A significant challenge in Drosophila centriole biology is its small size. Advanced super-resolution techniques have provided valuable insights, but require specialized equipment and can be difficult to implement in tissues. Expansion Microscopy (ExM) offers an accessible alternative, yet its application in Drosophila centriole research has been sparse. We provide an ExM protocol for cultured S2 cells and fly tissues that revealed new insights into pro-centriole biology. In S2 cells we document overduplication in the form of the classic "rosettes", while in spermatids we uncover an unexpected movement of the pro-centriole-like structure (PCL). ExM has also refined existing molecular models. In S2 cells we document the distal tip protein Cep97 as a ring, which clarifies its role in capping the growing centriole. In spermatids, we spatially segregated the inner nuclear membrane protein Spag4 and the cytoplasmic protein Yuri, which led to the new hypothesis that they play independent roles at the centriole-nucleus contact site. Finally, we show that our ExM protocol is a hypothesis-generator and discovery tool applicable beyond Drosophila centrioles by imaging synaptonemal complexes in the Plodia interpunctella moth.

Multi-omics analysis reveals fluorinated liquid crystal monomers BDPrB/EDPrB exposure disrupts the steroid biosynthesis function via inhibition of CYP17A1.

The synaptonemal complex (SC) is a highly conserved proteinaceous structure that physically links homologous chromosomes during meiosis, thereby orchestrating chromosome pairing, recombination, and segregation . Recent advances in super-resolution microscopy, cryo-electron microscopy, and multi-omics approaches have substantially deepened our understanding of the SC’s molecular architecture, dynamic assembly–disassembly process, and its pivotal roles in reproductive biology. In this review, we synthesize current knowledge on the structural organization of the SC, elucidate how mutations in SC-associated genes perturb meiosis and contribute to reproductive disorders, and summarize recent progress in methodological innovations that facilitate SC research. We further highlight unresolved questions and propose future research directions, aiming to bridge basic mechanistic insights with potential clinical applications in the diagnosis and management of infertility.

Meiotic crossover interference is a one-dimensional spatial patterning process that produces evenly-spaced crossovers. Quantitative analysis of diagnostic molecules along budding yeast chromosomes reveals that this process sets up two interdigitated patterns, of shorter and longer periodicity, by “two-tiered” patterning. Both tiers comprise clustered assemblies of three types of molecules (“triads”) representing the three major components of meiotic chromosomes (crossover recombination, axes, and the synaptonemal complex). One tier of triads occurs at sites of majority (“canonical”) crossovers. Second tier triads are more widely spaced but also exhibit interference, dependent on the same functions as canonical crossover interference. Diverse lines of evidence suggest that second tier triads arise at sites of previously mysterious “minority” crossovers. Finally, conserved protein remodeler Pch2/TRIP13 modulates the abundance of triad components, specifically in longer periodicity triads, dynamically in real time. Potential roles of triad structure, mechanisms of two-tiered patterning, and the nature of minority crossovers are discussed.

The germline-restricted chromosome (GRC) is a unique and enigmatic element found exclusively in the germ cells of passerine birds, with its function and evolutionary dynamics still largely unresolved. This study utilizes cytogenetic analysis of the Eurasian bullfinch (Pyrrhula pyrrhula) to explore the meiotic behavior of the GRC. We report the novel discovery of naturally occurring tetraploid and octoploid spermatocytes in this species. Remarkably, in these polyploid cells, the GRC exhibited normal meiotic processes, including full synapsis and recombination. Recombination was restricted to the H3K9me3-negative proximal half of the GRC bivalent, implicating a chromatin-based regulation mechanism. The standard chromosome set in the polyploid cells showed orderly chromosome synapsis. The number of recombination nodules in tetraploid and octoploid nuclei was approximately equal to the standard value for diploids multiplied by 2 and 4, respectively. These findings suggest that polyploidy does not completely hinder meiotic progression in birds and offer new insights into GRC regulation during meiosis.

Several proteins collaborate to promote the crossover recombination events critical for accurate chromosome segregation during meiosis. How these “ZMM” factors (Zip2, Zip3, Zip4, Spo16, Mer3 and MutSγ) collaboratively function remains incompletely understood. We previously reported that Zip3’s abundance and activity rely on the synaptonemal complex (SC) component Zip1, and specifically on Zip1’s N-terminal residues associated with crossovers and coupling SC assembly to the crossover pathway. Here, we demonstrate that Zip3 co-immunoprecipitates Zip1 from meiotic cells independent of recombination initiation and other ZMMs, and that Zip3’s interaction with Zip1 relies on Zip1’s N terminal residues. Co-expression and pull-down experiments in bacterial cells demonstrate that Zip1 and Zip3 interact directly. Experiments to identify Zip3 regions required for the Zip1 interaction unexpectedly revealed an incorrectly annotated translational start; we also determined that Zip3’s N-terminal structured region is necessary and sufficient for the interaction, and a predicted coil downstream of Zip3’s RING domain is essential for specific activities attributed to Zip1’s N-terminal tip such as proximity labeling of Zip3 by Zip2 and the coupling of crossover recombination to SC assembly. Finally, we discovered that interaction with Zip1 protects Zip3 not only from proteasome-mediated degradation but also from post-translational modification when another ZMM is absent. We propose that direct interaction with Zip1’s N terminus orients Zip3 within a nascent ZMM ensemble in a manner that facilitates crossover formation and the coupling of crossover intermediates to SC assembly, and furthermore ensures Zip3 remains both abundant and unmodified until all requisite ZMMs have joined the group.

The synaptonemal complex (SC) is a meiosis-specific tripartite proteinaceous structure that regulates the number and positions of crossovers (COs). Here we characterize SCEP3, a new Arabidopsis SC component that is essential for CO assurance, promoting positive CO interference and preventing negative CO interference. SCEP3 localizes to the chromosome axes as numerous foci at leptotene, of which a small proportion cluster as large foci that initiate synapsis. SCEP3 then relocates to the central region of the SC as ZYP1 polymerizes. In the absence of SCEP3, homologues align but do not synapse. In the scep3 mutants, COs increase in number towards the chromosome ends and are more likely to cluster together. SCEP3 encodes an 801-amino-acid intrinsically disordered protein that is structurally similar to SIX6OS1 in mammals and SYP-4 in nematodes, containing phenylalanine repeats at the amino terminus and a carboxy-terminal coiled-coil, suggesting that it is a fundamentally conserved SC component across kingdoms.

Holliday junctions (HJs) are branched four-way DNA structures that link recombining chromosomes during double-strand break repair1. Despite posing a risk to chromosome segregation, HJs accumulate during meiotic prophase I as intermediates in the process of crossing-over2,3. Whether HJs have additional regulatory functions remains unclear. Here we establish an experimental system in budding yeast that enables conditional nucleolytic resolution of HJs after the establishment of meiotic chromosome synapsis. We find that HJ resolution triggers complete disassembly of the synaptonemal complex without disrupting the axis-loop organization of chromosomes. Mechanistically, HJs mediate the continued association of ZMM proteins with recombination nodules that form at the axes interface of homologous chromosome pairs. ZMM proteins, in turn, promote polymerization of the synaptonemal complex while simultaneously protecting HJs from processing by non-crossover pathways. Thus, reciprocal feedback between ZMMs, which stabilize HJs, and HJs, which retain ZMM proteins at future crossover sites, maintains chromosome synapsis until HJ-resolving enzymes are activated during exit from prophase I. Notably, by polymerizing and maintaining the synaptonemal complex structure, the HJ-ZMM interplay suppresses de novo double-strandbreak formation and recombination reinitiation. In doing so, this interplay suppresses the DNAdamage response, enabling meiotic progression without unrepaired breaks and supporting crossover assurance.

The transverse filament (TF) protein of the synaptonemal complex (SC) is crucial for rice meiosis, but its role in male sterility remains unexplored. We used Yuanjiang common wild rice (YJCWR) as a donor parent and developed a series of introgression lines (ILs, BC4) in the genetic background of YUNDAO 1. Male Sterile 4 (MS4) was mapped to a 49.8 kb region containing OsZEP1 (Os04g37960), which encodes a transverse filament (TF) protein. Homozygous sterile lines exhibited shorter, weaker, pale yellow anthers and sterile pollens compared to fertile lines. SEM analysis reveals that OsZEP1-CWR impairs anther cuticle and pollen wall formation. Transverse semithin sections of sterile anthers show abnormal cuticle, pollen exine formation, delayed tapetum degradation, and defective microspores. CRISPR/Cas9 OsZEP1 knockouts validated OsZEP1 as the MS4 candidate gene, causing complete male sterility. This study highlights the role of MS4 in male fertility, and the newly developed KASP markers will be useful for marker-assisted breeding.

Meiotic crossovers (COs) are needed to produce genetically balanced gametes. In mammals, CO formation is mediated by a conserved set of pro-CO proteins via mechanisms that remain unclear. Here, we characterize a mammalian pro-CO factor HEIP1. In mouse HEIP1 is essential for crossover and fertility of both sexes. HEIP1 promotes crossover by orchestrating the recruitment of other pro-CO proteins, including the MutSγ complex (MSH4-MSH5) and E3 ligases (HEI10, RNF212, and RNF212B), that are required to mature CO sites and recruit the CO-specific resolution complex MutLγ. Moreover, HEIP1 directly interacts with HEI10, suggesting a direct role in controlling the recruitment of pro-CO E3 ligases. During early stages of meiotic prophase I, HEIP1 interacts with the chromosome axes, independently of recombination, before relocalizing to the central region of the synaptonemal complex. We propose that HEIP1 is a conserved master regulator of CO proteins that controls different CO maturation steps.

Organisms use a specialized cell division called meiosis for the creation of haploid gametes. Multiple carefully orchestrated steps must occur at specific times and places for meiosis to be successful, including chromosome pairing, meiotic entry, recombination, synapsis, and two rounds of chromosome segregation. The regulation and molecular mechanisms for many of the steps of meiosis have not been fully elucidated. During synapsis, the synaptonemal complex (SC) builds along the entire lengths of the homologs to maintain the pairing of the homologs and promote the formation of the crossovers that help ensure proper segregation of homologs at the meiosis I division in many organisms. The SC is a large tripartite structure that is believed to function as a biomolecular condensate. To attempt to identify proteins that interact with SC components during female meiosis in Drosophila melanogaster, a protein of the lateral element, C(2)M, and a protein of the central element, Cona, were tagged with the APEX2 enzyme, which can biotinylate nearby proteins under the appropriate conditions. Under biotinylating promoting conditions, biotin labeled proteins were observed to be associated with the SC by immunofluorescence. Biotinylated proteins were isolated for mass spectrometry analysis, and multiple proteins were found to be enriched compared to control samples. RNAi knockdown lines targeting a subset of enriched proteins were examined for phenotypes in early Drosophila female meiosis. RNAi knockdown of Cpsf5, an mRNA cleavage factor, caused delayed or defective SC formation, as well as additional meiotic defects, indicating a role for maturation of mRNA in regulating processes of female meiosis. These results support proximity labeling as a strategy for identifying additional meiotic proteins.

A key event in meiosis is the conversion of a small subset of double strand breaks into interhomolog crossovers. In this study, we demonstrate that Caenorhabditis elegans male spermatogenesis has less robust mechanisms than hermaphrodite oogenesis in regulating crossover numbers. This is not a consequence of differences in meiotic prophase timing, sex chromosome genotype, or the presence or absence of germline apoptosis. Using the cyclin-like crossover marker COSA-1, we show that males are less efficient in both converting double strand breaks into crossover designated events and limiting their number, suggesting weakened crossover homeostasis. Surprisingly, we discovered that significant numbers of COSA-1 foci form at the very end of meiotic prophase in the absence of SPO-11 during spermatogenesis. These COSA-1-marked sites are also independent of homologous recombination, and Topoisomerases I and II. We find that the synaptonemal complex, which holds homologs in proximity, differently modulates COSA-1 enrichment to chromosomes in the absence of SPO-11 in males and hermaphrodites. Together, these findings suggest that males have less robust crossover control and that there are previously unrecognized lesions or structures at the end of meiotic prophase in spermatocytes that can accumulate crossover markers.

Chromosomal linkages formed through crossover recombination are essential for the accurate segregation of homologous chromosomes during meiosis1. The DNA events of recombination are linked to structural components of meiotic chromosomes2. Imperatively, the biased resolution of double Holliday junction (dHJ) intermediates into crossovers3,4 occurs within the synaptonemal complex (SC), the meiosis-specific structure that mediates end-to-end synapsis of homologues during the pachytene stage5,6. However, the role of the SC in crossover-specific dHJ resolution remains unclear. Here we show that key SC components function through dependent and interdependent relationships to protect dHJs from aberrant dissolution into non-crossover products. Conditional ablation experiments reveal that cohesin, the core of SC lateral elements, is required to maintain both synapsis and dHJ-associated crossover recombination complexes (CRCs) during pachytene. The SC central region transverse-filament protein is also required to maintain CRCs. Reciprocally, the stability of the SC central region requires the continuous presence of CRCs effectively coupling synapsis to dHJ formation and desynapsis to resolution. However, dHJ protection and CRC maintenance can occur without end-to-end homologue synapsis mediated by the central element of the SC central region. We conclude that local ensembles of SC components are sufficient to enable crossover-specific dHJ resolution to ensure the linkage and segregation of homologous chromosomes.

Meiosis is a pivotal biological process essential for the generation of haploid gametes and the preservation of genomic integrity across generations. Caenorhabditis elegans has emerged as a powerful model for dissecting the molecular and cellular mechanisms of meiosis due to its unique combination of genetic tractability, optical transparency, and synchronized gonad architecture. This review summarizes the key advantages of Caenorhabditis elegans, including its compact and well-annotated genome, versatile genetic manipulation tools (e.g., RNAi, CRISPR/Cas9), and in vivo imaging capabilities that enable high-resolution observation of meiotic events. We highlight recent insights into homolog pairing, synapsis, crossover control, spindle assembly, and meiotic checkpoint regulation, emphasizing the coordinated actions of conserved and nematode-specific factors such as pairing centers, synaptonemal complex components, and regulatory networks involving CHK-2, MAD-1/MAD-2, and PCH-2. While limitations existsuch as its self-fertilizing reproductive mode and holocentric chromosome structureongoing advancements in imaging, multi-omics, and synthetic biology continue to enhance its applicability. Caenorhabditis elegans thus remains a valuable and forward-looking system for unraveling the principles of meiosis with implications for fertility, genome maintenance, and developmental biology.

Meiotic recombination plays an important role in ensuring proper chromosome segregation during meiosis I through the creation of chiasmata that connect homologous chromosomes. Recombination plays an additional role in evolution by creating new allelic combinations. Organisms display species-specific crossover patterns, but how these patterns are established is poorly understood. Drosophila mauritiana displays a different meiotic recombination pattern compared to Drosophila melanogaster, with D. mauritiana experiencing a reduced centromere effect, the suppression of recombination emanating from the centromeres. To evaluate the contribution of the synaptonemal complex (SC) C(3)G protein to these recombination rate differences, the D. melanogaster allele was replaced with D. mauritiana c(3)G coding sequence. We found that the D. mauritiana C(3)G could interact with the D. melanogaster SC machinery to build full length tripartite SC and chromosomes segregated accurately, indicating sufficient crossovers were generated. However, the placement of crossovers was altered, displaying an increase in frequency in the centromere-proximal euchromatin indicating a decrease in the centromere effect, similar to that observed in D. mauritiana females. Recovery of chromatids with more than one crossover was also increased, likely due to the larger chromosome span now available for crossovers. As replacement of a single gene mediated a strong shift of one species' crossover pattern towards another species, it indicates a small number of discrete factors may have major influence on species-specific crossover patterning. Additionally, it demonstrates the SC, a structure known to be required for crossover formation in many species, is likely one of these discrete factors.

Blue-eyed red-fin pleco Hypostomus soniae (family Loricariidae) presented a putative sex system XX/XY in early stage. Aiming to explore the inter-populational karyotypic variation and proposed emergence of the XX/XY system, we studied 13 H. soniae individuals (6 males, 7 females) from the Tapajós River. Mitotic karyotypes and meiotic cells were analysed using C-banding, site verification Ag-NOR (nucleolar organizing region), chromomycin A3 (CMA3) and fluorescent in situ hybridization with repetitive DNA probes (DNAr 18s, 5S, histone H1, H3, telomere). The synaptonemal complex on meiocytes was studied using immunodetection with anti-structural maintenance of chromosomes protein 3, anti-γH2AX and anti-H3K9ac. The karyotype presented was 2n = 64, with clear size heteromorphism varying between male and female. The species presented multiple NORs colocalized with 18S and CMA3-positive marks in two pairs of acrocentric chromosomes. The centromeric region of pairs 25 and 26 carries the repeats of histones H1 and H3. All the bivalents at pachytene exhibited tip-to-tip pairing, revealing the absence of an XY pair with partial synapsis. The synaptic behaviour of the putative sexual pair failed to corroborate the XX/XY system hypothesis in H. soniae. Further cytogenetic and molecular investigations are necessary to determine the proposed emergence of an XX/XY sexual system in H. soniae and the regulatory mechanisms underlying its atypical meiotic behaviour.

The synaptonemal complex (SC) is a meiosis-specific structure that aligns homologous chromosomes and promotes the repair of meiotic DNA double-strand breaks (DSBs). To investigate how defects in SC formation affect gametogenesis in zebrafish, we analyzed mutations in two genes encoding core SC components: syce2 and sycp1. In syce2 mutants, chromosomes exhibit partial synapsis, primarily at sub-telomeric regions, whereas sycp1 mutant chromosomes display early prophase co-alignment but fail to synapse. Both mutants exhibit reduced efficiency in repairing meiotic DSBs compared to wild type. Despite these defects, syce2 and sycp1 mutant females are fertile. However, sycp1 mutant females produce a higher proportion of malformed progeny, correlating with increased univalent formation. While syce2 mutant males are fertile and produce normal offspring, sycp1 mutant males are sterile, with spermatocytes that transit prophase I but arrest at metaphase I or II. Additionally, sycp1 mutants display a male-biased sex ratio that can be suppressed by extending the developmental window for sex determination, suggesting that the absence of synapsis delays-but does not completely block-meiotic progression. Notably, embryos from syce2 and sycp1 mutant females exhibit widespread somatic mosaic aneuploidy, indicating that impaired meiotic chromosome dynamics can compromise genome stability during early development. In contrast to mouse SC mutants, the zebrafish syce2 and sycp1 mutants examined in this study progress through meiotic prophase I with minimal disruption, suggesting a less stringent surveillance mechanism for synapsis errors in zebrafish.

Treatment of patients with platinum-resistant ovarian cancer is a major clinical challenge. We found that high expression of a meiotic protein, Synaptonemal Complex Protein 2 (SYCP2), is associated with platinum resistance and tyrosine kinase ABL1 inhibitor sensitivity in ovarian cancer. We demonstrate that tyrosine kinase ABL1 inhibitors inhibit cancer cell proliferation more efficiently in ovarian cancer cell lines with SYCP2 overexpression. Moreover, ABL1 inhibition effectively prevents tumor growth in vivo. Mechanistically, we identified a phosphorylation motif [RK]-x(2,3)-[DE]-x(2,3)-Y in SYCP2 and found that abolishing ABL1-mediated phosphorylation of SYCP2 at its tyrosine (Y) 739 within this motif renders ABL1 sensitivity of cancer cells. Importantly, ABL1 and SYCP2 colocalize at sites of R-loops after damage and promote transcription-coupled homologous recombination. Moreover, ABL1-mediated Y739 phosphorylation of SYCP2 promotes function of SYCP2 at sites of R-loops by facilitating RAD51 localization and repair, contributing to ovarian cancer cell survival. Overall, these findings highlight a novel therapeutic mechanism where ABL1 inhibitors induce cell death in platinum-resistant ovarian cancer by impairing transcription-coupled homologous recombination repair.

The goal of meiosis is typically to produce haploid gametes (eggs or sperm). Failure to do so is catastrophic for fertility and offspring health. However, Lepidopteran (moths and butterflies) males produce two morphs of sperm: nucleated (eupyrene) sperm which fertilize the egg, and anucleated (apyrene) sperm, both of which are essential for fertilization. The meiotic differences in the two types of spermatogenesis have not been well characterized, and our knowledge of the molecular differences between eupyrene and apyrene spermatogenesis are extremely limited in all systems. The only factor identified as being required for apyrene spermatogenesis is Sex-lethal (Sxl). Here, we show through cytological analysis of early meiotic events that there are several key differences in the genesis of apyrene sperm and eupyrene sperm. Specifically, apyrene spermatocytes fail to condense and pair their chromosomes during meiotic prophase I. In addition, telomeres do not attach to the nuclear envelope. Due to these differences, full-length synaptonemal complex does not form. RNA sequencing of both eupyrene- and apyrene-producing testes revealed distinct changes in transcriptional programs, including down-regulation of a myriad of meiotic genes and cell cycle checkpoint factors during apyrene meiosis. By comparing wild-type and Sxl-knockout apyrene testes, we found that Sxl is not required for the changes in the expression of the meiotic genes but instead plays a role in checkpoint inactivation to allow this error-prone meiosis to proceed. Together, our findings reveal significant insights into two converging molecular pathways that promote the formation of dimorphic sperm in Lepidoptera.

Meiotic crossovers are needed to produce genetically balanced gametes. In mammals, crossover formation is mediated by a conserved set of pro-crossover proteins via mechanisms that remain unclear. Here, we characterize a mammalian pro-crossover factor HEIP1. In mouse HEIP1 is essential for crossing over and fertility of both sexes. HEIP1 promotes crossing over by orchestrating the recruitment of other pro-crossover proteins, including the MutSγ complex (MSH4-MSH5) and E3 ligases (HEI10, RNF212, and RNF212B), that are required to mature crossover sites and recruit the crossover-specific resolution complex MutLγ. Moreover, HEIP1 directly interacts with HEI10, suggesting a direct role in controlling the recruitment of pro-crossover E3 ligases. During early stages of meiotic prophase I, HEIP1 interacts with the chromosome axes, independently of recombination, before relocalizing to the central region of the synaptonemal complex. We propose that HEIP1 is a new conserved master regulator of crossover proteins that controls different crossover maturation steps.