Simple SummaryThe rough periwinkle Littorina saxatilis has become a model species for evolutionary biology in recent decades. The reproductive strategy of this species, including meiosis, is of particular importance. A cytogenetic study using nuclear spreads and live cell observations revealed for the first time some non-canonical features of male meiosis in this marine snail. The most intriguing are the diffuse stage and the karyosome stage during prophase I, as well as the delay in the appearance of chiasmata and chromosome-specific chromomeric patterns of bivalents during diakinesis—early anaphase I.An atypical course of male meiosis in Littorina saxatilis from zygotene to early anaphase I has been established, which includes non-canonical stages—diffuse and karyosomal. In diakinesis, a structural stepwise transition of bivalents from a single-thread, homogeneously colored form to a double-thread and banded form was discovered. In early diakinesis, in addition to bivalents without pronounced chiasmata, which constitute the majority, rare cruciform short bivalents with one chiasma are revealed. In mid-diakinesis, two or three types of bivalents with one or two chiasmata and several achiasmatic bivalents are identified. In late diakinesis—metaphase I—seven types of bivalents were distinguished, bearing from one to three chiasmata. Some bivalents of the set showed noticeable chiasmata only in early anaphase I. Therefore, the course of diakinesis in L. saxatilis male meiosis is rather atypical. In pachytene, mid- and late diakinesis, and partly in metaphase I, individual bivalents of the L. saxatilis set are reliably identified because they exhibit chromomeric patterns similar to those of the G-banded prometaphase chromosomes of early embryos and spermatogonial mitotic cells. Our research provides a cytological basis for further studies of conservation/variability and evolution of male meiosis.

KNUCKLES promotes meiotic cell cycle progression by directly repressing the expression of KRP1 and KRP3 to ensure male fertility.

The mealybug Pseudococcus viburni is a plant-feeding insect with a non-Mendelian genetic system known as paternal genome elimination (PGE). In PGE, males eliminate their paternally inherited chromosomes during meiosis, transmitting only the maternal genome to the next generation. This involves genome-wide imprinting, where paternal chromosomes are heterochromatinised in embryogenesis and throughout adulthood. In this species, a non-essential B chromosome can escape paternal genome elimination, thereby enhancing its transmission rate to the next generation. Previous studies show that the B chromosome escapes elimination by changing its chromatin compaction during meiosis to resemble that of maternal chromosomes. Although the exact mechanism underlying this change is poorly understood. Here we investigated histone methylation and acetylation modifications, as well as the Heterochromatin Protein 1 (HP1), to characterise differences between maternal, paternal and B chromosomes during male meiosis of P. viburni. Maternal and paternal chromosomes show distinct histone modification patterns, with marks associated with euchromatin present on maternal chromosomes and marks associated with heterochromatin present on paternal chromosomes. We then identified key histone modification changes that coincide with chromatin remodelling of the B chromosome, which allows it to segregate with maternal chromosomes. In addition, we showed that these chromatin modifications occur regardless of the parental origin of the B chromosome. Overall, our findings support the role of histone modifications for proper chromosome segregation during meiosis in mealybugs and provide insight into the mechanisms by which the B chromosome exploits PGE for its preferential transmission.

The inherent differences between sex chromosomes in males and females create conflicts in gene expression, driving the evolution of regulatory mechanisms such as Meiotic Sex Chromosome Inactivation (MSCI), a process that transcriptionally silences the sex chromosomes during male meiosis. In this study, we explore the evolutionary dynamics of MSCI within the Drosophila genus by analyzing transcriptomes across different stages of spermatogenesis in D. melanogaster and its progressively more distant relatives, D. simulans, D. willistoni, and D. mojavensis. Stage-enriched bulk RNA sequencing, showing a strong correlation in spermatogenic gene expression patterns among these species, revealed that MSCI dates back to the early evolution of the Drosophila genus, impacting the regulation of both coding and long non-coding RNAs. Notably, for newly evolved genes, X-linked genes show higher expression levels than autosomal genes during mitosis and meiosis, indicating that MSCI predominantly regulates older genes. In contrast, newly evolved autosomal genes exhibit a gradual increase in expression throughout spermatogenesis, reaching their peak in the post-meiotic phase. During this phase, the expression of X-linked new genes decreases, eventually aligning with that of autosomal genes. This expression pattern suggests that haploid selection plays a crucial role in the regulation of new genes, with monoallelic expression of the X chromosome providing an advantage across all stages of germline development, while autosomal gene expression gains a selective edge primarily in the post-meiotic phase. Together, these findings provide new insights into the evolution of sex chromosomes and highlight the critical role of MSCI in shaping gene expression profiles in Drosophila.

The timing of male meiosis plays a pivotal role in ensuring successful pollination and may represent a critical window during which environmental stressors can significantly impact reproductive outcomes. In anemophilous plants, both the quantity of microspores produced and the development of viable pollen are particularly susceptible to external influences, such as fluctuating climatic conditions. This study undertakes a comprehensive analysis of reproductive features, focusing on the meiotic events of male gametogenesis and the phenological phases from the onset to full flowering in olive from central Italy. Utilizing a robust 11-year database (2012–2022), the research incorporates detailed micro- and macro-phenological observations alongside systematic pollen monitoring. The temporal regulation of male meiosis directs the phenological plasticity of the olive tree (Olea europaea L.) by transforming it into maladaptive phenological plasticity, effectively making the plant insensitive to thermal changes. This remarkable physiological trait underscores the resilience of this woody species to climate change. The results obtained will help to elucidate the interaction between climatic factors and reproductive dynamics, contributing valuable insights into the broader context of phenological responses to environmental changes.

In eukaryotes, the cellular processes contributing to gamete formation form the building blocks of genetic inheritance across generations. While traditionally viewed as conserved across model organisms, emerging studies reveal significant variation in meiotic and post-meiotic processes. Extending our knowledge to non-model organisms is therefore critical to improve our understanding of the evolutionary origin and significance of modifications associated with gamete formation. We describe the cytological patterns underlying chromosome segregation, recombination, and meiotic sex chromosome inactivation during male meiosis in the stick insect group Timema. Our results provide a detailed description of centromere protein loading dynamics during spermatogenesis, and further reveal that 1) recombination initiates before synapsis (unlike Drosophila meiosis), and 2) that the X remains actively silenced despite two waves of transcriptional activation in autosomes during spermatogenesis. Together, our observations help understand the evolutionary significance of key cellular events related to spermatogenesis and shed light on the diversity of their associated molecular processes among species, including Timema stick insects.

ABSTRACTIt is known that various testis-specific mitochondrial proteins are associated with energy metabolism and male meiosis. PDHA2 is a testis-specific mitochondrial protein, and its encoding gene is speculated to be an autosomal retrogene of the progenitor X-linked Pdha1. Here, we show that Pdha2 knockout (KO) mice exhibit azoospermia due to failure at the late pachytene-diplotene transition. We found that PDHA2 interacts with PDHB and PDHA1. PDHA2 absence leads to decreased PDHB amounts and ATP levels in male germ cells. ATP reduction impairs the function of the ATPase recombination proteins RAD51 and DMC1, causing crossover formation deficiency, further resulting in double-strand break repair failure at the pachytene stage. Pdha1 expression by transgenes in Pdha2 KO germ cells rescues fertility and PDHB expression in Pdha2 KO males, confirming the functional equivalence of PDHA1 and PDHA2. Because X-linked Pdha1 expression is silenced during meiotic sex chromosome inactivation, our findings also support the hypothesis that Pdha2 was transposed from Pdha1. In summary, PDHA2 compensates for silenced PDHA1 in male germ cells, and plays a crucial role in maintaining efficient double-strand break repair for proper meiotic progression.

Establishment of correct chromatin configuration in male meiosis is essential for sperm formation and male fertility. However, how chromatin remodeling contributes to meiotic progression in male germ cells is not well understood. Here, we find that the ISWI family ATP-dependent chromatin remodeling factor SMARCA5 (SNF2H) plays a critical role in regulating meiotic prophase progression during spermatogenesis in mice. Male mice with germ cell-specific depletion of SMARCA5 are infertile and unable to form sperm. Conditional knockout of Smarca5 results in meiotic progression failure, with abnormal spermatocytes appearing at the pachytene stage of meiosis I and subsequent accumulation of defects in chromosome synapsis, DNA repair, and transposon control, along with elevated rates of apoptosis. SMARCA5 interacts with known cofactors BAZ1A/ACF and BAZ2A/TIP5, as well as numerous DNA repair and recombination factors, in the testis. Single cell RNA sequencing confirmed failure to achieve a normal transcriptional state in premeiotic spermatogonia and during meiotic prophase, with reduced levels of meiotic gene transcripts and increasingly aberrant transcriptional states at later stages of spermatogenic development. Transcriptional misregulation in meiotic prophase was preceded by a widespread increase in chromatin accessibility in spermatogonia at promoters and repeat elements. Our findings suggest that SMARCA5 restricts chromatin accessibility in male germ cells to guide appropriate chromatin remodeling during meiotic recombination, contrasting with its role promoting chromatin accessibility during female meiosis.

The generation of haploid gametes is a hallmark of sexual reproduction achieved through a complex, albeit tightly regulated, reductional cell division known as meiosis. While the molecular underpinnings of meiosis have been extensively characterized in eutherian mammalian models, key aspects-particularly those governing chromosome synapsis and recombination-remain poorly understood in non-eutherian mammals and non-model vertebrates. This knowledge gap is especially relevant for understanding genome evolution, with a focus on sex chromosomes. Comparative studies across diverse vertebrate lineages are therefore essential to uncover conserved and lineage-specific features of meiotic regulation. In this review, we explore the evolutionary dynamics of meiosis in vertebrates, emphasizing how the meiotic program influences genome architecture and the evolution of heteromorphic sex chromosomes, especially the Y chromosome. As research in non-model species gains momentum, dissecting the diversity of meiotic mechanisms across taxa emerges as a key to understanding genome plasticity and evolutionary innovation.

N6‐methyladenosine (m6A) reader proteins have been demonstrated to be involved in numerous biological processes. However, the regulatory mechanism of specific m6A reader proteins during mammalian meiotic processes remains largely elusive. Here, this study identified hnRNPA2B1 as an m6A reader protein that plays a critical role in meiotic pachytene progression using a tamoxifen‐induced knockout mouse model. Deletion of hnRNPA2B1 in spermatocytes disrupts homologous recombination and synapsis, with the mislocalization of double‐strand break (DSB) repair proteins beyond the chromosome axes in pachytene spermatocytes. Multi‐omics analyses revealed extensive dysregulation of the transcriptome and proteome in hnRNPA2B1‐deficient spermatocytes, particularly affecting genes involved in chromosome organization, meiotic cell cycle, and DNA damage response, thereby triggering the pachytene checkpoint for cell elimination. In vitro luciferase assays confirmed that hnRNPA2B1 directly targets several meiosis‐related transcripts (e.g., Ep400, Rrs1, etc.) in an m6A‐dependent manner to regulate their expression. Furthermore, this finding demonstrates that hnRNPA2B1 biologically interacts with mRNA processing regulators and translation factors (e.g., eIF4G3, RPS3, RPL13, DDX5, YTHDC2) and functions as a post‐transcriptional factor essential for pachytene progression during male meiosis. Collectively, this study underscores the critical role of the m6A reader hnRNPA2B1 in the pachytene checkpoint and advances our understanding of the regulatory mechanisms underlying male meiosis.

An adult tree of Larix decidua Mill., European larch, was produced from doubling one haploid female gametophyte. Whether this tree can produce normal male meiocytes is the crucial question. This adult’s pollen mother cells (PMCs), or male meiocytes, were squashed and stained. Male meiosis was normal and no abortive pollen grains were observed. This female gametophytic apomict of a conifer, a dihaploid adult, is 100% homozygous yet also reproductively competent with normal male meiosis and functional male pollen. Here we show that doubled female gametophytes can produce embryos and reproductively competent adult trees. This shows a way to gain rapid homozygosity and produce completely inbred lines for larch. This is a novel breeding shortcut reported for the first time for a conifer species.

Programmed DNA elimination, a phenomenon wherein cells eliminate a subset of genetic material during certain stages of development, is observed in a broad range of organisms. The fungus gnat Bradysia (formerly Sciara) coprophila undergoes a series of programmed DNA elimination events during their development, including elimination of germline-restricted chromosomes (called L chromosomes) in the soma and elimination of paternal chromosomes during male meiosis. However, a lack of understanding surrounding the nature of eliminated chromosomes poses a barrier to studying programmed DNA elimination in this system. Highly repetitive satellite DNA, which often shows chromosome-specific distribution, is a possible candidate for sequences involved in programmed DNA elimination. In this study, we utilized recent genomic data and genome assemblies to identify new satellite DNA sequences of B. coprophila, and characterized their distribution on chromosomes. The results imply that the X and autosomes do not share centromeric satellite DNA sequence (BcopSat-155) with the L chromosomes. We further provide cytological evidence that confirms a recent finding based on the genome assembly that there are 2 distinct L chromosomes that were not previously distinguished cytologically. Together, our work lays a foundation for future studies to explore the possible connection between satellite DNA and the mechanism of programmed DNA elimination in B. coprophila.

Genetic deficiency of EXOSC10 ribonuclease disrupts spermatogenesis and male fertility in mice.

Delta tubulin (TUBD1) is a noncanonical tubulin protein that has been linked to complex microtubule structures in somatic cell lines and unicellular species. Its role in mammals remains enigmatic; however, TUBD1 is enriched within mammalian male germ cells. Herein, we have defined new roles for TUBD1 during male germ cell development in vivo using a conditional knockout mouse model and shown that spermatogenesis in the absence of TUBD1 causes sterility. We show TUBD1 stabilizes kinetochores during male mouse meiosis, enabling meiotic progression, and that it is required for appropriate spindle polarity and cytokinesis. Subsequently, in haploid cells, TUBD1 works in partnership with the microtubule-severing enzymes KATNAL2 and KATNB1 to regulate manchette remodeling and shape the sperm head. Collectively, these findings reveal TUBD1 plays a key role in the formation and function of highly specialized microtubule structures in mammalian spermatogenesis. Advanced knowledge of TUBD1 may generate new insights into underlying causes of diseases associated with infertility or development.

Male meiosis and chromosome numbers in seventeen species of tribe Cardueae from northwest Himalayas

Abstract Chromosome arm-level aneuploidies (CAAs) are a common consequence of genomic instability during the evolution of solid tumor and often associated with metastasis and therapy resistance. In breast cancer, chromosome arms 1q and 8q are the frequent amplified regions for CAAs. This study aims to investigate whether critical genes amplified in genomic regions with CAAs contribute to the pathophysiology in triple-negative breast cancer (TNBC). We established multiple long-term culturable TNBC patient-derived cells (PDCs) using spheroid culture technique, which is favorable for the enrichment of cell population with cancer stemness. We performed a genome-wide chromatin immunoprecipitation study for histone H3K27ac and super-enhancer analysis in these TNBC PDCs and cell lines. Super-enhancer analysis revealed that 8q is a substantial chromosomal region with a high density of super-enhancers. Among genes in the vicinity of 8q super-enhancers, we identified that MYB proto-oncogene like 1 (MYBL1) is a transcription factor with high abundance in TNBC PDCs as well as in basal-like BT549 cells. In TCGA breast cancer database, 8q gain is a common genomic feature in MYBL1 gene-amplified breast cancer tissues and MYBL1 expression is substantially correlated with 8q-related genes and proliferation-related genes like MKI67 and BUB1. In TNBC PDCs and cell lines, we showed that MYBL1-specific siRNAs significantly repressed TNBC cell proliferation and migration. RNA sequencing analysis in TNBC cells revealed that MYBL1 silencing substantially downregulated genes involved in the transcription machinery such as nuclear envelope, chromatin dynamics, and DNA replication. In immunohistochemical analysis of clinical TNBC tissues from a Japanese cohort, we showed that a positive MYBL1 immunoreactivity (IR) was significantly associated with shorter disease-free survival (DFS). Among candidate MYBL1 targets, we demonstrated that NCAPH, a subunit of condensin I complex, was a prognostic factor for patients with TNBC based on the immunohistochemistry of clinical TNBC tissues. Notably, patients with double IR positivity for MYBL1 and NCAPH showed shorter DFS than those with MYBL1 or NCAPH positivity alone. MYBL1 has been characterized as an essential transcriptional regulator in male meiosis and in female mammary gland development. Our findings indicate that MYBL1 plays an essential role in the pathophysiology for advanced breast cancers such as TNBC, and MYBL1 and its downstream genes can be potential diagnostic and therapeutic targets for the disease. Citation Format: Akihiro Fujimoto, Kazuhiro Ikeda, Keiichi Kinowaki, Hidetaka Kawabata, Akihiko Osaki, Satoshi Inoue, Kuniko Horie. Chromosome 8q gain-related transcription factor MYBL1 is a critical regulator in triple-negative breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P2-02-08.

BackgroundThe lower Dipteran fungus gnat, Bradysia (aka Sciara) coprophila, has compelling chromosome biology. Paternal chromosomes are eliminated during male meiosis I and both maternal X sister chromatids are retained in male meiosis II. Embryos start with three copies of the X chromosome, but 1–2 copies are eliminated from somatic cells as part of sex determination, and one is eliminated in the germline to restore diploidy. In addition, there is gene amplification in larval polytene chromosomes, and the X polytene chromosome folds back on itself mediated by extremely long-range interactions between three loci. These developmentally normal events present opportunities to study chromosome behaviors that are unusual in other systems. Moreover, little is known about the centromeric and telomeric sequences of lower Dipterans in general, and there are recent claims of horizontally-transferred genes in fungus gnats. Overall, there is a pressing need to learn more about the fungus gnat chromosome sequences.ResultsWe produced the first chromosome-scale models of the X and autosomal chromosomes where each somatic chromosome is represented by a single scaffold. Extensive analysis supports the chromosome identity and structural accuracy of the scaffolds, demonstrating they are co-linear with historical polytene maps, consistent with evolutionary expectations, and have accurate centromere positions, chromosome lengths, and copy numbers. The positions of alleged horizontally-transferred genes in the nuclear chromosomes were broadly confirmed by genomic analyses of the chromosome scaffolds using Hi-C and single-molecule long-read datasets. The chromosomal context of repeats shows family-specific biases, such as retrotransposons correlated with the centromeres. Moreover, scaffold termini were enriched with arrays of retrotransposon-related sequence as well as nucleosome-length (~ 175 bp) satellite repeats. Finally, the Hi-C data captured Mb-scale physical interactions on the X chromosome that are seen in polytene spreads, and we characterize these interesting “fold-back regions” at the sequence level for the first time.ConclusionsThe chromosome scaffolds were shown to be of exceptional quality, including loci harboring horizontally-transferred genes. Repeat analyses demonstrate family-specific biases and telomere repeat candidates. Hi-C analyses revealed the sequences of ultra-long-range interactions on the X chromosome. The chromosome-scale scaffolds pave the way for further studies of the unusual chromosome movements in Bradysia coprophila.

Feedback regulation of m6A modification creates local auxin maxima essential for rice microsporogenesis.

Meiosis plays a pivotal role in plant reproduction, which is also crucial for enhancing genetic diversity. Although the impact of MOF1 on floral organ development and its negative regulation of the key tapetal gene PKS2 have been established, the specific function of MOF1 in male meiotic process remains elusive. In this study, we identified two mutant lines of MOF1 in Nipponbare background. Compared to the wild-type controls, MOF1 mutations resulted in significant reductions in seed setting rate and pollen fertility, partially attributed to its defects in the formation of male meiotic bivalents. RNA-seq analyses and RT-qPCR assays revealed that loss-of-function mutation of MOF1 didn’t alter expression levels of 60 known meiotic-regulated genes, suggesting that MOF1 may not function as a transcriptional factor in its meiotic regulation. Yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated the protein-protein interactions among MOF1, RPA2c, RPA1c, as well as FAR1, among which RPA1c and RPA2c involved in meiotic bivalent formation. Furthermore, gene expression pattern analyses and subcellular localization studies indicated the co-expression among above interacted proteins in nucleus during anther development. Our findings provide a mechanistic insight into how MOF1 modulate male meiosis possibly through interactions with key meiotic proteins, facilitating a better understanding of male reproductive regulation.

Meiotic drivers are selfish genetic elements that bias their own transmission during meiosis or gamete formation. Due to the fundamental differences between male and female meiosis in animals and plants, meiotic drivers operate through distinct mechanisms in the two sexes: In females, they exploit the asymmetry of meiosis to ensure their inclusion in the egg, whereas in males, they eliminate competing gametes after symmetric meiosis. Meiotic drive is commonly reported in males, where it strongly influences the evolution of spermatogenesis, while the few known cases in females have highlighted its crucial role in centromere evolution. Despite a growing number of examples in a wide range of organisms, meiotic drive has so far only been observed in one sex or the other since its discovery nearly 100 y ago. Here, we show that a selfish X chromosome known to cause meiotic drive in male Drosophila testacea flies also causes meiotic drive in females. We find that this X chromosome has supergene architecture, harboring extensive structural rearrangements that suppress recombination between the two X chromosomes. This has contributed to a substantial expansion of its size compared to the wild-type chromosome, partly due to the accumulation of species-specific repetitive elements. Our findings suggest that female meiotic drive may play an important role in the evolutionary dynamics of polymorphic structural variants that suppress recombination, including inversions, translocations, and supergenes.