An egg sabotaging mechanism drives non-Mendelian transmission in mice.

During meiosis, homologous chromosomes segregate so that alleles are transmitted equally to haploid gametes, following Mendel's Law of Segregation. However, some selfish genetic elements drive in meiosis to distort the transmission ratio and increase their representation in gametes. The established paradigms for drive are fundamentally different for female vs male meiosis. In male meiosis, selfish elements typically kill gametes that do not contain them. In female meiosis, killing is predetermined, and selfish elements bias their segregation to the single surviving gamete (i.e., the egg in animal meiosis). Here we show that a selfish element on mouse chromosome 2, R2d2, drives using a hybrid mechanism in female meiosis, incorporating elements of both male and female drivers. If R2d2 is destined for the polar body, it manipulates segregation to sabotage the egg by causing aneuploidy that is subsequently lethal in the embryo, so that surviving progeny preferentially contain R2d2. In heterozygous females, R2d2 orients randomly on the metaphase spindle but lags during anaphase and preferentially remains in the egg, regardless of its initial orientation. Thus, the egg genotype is either euploid with R2d2 or aneuploid with both homologs of chromosome 2, with only the former generating viable embryos. Consistent with this model, R2d2 heterozygous females produce eggs with increased aneuploidy for chromosome 2, increased embryonic lethality, and increased transmission of R2d2. In contrast to a male meiotic driver, which kills its sister gametes produced as daughter cells in the same meiosis, R2d2 eliminates "cousins" produced from meioses in which it should have been excluded from the egg.

Condensin dysfunction is a reproductive isolating barrier in mice.

Reproductive isolation occurs when the genomes of two populations accumulate genetic incompatibilities that prevent interbreeding1,2. Understanding of hybrid incompatibility at the cell biology level is limited, particularly in the case of hybrid female sterility3. Here we find that species divergence in condensin regulation and centromere organization between two mouse species, Mus musculus domesticus and Mus spretus, drives chromosome decondensation and mis-segregation in their F1 hybrid oocytes, reducing female fertility. The decondensation in hybrid oocytes was especially prominent at pericentromeric major satellites, which are highly abundant at M. m. domesticus centromeres4-6, leading to species-specific chromosome mis-segregation and egg aneuploidy. Consistent with the condensation defects, a chromosome structure protein complex, condensin II7,8, was reduced on hybrid oocyte chromosomes. We find that the condensin II subunit NCAPG2 was specifically reduced in the nucleus in prophase and that overexpressing NCAPG2 rescued both the decondensation and egg aneuploidy phenotypes. In addition to the overall reduction in condensin II on chromosomes, major satellites further reduced condensin II levels locally, explaining why this region is particularly prone to decondensation. Together, this study provides cell biological insights into hybrid incompatibility in female meiosis and demonstrates that condensin misregulation and pericentromeric satellite expansion can establish a reproductive isolating barrier in mammals.

MRCK ensures cortex-chromatin "social distancing" to enable egg spindle rotation.

During the second meiotic cell division, egg cells discard one set of chromatids to the polar body to produce a large haploid gamete. Meiotic spindle rotation is a critical step to ensure proper polar body extrusion. In this issue, Bourdais et al. (2023. J. Cell Biol.https://doi.org/10.1083/jcb.202211029) have identified MRCKß as an essential kinase for efficient spindle rotation. MRCK activates cortical myosin II rings overlying the spindle to prevent the notoriously sticky interaction between the cell cortex and chromatin to facilitate spindle rotation. Furthermore, Bourdais et al. found that the same MRCK-myosin II pathway also operates in zygotes to promote parental genome unification.

Meiotic drive of noncentromeric loci in mammalian meiosis II eggs.

The germline produces haploid gametes through a specialized cell division called meiosis. In general, homologous chromosomes from each parent segregate randomly to the daughter cells during meiosis, providing parental alleles with an equal chance of transmission. Meiotic drivers are selfish elements who cheat this process to increase their transmission rate. In female meiosis, selfish centromeres and noncentromeric drivers cheat by preferentially segregating to the egg cell. Selfish centromeres cheat in meiosis I (MI), while noncentromeric drivers can cheat in both meiosis I and meiosis II (MII). Here, we highlight recent advances on our understanding of the molecular mechanisms underlying these genetic cheating strategies, especially focusing on mammalian systems, and discuss new models of how noncentromeric selfish drivers can cheat in MII eggs.

Initial spindle positioning at the oocyte center protects against incorrect kinetochore-microtubule attachment and aneuploidy in mice.

Spindle positioning within the oocyte must be tightly regulated. In mice, the spindle is predominantly assembled at the oocyte center before its migration toward the cortex to achieve the highly asymmetric division, a characteristic of female meiosis. The significance of the initial central positioning of the spindle is largely unknown. We show that initial spindle positioning at the oocyte center is an insurance mechanism to avoid the premature exposure of the spindle to cortical CDC42 signaling, which perturbs proper kinetochore-microtubule attachments, leading to the formation of aneuploid gametes. These findings contribute to understanding why female gametes are notoriously associated with high rates of aneuploidy, the leading genetic cause of miscarriage and congenital abnormalities.

Tubulin post-translational modifications in meiosis.

Haploid gametes are produced from diploid parents through meiosis, a process inherent to all sexually reproducing eukaryotes. Faithful chromosome segregation in meiosis is essential for reproductive success, although it is less clear how the meiotic spindle achieves this compared to the mitotic spindle. It is becoming increasingly clear that tubulin post-translational modifications (PTMs) play critical roles in regulating microtubule functions in many biological processes, and meiosis is no exception. Here, I review recent advances in the understanding of tubulin PTMs in meiotic spindles, especially focusing on their roles in spindle integrity, oocyte aging, and non-Mendelian transmission.

Unravelling the mystery of female meiotic drive: where we are.

Female meiotic drive is the phenomenon where a selfish genetic element alters chromosome segregation during female meiosis to segregate to the egg and transmit to the next generation more frequently than Mendelian expectation. While several examples of female meiotic drive have been known for many decades, a molecular understanding of the underlying mechanisms has been elusive. Recent advances in this area in several model species prompts a comparative re-examination of these drive systems. In this review, we compare female meiotic drive of several animal and plant species, highlighting pertinent similarities.

Molecular Strategies of Meiotic Cheating by Selfish Centromeres.

Asymmetric division in female meiosis creates selective pressure favoring selfish centromeres that bias their transmission to the egg. This centromere drive can explain the paradoxical rapid evolution of both centromere DNA and centromere-binding proteins despite conserved centromere function. Here, we define a molecular pathway linking expanded centromeres to histone phosphorylation and recruitment of microtubule destabilizing factors, leading to detachment of selfish centromeres from spindle microtubules that would direct them to the polar body. Exploiting centromere divergence between species, we show that selfish centromeres in two hybrid mouse models use the same molecular pathway but modulate it differently to enrich destabilizing factors. Our results indicate that increasing microtubule destabilizing activity is a general strategy for drive in both models, but centromeres have evolved distinct mechanisms to increase that activity. Furthermore, we show that drive depends on slowing meiotic progression, suggesting that selfish centromeres can be suppressed by regulating meiotic timing.

Chromosome Segregation: Poor Supervision in the Early Stage of Life.

Abnormal chromosome number, or aneuploidy, is common in early mammalian embryos, although the underlying cell biological basis is still incompletely understood. New research reveals that cells often fail to wait for all chromosomes to properly attach to the spindle machinery before segregation, explaining why early embryonic cell cycles are so error-prone.

Optogenetic Manipulation of Mouse Oocytes.

Like many biological processes, oocyte development depends on careful orchestration of protein localization. Optogenetic approaches have the potential to manipulate this dynamic system with spatial and temporal precision and molecular specificity. This chapter describes the use of a photocaged chemical inducer of dimerization to control localization of genetically tagged proteins with light. As an example, we recruit a fluorescently tagged protein to one spindle pole in metaphase.

Spindle asymmetry drives non-Mendelian chromosome segregation.

Genetic elements compete for transmission through meiosis, when haploid gametes are created from a diploid parent. Selfish elements can enhance their transmission through a process known as meiotic drive. In female meiosis, selfish elements drive by preferentially attaching to the egg side of the spindle. This implies some asymmetry between the two sides of the spindle, but the molecular mechanisms underlying spindle asymmetry are unknown. Here we found that CDC42 signaling from the cell cortex regulated microtubule tyrosination to induce spindle asymmetry and that non-Mendelian segregation depended on this asymmetry. Cortical CDC42 depends on polarization directed by chromosomes, which are positioned near the cortex to allow the asymmetric cell division. Thus, selfish meiotic drivers exploit the asymmetry inherent in female meiosis to bias their transmission.

Expanded Satellite Repeats Amplify a Discrete CENP-A Nucleosome Assembly Site on Chromosomes that Drive in Female Meiosis.

Female meiosis provides an opportunity for selfish genetic elements to violate Mendel's law of segregation by increasing the chance of segregating to the egg [1]. Centromeres and other repetitive sequences can drive in meiosis by cheating the segregation process [2], but the underlying mechanisms are unknown. Here, we show that centromeres with more satellite repeats house more nucleosomes that confer centromere identity, containing the histone H3 variant CENP-A, and bias their segregation to the egg relative to centromeres with fewer repeats. CENP-A nucleosomes predominantly occupy a single site within the repeating unit that becomes limiting for centromere assembly on smaller centromeres. We propose that amplified repetitive sequences act as selfish elements by promoting expansion of CENP-A chromatin and increased transmission through the female germline.

Freewheeling sisters cause problems.

The factors that lead to errors in chromosome segregation during the production of egg cells in humans are becoming clearer.

The spindle assembly checkpoint promotes chromosome bi-orientation: A novel Mad1 role in chromosome alignment.

Faithful chromosome segregation relies on dynamic interactions between spindle microtubules and chromosomes. Especially, all chromosomes must be aligned at the equator of the spindle to establish bi-orientation before they start to segregate. The spindle assembly checkpoint (SAC) monitors this process, inhibiting chromosome segregation until all chromosomes achieve bi-orientation. The original concept of 'checkpoints' was proposed as an external surveillance system that does not play an active role in the process it monitors. However, accumulating evidence from recent studies suggests that SAC components do play an active role in chromosome bi-orientation. In this review, we highlight a novel Mad1 role in chromosome alignment, which is the first conserved mechanism that links the SAC and kinesin-mediated chromosome gliding.

Mad1 promotes chromosome congression by anchoring a kinesin motor to the kinetochore.

For proper partitioning of genomes in mitosis, all chromosomes must be aligned at the spindle equator before the onset of anaphase. The spindle assembly checkpoint (SAC) monitors this process, generating a 'wait anaphase' signal at unattached kinetochores of misaligned chromosomes. However, the link between SAC activation and chromosome alignment is poorly understood. Here we show that Mad1, a core SAC component, plays a hitherto concealed role in chromosome alignment. Protein-protein interaction screening revealed that fission yeast Mad1 binds the plus-end-directed kinesin-5 motor protein Cut7 (Eg5 homologue), which is generally thought to promote spindle bipolarity. We demonstrate that Mad1 recruits Cut7 to kinetochores of misaligned chromosomes and promotes chromosome gliding towards the spindle equator. Similarly, human Mad1 recruits another kinetochore motor CENP-E, revealing that Mad1 is the conserved dual-function protein acting in SAC activation and chromosome gliding. Our results suggest that the mitotic checkpoint has co-evolved with a mechanism to drive chromosome congression.

Interpolar microtubules are dispensable in fission yeast meiosis II.

The mitotic spindle consists of two types of microtubules. Dynamic kinetochore microtubules capture kinetochores, whereas stable interpolar microtubules serve as the structural backbone that connects the two spindle poles. Both have been believed to be indispensable for cell division in eukaryotes. Here we demonstrate that interpolar microtubules are dispensable for the second division of meiosis in fission yeast. Even when interpolar microtubules are disrupted by a microtubule-depolymerizing drug, spindle poles separate and chromosomes segregate poleward in second division of meiosis in most zygotes, producing viable spores. The forespore membrane, which encapsulates the nucleus in second division of meiosis and is guided by septins and the leading-edge proteins, is responsible for carrying out meiotic events in the absence of interpolar microtubules. Furthermore, during physiological second division of meiosis without microtubule perturbation, the forespore membrane assembly contributes structurally to spindle pole separation and nuclear division, generating sufficient force for spindle pole separation and subsequent events independently of interpolar microtubules.