Periodic Reporting for period 4 - ChromoSOMe (Canonical and Non-canonical modes of Chromosome Segregation in Oocyte Meiosis )
Reporting period: 2024-07-01 to 2024-12-31
Cell division is essential for complex organism development, tissue and organ maintenance, and adult reproductive capacity. In sexually reproducing species, gametes—spermatozoa in males and oocytes in females—are produced via meiosis, a specialized form of cell division. Meiosis involves a single round of DNA replication followed by two successive divisions (meiosis I and II), leading to genome haploidization. Errors in meiotic chromosome segregation can cause gamete and embryonic aneuploidy, a major contributor to birth defects and pregnancy loss in humans.
Despite its fundamental role in species continuity, the mechanisms governing meiotic chromosome segregation, particularly in oocytes, remain poorly understood. This knowledge gap stems from the evolutionary diversity of regulatory mechanisms controlling oocyte chromosome segregation, despite the overall conservation of meiosis across eukaryotes. Notably, in humans, the frequency of chromosome segregation errors in oocytes increases sharply with maternal age, posing a significant reproductive barrier, yet the underlying causes remain elusive.
Our project aimed to elucidate the mechanisms ensuring accurate chromosome segregation in oocytes. Building on our prior contributions, we adopted an interdisciplinary, multi-organism, and multi-scale approach to achieve a comprehensive understanding of female meiosis. By dissecting the molecular players involved while embracing the diversity of meiotic mechanisms, our work provided key insights into the essential and universal principles governing chromosome segregation in oocytes.
Despite its fundamental role in species continuity, the mechanisms governing meiotic chromosome segregation, particularly in oocytes, remain poorly understood. This knowledge gap stems from the evolutionary diversity of regulatory mechanisms controlling oocyte chromosome segregation, despite the overall conservation of meiosis across eukaryotes. Notably, in humans, the frequency of chromosome segregation errors in oocytes increases sharply with maternal age, posing a significant reproductive barrier, yet the underlying causes remain elusive.
Our project aimed to elucidate the mechanisms ensuring accurate chromosome segregation in oocytes. Building on our prior contributions, we adopted an interdisciplinary, multi-organism, and multi-scale approach to achieve a comprehensive understanding of female meiosis. By dissecting the molecular players involved while embracing the diversity of meiotic mechanisms, our work provided key insights into the essential and universal principles governing chromosome segregation in oocytes.
Our achievements resulted in the publication of six original research articles in top-tier peer-reviewed journals. Below, we summarize our main findings along with the corresponding journals:
1- Kinetochore Component Function in C. elegans Oocytes Revealed by 4D Tracking of Holocentric Chromosomes (Nature Communications, 2023)
Using 4D live microscopy, we systematically perturbed kinetochore sub-complexes in C. elegans oocytes. We found that accurate chromosome segregation depends on a combination of facultative Ndc80-mediated end-on attachments generating pulling forces and essential BHC-dependent pushing forces from central spindle microtubules. Unlike in most systems, C. elegans oocytes achieve accurate segregation without strict reliance on the Ndc80 complex or chromosome bi-orientation.
2- SSynergistic Stabilization of Microtubules by BUB-1, HCP-1, and CLS-2 Controls Microtubule Pausing and Meiotic Spindle Assembly (eLife, 2023)
To understand BHC-dependent pushing forces, we studied BHC components (BUB-1, HCP-1/2/CENP-F, and CLS-2/CLASP) in C. elegans oocytes. Using in vivo and in vitro assays with TIRF microscopy, we showed that BHC components synergistically regulate microtubule growth, rescue, and pausing while inhibiting catastrophe. This regulation is essential for meiotic spindle formation and accurate chromosome segregation.
3- An Unconventional TOG Domain is Required for CLASP Localization (Current Biology, 2023)
We identified a conserved unconventional TOG domain in the CLASP-family proteins' C-terminal domain. Through structural analysis, yeast-two-hybrid screens, and mass spectrometry, we demonstrated that a single conserved arginine residue is critical for all known CLASP localizations and interactions with partner proteins. These findings support the idea that CLASPs use an evolutionarily repurposed TOG domain for subcellular targeting.
4- An Interkinetic Envelope Surrounds Chromosomes Between Meiosis I and II in C. elegans Oocytes (Journal of Cell Biology, 2025)
Contrary to the belief that the nuclear envelope does not reform between meiosis I and II, we discovered a transient double membrane, the interkinetic envelope, covering chromosomes during interkinesis in C. elegans oocytes. This novel organelle, distinct in composition and assembly from canonical nuclear envelopes, contributes to normal chromosome segregation.
5- Regulation of Outer Kinetochore Assembly During Meiosis I and II by CENP-A and KNL-2/M18BP1 in C. elegans Oocytes (Current Biology, 2024)
We demonstrated that kinetochore formation in C. elegans oocytes requires both CENP-A and an independent KNL-2/M18BP1-MEL-28/ELYS pathway. Codepletion of CENP-A (or CENP-C) and KNL-2/M18BP1 (or MEL-28/ELYS) prevented outer kinetochore assembly during meiosis I. We further identified a KNL-2/M18BP1 N-terminal domain responsible for MEL-28/ELYS recruitment, independent of its role in CENP-A loading. Meiosis II kinetochore assembly relied solely on the canonical CENP-A/CENP-C pathway.
6- Maternal inheritance of functional centriolesin two parthenogenetic nematodes. (Nature Communications, 2024)
Centrioles are the core constituent of centrosomes, microtubule-organizing centers involved in directing mitotic spindle assembly and chromosome segregation in animal cells. In sexually reproducing species, centrioles degenerate during oogenesis such that female meiosis is usually acentrosomal and anastral. Centrioles are retained during male meiosis and, in most species, are reintroduced with the sperm during fertilization, restoring centriole numbers in embryos. In contrast, the presence, origin, and function of centrioles in parthenogenetic species is unknown. We found that centrioles are maternally inherited in two species of parthenogenetic nematodes and identified two different strategies for maternal inheritance evolved in the two species. In Rabditophanes diutinus, centrioles organize the poles of the meiotic spindle and are inherited by both the polar body and embryo. In Disploscapter pachys, the two pairs of centrioles remain close together and are inherited by the embryo only. Maternally-inherited centrioles organize the embryonic spindle poles and act as a symmetry-breaking cue to induce embryo polarization. Thus, in parthenogenetic nematodes, centrioles are maternally-inherited and functionally replace their sperm-inherited counterparts in sexually reproducing species.
Together, our findings significantly advance the understanding of chromosome segregation mechanisms in oocytes, revealing both conserved and divergent pathways that regulate meiotic fidelity.
1- Kinetochore Component Function in C. elegans Oocytes Revealed by 4D Tracking of Holocentric Chromosomes (Nature Communications, 2023)
Using 4D live microscopy, we systematically perturbed kinetochore sub-complexes in C. elegans oocytes. We found that accurate chromosome segregation depends on a combination of facultative Ndc80-mediated end-on attachments generating pulling forces and essential BHC-dependent pushing forces from central spindle microtubules. Unlike in most systems, C. elegans oocytes achieve accurate segregation without strict reliance on the Ndc80 complex or chromosome bi-orientation.
2- SSynergistic Stabilization of Microtubules by BUB-1, HCP-1, and CLS-2 Controls Microtubule Pausing and Meiotic Spindle Assembly (eLife, 2023)
To understand BHC-dependent pushing forces, we studied BHC components (BUB-1, HCP-1/2/CENP-F, and CLS-2/CLASP) in C. elegans oocytes. Using in vivo and in vitro assays with TIRF microscopy, we showed that BHC components synergistically regulate microtubule growth, rescue, and pausing while inhibiting catastrophe. This regulation is essential for meiotic spindle formation and accurate chromosome segregation.
3- An Unconventional TOG Domain is Required for CLASP Localization (Current Biology, 2023)
We identified a conserved unconventional TOG domain in the CLASP-family proteins' C-terminal domain. Through structural analysis, yeast-two-hybrid screens, and mass spectrometry, we demonstrated that a single conserved arginine residue is critical for all known CLASP localizations and interactions with partner proteins. These findings support the idea that CLASPs use an evolutionarily repurposed TOG domain for subcellular targeting.
4- An Interkinetic Envelope Surrounds Chromosomes Between Meiosis I and II in C. elegans Oocytes (Journal of Cell Biology, 2025)
Contrary to the belief that the nuclear envelope does not reform between meiosis I and II, we discovered a transient double membrane, the interkinetic envelope, covering chromosomes during interkinesis in C. elegans oocytes. This novel organelle, distinct in composition and assembly from canonical nuclear envelopes, contributes to normal chromosome segregation.
5- Regulation of Outer Kinetochore Assembly During Meiosis I and II by CENP-A and KNL-2/M18BP1 in C. elegans Oocytes (Current Biology, 2024)
We demonstrated that kinetochore formation in C. elegans oocytes requires both CENP-A and an independent KNL-2/M18BP1-MEL-28/ELYS pathway. Codepletion of CENP-A (or CENP-C) and KNL-2/M18BP1 (or MEL-28/ELYS) prevented outer kinetochore assembly during meiosis I. We further identified a KNL-2/M18BP1 N-terminal domain responsible for MEL-28/ELYS recruitment, independent of its role in CENP-A loading. Meiosis II kinetochore assembly relied solely on the canonical CENP-A/CENP-C pathway.
6- Maternal inheritance of functional centriolesin two parthenogenetic nematodes. (Nature Communications, 2024)
Centrioles are the core constituent of centrosomes, microtubule-organizing centers involved in directing mitotic spindle assembly and chromosome segregation in animal cells. In sexually reproducing species, centrioles degenerate during oogenesis such that female meiosis is usually acentrosomal and anastral. Centrioles are retained during male meiosis and, in most species, are reintroduced with the sperm during fertilization, restoring centriole numbers in embryos. In contrast, the presence, origin, and function of centrioles in parthenogenetic species is unknown. We found that centrioles are maternally inherited in two species of parthenogenetic nematodes and identified two different strategies for maternal inheritance evolved in the two species. In Rabditophanes diutinus, centrioles organize the poles of the meiotic spindle and are inherited by both the polar body and embryo. In Disploscapter pachys, the two pairs of centrioles remain close together and are inherited by the embryo only. Maternally-inherited centrioles organize the embryonic spindle poles and act as a symmetry-breaking cue to induce embryo polarization. Thus, in parthenogenetic nematodes, centrioles are maternally-inherited and functionally replace their sperm-inherited counterparts in sexually reproducing species.
Together, our findings significantly advance the understanding of chromosome segregation mechanisms in oocytes, revealing both conserved and divergent pathways that regulate meiotic fidelity.
Together, our findings significantly advance the understanding of chromosome segregation mechanisms in oocytes, revealing both conserved and divergent pathways that regulate meiotic fidelity.