Summary 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.

Highlights d High microtubule-destabilizing activity makes mouse centromeres selfish in meiosis d Amplified BUB1 signaling enriches destabilizing activity on selfish centromeres d Selfish centromeres can modulate the BUB1 pathway by different mechanisms d Rapid progression through meiosis I can suppress centromere drive

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 molecular mechanisms underlying spindle asymmetry are unknown. Here we found that CDC42 signaling from the cell cortex regulated microtubule tyrosination to induce spindle asymmetry. NonMendelian 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.Genetic conflict is inherent in any haploid-diploid life cycle because genetic elements compete for transmission through meiosis. Mendel's Law of Segregation states that alleles of a gene are transmitted with equal probability, but this law can be violated by selfish genetic elements through meiotic drive, for example by eliminating competing gametes (e.g., sperm killing or spore killing) or by exploiting the asymmetry in female meiosis to increase transmission to the egg. Despite the impact of meiotic drive on evolution and genetics (1-4), the underlying mechanisms are largely unknown. Female meiosis provides a clear opportunity for selfish elements to cheat because only chromosomes that segregate to the egg can be transmitted to offspring, while the rest are degraded in polar bodies. Conceptually, female meiotic drive depends on three conditions: asymmetry in cell fate, a functional difference between homologous chromosomes that influences their segregation, and asymmetry within the meiotic spindle (5). The asymmetry in cell fate is well established (6), and chromosomal rearrangements and amplifications of repetitive sequences (e.g., centromeres) are associated with biased segregation (7-10). Asymmetry within the meiotic spindle was noted in grasshopper in 1976 (11) but not studied further.Oocyte spindles are positioned close to the cortex and oriented perpendicular to the cortex so that cytokinesis produces a large egg and a small polar body. A selfish element drives by preferentially attaching to the egg side of the spindle, implying some difference in microtubules (MTs) between the egg and cortical sides. To determine how such spindle asymmetry is regulated, using mouse oocytes as a model for meiotic drive (10, 12), we tested for asymmetry in post-translational modifications that functionally diversify MTs ( To distinguish between these possibilities, we manipulated spindle position by treating oocytes with cytochalasin B (CCB) before maturation to inhibit actin polymerization. The nucleus drifted to the cortex in 24% of these oocytes, with the spindle positioned near the cortex by 3 h after germinal vesicle breakdown (GVBD) vs. migration at 6 h under norma...

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.