Sexual dimorphism in the width of the mouse synaptonemal complex

Sexual dimorphic regulation of recombination by the synaptonemal complex in C. elegans

In sexually reproducing organisms, germ cells faithfully transmit the genome to the next generation by forming haploid gametes, such as eggs and sperm. Although most meiotic proteins are conserved between eggs and sperm, many aspects of meiosis are sexually dimorphic, including the regulation of recombination. The synaptonemal complex (SC), a large ladder-like structure that forms between homologous chromosomes, is essential for regulating meiotic chromosome organization and promoting recombination. To assess whether sex-specific differences in the SC underpin sexually dimorphic aspects of meiosis, we examined Caenorhabditis elegans SC central region proteins (known as SYP proteins) in oogenesis and spermatogenesis and uncovered sex-specific roles for the SYPs in regulating meiotic recombination. We find that SC composition, specifically SYP-2, SYP-3, SYP-5, and SYP-6, is regulated by sex-specific mechanisms throughout meiotic prophase I. During pachytene, both oocytes and spermatocytes differentially regulate the stability of SYP-2 and SYP-3 within an assembled SC. Further, we uncover that the relative amount of SYP-2 and SYP-3 within the SC is independently regulated in both a sex-specific and a recombination-dependent manner. Specifically, we find that SYP-2 regulates the early steps of recombination in both sexes, while SYP-3 controls the timing and positioning of crossover recombination events across the genomic landscape in only oocytes. Finally, we find that SYP-2 and SYP-3 dosage can influence the composition of the other SYPs in the SC via sex-specific mechanisms during pachytene. Taken together, we demonstrate dosage-dependent regulation of individual SC components with sex-specific functions in recombination. These sexual dimorphic features of the SC provide insights into how spermatogenesis and oogenesis adapted similar chromosome structures to differentially regulate and execute recombination.

Meiotic Chromosome Structure, the Synaptonemal Complex, and Infertility

In meiosis, homologous chromosome synapsis is mediated by a supramolecular protein structure, the synaptonemal complex (SC), that assembles between homologous chromosome axes. The mammalian SC comprises at least eight largely coiled-coil proteins that interact and self-assemble to generate a long, zipper-like structure that holds homologous chromosomes in close proximity and promotes the formation of genetic crossovers and accurate meiotic chromosome segregation. In recent years, numerous mutations in human SC genes have been associated with different types of male and female infertility. Here, we integrate structural information on the human SC with mouse and human genetics to describe the molecular mechanisms by which SC mutations can result in human infertility. We outline certain themes in which different SC proteins are susceptible to different types of disease mutation and how genetic variants with seemingly minor effects on SC proteins may act as dominant-negative mutations in which the heterozygous state is pathogenic.

ATM controls meiotic DNA double-strand break formation and recombination and affects synaptonemal complex organization in plants

Meiosis is a specialized cell division that gives rise to genetically distinct gametic cells. Meiosis relies on the tightly controlled formation of DNA double-strand breaks (DSBs) and their repair via homologous recombination for correct chromosome segregation. Like all forms of DNA damage, meiotic DSBs are potentially harmful and their formation activates an elaborate response to inhibit excessive DNA break formation and ensure successful repair. Previous studies established the protein kinase ATM as a DSB sensor and meiotic regulator in several organisms. Here we show that Arabidopsis ATM acts at multiple steps during DSB formation and processing, as well as crossover (CO) formation and synaptonemal complex (SC) organization, all vital for the successful completion of meiosis. We developed a single-molecule approach to quantify meiotic breaks and determined that ATM is essential to limit the number of meiotic DSBs. Local and genome-wide recombination screens showed that ATM restricts the number of interference-insensitive COs, while super-resolution STED nanoscopy of meiotic chromosomes revealed that the kinase affects chromatin loop size and SC length and width. Our study extends our understanding of how ATM functions during plant meiosis and establishes it as an integral factor of the meiotic program.

Oligopaint DNA FISH reveals telomere-based meiotic pairing dynamics in the silkworm, Bombyx mori

Accurate chromosome segregation during meiosis is essential for reproductive success. Yet, many fundamental aspects of meiosis remain unclear, including the mechanisms regulating homolog pairing across species. This gap is partially due to our inability to visualize individual chromosomes during meiosis. Here, we employ Oligopaint FISH to investigate homolog pairing and compaction of meiotic chromosomes and resurrect a classical model system, the silkworm Bombyx mori. Our Oligopaint design combines multiplexed barcoding with secondary oligo labeling for high flexibility and low cost. These studies illustrate that Oligopaints are highly specific in whole-mount gonads and on meiotic squashes. We show that meiotic pairing is robust in both males and females and that pairing can occur through numerous partially paired intermediate structures. We also show that pairing in male meiosis occurs asynchronously and seemingly in a transcription-biased manner. Further, we reveal that meiotic bivalent formation in B. mori males is highly similar to bivalent formation in C. elegans, with both of these pathways ultimately resulting in the pairing of chromosome ends with non-paired ends facing the spindle pole. Additionally, microtubule recruitment in both C. elegans and B. mori is likely dependent on kinetochore proteins but independent of the centromere-specifying histone CENP-A. Finally, using super-resolution microscopy in the female germline, we show that homologous chromosomes remain associated at telomere domains in the absence of chiasma and after breakdown and modification to the synaptonemal complex in pachytene. These studies reveal novel insights into mechanisms of meiotic homolog pairing both with or without recombination.

High Resolution View on the Regulation of Recombinase Accumulation in Mammalian Meiosis

A distinguishing feature of meiotic DNA double-strand breaks (DSBs), compared to DSBs in somatic cells, is the fact that they are induced in a programmed and specifically orchestrated manner, which includes chromatin remodeling prior to DSB induction. In addition, the meiotic homologous recombination (HR) repair process that follows, is different from HR repair of accidental DSBs in somatic cells. For instance, meiotic HR involves preferred use of the homolog instead of the sister chromatid as a repair template and subsequent formation of crossovers and non-crossovers in a tightly regulated manner. An important outcome of this distinct repair pathway is the pairing of homologous chromosomes. Central to the initial steps in homology recognition during meiotic HR is the cooperation between the strand exchange proteins (recombinases) RAD51 and its meiosis-specific paralog DMC1. Despite our understanding of their enzymatic activity, details on the regulation of their assembly and subsequent molecular organization at meiotic DSBs in mammals have remained largely enigmatic. In this review, we summarize recent mouse data on recombinase regulation via meiosis-specific factors. Also, we reflect on bulk “omics” studies of initial meiotic DSB processing, compare these with studies using super-resolution microscopy in single cells, at single DSB sites, and explore the implications of these findings for our understanding of the molecular mechanisms underlying meiotic HR regulation.

Leagues of their own: sexually dimorphic features of meiotic prophase I

Meiosis is a conserved cell division process that is used by sexually reproducing organisms to generate haploid gametes. Males and females produce different end products of meiosis: eggs (females) and sperm (males). In addition, these unique end products demonstrate sex-specific differences that occur throughout meiosis to produce the final genetic material that is packaged into distinct gametes with unique extracellular morphologies and nuclear sizes. These sexually dimorphic features of meiosis include the meiotic chromosome architecture, in which both the lengths of the chromosomes and the requirement for specific meiotic axis proteins being different between the sexes. Moreover, these changes likely cause sex-specific changes in the recombination landscape with the sex that has the longer chromosomes usually obtaining more crossovers. Additionally, epigenetic regulation of meiosis may contribute to sexually dimorphic recombination landscapes. Here we explore the sexually dimorphic features of both the chromosome axis and crossing over for each stage of meiotic prophase I in Mus musculus, Caenorhabditis elegans, and Arabidopsis thaliana. Furthermore, we consider how sex-specific changes in the meiotic chromosome axes and the epigenetic landscape may function together to regulate crossing over in each sex, indicating that the mechanisms controlling crossing over may be different in oogenesis and spermatogenesis.

Structural basis of meiotic chromosome synapsis through SYCP1 self-assembly

Meiotic chromosomes adopt unique structures in which linear arrays of chromatin loops are bound together in homologous chromosome pairs by a supramolecular protein assembly, the synaptonemal complex. This three-dimensional scaffold provides the essential structural framework for genetic exchange by crossing over and subsequent homolog segregation. The core architecture of the synaptonemal complex is provided by SYCP1. Here we report the structure and self-assembly mechanism of human SYCP1 through X-ray crystallographic and biophysical studies. SYCP1 has an obligate tetrameric structure in which an N-terminal four-helical bundle bifurcates into two elongated C-terminal dimeric coiled-coils. This building block assembles into a zipper-like lattice through two self-assembly sites. N-terminal sites undergo cooperative head-to-head assembly in the midline, while C-terminal sites interact back to back on the chromosome axis. Our work reveals the underlying molecular structure of the synaptonemal complex in which SYCP1 self-assembly generates a supramolecular lattice that mediates meiotic chromosome synapsis.

Pronounced maternal parent-of-origin bias for type-1 NF1 microdeletions

Neurofibromatosis type 1 (NF1) is caused, in 4.7-11% of cases, by large deletions encompassing the NF1 gene and its flanking regions within 17q11.2. Different types of large NF1 deletion occur which are distinguishable by their breakpoint location and underlying mutational mechanism. Most common are the type-1 NF1 deletions of 1.4 Mb which exhibit recurrent breakpoints caused by nonallelic homologous recombination (NAHR), also termed unequal crossover. Here, we analyzed 37 unrelated families of patients with de novo type-1 NF1 deletions by means of short tandem repeat (STR) profiling to determine the parental origin of the deletions. We observed that 33 of the 37 type-1 deletions were of maternal origin (89.2% of cases; p < 0.0001). Analysis of the patients' siblings indicated that, in 14 informative cases, ten (71.4%) deletions resulted from interchromosomal unequal crossover during meiosis I. Our findings indicate a strong maternal parent-of-origin bias for type-1 NF1 deletions. A similarly pronounced maternal transmission bias has been reported for recurrent copy number variants (CNVs) within 16p11.2 associated with autism, but not so far for any other NAHR-mediated pathogenic CNVs. Region-specific genomic features are likely to be responsible for the maternal bias in the origin of both the 16p11.2 CNVs and type-1 NF1 deletions.