elements mediating the meiotic process.     

The structure of condensed chromosomes in mitosis and meiosis of insects

This article revolves around the structure of condensed mitotic and meiotic chromosomes in insects. In the first section, the potential of cytological approaches in the field of chromosome biology is described. Emphasis is on immunolabeling, and transmission and scanning electron microscopy. In particular, the latter technology revealed a series of unusual components in association with the chromosomes, such as membranes and non-chromatin material, which is presumably responsible for the formation of achiasmatic bivalents. Virus-like particles were found scattered throughout the chromatin in a Lepidoptera species. This association is possibly responsible for the transmission of the particles into the next generation. Then, the cytology and the molecular make-up of the key components of insect chromosomes are described. These are the centromeres, telomeres, and nucleoli. In any case, the situation in insects is compared briefly with that in mammals. The general structure of the centromeres in terms of centromere-specific repetitive DNA and proteins is similar in insects and mammals. This applies also to telomeres of most insect orders, but the chromosome ends of Diptera species differ from those in mammals. Fine structure observations raise also the possibility that insect nucleoli have a specific architecture. Chromosomal proteins—and emphasis is on histone acetylation—are addressed in an individual section. Evidence is accumulating that histone H4 acetylation plays a role in dosage compensation and is a cytogenetic marker of constitutive heterochromatin in insects. In the final section, the characteristics of holokinetic chromosomes are listed. A series of insect orders, where direct or indirect evidence points to chromosomes with relatively large centromeres, is presented. These are Lepidoptera, Trichoptera, Hemiptera, Homoptera, Odonata, and Dermaptera.     

The role of centromere alignment in meiosis I segregation of homologous chromosomes in Saccharomyces cerevisiae

During meiosis, homologous chromosomes pair and then segregate from each other at the first meiotic division. Homologous centromeres appear to be aligned when chromosomes are paired. The role of centromere alignment in meiotic chromosome segregation was investigated in Saccharomyces cerevisiae diploids that contained one intact copy of chromosome I and one copy bisected into two functional centromere-containing fragments. The centromere on one fragment was aligned with the centromere on the intact chromosome while the centromere on the other fragment was either aligned or misaligned. Fragments containing aligned centromeres segregated efficiently from the intact chromosome, while fragments containing misaligned centromeres segregated much less efficiently from the intact chromosome. Less efficient segregation was correlated with crossing over in the region between the misaligned centromeres. Models that suggest that these crossovers impede proper segregation by preventing either a segregation-promoting chromosome alignment on the meiotic spindle or some physical interaction between homologous centromeres are proposed.     

glp-1 is required in the germ line for regulation of the decision between mitosis and meiosis in C. elegans

In the wild-type C. elegans germ line there are both mitotic and meiotic germ cells. Mutations in glp-1 cause germ cells that would normally divide mitotically to enter meiosis. This mutant phenotype mimics the effect of killing the distal tip cell, a somatic cell that interacts with the germ line to regulate the mitotic/meiotic decision. In addition, wild-type glp-1 product is required maternally for embryogenesis. Temperature-shift experiments indicate that the temporal requirement for glp-1 activity in the germ line is the same as that for distal tip cell regulation. Mosaic analyses suggest that glp-1 is produced in the germ line. We propose that glp-1 acts as part of the receiving mechanism in the interaction between the distal tip cell and germ line.     

RAD‐seq linkage mapping and patterns of segregation distortion in sedges: meiosis as a driver of karyotypic evolution in organisms with holocentric chromosomes

Meiotic drive, the class of meiotic mechanisms that drive unequal segregation of alleles among gametes, may be an important force in karyotype evolution. Its role in holocentric organisms, whose chromosomes lack localized centromeres, is poorly understood. We crossed two individuals of Carex scoparia (Cyperaceae) with different chromosome numbers (2n = 33^II = 66 × 2n = 32^II = 64) to obtain F1 individuals, which we then self‐pollinated to obtain second‐generation (F2) crosses. RAD‐seq was performed for 191 individuals (including the parents, five F1 individuals and 184 F2 individuals). Our F2 linkage map based on stringent editing of the RAD‐seq data set yielded 32 linkage groups. In the final map, 865 loci were located on a linkage map of 3966.99 cM (linkage groups ranged from 24.39 to 193.31 cM in length and contained 5–51 loci each). Three linkage groups exhibit more loci under segregation distortion than expected by chance; within linkage groups, loci exhibiting segregation distortion are clustered. This finding implicates meiotic drive in the segregation of chromosome variants, suggesting that selection of chromosome variants in meiosis may contribute to the establishment and fixation of chromosome variants in Carex, which is renowned for high chromosomal and species diversity. This is an important finding as previous studies demonstrate that chromosome divergence may play a key role in differentiation and speciation in Carex.     

Differential staining and segregation of parental chromosomes in mouse-rabbit hybridomas.

Hoechst 33258 fluorescent staining can be coupled with G-banding to identify the chromosomal contribution of each parent in mouse-rabbit hybridomas. A fast and essentially complete segregation of rabbit chromosomes is obtained in these cells. The rabbit X chromosome is preferentially maintained in media imposing HGPRT activity for cell growth. Mouse-rabbit hybridomas, some of which secrete rabbit immunoglobulin chains, should be a convenient material for the identification of chromosomes governing rabbit Ig chain synthesis.     

Differential function and expression of Saccharomyces cerevisiae B-type cyclins in mitosis and meiosis.

We have studied the patterns of expression of four B-type cyclins (Clbs), Clb1, Clb2, Clb3, and Clb4, and their ability to activate p34cdc28 during the mitotic and meiotic cell cycles of Saccharomyces cerevisiae. During the mitotic cell cycle, Clb3 and Clb4 were expressed and induced a kinase activity in association with p34cdc28 from early S phase up to mitosis. On the other hand, Clb1 and Clb2 were expressed and activated p34cdc28 later in the mitotic cell cycle, starting in late S phase and continuing up to mitosis. The pattern of expression of Clb3 and Clb4 suggests a possible role in the regulation of DNA replication as well as mitosis. Clb1 and Clb2, whose pattern of expression is similar to that of other known Clbs, are likely to have a role predominantly in the regulation of M phase. During the meiotic cell cycle, Clb1, Clb3, and Clb4 were expressed and induced a p34cdc28-associated kinase activity just before the first meiotic division. The fact that Clb3 and Clb4 were not synthesized earlier, in S phase, suggests that these cyclins, which probably have a role in S phase during the mitotic cell cycle, are not implicated in premeiotic S phase. Clb2, the primary mitotic cyclin in S. cerevisiae, was not detectable during meiosis. Sporulation experiments on strains deleted for one, two, or three Clbs indicate, in agreement with the biochemical data, that Clb1 is the primary cyclin for the regulation of meiosis, while Clb2 is not involved at all.