Meiotic Silencing in Mammals

Non-homologous sequence interactions during meiosis: meiotic challenges and evolutionary opportunities

A hallmark of meiosis is pairing of homologous chromosomes, an event that ensures proper segregation into the gametes. Homology pairing is crucial to the formation of normal gametes, the maintenance of genomic integrity, and avoidance of aneuploidy. However, chromosomes are not completely homologous. Here we discuss two notable exceptions to homology: the mammalian sex chromosomes and centromeres. In themselves, these exceptions illustrate meiotic adaptations that both ensure correct chromosome segregation and present evolutionary opportunities. More broadly, such examples of non-homology provide a window for viewing normal mechanisms of meiotic pairing and chromosome modifications. Current analyses of mammalian meiotic chromosome dynamics suggest that the basis for the initial recognition of homology early in meiosis may be based in epigenetic chromatin modifications. Chromatin units may both form pairing sites and provide the modifications that allow non-homologous sequences to be tolerated. Despite recent research progress, we have yet to understand why some non-homologies are tolerated, while others lead to aneuploidy. Understanding how genomes evolve strategies to subvert the usual rules of meiosis will benefit from studies focused on the identification and characterization of meiosis in species with recently acquired non-homology. Looking forward, we are now armed with technologies and tools suited to precisely measure the extent of nonhomology across mammalian chromosomes and to probe the molecular and biophysical steps required for the initiation of homologous chromosome recognition and pairing. These goals are important for elucidating an essential mechanism of meiosis and ultimately for advancing the clinical diagnosis of gametic and embryo aneuploidy.

Mechanisms of Hormonal, Genetic, and Temperature Regulation of Germ Cell Proliferation, Differentiation, and Death During Spermatogenesis

Spermatogenesis is a complex and highly regulated process involving the proliferation, differentiation, and apoptosis of germ cells. This process is controlled by various hormonal, genetic, and environmental factors, including temperature. In hormonal regulation, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone (T) are essential for correct spermatogenesis development from the early stages and spermatogonia proliferation to germ cell maturation. Other hormones, like inhibin and activin, finely participate tuning the process of spermatogenesis. Genetic regulation involves various transcription factors, such as SOX9, SRY, and DMRT1, which are crucial for the development and maintenance of the testis and germ cells. MicroRNAs (miRNAs) play a significant role by regulating gene expression post-transcriptionally. Epigenetic modifications, including DNA methylation, histone modifications, and chromatin remodelling, are also vital. Temperature regulation is another critical aspect, with the testicular temperature maintained around 2-4 °C below body temperature, essential for efficient spermatogenesis. Heat shock proteins (HSPs) protect germ cells from heat-induced damage by acting as molecular chaperones, ensuring proper protein folding and preventing the aggregation of misfolded proteins during thermal stress. Elevated testicular temperature can impair spermatogenesis, increasing germ cell apoptosis and inducing oxidative stress, DNA damage, and the disruption of the blood-testis barrier, leading to germ cell death and impaired differentiation. The cellular mechanisms of germ cell proliferation, differentiation, and death include the mitotic divisions of spermatogonia to maintain the germ cell pool and produce spermatocytes. Spermatocytes undergo meiosis to produce haploid spermatids, which then differentiate into mature spermatozoa. Apoptosis, or programmed cell death, ensures the removal of defective germ cells and regulates the germ cell population. Hormonal imbalance, genetic defects, and environmental stress can trigger apoptosis during spermatogenesis. Understanding these mechanisms is crucial for addressing male infertility and developing therapeutic interventions. Advances in molecular biology and genetics continue to uncover the intricate details of how spermatogenesis is regulated at multiple levels, providing new insights and potential targets for treatment.

KRAB zinc-finger proteins regulate endogenous retroviruses to sculpt germline transcriptomes and genome evolution

As transposable elements (TEs) coevolved with the host genome, the host genome exploited TEs as functional regulatory elements of gene expression. Here we show that a subset of KRAB domain-containing zinc-finger proteins (KZFPs), which are highly expressed in mitotically dividing spermatogonia, repress the enhancer function of endogenous retroviruses (ERVs) and that the release from KZFP-mediated repression allows activation of ERV enhancers upon entry into meiosis. This regulatory feature is observed for independently evolved KZFPs and ERVs in mice and humans, suggesting evolutionary conservation in mammals. Further, we show that KZFP-targeted ERVs are underrepresented on the sex chromosomes in meiosis, suggesting that meiotic sex chromosome inactivation (MSCI) may antagonize the coevolution of KZFPs and ERVs in mammals. Our study uncovers a mechanism by which a subset of KZFPs regulate ERVs to sculpt germline transcriptomes. We propose that epigenetic programming during the transition from mitotic spermatogonia to meiotic spermatocytes facilitates the coevolution of KZFPs and TEs on autosomes and is antagonized by MSCI.

Aberrant activation of chromosome asynapsis checkpoint triggers oocyte elimination

Chromosome synapsis is an evolutionarily conserved process essential for meiotic recombination. HORMAD1 and HORMAD2, which monitor chromosome asynapsis by localizing to unsynapsed chromosome axes, are removed from synapsed chromosome axes by TRIP13, though the biological significance of this process remains unclear. We show that when HORMAD1 and HORMAD2 are retained on synapsed chromosome axes, they recruit BRCA1, activate chromosome asynapsis checkpoint, and trigger oocyte elimination. Unexpectedly, N-terminal tagging retains HORMAD1 and HORMAD2 on synapsed chromosome axes without triggering oocyte elimination due to defective BRCA1 recruitment. Mechanistically, HORMAD1 co-immunoprecipitates with BRCA1 readily, not through the canonical closure motif-binding mode but via an interface on its HORMA domain near the N-terminus. HORMAD2 co-immunoprecipitates with BRCA1 weakly but also regulates its recruitment. Collectively, the TRIP13-dependent removal of HORMAD1 and HORMAD2 from synapsed chromosome axes is essential for female fertility, preventing aberrant chromosome asynapsis checkpoint activation and unintended oocyte elimination.

CTCF-mediated 3D chromatin sets up the gene expression program in the male germline

Spermatogenesis is a unidirectional differentiation process that generates haploid sperm, but how the gene expression program that directs this process is established is largely unknown. Here we determine the high-resolution three-dimensional (3D) chromatin architecture of mouse male germ cells during spermatogenesis and show that CTCF-mediated 3D chromatin dictates the gene expression program required for spermatogenesis. In undifferentiated spermatogonia, CTCF-mediated chromatin interactions between meiosis-specific super-enhancers (SEs) and their target genes precede activation of these SEs on autosomes. These meiotic SEs recruit the master transcription factor A-MYB (MYBL1) in meiotic spermatocytes, which strengthens their 3D contacts and instructs a burst of meiotic gene expression. We also find that at the mitosis-to-meiosis transition, the germline-specific Polycomb protein SCML2 facilitates the resolution of chromatin loops that are specific to mitotic spermatogonia. Moreover, SCML2 and A-MYB help shape the unique 3D chromatin organization of sex chromosomes during meiotic sex chromosome inactivation. We propose that CTCF-mediated 3D chromatin organization regulates epigenetic priming that directs unidirectional differentiation, thereby determining the cellular identity of the male germline.

Dynamics of X chromosome hyper-expression and inactivation in male tissues during stick insect development

Differentiated sex chromosomes are frequently associated with major transcriptional changes: the evolution of dosage compensation (DC) to equalize gene expression between the sexes and the establishment of meiotic sex chromosome inactivation (MSCI). Our study investigates the mechanisms and developmental dynamics of dosage compensation and meiotic sex chromosome inactivation in the stick insect species T. poppense. Stick insects are characterized by XX/X0 sex determination, with an X chromosome that likely evolved prior to the diversification of insects over 450 Mya. We generated a chromosome-level genome assembly and analyzed gene expression from various tissues (brain, gut, antennae, leg, and reproductive tract) across developmental stages in both sexes. Our results show that complete dosage compensation is maintained in male somatic tissues throughout development, mediated by upregulation of the single X chromosome. Contrarily, in male reproductive tissues, dosage compensation is present only in the early nymphal stages. As males reach the 4th nymphal stage and adulthood, X-linked gene expression diminishes, coinciding with the onset of meiosis and MSCI, which involves classical silencing histone modifications. These findings reveal the dynamic regulation of X-linked gene expression in T. poppense, and suggest that reduced X-expression in insect testes is generally driven by MSCI rather than an absence of dosage compensation mechanisms. Our work provides critical insights into sex chromosome evolution and the complex interplay of dosage compensation and MSCI across tissues and developmental stages.

Genome-wide transcriptional silencing and mRNA stabilization allow the coordinated expression of the meiotic program in mice

The transcriptional dynamic of mammalian cells when these transit from the ubiquitous mitotic to a meiotic-specific program is key to understand this switch central to sexual reproduction. By quantifying active RNA polymerase II and nascent transcripts using single cell dataset and ethynyl-uridine pool-down with sorted cells from synchronized testes, we detailed the transcriptional activity of murine male germ cells. When spermatogonia differentiate, transcription slows down, reaching minimal activity at meiotic entry and resumes during pachytene stage. This event, we termed EMLT (for early meiotic low transcription), is distinct from the silencing of sex chromosomes as it is independent of Setdb1, though it is accompanied by the same chromatin mark, H3K9me3. EMLT is delayed in Stra8KO but occurs in mutants altering meiotic chromosome structure or double-strand break formation or repair. By comparing transcript abundance and nascent transcription we unveil a massive event of messenger RNA stabilization that parallels EMLT. Altogether our data indicate that meiosis is initiated with a nearly silent genome, and we propose that the stabilization of transcripts at that time facilitates the meiotic entry by synchronizing the expression of several meiotic subprograms.

Testis-Specific PDHA2 Is Required for Proper Meiotic Recombination and Chromosome Organisation During Spermatogenesis

Proper segregation of homologous chromosomes during meiosis requires crossovers that are tightly regulated by the chromosome structure. PDHA2 is the testis-specific paralog of PDHA1, a core subunit of pyruvate dehydrogenase. However, its role during spermatogenesis is unclear. We show that PDHA2 knockout results in male infertility in mice, but meiotic DSBs in spermatocytes occur normally and are efficiently repaired. Detailed analysis reveals that mid/late recombination intermediates are moderately reduced, resulting in fewer crossovers and many chromosomes without a crossover. Furthermore, defective chromosome structure is observed, including aberrant histone modifications, defective chromosome ends, precocious release of REC8 from chromosomes and fragmented chromosome axes after pachytene. These defects contribute to the failure of pyruvate conversion to acetyl-CoA, resulting in decreased acetyl-CoA and precursors for metabolites and energy in the absence of PDHA2. These findings reveal the important functions of PDHA2 in ensuring proper crossover formation and in modulating chromosome structure during spermatogenesis.

Different complex regulatory phenotypes underlie hybrid male sterility in divergent rodent crosses

Hybrid incompatibilities are a critical component of species barriers and may arise due to negative interactions between divergent regulatory elements in parental species. We used a comparative approach to identify common themes in the regulatory phenotypes associated with hybrid male sterility in two divergent rodent crosses, dwarf hamsters and house mice. We investigated three potential characteristic gene expression phenotypes in hybrids including the propensity of transgressive differentially expressed genes toward over or underexpression, the influence of developmental stage on patterns of misexpression, and the role of the sex chromosomes on misexpression phenotypes. In contrast to near pervasive overexpression in hybrid house mice, we found that misexpression in hybrid dwarf hamsters was dependent on developmental stage. In both house mouse and dwarf hamster hybrids, however, misexpression increased with the progression of spermatogenesis, although to varying extents and with potentially different consequences. In both systems, we detected sex chromosome-specific overexpression in stages of spermatogenesis where inactivated X chromosome expression was expected, but the hybrid overexpression phenotypes were fundamentally different. Importantly, misexpression phenotypes support the presence of multiple developmental blocks to spermatogenesis in dwarf hamster hybrids, including a potential role of meiotic stalling or breakdown early in spermatogenesis. Collectively, we demonstrate that while there are some similarities in hybrid expression phenotypes of house mice and dwarf hamsters, there are also clear differences that point toward unique mechanisms underlying hybrid male sterility. Our results highlight the potential of comparative approaches in helping to understand the causes and consequences of disrupted gene expression in speciation.

Chromatin environment-dependent effects of DOT1L on gene expression in male germ cells

The H3K79 methyltransferase DOT1L is essential for multiple aspects of mammalian development where it has been shown to regulate gene expression. Here, by producing and integrating epigenomic and spike-in RNA-seq data, we decipher the molecular role of DOT1L during mouse spermatogenesis and show that it has opposite effects on gene expression depending on chromatin environment. On one hand, DOT1L represses autosomal genes that are devoid of H3K79me2 at their bodies and located in H3K27me3-rich/H3K27ac-poor environments. On the other hand, it activates the expression of genes enriched in H3K79me2 and located in H3K27me3-poor/H3K27ac-rich environments, predominantly X chromosome-linked genes, after meiosis I. This coincides with a significant increase in DOT1L expression at this stage and a genome-wide acquisition of H3K79me2, particularly on the sex chromosomes. Taken together, our results show that H3K79me2 positively correlates with male germ cell genetic program throughout spermatogenesis, with DOT1L predominantly inhibiting rather than activating gene expression. Interestingly, while DOT1L appears to directly regulate the (re)activation of X genes following meiotic sex chromosome inactivation, it also controls the timely expression of (autosomal) differentiation genes during spermatogenesis.

NKAPL facilitates transcription pause-release and bridges elongation to initiation during meiosis exit

Transcription elongation, especially RNA polymerase II (Pol II) pause-release, is less studied than transcription initiation in regulating gene expression during meiosis. It is also unclear how transcription elongation interplays with transcription initiation. Here, we show that depletion of NKAPL, a testis-specific protein distantly related to RNA splicing factors, causes male infertility in mice by blocking the meiotic exit and downregulating haploid genes. NKAPL binds to promoter-associated nascent transcripts and co-localizes with DNA-RNA hybrid R-loop structures at GAA-rich loci to enhance R-loop formation and facilitate Pol II pause-release. NKAPL depletion prolongs Pol II pauses and stalls the SOX30/HDAC3 transcription initiation complex on the chromatin. Genetic variants in NKAPL are associated with azoospermia in humans, while mice carrying an NKAPL frameshift mutation (M349fs) show defective meiotic exit and transcriptomic changes similar to NKAPL depletion. These findings identify NKAPL as an R-loop-recognizing factor that regulates transcription elongation, which coordinates the meiotic-to-postmeiotic transcriptome switch in alliance with the SOX30/HDAC3-mediated transcription initiation.

METTL16 is Required for Meiotic Sex Chromosome Inactivation and DSB Formation and Recombination during Male Meiosis

Meiosis in males is a critical process that ensures complete spermatogenesis and genetic diversity. However, the key regulators involved in this process and the underlying molecular mechanisms remain unclear. Here, we report an essential role of the m6A methyltransferase METTL16 in meiotic sex chromosome inactivation (MSCI), double-strand break (DSB) formation, homologous recombination and SYCP1 deposition during male meiosis. METTL16 depletion results in a significantly upregulated transcriptome on sex chromosomes in pachytene spermatocytes and leads to reduced DSB formation and recombination, and increased SYCP1 depositioin during the first wave of spermatogenesis. Mechanistically, in pachytene spermatocytes, METTL16 interacts with MDC1/SCML2 to coordinate DNA damage response (DDR) and XY body epigenetic modifications that establish and maintain MSCI, and in early meiotic prophase I, METTL16 regulates DSB formation and recombination by regulating protein levels of meiosis-related genes. Furthermore, multi-omics analyses reveal that METTL16 interacts with translational factors and controls m6A levels in the RNAs of meiosis-related genes (e.g., Ubr2) to regulate the expression of critical meiotic regulators. Collectively, this study identified METTL16 as a key regulator of male meiosis and demonstrated that it modulates meiosis by interacting with MSCI-related factors and regulating m6A levels and translational efficiency (TE) of meiosis-related genes.

Identification of candidate genes related to hybrid sterility by genomic structural variation and transcriptome analyses in cattle-yak

Hybrids between closely related but genetically incompatible species are often inviable or sterile. Cattle-yak, an interspecific hybrid of yak and cattle, exhibits male-specific sterility, which limits the fixation of its desired traits and prevents genetic improvement in yak through crossbreeding. Transcriptome profiles of testicular tissues have been generated in cattle, yak, and cattle-yak; however, the genetic variations underlying differential gene expression associated with hybrid sterility have yet to be elucidated. We detected differences in the cellular composition and gene expression of testes from yak and cattle-yak at 3 mo of age, 10 mo of age, and adulthood. Histological analysis revealed that the most advanced germ cells were gonocytes (prospermatogonia) at 3 mo and spermatocytes at 10 mo. Complete spermatogenesis occurred in the seminiferous tubules of adult yak, whereas only spermatogonia and a limited number of spermatocytes were detected in the testis of adult cattle-yak. Transcriptome analysis revealed 180, 6,310, and 6,112 differentially expressed genes (DEG) in yak and cattle-yak at each stage, respectively. Next, we examined the spermatogenic cell types in the backcross generation (BC1) and detected the appearance of round spermatids, indicating the partial recovery of spermatogenesis in these animals. Compared with those in cattle-yak, 272 DEG were identified in the testes of BC1 animals. Notably, we discovered that the expression of X chromosome-linked genes was upregulated in the testis of cattle-yak compared with yak, suggesting a possible abnormality in the process of meiotic sex chromosome inactivation in hybrid animals. We next screened DEG harboring structural variations (SV) and identified a list of SV genes associated with spermatogonial development, meiotic recombination, and double-strand break repair. Furthermore, we found that the SV genes ESCO2 (establishment of sister chromatid cohesion N-acetyltransferase 2) and BRDT (bromodomain testis associated) may be involved in meiotic arrest of cattle-yak spermatocytes. Overall, our research provides a valuable database for identifying structural variant loci that contribute to hybrid sterility.

LARP7 is required for sex chromosome silencing during meiosis in mice

Meiotic sex chromosome inactivation (MSCI) is an essential event in meiotic progression in mammalian spermatogenesis. We found that La Ribonucleoprotein 7 (LARP7) is involved in MSCI. LARP7 plays a role in fetal germ cells to promote their proliferation, but is once abolished in postnatal gonocytes and re-expressed in spermatocytes at the onset of meiosis. In spermatocytes, LARP7 localizes to the XY body, a compartmentalized chromatin domain on sex chromosomes. In germline-specific Larp7-deficient mice, spermatogenesis is arrested in spermatocytes, and transcription of the genes on sex chromosomes remained active, which suggests failure of meiotic sex chromosome inactivation (MSCI). Furthermore, the XY body in spermatocytes lacking Larp7 shows accumulation of H4K12ac and elimination of H3K9me2, suggesting defective chromatin silencing by abnormal epigenetic controls. These results indicate a new functional role for LARP7 in MSCI.

Supernumerary Chromosomes Enhance Karyotypic Diversification of Narrow-Headed Voles of the Subgenus Stenocranius (Rodentia, Mammalia)

The subgenus Stenocranius contains two cryptic species: Lasiopodomys gregalis (subdivided into three allopatrically distributed and genetically well-isolated lineages A, B, and C) and Lasiopodomys raddei. To identify karyotype characteristics of this poorly studied cryptic species complex, we used comparative cytogenetic analysis of 138 individuals from 41 localities in South Siberia and Mongolia. A detailed description of the L. raddei karyotype and of the L. gregalis lineage С karyotype is presented for the first time. The A chromosome complement of all examined narrow-headed voles consisted of 2n = 36 and a fundamental number of autosomal arms (FNa) of 50. Between species, patterns of differential staining were similar, though additional C-heterochromatic blocks were found in L. gregalis lineages; Ag-positive nucleolar organizers and ribosomal DNA (rDNA) clusters are located on eight and nine acrocentric pairs, respectively. No B chromosomes (Bs) were found in the Early Pleistocene relic L. raddei, while one to five small heterochromatic acrocentric Bs were detected in all L. gregalis lineages; the number and frequency of Bs varied considerably within lineages, but no intraindividual variation was observed. In both species, telomeric repeats were visualized at termini of all chromosomes, including Bs. The number and localization of rDNA clusters on Bs varied among B-carriers. Immunodetection of several meiotic proteins indicated that meio-Bs are transcriptionally inactive and have a pattern of meiotic behavior similar to that of sex chromosomes (some homology of Bs to sex chromosomes is supposed). The nature, mechanisms of inheritance and stability of Bs in L. gregalis require further investigation.

Single cell RNA-sequencing reveals no evidence for meiotic sex chromosome inactivation in the threespine stickleback fish

Sex chromosomes often evolve unique patterns of gene expression in spermatogenesis. In many species, sex-linked genes are downregulated during meiosis in response to asynapsis of the heterogametic sex chromosome pair (meiotic sex chromosome inactivation; MSCI). Our understanding of this process has been limited to a handful of species, including mammals, Drosophila , and C. elegans. Based on findings from these taxa, MSCI has been viewed as likely a conserved process. However, in other groups like teleost fish, our understanding of this process is limited. Teleost fish are a noteworthy group to investigate because sex chromosomes can rapidly evolve between closely related species. Transcriptional profiling of spermatogenesis at the single-cell level is a useful approach to investigate whether MSCI occurs in other species with independently derived sex chromosomes. Here, we investigate whether MSCI occurs in the threespine stickleback fish ( Gasterosteus aculeatus ), which have an X and Y chromosome that evolved less than 26 million years ago. Using single-cell RNA-seq, we found that the X and Y chromosomes do not have a signature of MSCI, maintaining gene expression across meiosis. Using immunofluorescence, we also show the threespine stickleback do not form a condensed sex body around the X and Y, a conserved feature of MSCI in many species. We did not see patterns of gene content evolution documented in other species with MSCI. Y-linked ampliconic gene families were expressed across multiple stages of spermatogenesis, rather than being restricted to post-meiotic stages, like in mammals. Our work shows MSCI does not occur in the threespine stickleback fish and has not shaped the evolution of the Y chromosome. In addition, the absence of MSCI in the threespine stickleback suggests this process may not be a conserved feature of teleost fish and argues for additional investigation in other species.

Author Summary

As male germ cells enter meiosis, the X and Y chromosome of many species undergo a drastic repression of gene expression. In mammals, this process has been shown to be essential for fertility, and the expression of sex-linked genes can lead to meiotic arrest and cell death. This process has only been studied in a handful of organisms, which limits our understanding how conserved MSCI is across the tree of life. Teleost fish are an understudied group with many examples of independently derived sex chromosomes across closely related species. Here, we investigate whether MSCI occurs in the threespine stickleback fish, using single-cell transcriptional profiling. We found gene expression remains active throughout meiosis on the sex chromosomes, indicating MSCI does not occur. This indicates that MSCI is not a conserved feature of all taxa and is not an inevitable outcome of degenerating Y chromosomes.

B Chromosome Transcriptional Inactivation in the Spermatogenesis of the Grasshopper Eyprepocnemis plorans

BACKGROUND/OBJECTIVES

We analyzed the relationship between synapsis, recombination, and transcription during the spermatogenesis of the grasshopper Eyprepocnemis plorans carrying B chromosomes (type B1).

METHODS

The progression of synapsis was interpreted according to the dynamics of the cohesin subunit SMC3 axes. DNA double-strand breaks were revealed by RAD51 immunolabeling, while transcriptional activity was determined by the presence of RNA polymerase II phosphorylated at serine 2 (pRNApol II) immunolabeling. The two repressive epigenetic modifications, histone H3 methylated at lysine 9 (H3K9me3) and histone H2AX phosphorylated at serine 139 (γ-H2AX), were employed to reveal transcriptional inactivity.

RESULTS

During prophase I, spermatocytes with one B1 chromosome showed overall transcription except in the regions occupied by both the X and the B1 chromosomes. This transcriptional inactivity was accompanied by the accumulation of repressive epigenetic modifications. When two B1 chromosomes were present, they could appear as a fully synapsed monochiasmatic bivalent, showing intense H3K9me3 labeling and absence of pRNApol II, while γ-H2AX labeling was similar to that shown by the autosomes.

CONCLUSIONS

According to our results, B1 transcriptional inactivation was triggered in spermatogonia, long before the beginning of meiosis, and was accompanied by H3K9me3 heterochromatinization that was maintained throughout spermatogenesis. Moreover, when two B1 were present, the transcriptional inactivation did not preclude synapsis and recombination achievement by these chromosomes.

Different complex regulatory phenotypes underlie hybrid male sterility in divergent rodent crosses

Hybrid incompatibilities are a critical component of species barriers and may arise due to negative interactions between divergent regulatory elements in parental species. We used a comparative approach to identify common themes in the regulatory phenotypes associated with hybrid male sterility in two divergent rodent crosses, dwarf hamsters and house mice. We investigated three potential characteristic gene expression phenotypes in hybrids including the propensity of transgressive differentially expressed genes towards over or underexpression, the influence of developmental stage on patterns of misexpression, and the role of the sex chromosomes on misexpression phenotypes. In contrast to near pervasive overexpression in hybrid house mice, we found that misexpression in hybrid dwarf hamsters was dependent on developmental stage. In both house mouse and dwarf hamster hybrids, however, misexpression increased with the progression of spermatogenesis, although to varying extents and with potentially different consequences. In both systems, we detected sex-chromosome specific overexpression in stages of spermatogenesis where inactivated X chromosome expression was expected, but the hybrid overexpression phenotypes were fundamentally different. Importantly, misexpression phenotypes support the presence of multiple developmental blocks to spermatogenesis in dwarf hamster hybrids, including a potential role of meiotic stalling or breakdown early in spermatogenesis. Collectively, we demonstrate that while there are some similarities in hybrid expression phenotypes of house mice and dwarf hamsters, there are also clear differences that point towards unique mechanisms underlying hybrid male sterility. Our results highlight the potential of comparative approaches in helping to understand the causes and consequences of disrupted gene expression in speciation.

DNA cytosine methylation suppresses meiotic recombination at the sex-determining region

Meiotic recombination between homologous chromosomes is vital for maximizing genetic variation among offspring. However, sex-determining regions are often rearranged and blocked from recombination. It remains unclear whether rearrangements or other mechanisms might be responsible for recombination suppression. Here, we uncover that the deficiency of the DNA cytosine methyltransferase DNMT1 in the green alga Chlamydomonas reinhardtii causes anomalous meiotic recombination at the mating-type locus (MT), generating haploid progeny containing both plus and minus mating-type markers due to crossovers within MT. The deficiency of a histone methyltransferase for H3K9 methylation does not lead to anomalous recombination. These findings suggest that DNA methylation, rather than rearrangements or histone methylation, suppresses meiotic recombination, revealing an unappreciated biological function for DNA methylation in eukaryotes.

A Minimal Hybrid Sterility Genome Assembled by Chromosome Swapping Between Mouse Subspecies (Mus musculus)

Hybrid sterility is a reproductive isolation barrier between diverging taxa securing the early steps of speciation. Hybrid sterility is ubiquitous in the animal and plant kingdoms, but its genetic control is poorly understood. In our previous studies, we have uncovered the sterility of hybrids between musculus and domesticus subspecies of the house mouse, which is controlled by the Prdm9 gene, the X-linked Hstx2 locus, and subspecific heterozygosity for genetic background. To further investigate this form of genic-driven chromosomal sterility, we constructed a simplified hybrid sterility model within the genome of the domesticus subspecies by swapping domesticus autosomes with their homologous partners from the musculus subspecies. We show that the "sterility" allelic combination of Prdm9 and Hstx2 can be activated by a musculus/domesticus heterozygosity of as few as two autosomes, Chromosome 17 (Chr 17) and Chr 18 and is further enhanced when another heterosubspecific autosomal pair is present, whereas it has no effect on meiotic progression in the pure domesticus genome. In addition, we identify a new X-linked hybrid sterility locus, Hstx3, at the centromeric end of Chr X, which modulates the incompatibility between Prdm9 and Hstx2. These results further support our concept of chromosomal hybrid sterility based on evolutionarily accumulated divergence between homologous sequences. Based on these and previous results, we believe that future studies should include more information on the mutual recognition of homologous chromosomes at or before the first meiotic prophase in interspecific hybrids, as this may serve as a general reproductive isolation checkpoint in mice and other species.

An anciently diverged family of RNA binding proteins maintain correct splicing of a class of ultra-long exons through cryptic splice site repression

Previously, we showed that the germ cell-specific nuclear protein RBMXL2 represses cryptic splicing patterns during meiosis and is required for male fertility (Ehrmann et al., 2019). Here, we show that in somatic cells the similar yet ubiquitously expressed RBMX protein has similar functions. RBMX regulates a distinct class of exons that exceed the median human exon size. RBMX protein-RNA interactions are enriched within ultra-long exons, particularly within genes involved in genome stability, and repress the selection of cryptic splice sites that would compromise gene function. The RBMX gene is silenced during male meiosis due to sex chromosome inactivation. To test whether RBMXL2 might replace the function of RBMX during meiosis we induced expression of RBMXL2 and the more distantly related RBMY protein in somatic cells, finding each could rescue aberrant patterns of RNA processing caused by RBMX depletion. The C-terminal disordered domain of RBMXL2 is sufficient to rescue proper splicing control after RBMX depletion. Our data indicate that RBMX and RBMXL2 have parallel roles in somatic tissues and the germline that must have been conserved for at least 200 million years of mammalian evolution. We propose RBMX family proteins are particularly important for the splicing inclusion of some ultra-long exons with increased intrinsic susceptibility to cryptic splice site selection.

INO80 regulates chromatin accessibility to facilitate suppression of sex-linked gene expression during mouse spermatogenesis

The INO80 protein is the main catalytic subunit of the INO80-chromatin remodeling complex, which is critical for DNA repair and transcription regulation in murine spermatocytes. In this study, we explored the role of INO80 in silencing genes on meiotic sex chromosomes in male mice. INO80 immunolocalization at the XY body in pachytene spermatocytes suggested a role for INO80 in the meiotic sex body. Subsequent deletion of Ino80 resulted in high expression of sex-linked genes. Furthermore, the active form of RNA polymerase II at the sex chromosomes of Ino80-null pachytene spermatocytes indicates incomplete inactivation of sex-linked genes. A reduction in the recruitment of initiators of meiotic sex chromosome inhibition (MSCI) argues for INO80-facilitated recruitment of DNA repair factors required for silencing sex-linked genes. This role of INO80 is independent of a common INO80 target, H2A.Z. Instead, in the absence of INO80, a reduction in chromatin accessibility at DNA repair sites occurs on the sex chromosomes. These data suggest a role for INO80 in DNA repair factor localization, thereby facilitating the silencing of sex-linked genes during the onset of pachynema.

Single-nucleus atlas of the Artemia female reproductive system suggests germline repression of the Z chromosome

Our understanding of the molecular pathways that regulate oogenesis and define cellular identity in the Arthropod female reproductive system and the extent of their conservation is currently very limited. This is due to the focus on model systems, including Drosophila and Daphnia, which do not reflect the observed diversity of morphologies, reproductive modes, and sex chromosome systems. We use single-nucleus RNA and ATAC sequencing to produce a comprehensive single nucleus atlas of the adult Artemia franciscana female reproductive system. We map our data to the Fly Cell Atlas single-nucleus dataset of the Drosophila melanogaster ovary, shedding light on the conserved regulatory programs between the two distantly related Arthropod species. We identify the major cell types known to be present in the Artemia ovary, including germ cells, follicle cells, and ovarian muscle cells. Additionally, we use the germ cells to explore gene regulation and expression of the Z chromosome during meiosis, highlighting its unique regulatory dynamics and allowing us to explore the presence of meiotic sex chromosome silencing in this group.

INO80 regulates chromatin accessibility to facilitate suppression of sex-linked gene expression during mouse spermatogenesis

The INO80 protein is the main catalytic subunit of the INO80-chromatin remodeling complex, which is critical for DNA repair and transcription regulation in murine spermatocytes. In this study, we explored the role of INO80 in silencing genes on meiotic sex chromosomes in male mice. INO80 immunolocalization at the XY body in pachytene spermatocytes suggested a role for INO80 in the meiotic sex body. Subsequent deletion of Ino80 resulted in high expression of sex-linked genes. Furthermore, the active form of RNA polymerase II at the sex chromosomes of Ino80 -null pachytene spermatocytes indicates incomplete inactivation of sex-linked genes. A reduction in the recruitment of initiators of meiotic sex chromosome inhibition (MSCI) argues for INO80-facilitated recruitment of DNA repair factors required for silencing sex-linked genes. This role of INO80 is independent of a common INO80 target H2A.Z. Instead, in the absence of INO80, a reduction in chromatin accessibility at DNA repair sites occurs on the sex chromosomes. These data suggest a role for INO80 in DNA repair factor localization, thereby facilitating the silencing of sex-linked genes during the onset of pachynema.

Summary Statement

Chromatin accessibility and DNA repair factor localization at the sex chromosomes are facilitated by INO80, which regulates sex-linked gene silencing during meiotic progression in spermatocytes.

Differential phosphorylation of two serine clusters in mouse HORMAD1 during meiotic prophase I progression

Mouse HORMAD1 is a phospho-protein involved in multiple functions during meiotic prophase I. To obtain insight into the significance of its phosphorylation, we generated phospho-specific antibodies against two serine residues, Ser307 and Ser378, representing each of two serine clusters in mouse HORMAD1. The Ser307 phosphorylation is detectable from early leptotene substage in both wild-type and Spo11-/- spermatocytes, indicating that Ser307 is a primary and SPO11-independent phosphorylation site. In contrast, the Ser378 phosphorylation is negligible at earlier substages in wild-type and Spo11-/- spermatocytes. After mid-zygotene substage, the Ser378 phosphorylation is abundant on unsynapsed chromosome axes in wild-type spermatocytes and is detected only in a part of unsynapsed chromosome axes in Spo11-/- spermatocytes. We also generated a non-phosphorylated Ser307-specific antibody and found that Ser307 is phosphorylated on sex chromosome axes but is almost entirely unphosphorylated on desynapsed chromosome axes in diplotene spermatocytes. These results demonstrated a substage-specific phosphorylation status of mouse HORMAD1, which might be associated with multiple substage-specific functions.

Mechanisms of meiosis initiation and meiotic prophase progression during spermatogenesis

Meiosis is a critical step for spermatogenesis and oogenesis. Meiosis commences with pre-meiotic S phase that is subsequently followed by meiotic prophase. The meiotic prophase is characterized by the meiosis-specific chromosomal events such as chromosome recombination and homolog synapsis. Meiosis initiator (MEIOSIN) and stimulated by retinoic acid gene 8 (STRA8) initiate meiosis by activating the meiotic genes by installing the meiotic prophase program at pre-meiotic S phase. This review highlights the mechanisms of meiotic initiation and meiotic prophase progression from the point of the gene expression program and its relevance to infertility. Furthermore, upstream pathways that regulate meiotic initiation will be discussed in the context of spermatogenic development, indicating the sexual differences in the mode of meiotic entry.

Atypical heat shock transcription factor HSF5 is critical for male meiotic prophase under non-stress conditions

Meiotic prophase progression is differently regulated in males and females. In males, pachytene transition during meiotic prophase is accompanied by robust alteration in gene expression. However, how gene expression is regulated differently to ensure meiotic prophase completion in males remains elusive. Herein, we identify HSF5 as a male germ cell-specific heat shock transcription factor (HSF) for meiotic prophase progression. Genetic analyzes and single-cell RNA-sequencing demonstrate that HSF5 is essential for progression beyond the pachytene stage under non-stress conditions rather than heat stress. Chromatin binding analysis in vivo and DNA-binding assays in vitro suggest that HSF5 binds to promoters in a subset of genes associated with chromatin organization. HSF5 recognizes a DNA motif different from typical heat shock elements recognized by other canonical HSFs. This study suggests that HSF5 is an atypical HSF that is required for the gene expression program for pachytene transition during meiotic prophase in males.

Gene expression programs in mammalian spermatogenesis

Mammalian spermatogenesis, probably the most complex of all cellular developmental processes, is an ideal model both for studying the specific mechanism of gametogenesis and for understanding the basic rules governing all developmental processes, as it entails both cell type-specific and housekeeping molecular processes. Spermatogenesis can be viewed as a mission with many tasks to accomplish, and its success is genetically programmed and ensured by the collaboration of a large number of genes. Here, I present an overview of mammalian spermatogenesis and the mechanisms underlying each step in the process, covering the cellular and molecular activities that occur at each developmental stage and emphasizing their gene regulation in light of recent studies.

Roles of histone post-translational modifications in meiosis†

Histone post-translational modifications, such as phosphorylation, methylation, acetylation, and ubiquitination, play vital roles in various chromatin-based cellular processes. Meiosis is crucial for organisms that depend on sexual reproduction to produce haploid gametes, during which chromatin undergoes intricate conformational changes. An increasing body of evidence is clarifying the essential roles of histone post-translational modifications during meiotic divisions. In this review, we concentrate on the post-translational modifications of H2A, H2B, H3, and H4, as well as the linker histone H1, that are required for meiosis, and summarize recent progress in understanding how these modifications influence diverse meiotic events. Finally, challenges and exciting open questions for future research in this field are discussed. Summary Sentence  Diverse histone post-translational modifications exert important effects on the meiotic cell cycle and these "histone codes" in meiosis might lead to the development of novel therapeutic strategies against reproductive diseases.

ATF7IP2/MCAF2 directs H3K9 methylation and meiotic gene regulation in the male germline

H3K9 trimethylation (H3K9me3) plays emerging roles in gene regulation, beyond its accumulation on pericentric constitutive heterochromatin. It remains a mystery why and how H3K9me3 undergoes dynamic regulation in male meiosis. Here, we identify a novel, critical regulator of H3K9 methylation and spermatogenic heterochromatin organization: the germline-specific protein ATF7IP2 (MCAF2). We show that in male meiosis, ATF7IP2 amasses on autosomal and X-pericentric heterochromatin, spreads through the entirety of the sex chromosomes, and accumulates on thousands of autosomal promoters and retrotransposon loci. On the sex chromosomes, which undergo meiotic sex chromosome inactivation (MSCI), the DNA damage response pathway recruits ATF7IP2 to X-pericentric heterochromatin, where it facilitates the recruitment of SETDB1, a histone methyltransferase that catalyzes H3K9me3. In the absence of ATF7IP2, male germ cells are arrested in meiotic prophase I. Analyses of ATF7IP2-deficient meiosis reveal the protein's essential roles in the maintenance of MSCI, suppression of retrotransposons, and global up-regulation of autosomal genes. We propose that ATF7IP2 is a downstream effector of the DDR pathway in meiosis that coordinates the organization of heterochromatin and gene regulation through the spatial regulation of SETDB1-mediated H3K9me3 deposition.

A TOPBP1 allele causing male infertility uncouples XY silencing dynamics from sex body formation

Meiotic sex chromosome inactivation (MSCI) is a critical feature of meiotic prophase I progression in males. While the ATR kinase and its activator TOPBP1 are key drivers of MSCI within the specialized sex body (SB) domain of the nucleus, how they promote silencing remains unclear given their multifaceted meiotic functions that also include DNA repair, chromosome synapsis, and SB formation. Here we report a novel mutant mouse harboring mutations in the TOPBP1-BRCT5 domain. Topbp1B5/B5 males are infertile, with impaired MSCI despite displaying grossly normal events of early prophase I, including synapsis and SB formation. Specific ATR-dependent events are disrupted, including phosphorylation and localization of the RNA:DNA helicase Senataxin. Topbp1B5/B5 spermatocytes initiate, but cannot maintain ongoing, MSCI. These findings reveal a non-canonical role for the ATR-TOPBP1 signaling axis in MSCI dynamics at advanced stages in pachynema and establish the first mouse mutant that separates ATR signaling and MSCI from SB formation.

A Comparative Analysis of Mouse Imprinted and Random X-Chromosome Inactivation

The mammalian sexes are distinguished by the X and Y chromosomes. Whereas males harbor one X and one Y chromosome, females harbor two X chromosomes. To equalize X-linked gene expression between the sexes, therian mammals have evolved X-chromosome inactivation as a dosage compensation mechanism. During X-inactivation, most genes on one of the two X chromosomes in females are transcriptionally silenced, thus equalizing X-linked gene expression between the sexes. Two forms of X-inactivation characterize eutherian mammals, imprinted and random. Imprinted X-inactivation is defined by the exclusive inactivation of the paternal X chromosome in all cells, whereas random X-inactivation results in the silencing of genes on either the paternal or maternal X chromosome in individual cells. Both forms of X-inactivation have been studied intensively in the mouse model system, which undergoes both imprinted and random X-inactivation early in embryonic development. Stable imprinted and random X-inactivation requires the induction of the Xist long non-coding RNA. Following its induction, Xist RNA recruits proteins and complexes that silence genes on the inactive-X. In this review, we present a current understanding of the mechanisms of Xist RNA induction, and, separately, the establishment and maintenance of gene silencing on the inactive-X by Xist RNA during imprinted and random X-inactivation.

The male pachynema-specific protein MAPS drives phase separation in vitro and regulates sex body formation and chromatin behaviors in vivo

Dynamic chromosome remodeling and nuclear compartmentalization take place during mammalian meiotic prophase I. We report here that the crucial roles of male pachynema-specific protein (MAPS) in pachynema progression might be mediated by its liquid-liquid phase separation in vitro and in cellulo. MAPS forms distinguishable liquid phases, and deletion or mutations of its N-terminal amino acids (aa) 2-9 disrupt its secondary structure and charge properties, impeding phase separation. Maps-/- pachytene spermatocytes exhibit defects in nucleus compartmentalization, including defects in forming sex bodies, altered nucleosome composition, and disordered chromatin accessibility. MapsΔ2-9/Δ2-9 male mice expressing MAPS protein lacking aa 2-9 phenocopy Maps-/- mice. Moreover, a frameshift mutation in C3orf62, the human counterpart of Maps, is correlated with nonobstructive azoospermia in a patient exhibiting pachynema arrest in spermatocyte development. Hence, the phase separation property of MAPS seems essential for pachynema progression in mouse and human spermatocytes.

3D genome remodeling and homologous pairing during meiotic prophase of mouse oogenesis and spermatogenesis

During meiosis, the chromatin and transcriptome undergo prominent switches. Although recent studies have explored the genome reorganization during spermatogenesis, the chromatin remodeling in oogenesis and characteristics of homologous pairing remain largely elusive. We comprehensively compared chromatin structures and transcriptomes at successive substages of meiotic prophase in both female and male mice using low-input high-through chromosome conformation capture (Hi-C) and RNA sequencing (RNA-seq). Compartments and topologically associating domains (TADs) gradually disappeared and slowly recovered in both sexes. We found that homologs adopted different sex-conserved pairing strategies prior to and after the leptotene-to-zygotene transition, changing from long interspersed nuclear element (LINE)-enriched compartments B to short interspersed nuclear element (SINE)-enriched compartments A. We complemented marker genes and predicted the sex-specific meiotic sterile genes for each substage. This study provides valuable insights into the similarities and distinctions between sexes in chromosome architecture, homologous pairing, and transcriptome during meiotic prophase of both oogenesis and spermatogenesis.

CTCF-mediated 3D chromatin predetermines the gene expression program in the male germline

Spermatogenesis is a unidirectional differentiation process that generates haploid sperm, but how the gene expression program that directs this process is established is largely unknown. Here we determine the high-resolution 3D chromatin architecture of male germ cells during spermatogenesis and show that CTCF-mediated 3D chromatin predetermines the gene expression program required for spermatogenesis. In undifferentiated spermatogonia, CTCF-mediated chromatin contacts on autosomes pre-establish meiosis-specific super-enhancers (SE). These meiotic SE recruit the master transcription factor A-MYB in meiotic spermatocytes, which strengthens their 3D contacts and instructs a burst of meiotic gene expression. We also find that at the mitosis-to-meiosis transition, the germline-specific Polycomb protein SCML2 resolves chromatin loops that are specific to mitotic spermatogonia. Moreover, SCML2 and A-MYB establish the unique 3D chromatin organization of sex chromosomes during meiotic sex chromosome inactivation. We propose that CTCF-mediated 3D chromatin organization enforces epigenetic priming that directs unidirectional differentiation, thereby determining the cellular identity of the male germline.

Chromosome-specific maturation of the epigenome in the Drosophila male germline

Spermatogenesis in the Drosophila male germline proceeds through a unique transcriptional program controlled both by germline-specific transcription factors and by testis-specific versions of core transcriptional machinery. This program includes the activation of genes on the heterochromatic Y chromosome, and reduced transcription from the X chromosome, but how expression from these sex chromosomes is regulated has not been defined. To resolve this, we profiled active chromatin features in the testes from wildtype and meiotic arrest mutants and integrate this with single-cell gene expression data from the Fly Cell Atlas. These data assign the timing of promoter activation for genes with germline-enriched expression throughout spermatogenesis, and general alterations of promoter regulation in germline cells. By profiling both active RNA polymerase II and histone modifications in isolated spermatocytes, we detail widespread patterns associated with regulation of the sex chromosomes. Our results demonstrate that the X chromosome is not enriched for silencing histone modifications, implying that sex chromosome inactivation does not occur in the Drosophila male germline. Instead, a lack of dosage compensation in spermatocytes accounts for the reduced expression from this chromosome. Finally, profiling uncovers dramatic ubiquitinylation of histone H2A and lysine-16 acetylation of histone H4 across the Y chromosome in spermatocytes that may contribute to the activation of this heterochromatic chromosome.

A TOPBP1 Allele Causing Male Infertility Uncouples XY Silencing Dynamics From Sex Body Formation

Meiotic sex chromosome inactivation (MSCI) is a critical feature of meiotic prophase I progression in males. While the ATR kinase and its activator TOPBP1 are key drivers of MSCI within the specialized sex body (SB) domain of the nucleus, how they promote silencing remains unclear given their multifaceted meiotic functions that also include DNA repair, chromosome synapsis and SB formation. Here we report a novel mutant mouse harboring mutations in the TOPBP1-BRCT5 domain. Topbp1 B5/B5 males are infertile, with impaired MSCI despite displaying grossly normal events of early prophase I, including synapsis and SB formation. Specific ATR-dependent events are disrupted including phosphorylation and localization of the RNA:DNA helicase Senataxin. Topbp1 B5/B5 spermatocytes initiate, but cannot maintain ongoing, MSCI. These findings reveal a non-canonical role for the ATR-TOPBP1 signaling axis in MSCI dynamics at advanced stages in pachynema and establish the first mouse mutant that separates ATR signaling and MSCI from SB formation.

A homozygous frameshift variant in SYCP2 caused meiotic arrest and non-obstructive azoospermia

Genetic causation for the majority of non-obstructive azoospermia (NOA) remains unclear. Mutations in synaptonemal complex (SC)-associated genes could cause meiotic arrest and NOA. Previous studies showed that heterozygous truncating variants in SYCP2 encoding a protein essential for SC formation, are associated with non-obstructive azoospermia and severe oligozoospermia. Herein, we showed a homozygous loss-of-function variant in SYCP2 (c.2689_2690insT) in an NOA-affected patient. And this variant was inherited from heterozygous parental carriers by natural reproduction. HE, IF, and meiotic chromosomal spread analyses demonstrated that spermatogenesis was arrested at the zygotene stage in the proband with NOA. Thus, this study revealed that SYCP2 associated with NOA segregates in an autosomal recessive inheritance pattern, rather than an autosomal dominant pattern. Furthermore, our study expanded the knowledge of variants in SYCP2 and provided new insight into understanding the genetic etiology of NOA.

Chromosome-specific maturation of the epigenome in the Drosophila male germline

Spermatogenesis in the Drosophila male germline proceeds through a unique transcriptional program controlled both by germline-specific transcription factors and by testis-specific versions of core transcriptional machinery. This program includes the activation of genes on the heterochromatic Y chromosome, and reduced transcription from the X chromosome, but how expression from these sex chromosomes is regulated has not been defined. To resolve this, we profiled active chromatin features in the testes from wildtype and meiotic arrest mutants and integrate this with single-cell gene expression data from the Fly Cell Atlas. These data assign the timing of promoter activation for genes with germline-enriched expression throughout spermatogenesis, and general alterations of promoter regulation in germline cells. By profiling both active RNA polymerase II and histone modifications in isolated spermatocytes, we detail widespread patterns associated with regulation of the sex chromosomes. Our results demonstrate that the X chromosome is not enriched for silencing histone modifications, implying that sex chromosome inactivation does not occur in the Drosophila male germline. Instead, a lack of dosage compensation in spermatocytes accounts for the reduced expression from this chromosome. Finally, profiling uncovers dramatic ubiquitinylation of histone H2A and lysine-16 acetylation of histone H4 across the Y chromosome in spermatocytes that may contribute to the activation of this heterochromatic chromosome.

TAF7L REGULATES EARLY STAGES OF MALE GERM CELL DEVELOPMENT

Male germ cell development is dependent on the orchestrated regulation of gene networks. TATA-box binding protein associated factors (TAFs) facilitate interactions of TATA-binding protein with the TATA element, which is known to coordinate gene transcription during organogenesis. TAF7 like (Taf7l) is situated on the X chromosome and has been implicated in testis development. We examined the biology of TAF7L in testis development using the rat. Taf7l was prominently expressed in preleptotene to leptotene spermatocytes. To study the impact of TAF7L on the testis we generated a global loss-of-function rat model using CRISPR/Cas9 genome editing. Exon 3 of the Taf7l gene was targeted. A founder was generated possessing a 110 bp deletion within the Taf7l locus, which resulted in a frameshift and the premature appearance of a stop codon. The mutation was effectively transmitted through the germline. Deficits in TAF7L did not adversely affect pregnancy or postnatal survival. However, the Taf7l disruption resulted in male infertility due to compromised testis development and failed sperm production. Mutant germ cells suffer meiotic arrest at the zygotene stage, with defects in sex body formation and meiotic sex chromosome inactivation. This testis phenotype was more pronounced than previously described for the subfertile Taf7l null mouse. We conclude that TAF7L is essential for male germ cell development in the rat.

ATF7IP2/MCAF2 directs H3K9 methylation and meiotic gene regulation in the male germline

H3K9 tri-methylation (H3K9me3) plays emerging roles in gene regulation, beyond its accumulation on pericentric constitutive heterochromatin. It remains a mystery why and how H3K9me3 undergoes dynamic regulation in male meiosis. Here, we identify a novel, critical regulator of H3K9 methylation and spermatogenic heterochromatin organization: the germline-specific protein ATF7IP2 (MCAF2). We show that, in male meiosis, ATF7IP2 amasses on autosomal and X pericentric heterochromatin, spreads through the entirety of the sex chromosomes, and accumulates on thousands of autosomal promoters and retrotransposon loci. On the sex chromosomes, which undergo meiotic sex chromosome inactivation (MSCI), the DNA damage response pathway recruits ATF7IP2 to X pericentric heterochromatin, where it facilitates the recruitment of SETDB1, a histone methyltransferase that catalyzes H3K9me3. In the absence of ATF7IP2, male germ cells are arrested in meiotic prophase I. Analyses of ATF7IP2-deficient meiosis reveal the protein's essential roles in the maintenance of MSCI, suppression of retrotransposons, and global upregulation of autosomal genes. We propose that ATF7IP2 is a downstream effector of the DDR pathway in meiosis that coordinates the organization of heterochromatin and gene regulation through the spatial regulation of SETDB1-mediated H3K9me3 deposition.

The molecular evolution of mammalian spermatogenesis

The testis is a key male reproductive organ that produces gametes through the process of spermatogenesis. Testis morphologies, sperm phenotypes, and the process of spermatogenesis evolve rapidly in mammals, presumably due to the evolutionary pressure on males to give rise to their own offspring. Here, we review studies illuminating the molecular evolution of the testis, in particular large-scale transcriptomic studies, which were based on bulk tissue samples and, more recently, individual cells. Together with various genomic and epigenomic data, these studies have unveiled the cellular source, molecular mechanisms, and evolutionary forces that underlie the rapid phenotypic evolution of the testis. They also revealed shared (ancestral) and species-specific spermatogenic gene expression programs. The insights and available data that have accumulated also provide a valuable resource for the investigation and treatment of male fertility disorders - a dramatically increasing problem in modern industrial societies.

Study on the changes of LHR, FSHR and AR with the development of testis cells in Hu sheep

The process of testis development in mammals is accompanied by the proliferation and maturation of Sertoli, Leydig and germ cells. Spermatogenesis depends on hormone regulation, which must bind to a receptor to exert its biological effects. The changes in Hu sheep testis cell composition and FSHR, LHR and AR expression during different developmental stages are unclear (newborn, puberty and adulthood). To address this, using single-cell RNA sequencing, we analyzed testis cell composition and hormone receptor expression changes during three important developmental stages of Hu sheep. We observed significant changes in the composition of somatic and germ cells in different Hu sheep testis developmental stages. Furthermore, we analyzed the FSHR, LHR and AR distribution and expression changes at three important periods and verified them by qRT-PCR and immunofluorescence. Our results suggest that after birth, the proportion of germ cells increased gradually, peaking in adulthood; the proportion of Sertoli cells decreased gradually, reaching the lowest in adulthood; and the proportion of Leydig cells increased and then decreased, reaching the lowest in adulthood. In addition, FSHR, LHR and AR are mainly located in Sertoli, Leydig and germ cells. LHR and FSHR expression decreased with increasing age, while AR expression increased and then decreased with increasing age.

KRAB-zinc-finger proteins regulate endogenous retroviruses to sculpt germline transcriptomes and genome evolution

As transposable elements (TEs) coevolved with the host genome, the host genome exploited TEs as functional regulatory elements. What remains largely unknown are how the activity of TEs, namely, endogenous retroviruses (ERVs), are regulated and how TEs evolved in the germline. Here we show that KRAB domain-containing zinc-finger proteins (KZFPs), which are highly expressed in mitotically dividing spermatogonia, bind to suppressed ERVs that function following entry into meiosis as active enhancers. These features are observed for independently evolved KZFPs and ERVs in mice and humans, i.e., are evolutionarily conserved in mammals. Further, we show that meiotic sex chromosome inactivation (MSCI) antagonizes the coevolution of KZFPs and ERVs in mammals. Our study uncovers a mechanism by which KZFPs regulate ERVs to sculpt germline transcriptomes. We propose that epigenetic programming in the mammalian germline during the mitosis-to-meiosis transition facilitates coevolution of KZFPs and TEs on autosomes and is antagonized by MSCI.

Casting histone variants during mammalian reproduction

During mammalian reproduction, germ cell chromatin packaging is key to prepare parental genomes for fertilization and to initiate embryonic development. While chromatin modifications such as DNA methylation and histone post-translational modifications are well known to carry regulatory information, histone variants have received less attention in this context. Histone variants alter the stability, structure and function of nucleosomes and, as such, contribute to chromatin organization in germ cells. Here, we review histone variants expression dynamics during the production of male and female germ cells, and what is currently known about their parent-of-origin effects during reproduction. Finally, we discuss the apparent conundrum behind these important functions and their recent evolutionary diversification.

Chromosome Asynapsis Is the Main Cause of Male Sterility in the Interspecies Hybrids of East Asian Voles (Alexandromys, Rodentia, Arvicolinae)

Closely related mammalian species often have differences in chromosome number and morphology, but there is still a debate about how these differences relate to reproductive isolation. To study the role of chromosome rearrangements in speciation, we used the gray voles in the Alexandromys genus as a model. These voles have a high level of chromosome polymorphism and substantial karyotypic divergence. We investigated testis histology and meiotic chromosome behavior in the captive-bred colonies of Alexandromys maximowiczii, Alexandromys mujanensis, two chromosome races of Alexandromys evoronensis, and their interracial and interspecies hybrids, to explore the relationship between karyotypic differences and male hybrid sterility. We found that the seminiferous tubules of the males of the parental species and the interracial hybrids, which were simple heterozygotes for one or more chromosome rearrangements, contained germ cells at all stages of spermatogenesis, indicating their potential fertility. Their meiotic cells displayed orderly chromosome synapsis and recombination. In contrast, all interspecies male hybrids, which were complex heterozygotes for a series of chromosome rearrangements, showed signs of complete sterility. Their spermatogenesis was mainly arrested at the zygotene- or pachytene-like stages due to the formation of complex multivalent chains, which caused extended chromosome asynapsis. The asynapsis led to the silencing of unsynapsed chromatin. We suggest that chromosome asynapsis is the main cause of meiotic arrest and male sterility in the interspecies hybrids of East Asian voles.

RPL3L-containing ribosomes determine translation elongation dynamics required for cardiac function

Although several ribosomal protein paralogs are expressed in a tissue-specific manner, how these proteins affect translation and why they are required only in certain tissues have remained unclear. Here we show that RPL3L, a paralog of RPL3 specifically expressed in heart and skeletal muscle, influences translation elongation dynamics. Deficiency of RPL3L-containing ribosomes in RPL3L knockout male mice resulted in impaired cardiac contractility. Ribosome occupancy at mRNA codons was found to be altered in the RPL3L-deficient heart, and the changes were negatively correlated with those observed in myoblasts overexpressing RPL3L. RPL3L-containing ribosomes were less prone to collisions compared with RPL3-containing canonical ribosomes. Although the loss of RPL3L-containing ribosomes altered translation elongation dynamics for the entire transcriptome, its effects were most pronounced for transcripts related to cardiac muscle contraction and dilated cardiomyopathy, with the abundance of the encoded proteins being correspondingly decreased. Our results provide further insight into the mechanisms and physiological relevance of tissue-specific translational regulation.

The m6A reader PRRC2A is essential for meiosis I completion during spermatogenesis

N6-methyladenosine (m6A) and its reader proteins YTHDC1, YTHDC2, and YTHDF2 have been shown to exert essential functions during spermatogenesis. However, much remains unknown about m6A regulation mechanisms and the functions of specific readers during the meiotic cell cycle. Here, we show that the m6A reader Proline rich coiled-coil 2A (PRRC2A) is essential for male fertility. Germ cell-specific knockout of Prrc2a causes XY asynapsis and impaired meiotic sex chromosome inactivation in late-prophase spermatocytes. Moreover, PRRC2A-null spermatocytes exhibit delayed metaphase entry, chromosome misalignment, and spindle disorganization at metaphase I and are finally arrested at this stage. Sequencing data reveal that PRRC2A decreases the RNA abundance or improves the translation efficiency of targeting transcripts. Specifically, PRRC2A recognizes spermatogonia-specific transcripts and downregulates their RNA abundance to maintain the spermatocyte expression pattern during the meiosis prophase. For genes involved in meiotic cell division, PRRC2A improves the translation efficiency of their transcripts. Further, co-immunoprecipitation data show that PRRC2A interacts with several proteins regulating mRNA metabolism or translation (YBX1, YBX2, PABPC1, FXR1, and EIF4G3). Our study reveals post-transcriptional functions of PRRC2A and demonstrates its critical role in the completion of meiosis I in spermatogenesis.

Genetic control of meiosis surveillance mechanisms in mammals

Meiosis is a specialized cell division that generates haploid gametes and is critical for successful sexual reproduction. During the extended meiotic prophase I, homologous chromosomes progressively pair, synapse and desynapse. These chromosomal dynamics are tightly integrated with meiotic recombination (MR), during which programmed DNA double-strand breaks (DSBs) are formed and subsequently repaired. Consequently, parental chromosome arms reciprocally exchange, ultimately ensuring accurate homolog segregation and genetic diversity in the offspring. Surveillance mechanisms carefully monitor the MR and homologous chromosome synapsis during meiotic prophase I to avoid producing aberrant chromosomes and defective gametes. Errors in these critical processes would lead to aneuploidy and/or genetic instability. Studies of mutation in mouse models, coupled with advances in genomic technologies, lead us to more clearly understand how meiosis is controlled and how meiotic errors are linked to mammalian infertility. Here, we review the genetic regulations of these major meiotic events in mice and highlight our current understanding of their surveillance mechanisms. Furthermore, we summarize meiotic prophase genes, the mutations that activate the surveillance system leading to meiotic prophase arrest in mouse models, and their corresponding genetic variants identified in human infertile patients. Finally, we discuss their value for the diagnosis of causes of meiosis-based infertility in humans.