Control mechanisms of spindle assembly and chromosome segregation are vital for preventing aneuploidy during cell division. The mammalian germ cells and embryos are prone to chromosome segregation errors, and the resulting aneuploidy is a major cause of termination of development or severe developmental disorders. Here we focused on early mouse embryos, and using combination of methods involving microinjection, immunodetection and confocal live cell imaging, we concentrated on the Spindle Assembly Checkpoint (SAC) and Anaphase Promoting Complex/Cyclosome (APC/C). These are two important mechanisms cooperating during mitosis to ensure accurate chromosome segregation, and assessed their activity during the first two mitoses after fertilization. Our results showed, that in zygotes and 2-cell embryos, the SAC core protein Mad1 shows very low levels on kinetochores in comparison to oocytes and its interaction with chromosomes is restricted to a short time interval after nuclear membrane disassembly (NEBD). Exposure of 2-cell embryos to low levels of spindle poison does not prevent anaphase, despite the spindle damage induced by the drug. Lastly, the APC/C is activated coincidentally with NEBD before the spindle assembly completion. This early onset of APC/C activity, together with precocious relocalization of Mad1 from chromosomes, prevents proper surveillance of spindle assembly by SAC. The results contribute to the understanding of the origin of aneuploidy in early embryos.

• Klíčová slova
• Mad1, anaphase, anaphase-promoting complex, chromosome segregation, embryo, spindle, spindle assembly checkpoint,
• Publikační typ
• časopisecké články MeSH

Chromosome segregation in female germ cells and early embryonic blastomeres is known to be highly prone to errors. The resulting aneuploidy is therefore the most frequent cause of termination of early development and embryo loss in mammals. And in specific cases, when the aneuploidy is actually compatible with embryonic and fetal development, it leads to severe developmental disorders. The main surveillance mechanism, which is essential for the fidelity of chromosome segregation, is the Spindle Assembly Checkpoint (SAC). And although all eukaryotic cells carry genes required for SAC, it is not clear whether this pathway is active in all cell types, including blastomeres of early embryos. In this review, we will summarize and discuss the recent progress in our understanding of the mechanisms controlling chromosome segregation and how they might work in embryos and mammalian embryos in particular. Our conclusion from the current literature is that the early mammalian embryos show limited capabilities to react to chromosome segregation defects, which might, at least partially, explain the widespread problem of aneuploidy during the early development in mammals.

• Klíčová slova
• CDK1, aneuploidy, cell size, chromosome division, embryo, segregation errors, spindle, spindle assembly checkpoint,
• MeSH
• aneuploidie MeSH
• chromozomy MeSH
• embryonální vývoj * genetika   MeSH
• lidé MeSH
• savci genetika   MeSH
• segregace chromozomů * MeSH
• velikost buňky MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• přehledy MeSH

The transition from sexual reproduction to asexuality is often triggered by hybridization. The gametogenesis of many hybrid asexuals involves premeiotic genome endoreplication leading to bypass hybrid sterility and forming clonal gametes. However, it is still not clear when endoreplication occurs, how many gonial cells it affects and whether its rate differs among clonal lineages. Here, we investigated meiotic and premeiotic cells of diploid and triploid hybrids of spined loaches (Cypriniformes: Cobitis) that reproduce by gynogenesis. We found that in naturally and experimentally produced F1 hybrids asexuality is achieved by genome endoreplication, which occurs in gonocytes just before entering meiosis or, rarely, one or a few divisions before meiosis. However, genome endoreplication was observed only in a minor fraction of the hybrid's gonocytes, while the vast majority of gonocytes were unable to duplicate their genomes and consequently could not proceed beyond pachytene due to defects in bivalent formation. We also noted that the rate of endoreplication was significantly higher among gonocytes of hybrids from natural clones than of experimentally produced F1 hybrids. Thus, asexuality and hybrid sterility are intimately related phenomena and the transition from sexual reproduction to asexuality must overcome significant problems with genome incompatibilities with a possible impact on reproductive potential.

• Klíčová slova
• Cobitis taenia complex, endoreplication, gynogenesis, hybrid sterility, meiosis, polyploidy,
• MeSH
• gametogeneze genetika   MeSH
• hybridizace genetická MeSH
• křížení genetické MeSH
• máloostní genetika   růst a vývoj   MeSH
• meióza genetika   MeSH
• nepohlavní rozmnožování genetika   MeSH
• rozmnožování genetika   MeSH
• Taenia genetika   růst a vývoj   MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH

The onset of an early development is, in mammals, characterized by profound changes of multiple aspects of cellular morphology and behavior. These are including, but not limited to, fertilization and the merging of parental genomes with a subsequent transition from the meiotic into the mitotic cycle, followed by global changes of chromatin epigenetic modifications, a gradual decrease in cell size and the initiation of gene expression from the newly formed embryonic genome. Some of these important, and sometimes also dramatic, changes are executed within the period during which the gene transcription is globally silenced or not progressed, and the regulation of most cellular activities, including those mentioned above, relies on controlled translation. It is known that the blastomeres within an early embryo are prone to chromosome segregation errors, which might, when affecting a significant proportion of a cell within the embryo, compromise its further development. In this review, we discuss how the absence of transcription affects the transition from the oocyte to the embryo and what impact global transcriptional silencing might have on the basic cell cycle and chromosome segregation controlling mechanisms.

• Klíčová slova
• cell cycle, embryo, oocyte, transcriptional repression, translation,
• MeSH
• buněčný cyklus genetika   MeSH
• chromatin genetika   MeSH
• embryo savčí fyziologie   MeSH
• embryonální vývoj genetika   MeSH
• genetická transkripce genetika   MeSH
• lidé MeSH
• segregace chromozomů genetika   MeSH
• umlčování genů fyziologie   MeSH
• vývojová regulace genové exprese genetika   MeSH
• zvířata MeSH
• Check Tag
• lidé MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• přehledy MeSH
• Názvy látek
• chromatin MeSH

According to the Dobzhansky-Muller model, hybrid sterility is a consequence of the independent evolution of related taxa resulting in incompatible genomic interactions of their hybrids. The model implies that the incompatibilities evolve randomly, unless a particular gene or nongenic sequence diverges much faster than the rest of the genome. Here we propose that asynapsis of heterospecific chromosomes in meiotic prophase provides a recurrently evolving trigger for the meiotic arrest of interspecific F1 hybrids. We observed extensive asynapsis of chromosomes and disturbance of the sex body in >95% of pachynemas of Mus m. musculus × Mus m. domesticus sterile F1 males. Asynapsis was not preceded by a failure of double-strand break induction, and the rate of meiotic crossing over was not affected in synapsed chromosomes. DNA double-strand break repair was delayed or failed in unsynapsed autosomes, and misexpression of chromosome X and chromosome Y genes was detected in single pachynemas and by genome-wide expression profiling. Oocytes of F1 hybrid females showed the same kind of synaptic problems but with the incidence reduced to half. Most of the oocytes with pachytene asynapsis were eliminated before birth. We propose the heterospecific pairing of homologous chromosomes as a preexisting condition of asynapsis in interspecific hybrids. The asynapsis may represent a universal mechanistic basis of F1 hybrid sterility manifested by pachytene arrest. It is tempting to speculate that a fast-evolving subset of the noncoding genomic sequence important for chromosome pairing and synapsis may be the culprit.

• MeSH
• apoptóza genetika   MeSH
• biologická evoluce MeSH
• biologické modely MeSH
• druhová specificita MeSH
• dvouřetězcové zlomy DNA MeSH
• inbrední kmeny myší klasifikace   genetika   fyziologie   MeSH
• infertilita genetika   patologie   patofyziologie   MeSH
• křížení genetické MeSH
• meióza genetika   MeSH
• myši inbrední BALB C MeSH
• myši inbrední C57BL MeSH
• myši MeSH
• oocyty patologie   MeSH
• párování chromozomů genetika   MeSH
• rekombinace genetická MeSH
• spermatocyty patologie   MeSH
• spermatogeneze genetika   MeSH
• těhotenství MeSH
• transkriptom MeSH
• vznik druhů (genetika) MeSH
• zvířata MeSH
• Check Tag
• mužské pohlaví MeSH
• myši MeSH
• těhotenství MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH

Chromosome segregation errors are highly frequent in mammalian female meiosis, and their incidence gradually increases with maternal age. The fate of aneuploid eggs is obviously dependent on the stringency of mechanisms for detecting unattached or repairing incorrectly attached kinetochores. In case of their failure, the newly formed embryo will inherit the impaired set of chromosomes, which will have severe consequences for its further development. Whether spindle assembly checkpoint (SAC) in oocytes is capable of arresting cell cycle progression in response to unaligned kinetochores was discussed for a long time. It is known that abolishing SAC increases frequency of chromosome segregation errors and causes precocious entry into anaphase; SAC, therefore, seems to be essential for normal chromosome segregation in meiosis I. However, it was also reported that for anaphase-promoting complex (APC) activation, which is a prerequisite for entering anaphase; alignment of only a critical mass of kinetochores on equatorial plane is sufficient. This indicates that the function of SAC and of cooperating chromosome attachment correction mechanisms in oocytes is different from somatic cells. To analyze this phenomenon, we used live cell confocal microscopy to monitor chromosome movements, spindle formation, APC activation and polar body extrusion (PBE) simultaneously in individual oocytes at various time points during first meiotic division. Our results, using oocytes from aged animals and interspecific crosses, demonstrate that multiple unaligned kinetochores and severe congression defects are tolerated at the metaphase to anaphase transition, although such cells retain sensitivity to nocodazole. This indicates that checkpoint mechanisms, operating in oocytes at this point, are essential for accurate timing of APC activation in meiosis I, but they are insufficient in detection or correction of unaligned chromosomes, preparing thus conditions for propagation of the aneuploidy to the embryo.

• MeSH
• anafáze MeSH
• anafázi podporující komplex MeSH
• aneuploidie MeSH
• časosběrné zobrazování metody   MeSH
• histony genetika   metabolismus   MeSH
• kinetochory metabolismus   MeSH
• komplexy ubikvitinligas genetika   metabolismus   MeSH
• konfokální mikroskopie metody   MeSH
• kontrolní body M fáze buněčného cyklu MeSH
• metafáze MeSH
• mikroinjekce MeSH
• myši inbrední C57BL MeSH
• myši MeSH
• oocyty cytologie   metabolismus   MeSH
• párování chromozomů * MeSH
• proteolýza MeSH
• savčí chromozomy genetika   metabolismus   MeSH
• savci MeSH
• segregace chromozomů * MeSH
• sekurin MeSH
• transportní proteiny genetika   metabolismus   MeSH
• tubulin genetika   metabolismus   MeSH
• zvířata MeSH
• Check Tag
• mužské pohlaví MeSH
• myši MeSH
• ženské pohlaví MeSH
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• anafázi podporující komplex MeSH
• histony MeSH
• komplexy ubikvitinligas MeSH
• sekurin MeSH
• transportní proteiny MeSH
• tubulin MeSH