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Oocyte spindle asymmetry in microtubule tubulin density. a Representative image of microtubules (α-tubulin-GFP, grey) and chromatin (H2BmCherry, cyan) in an oocyte at 7 h after NEBD. Purple line indicates mean-weighted position of the bivalents; blue and orange lines indicate 10 µm distance on the cortical and central side of the bivalents, respectively. Scale bar, 5 µm. b α-Tubulin intensity profile, in the x-axis, of the spindle (grey) and bivalents (cyan) is shown in a. Total α-tubulin intensity within 10 µm of the bivalents on the central (orange) and cortical (blue) spindles halves. c Cortical/central ratio of α-tubulin fluorescent intensity as shown in b (mean 1.274). *P < 0.05, 95% confidence intervals shown from 1.217 to 1.332 (i.e. >1.0); n = 10 oocytes, combined from two independent experiments
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In the first meiotic division (MI) of oocytes, the cortically positioned spindle causes bivalent segregation in which only the centre-facing homologue pairs are retained. 'Selfish' chromosomes are known to exist, which bias their spindle orientation and hence retention in the egg, a process known as 'meiotic drive'. Here we report on this phenomeno...
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... 7 h, a time during which spindle migration occurs (Fig. 3), and when the spindle direction is apparent, the difference in C-Kt separation was still present and resembled that observed at 4 h (cortex: 0.70 µm, centre: 0.46 µm, P < 0.0001, ANOVA with Tukey's post-hoc test; Fig. 4d). This difference was observed for 81% of all bivalents tested (Supplementary Fig. 5). The difference in C-Kt stretch between centre and cortical facing kinetochores also persisted when actin depolymerisation was induced by cytochalasin B suggesting that actin does not contribute to C-Kt stretch (cortical: 0.68 µm, central: 0.51 µm, P < 0.0001, ANOVA with Tukey's post-hoc test; Fig. 4d). ...
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... to exert an asymmetrical stretch on the bivalents during MI, which is evident both when it is at the centre and also when it is migrating to the cortex. Microtubules were examined initially because they form k-fibres, which exert tension across the chro- mosome 31 . Therefore, α-tubulin-GFP and H2B-mCherry were expressed in GV stage oocytes (Fig. 5a), and using the H2B signal to define the centre of the spindle we measured tubulin intensity at 7 h after NEBD. Consistent with the C-Kt distance asymmetry, there was a significantly greater tubulin density on the spindle half facing the cortex of the oocyte (tubulin density ratio cortex/ centre: 1.274, 95% CI: 1.217-1.332, n = 10 ...
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... 5a), and using the H2B signal to define the centre of the spindle we measured tubulin intensity at 7 h after NEBD. Consistent with the C-Kt distance asymmetry, there was a significantly greater tubulin density on the spindle half facing the cortex of the oocyte (tubulin density ratio cortex/ centre: 1.274, 95% CI: 1.217-1.332, n = 10 oocytes; Fig. 5b, c). Therefore, it is inferred that the greater kinetochore stretch is a result of the greater tubulin density found on the cortical ...
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Aurora kinases create phosphorylation gradients within the spindle during prometaphase and anaphase, thereby locally regulating factors that promote spindle organization, chromosome condensation and movement, and cytokinesis. We show that one such factor is the kinesin KIF4A, which is present along the chromosome axes throughout mitosis and the cen...
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... In taxa with a single centromere per chromosome (monocentric), the spindle fibers attach to the kinetochore structure around the localized centromere during meiotic division and differences between homologous chromosomes in kinetochore size may therefore cause meiotic drive . In this case, chromosomal rearrangements may induce meiotic drive since fused and unfused chromosomes can differ in the amount of centromeric DNA and the recruitment of kinetochore proteins (Wu et al. 2018). While such "centromere drive" can result in karyotypic change, selfish centromeres seem to occur rather frequently and not only in fission/ fusion heterokaryotypes (Henikoff et al. 2001;Dudka and Lampson 2022). ...
Species frequently differ in the number and structure of chromosomes they harbor, but individuals that are heterozygous for chromosomal rearrangements may suffer from reduced fitness. Chromosomal rearrangements like fissions and fusions can hence serve as a mechanism for speciation between incipient lineages, but their evolution poses a paradox. How can rearrangements get fixed between populations if heterozygotes have reduced fitness? One solution is that this process predominantly occurs in small and isolated populations, where genetic drift can override natural selection. However, fixation is also more likely if a novel rearrangement is favored by a transmission bias, such as meiotic drive. Here, we investigate chromosomal transmission distortion in hybrids between two wood white (Leptidea sinapis) butterfly populations with extensive karyotype differences. Using data from two different crossing experiments, we uncover that there is a transmission bias favoring the ancestral chromosomal state for derived fusions, a result that shows that chromosome fusions actually can fix in populations despite being counteracted by meiotic drive. This means that meiotic drive not only can promote runaway chromosome number evolution and speciation, but also that it can be a conservative force acting against karyotypic change and the evolution of reproductive isolation. Based on our results, we suggest a mechanistic model for why chromosome fusion mutations may be opposed by meiotic drive and discuss factors contributing to karyotype evolution in Lepidoptera.
... To further confirm whether the meiotic delay is mediated by SAC, mouse oocytes expressing TUBB8 were further treated with reversine, a small molecule inhibitor of the mitotic kinase MPS1 that initiates SAC response 31 , that can prevent SAC-mediated meiotic delay of mouse oocytes 32 . As shown in Fig. 2e, the mean time of PB1 extrusion in mouse oocytes expressing TUBB8 was accelerated with reversine treatment (5.3 h vs. 7.9 h). ...
Aneuploidy seriously compromises female fertility and increases incidence of birth defects. Rates of aneuploidy in human eggs from even young women are significantly higher than those in other mammals. However, intrinsic genetic factors underlying this high incidence of aneuploidy in human eggs remain largely unknown. Here, we found that ectopic expression of human TUBB8 in mouse oocytes increases rates of aneuploidy by causing kinetochore–microtubule (K–MT) attachment defects. Stretched bivalents in mouse oocytes expressing TUBB8 are under less tension, resulting in continuous phosphorylation status of HEC1 by AURKB/C at late metaphase I that impairs the established correct K–MT attachments. This reduced tension in stretched bivalents likely correlates with decreased recruitment of KIF11 on meiotic spindles. We also found that ectopic expression of TUBB8 without its C-terminal tail decreases aneuploidy rates by reducing erroneous K–MT attachments. Importantly, variants in the C-terminal tail of TUBB8 were identified in patients with recurrent miscarriages. Ectopic expression of an identified TUBB8 variant in mouse oocytes also compromises K–MT attachments and increases aneuploidy rates. In conclusion, our study provides novel understanding for physiological mechanisms of aneuploidy in human eggs as well as for pathophysiological mechanisms involved in recurrent miscarriages.
... 4 The aMTOCs migrate to the nucleus and decondense on the nuclear envelope. 5,6 And then, the aMTOCs are stretched and fragmented along the nuclear envelope by dynein. After nuclear envelope breakdown (NEBD), kinesin superfamily protein 11 further fragments the aMTOCs to assemble the acentrosomal bipolar spindle ( Figure 1). ...
... Similarly, recurrent changes in loops of the ancient AURKC protein kinase domain could modulate kinetochoremicrotubule attachment dynamics, specifically in meiosis I. Both centromere assembly and microtubule detachment (in meiosis I) represent mechanisms potentially hijacked by selfish centromeres (Akera et al., 2019;Henikoff et al., 2001;Rosin and Mellone, 2016;Wu et al., 2018). These analyses provide a starting point for future experiments probing the functional impacts of innovation, including testing whether positive selection reduces fitness costs associated with centromere drive (reviewed in Dudka and Lampson, 2022). ...
... CENP-O is expected to dock the CENP-OPQUR complex to centromeres (Eskat et al., 2012;Pesenti et al., 2018), promoting kinetochore-microtubule attachment stability (Amaro et al., 2010;Bancroft et al., 2015;Chen et al., 2021;Hori et al., 2008;Singh et al., 2021). Therefore, we propose that innovation in the CENP-O C-terminal RWD domain modulated interactions with other centromeric components (possibly CENP-Q and -U) necessary to form a stable CENP-OPQUR complex at centromeres (Foltz et al., 2006;Hori et al., 2008;Kagawa et al., 2014;Minoshima et al., 2005;Okada et al., 2006;Pesenti et al., 2018), potentially stabilizing kinetochore-microtubules to counteract destabilizing activities associated with driving centromeres (Akera et al., 2019;Wu et al., 2018). Future work using centromere drive models (reviewed in Dudka and Lampson, 2022) will be needed to experimentally test this idea. ...
Cell biologists typically focus on conserved regions of a protein, overlooking innovations that can shape its function over evolutionary time. Computational analyses can reveal potential innovations by detecting statistical signatures of positive selection that lead to rapid accumulation of beneficial mutations. However, these approaches are not easily accessible to non-specialists, limiting their use in cell biology. Here, we present an automated computational pipeline FREEDA that provides a simple graphical user interface requiring only a gene name; integrates widely used molecular evolution tools to detect positive selection in rodents, primates, carnivores, birds, and flies; and maps results onto protein structures predicted by AlphaFold. Applying FREEDA to >100 centromere proteins, we find statistical evidence of positive selection within loops and turns of ancient domains, suggesting innovation of essential functions. As a proof-of-principle experiment, we show innovation in centromere binding of mouse CENP-O. Overall, we provide an accessible computational tool to guide cell biology research and apply it to experimentally demonstrate functional innovation.
... Similarly, recurrent changes in loops of the ancient AURKC protein kinase domain could modulate kinetochore-microtubule attachment dynamics specifically in meiosis I. Both centromere assembly and microtubule detachment (in meiosis I) represent mechanisms potentially hijacked by selfish centromeres (Akera et al., 2019;Henikoff et al., 2001;Rosin and Mellone, 2016;Wu et al., 2018). ...
... /2023https://doi.org/10. .02.27.530329 doi: bioRxiv preprint al., 2005Okada et al., 2006;Pesenti et al., 2018), potentially stabilizing kinetochore-microtubules to counteract destabilizing activities associated with driving centromeres (Akera et al., 2019;Wu et al., 2018). Future work using centromere drive models (reviewed in (Dudka and Lampson, 2022)) will be needed to experimentally test this idea. ...
Cell biologists typically focus on conserved regions of a protein, overlooking innovations that can shape its function over evolutionary time. Computational analyses can reveal potential innovations by detecting statistical signatures of positive selection that leads to rapid accumulation of beneficial mutations. However, these approaches are not easily accessible to non-specialists, limiting their use in cell biology. Here, we present an automated computational pipeline FREEDA (Finder of Rapidly Evolving Exons in De novo Assemblies) that provides a simple graphical user interface requiring only a gene name, integrates widely used molecular evolution tools to detect positive selection, and maps results onto protein structures predicted by AlphaFold. Applying FREEDA to >100 mouse centromere proteins, we find evidence of positive selection in intrinsically disordered regions of ancient domains, suggesting innovation of essential functions. As a proof-of-principle experiment, we show innovation in centromere binding of CENP-O. Overall, we provide an accessible computational tool to guide cell biology research and apply it to experimentally demonstrate functional innovation.
... Centromeres are regions of the chromosomes composed of small satellite repeats that bind specialized nucleosomes and serve to assemble kinetochores that will attach the chromosomes to the spindle and ensure their segregation during anaphase. There is evidence that larger centromeres have preferential transmission to oocytes rather than to polar bodies during the first meiotic division [19] due to the asymmetry of the spindle [20,21]. ...
Mendel’s law of segregation states that the two alleles at a diploid locus should be transmitted equally to the progeny. A genetic segregation distortion, also referred to as transmission ratio distortion (TRD), is a statistically significant deviation from this rule. TRD has been observed in several mammal species and may be due to different biological mechanisms occurring at diverse time points ranging from gamete formation to lethality at post-natal stages. In this review, we describe examples of TRD and their possible mechanisms in mammals based on current knowledge. We first focus on the differences between TRD in male and female gametogenesis in the house mouse, in which some of the most well studied TRD systems have been characterized. We then describe known TRD in other mammals, with a special focus on the farmed species and in the peculiar common shrew species. Finally, we discuss TRD in human diseases. Thus far, to our knowledge, this is the first time that such description is proposed. This review will help better comprehend the processes involved in TRD. A better understanding of these molecular mechanisms will imply a better comprehension of their impact on fertility and on genome evolution. In turn, this should allow for better genetic counseling and lead to better care for human families.
... Another intraspecific study found initial unbiased kinetochore orientation in C57Bl6 x SJL hybrids, but there was orientation bias by anaphase I, with preferential segregation of a chromosome-17 variant with more minor satellite and less major satellite to the egg pole, although surprisingly it had less Spc24, a component of the HEC1/NDC80 complex [44]. Reorientation of the favored centromere depended on Aurora kinase B/C and occurred before spindle migration. ...
Centromeres are essential loci in eukaryotes that are necessary for the faithful segregation of chromosomes in mitosis and meiosis. Centromeres organize the kinetochore, the protein machine that attaches sister chromatids or homologous chromosomes to spindle microtubules and regulates their disjunction. Centromeres have both genetic and epigenetic determinants, which can come into conflict in asymmetric female meiosis in seed plants and animals. The centromere drive model was proposed to describe this conflict and explain how it leads to the rapid evolution of both centromeres and kinetochores. Recent studies confirm key aspects of the centromere drive model, clarify its mechanisms, and implicate rapid centromere/kinetochore evolution in hybrid inviability between species.
... The difference suggests that the mechanisms of female meiotic spindle assembly are not conserved between species. The aMTOC-directed meiotic spindle assembly was only elucidated in mouse oocytes (15,18,19). ...
... Upon meiotic resumption after prophase I arrest, multiple aMTOCs initiate microtubule nucleation around the nuclear envelope in mouse germinal vesicle (GV) oocytes. After nuclear envelope breakdown (NEBD), aMTOCs are clustered and then concentrated at the spindle poles for bipolar spindle organization (15,18,19). ...
Meiotic spindle assembly ensures proper chromosome segregation in oocytes. However, the mechanisms behind spindle assembly in human oocytes remain largely unknown. We used three-dimensional high-resolution imaging of more than 2000 human oocytes to identify a structure that we named the human oocyte microtubule organizing center (huoMTOC). The proteins TACC3, CCP110, CKAP5, and DISC1 were found to be essential components of the huoMTOC. The huoMTOC arises beneath the oocyte cortex and migrates adjacent to the nuclear envelope before nuclear envelope breakdown (NEBD). After NEBD, the huoMTOC fragments and relocates on the kinetochores to initiate microtubule nucleation and spindle assembly. Disrupting the huoMTOC led to spindle assembly defects and oocyte maturation arrest. These results reveal a physiological mechanism of huoMTOC-regulated spindle assembly in human oocytes.
... [28,46,47] The MI spindle assembly in human oocytes depends on the Ran-GTP pathway, similar to what was first observed in Xenopus egg extracts, [48,49] while microtubule nucleation from human oocyte chromatin occurs later (5 h after NEB) [28] and more slowly than that in mouse oocytes (immediately after NEB). [50] Over-expression of Ran T24N, a dominant-negative mutation of Ran, in human oocytes severely delayed the initiation of microtubule nucleation and blocked spindle assembly in meiosis I. [28] In summary, the unstable spindle microtubules cause incorrect kinetochore-microtubule attachments in human oocytes, resulting in frequent aneuploidy. ...
... Stable bipolar spindles are formed in mouse oocytes in early prometaphase by acentriolar microtubule organizing centers (aMTOCs) ( Figure 4A). [50][51][52] Loss of aMTOC-associated proteins impairs the stability of spindles in mouse oocytes. [53] Interestingly, aMTOCs are not observed on the spindles of human oocytes ( Figure 4B), indicating a different mechanism of spindle pole formation in human oocytes. ...
Meiotic defects cause abnormal chromosome segregation leading to aneuploidy in mammalian oocytes. Chromosome segregation is particularly error‐prone in human oocytes, but the mechanisms behind such errors remain unclear. To explain the frequent chromosome segregation errors, recent investigations have identified multiple meiotic defects and explained how these defects occur in female meiosis. In particular, we review the causes of cohesin exhaustion, leaky spindle assembly checkpoint (SAC), inherently unstable meiotic spindle, fragmented kinetochores or centromeres, abnormal aurora kinases (AURK), and clinical genetic variants in human oocytes. We mainly focus on meiotic defects in human oocytes, but also refer to the potential defects of female meiosis in mouse models. Multiple meiotic defects could cause the failure of female meiosis I, including cohesin exhaustion, lax spindle assembly checkpoint (SAC), inherently unstable meiotic spindle, fragmented kinetochore or damaged centromere, incorrect kinetochore‐microtubule (K‐MT) attachment, abnormal aurora kinases induced failure of K‐MT attachment error correction.
... F1 hybrids exhibit biased chromosome transmission through female meiosis, with the "stronger" centromere harboring a larger minor satellite array, reduced density of tubulin, weakened microtubule organizing center (MTOC) force, smaller centromerekinetochore distance, and lower SPC24 levels ( Fig. 2A). SPC24 is a marker of the outer kinetochore NDC80 complex; in turn, NDC80 complex size is thought to be directly proportional to minor satellite array size (Wu et al. 2018). Why "stronger" chromosomes with larger minor satellite arrays have lower SPC24 levels and apparently smaller NDC80 complex size is not yet understood. ...
Meiotic drive occurs when one allele at a heterozygous site cheats its way into a disproportionate share of functional gametes, violating Mendel’s law of equal segregation. This genetic conflict typically imposes a fitness cost to individuals, often by disrupting the process of gametogenesis. The evolutionary impact of meiotic drive is substantial, and the phenomenon has been associated with infertility and reproductive isolation in a wide range of organisms. However, cases of meiotic drive in humans remain elusive, a finding that likely reflects the inherent challenges of detecting drive in our species rather than unique features of human genome biology. Here, we make the case that house mice (Mus musculus) present a powerful model system to investigate the mechanisms and consequences of meiotic drive and facilitate translational inferences about the scope and potential mechanisms of drive in humans. We first detail how different house mouse resources have been harnessed to identify cases of meiotic drive and the underlying mechanisms utilized to override Mendel’s rules of inheritance. We then summarize the current state of knowledge of meiotic drive in the mouse genome. We profile known mechanisms leading to transmission bias at several established drive elements. We discuss how a detailed understanding of meiotic drive in mice can steer the search for drive elements in our own species. Lastly, we conclude with a prospective look into how new technologies and molecular tools can help resolve lingering mysteries about the prevalence and mechanisms of selfish DNA transmission in mammals.
