The molecular biology of meiosis in plants

The kinase ATR controls meiotic crossover distribution at the genome scale in Arabidopsis

Meiotic crossover, i.e. the reciprocal exchange of chromosome fragments during meiosis, is a key driver of genetic diversity. Crossover is initiated by the formation of programmed DNA double-strand breaks (DSBs). While the role of ATAXIA-TELANGIECTASIA AND RAD3-RELATED (ATR) kinase in DNA damage signaling is well-known, its impact on crossover formation remains understudied. Here, using measurements of recombination at chromosomal intervals and genome-wide crossover mapping, we showed that ATR inactivation in Arabidopsis (Arabidopsis thaliana) leads to dramatic crossover redistribution, with an increase in crossover frequency in chromosome arms and a decrease in pericentromeres. These global changes in crossover placement were not caused by alterations in DSB numbers, which we demonstrated by analyzing phosphorylated H2A.X foci in zygonema. Using the seed-typing technique, we found that hotspot usage remains mainly unchanged in atr mutants compared with wild-type individuals. Moreover, atr showed no change in the number of crossovers caused by two independent pathways, which implies no effect on crossover pathway choice. Analyses of genetic interaction indicate that while the effects of atr are independent of MMS AND UV SENSITIVE81 (MUS81), ZIPPER1 (ZYP1), FANCONI ANEMIA COMPLEMENTATION GROUP M (FANCM), and D2 (FANCD2), the underlying mechanism may be similar between ATR and FANCD2. This study extends our understanding of ATR's role in meiosis, uncovering functions in regulating crossover distribution.

Meiotic crossovers revealed by differential visualization of homologous chromosomes using enhanced haplotype oligo-painting in cucumber

The interaction dynamics of homologous chromosomes during meiosis, such as recognition, pairing, synapsis, recombination, and segregation are vital for species fertility and genetic diversity within populations. Meiotic crossover (CO), a prominent feature of meiosis, ensures the faithful segregation of homologous chromosomes and enriches genetic diversity within a population. Nevertheless, visually distinguishing homologous chromosomes and COs remains an intractable challenge in cytological studies, particularly in non-model or plants with small genomes, limiting insights into meiotic dynamics. In the present study, we developed a robust and reliable enhanced haplotype oligo-painting (EHOP) technique to image small amounts of oligos, enabling visual discrimination of homologous chromosomes. Using EHOP developed based on sequence polymorphisms and reconstructed oligonucleotides, we visually distinguished parental and most recombinant chromosomes in cucumber F1 hybrids and F2 populations. Results from EHOP revealed that meiotic CO events preferentially occur in the 30-60% intervals of chromosome arms with lower sequence polymorphisms and significant recombination bias exists between cultivated and ancestral chromosomes. Due to the occupation of extensive heterochromatin occupancy, it is not yet possible to precisely identify the meiotic COs present in the central portion of chr2 and chr4. Notably, CO accessibility was universally detected in the cytological centromere region in F2 populations, a feature rarely observed in crops with large genomes. EHOP demonstrated exceptional performance in distinguishing homologous chromosomes and holds significant potential for broad application in studying homologous chromosome interactions.

The plant early recombinosome: a high security complex to break DNA during meiosis

KEY MESSAGE

The formacion of numerous unpredictable DNA Double Strand Breaks (DSBs) on chromosomes iniciates meiotic recombination. In this perspective, we propose a 'multi-key lock' model to secure the risky but necesary breaks as well as a 'one per pair of cromatids' model for the topoisomerase-like early recombinosome. During meiosis, homologous chromosomes recombine at few sites of crossing-overs (COs) to ensure correct segregation. The initiation of meiotic recombination involves the formation of DNA double strand breaks (DSBs) during prophase I. Too many DSBs are dangerous for genome integrity: if these DSBs are not properly repaired, it could potentially lead to chromosomal fragmentation. Too few DSBs are also problematic: if the obligate CO cannot form between bivalents, catastrophic unequal segregation of univalents lead to the formation of sterile aneuploid spores. Research on the regulation of the formation of these necessary but risky DSBs has recently advanced in yeast, mammals and plants. DNA DSBs are created by the enzymatic activity of the early recombinosome, a topoisomerase-like complex containing SPO11. This opinion paper reviews recent insights on the regulation of the SPO11 cofactors necessary for the introduction of temporally and spatially controlled DSBs. We propose that a 'multi-key-lock' model for each subunit of the early recombinosome complex is required to secure the formation of DSBs. We also discuss the hypothetical implications that the established topoisomerase-like nature of the SPO11 core-complex can have in creating DSB in only one of the two replicated chromatids of early prophase I meiotic chromosomes. This hypothetical 'one per pair of chromatids' DSB formation model could optimize the faithful repair of the self-inflicted DSBs. Each DSB could use three potential intact homologous DNA sequences as repair template: one from the sister chromatid and the two others from the homologous chromosomes.

Analysis of the winter oilseed rape recombination landscape suggests maternal-paternal bias

Recombination, the reciprocal exchange of DNA between homologous chromosomes, is a mandatory step necessary for meiosis progression. Crossovers between homologous chromosomes generate new combinations of alleles and maintain genetic diversity. Due to genetic, epigenetic, and environmental factors, the recombination landscape is highly heterogeneous along the chromosomes and it also differs between populations and between sexes. Here, we investigated recombination characteristics across the 19 chromosomes of the model allopolyploid crop species oilseed rape (Brassica napus L.), using two unique multiparental populations derived from two genetically divergent founder pools, each of which comprised 50 genetically diverse founder accessions. A fully balanced, pairwise chain-crossing scheme was utilized to create each of the two populations. A total of 3213 individuals, spanning five successive generations, were genotyped using a 15K SNP array. We observed uneven distribution of recombination along chromosomes, with some genomic regions undergoing substantially more frequent recombination in both populations. In both populations, maternal recombination events were more frequent than paternal recombination. This study provides unique insight into the recombination landscape at chromosomal level and reveals a maternal-paternal bias for recombination number with implications for breeding.

FIGL1 attenuates meiotic interhomolog repair and is counteracted by the RAD51 paralog XRCC2 and the chromosome axis protein ASY1 during meiosis

Two recombinases, RAD51 and DMC1, catalyze meiotic break repair to ensure crossovers (COs) between homologous chromosomes (interhomolog) rather than between sisters (intersister). FIDGETIN-LIKE-1 (FIGL1) downregulates both recombinases. However, the understanding of how FIGL1 functions in meiotic repair remains limited. Here, we discover new genetic interactions of Arabidopsis thaliana FIGL1 that are important in vivo determinants of meiotic repair outcome. In figl1 mutants, compromising RAD51-dependent repair, either through the loss of RAD51 paralogs (RAD51B or XRCC2) or RAD54 or by inhibiting RAD51 catalytic activity, results in either unrepaired breaks or meiotic CO defects. Further, XRCC2 physically interacts with FIGL1 and partially counteracts FIGL1 activity for RAD51 focus formation. Our data indicate that RAD51-mediated repair mechanisms compensate FIGL1 dysfunction. FIGL1 is not necessary for intersister repair in dmc1 but is essential for the completion of meiotic repair in mutants such as asy1 that have impaired DMC1 functions and interhomolog bias. We show that FIGL1 attenuates interhomolog repair, and ASY1 counteracts FIGL1 to promote interhomolog recombination. Altogether, this study underlines that multiple factors can counteract FIGL1 activity to promote accurate meiotic repair.

Recombination landscape and karyotypic variations revealed by linkage mapping in the grapevine downy mildew pathogen Plasmopara viticola

Plasmopara viticola, the causal agent of grapevine downy mildew, is a biotrophic oomycete engaged in a tight coevolutionary relationship with its host. Rapid adaptation of the pathogen is favored by annual sexual reproduction that generates genotypic diversity. With the aim of studying the recombination landscape across the P. viticola genome, we generated 2 half-sibling F1 progenies (N = 189 and 162). Using targeted SNP sequencing, between 1,405 and 1,894 markers were included in parental linkage maps, and a consensus map was obtained by integrating 4,509 markers. The reference genome could be assembled into 17 pseudochromosomes, anchoring 88% of its physical length. We observed a strong collinearity between parental genomes and extensive synteny with the downy mildew Peronospora effusa. In the consensus map, the median recombination rate was 13.8 cM/Mb. The local recombination rate was highly variable along chromosomes, and recombination was suppressed in putative centromeric regions. Recombination rate was found negatively correlated with repeats' coverage and positively correlated with gene coverage. However, genes encoding secreted proteins and putative effectors were underrepresented in highly recombining regions. In both progenies, about 5% of the individuals presented karyotypic anomalies. Aneuploidies and triploidies almost exclusively originated from the male-transmitted chromosomes. Triploids resulted from fertilization by diploid gametes, but also from dispermy. Obligatory sexual reproduction each year may explain the lower level of karyotypic variation in P. viticola compared to other oomycetes. The linkage maps will be useful to guide future de novo chromosome-scale assemblies of P. viticola genomes and to perform forward genetics.

Identification and Gene Fine Mapping of the Bisexual Sterility Mutant Meiosis Abnormal Bisexual Sterility 1 in Rice

Exploring the genes regulating rice fertility is of great value for studying the molecular mechanisms of rice reproductive development and production practices. In this study, we identified a sterile mutant from the mutant library induced by ethyl methanesulfonate (EMS), designated as meiosis abnormal bisexual sterility 1 (mabs1). The mabs1 mutant exhibits no phenotypic differences from the wild-type during the vegetative growth phase but shows complete sterility during the reproductive growth phase. Phenotypic observations revealed that both pollen and embryo sac fertility are lost in mabs1. Notably, in mabs1, the development of the anther inner and outer walls, tapetum degeneration, and callose synthesis and degradation all proceed normally, yet meiosis fails to form normal tetrads. Genetic analysis indicated that this mutant trait is controlled by a single recessive nuclear gene. By constructing a genetic segregation population, we successfully mapped the MABS1 gene to a 49 kb region between primer markers Y7 and Y9 on chromosome 1. Resequencing revealed a single-nucleotide substitution in the exon of the LOC_Os01g66170 gene, which resulted in a change from Valine to Isoleucine. Subsequent sequencing of this locus in both wild-type and mabs1 mutants confirmed this mutation. Therefore, we have identified the gene at LOC_Os01g66170 as a candidate for MABS1, a previously unreported novel gene involved in rice meiosis. Through RT-qPCR, we found that the expression levels of multiple meiosis-related genes were significantly changed in the mabs1 mutant. Therefore, we believe that MABS1 is also involved in the process of rice meiosis. This study lays the groundwork for a functional study of MABS1.

Kinesin-1-like protein PSS1 is essential for full-length homologous pairing and synapsis in rice meiosis

The pairing and synapsis of homologous chromosomes are crucial for their correct segregation during meiosis. The LINC (Linker of Nucleoskeleton and Cytoskeleton) complex can recruit kinesin protein at the nuclear envelope, affecting telomere bouquet formation and homologous pairing. Kinesin-1-like protein Pollen Semi-Sterility1 (PSS1) plays a pivotal role in male meiotic chromosomal behavior and is essential for fertility in rice. However, its exact role in meiosis, especially as kinesin involved in homologous pairing and synapsis, has not been fully elucidated. Here, we generated three pss1 mutants by genome editing technology to dissect PSS1 biological functions in meiosis. The pss1 mutants exhibit alterations in the radial microtubule organization at pachytene and manifest a deficiency in telomere clustering, which is critical for full-length homologous pairing. We reveal that PSS1 serves as a key mediator between chromosomes and cytoskeleton, thereby regulating microtubule organization and transmitting the force to nuclei to facilitate homologous chromosome pairing and synapsis in meiosis.

Interference length reveals regularity of crossover placement across species

Crossover interference is a phenomenon that affects the number and positioning of crossovers in meiosis and thus affects genetic diversity and chromosome segregation. Yet, the underlying mechanism is not fully understood, partly because quantification is difficult. To overcome this challenge, we introduce the interference length Lint that quantifies changes in crossover patterning due to interference. We show that it faithfully captures known aspects of crossover interference and provides superior statistical power over previous measures such as the interference distance and the gamma shape parameter. We apply our analysis to empirical data and unveil a similar behavior of Lint across species, which hints at a common mechanism. A recently proposed coarsening model generally captures these aspects, providing a unified view of crossover interference. Consequently, Lint facilitates model refinements and general comparisons between alternative models of crossover interference.

Meiosis as a mechanism for epigenetic reprogramming and cellular rejuvenation

Meiosis is a hallmark of sexual reproduction because it represents the transition from one life cycle to the next and, in animals, meiosis produces gametes. Why meiosis evolved has been debated and most studies have focused on recombination of the parental alleles as the main function of meiosis. However, 40 years ago, Robin Holliday proposed that an essential function of meiosis is to oppose the consequence of successive mitoses that cause cellular aging. Cellular aging results from accumulated defective organelles and proteins and modifications of chromatin in the form of DNA methylation and histone modifications referred to collectively as epigenetic marks. Here, recent findings supporting the hypothesis that meiosis opposes cellular aging are reviewed and placed in the context of the diversity of the life cycles of eukaryotes, including animals, yeast, flowering plants and the bryophyte Marchantia.

Alternating between even and odd ploidy levels switches on and off the recombination control, even near the centromeres

Meiotic recombination is a key biological process in plant evolution and breeding, as it generates genetic diversity in each generation through the formation of crossovers (COs). However, due to their importance in genome stability, COs are highly regulated in frequency and distribution. We previously demonstrated that this strict regulation of COs can be modified, both in terms of CO frequency and distribution, in allotriploid Brassica hybrids (2n = 3x = 29; AAC) resulting from a cross between Brassica napus (2n = 4x = 38; AACC) and Brassica rapa (2n = 2x = 20; AA). Using the recently updated B. napus genome now including pericentromeres, we demonstrated that COs occur in these cold regions in allotriploids, as close as 375 kb from the centromere. Reverse transcription quantitative PCR (RT-qPCR) of various meiotic genes indicated that Class I COs are likely involved in the increased recombination frequency observed in allotriploids. We also demonstrated that this modified recombination landscape can be maintained via successive generations of allotriploidy (odd ploidy level). This deregulated meiotic behavior reverts to strict regulation in allotetraploid (even ploidy level) progeny in the second generation. Overall, we provide an easy way to manipulate tight recombination control in a polyploid crop.

FIGL1 prevents aberrant chromosome associations and fragmentation and limits crossovers in polyploid wheat meiosis

Meiotic crossovers (COs) generate genetic diversity and are crucial for viable gamete production. Plant COs are typically limited to 1-3 per chromosome pair, constraining the development of improved varieties, which in wheat is exacerbated by an extreme distal localisation bias. Advances in wheat genomics and related technologies provide new opportunities to investigate, and possibly modify, recombination in this important crop species. Here, we investigate the disruption of FIGL1 in tetraploid and hexaploid wheat as a potential strategy for modifying CO frequency/position. We analysed figl1 mutants and virus-induced gene silencing lines cytogenetically. Genetic mapping was performed in the hexaploid. FIGL1 prevents abnormal meiotic chromosome associations/fragmentation in both ploidies. It suppresses class II COs in the tetraploid such that CO/chiasma frequency increased 2.1-fold in a figl1 msh5 quadruple mutant compared with a msh5 double mutant. It does not appear to affect class I COs based on HEI10 foci counts in a hexaploid figl1 triple mutant. Genetic mapping in the triple mutant suggested no significant overall increase in total recombination across examined intervals but revealed large increases in specific individual intervals. Notably, the tetraploid figl1 double mutant was sterile but the hexaploid triple mutant was moderately fertile, indicating potential utility for wheat breeding.

HEI10 coarsening, chromatin and sequence polymorphism shape the plant meiotic recombination landscape

Meiosis is a conserved eukaryotic cell division that produces spores required for sexual reproduction. During meiosis, chromosomes pair and undergo programmed DNA double-strand breaks, followed by homologous repair that can result in reciprocal crossovers. Crossover formation is highly regulated with typically few events per homolog pair. Crossovers additionally show wider spacing than expected from uniformly random placement - defining the phenomenon of interference. In plants, the conserved HEI10 E3 ligase is initially loaded along meiotic chromosomes, before maturing into a small number of foci, corresponding to crossover locations. We review the coarsening model that explains these dynamics as a diffusion and aggregation process, resulting in approximately evenly spaced HEI10 foci. We review how underlying chromatin states, and the presence of interhomolog polymorphisms, shape the meiotic recombination landscape, in light of the coarsening model. Finally, we consider future directions to understand the control of meiotic recombination in plant genomes.

ASYNAPSIS3 has diverse dosage-dependent effects on meiotic crossover formation in Brassica napus

Crossovers create genetic diversity and are required for equal chromosome segregation during meiosis. Crossover number and distribution are highly regulated by different mechanisms that are not yet fully understood, including crossover interference. The chromosome axis is crucial for crossover formation. Here, we explore the function of the axis protein ASYNAPSIS3. To this end, we use the allotetraploid species Brassica napus; due to its polyploid nature, this system allows a fine-grained dissection of the dosage of meiotic regulators. The simultaneous mutation of all 4 ASY3 alleles results in defective synapsis and drastic reduction of crossovers, which is largely rescued by the presence of only one functional ASY3 allele. Crucially, while the number of class I crossovers in mutants with 2 functional ASY3 alleles is comparable to that in wild type, this number is significantly increased in mutants with only one functional ASY3 allele, indicating that reducing ASY3 dosage increases crossover formation. Moreover, the class I crossovers on each bivalent in mutants with 1 functional ASY3 allele follow a random distribution, indicating compromised crossover interference. These results reveal the distinct dosage-dependent effects of ASY3 on crossover formation and provide insights into the role of the chromosome axis in patterning recombination.

Contribution of homoeologous exchange to domestication of polyploid Brassica

BACKGROUND

Polyploidy is widely recognized as a significant evolutionary force in the plant kingdom, contributing to the diversification of plants. One of the notable features of allopolyploidy is the occurrence of homoeologous exchange (HE) events between the subgenomes, causing changes in genomic composition, gene expression, and phenotypic variations. However, the role of HE in plant adaptation and domestication remains unclear.

RESULTS

Here we analyze the whole-genome resequencing data from Brassica napus accessions representing the different morphotypes and ecotypes, to investigate the role of HE in domestication. Our findings demonstrate frequent occurrence of HEs in Brassica napus, with substantial HE patterns shared across populations, indicating their potential role in promoting crop domestication. HE events are asymmetric, with the A genome more frequently replacing C genome segments. These events show a preference for specific genomic regions and vary among populations. We also identify candidate genes in HE regions specific to certain populations, which likely contribute to flowering-time diversification across diverse morphotypes and ecotypes. In addition, we assemble a new genome of a swede accession, confirming the HE signals on the genome and their potential involvement in root tuber development. By analyzing HE in another allopolyploid species, Brassica juncea, we characterize a potential broader role of HE in allopolyploid crop domestication.

CONCLUSIONS

Our results provide novel insights into the domestication of polyploid Brassica species and highlight homoeologous exchange as a crucial mechanism for generating variations that are selected for crop improvement in polyploid species.

FIGNL1-FIRRM is essential for meiotic recombination and prevents DNA damage-independent RAD51 and DMC1 loading

During meiosis, nucleoprotein filaments of the strand exchange proteins RAD51 and DMC1 are crucial for repairing SPO11-generated DNA double-strand breaks (DSBs) by homologous recombination (HR). A balanced activity of positive and negative RAD51/DMC1 regulators ensures proper recombination. Fidgetin-like 1 (FIGNL1) was previously shown to negatively regulate RAD51 in human cells. However, FIGNL1's role during meiotic recombination in mammals remains unknown. Here, we decipher the meiotic functions of FIGNL1 and FIGNL1 Interacting Regulator of Recombination and Mitosis (FIRRM) using male germline-specific conditional knock-out (cKO) mouse models. Both FIGNL1 and FIRRM are required for completing meiotic prophase in mouse spermatocytes. Despite efficient recruitment of DMC1 on ssDNA at meiotic DSB hotspots, the formation of late recombination intermediates is defective in Firrm cKO and Fignl1 cKO spermatocytes. Moreover, the FIGNL1-FIRRM complex limits RAD51 and DMC1 accumulation on intact chromatin, independently from the formation of SPO11-catalyzed DSBs. Purified human FIGNL1ΔN alters the RAD51/DMC1 nucleoprotein filament structure and inhibits strand invasion in vitro. Thus, this complex might regulate RAD51 and DMC1 association at sites of meiotic DSBs to promote proficient strand invasion and processing of recombination intermediates.

Heating up meiosis - Chromosome recombination and segregation under high temperatures

Heat stress is one of the major constraints to plant growth and fertility. During the current climate crisis, heat waves have increased dramatically, and even more extreme conditions are predicted for the near future, considerably affecting ecosystems and seriously threatening world food security. Although heat is very well known to affect especially reproductive structures, little is known about how heat interferes with reproduction in comparison to somatic cells and tissues. Recently, the effect of heat on meiosis as a central process in sexual reproduction has been analyzed in molecular and cytological depth. Notably, these studies are not only important for applied research by laying the foundation for breeding heat-resilient crops, but also for fundamental research, revealing general regulatory mechanisms of recombination and chromosome segregation control.

MutL homolog 1 participates in interference-sensitive meiotic crossover formation in soybean

MutL homolog 1 (MLH1), a member of the MutL homolog family, is required for normal recombination in most organisms. However, its role in soybean (Glycine max) remains unclear to date. Here, we characterized the Glycine max female and male sterility 1 (Gmfms1) mutation that reduces pollen grain viability and increases embryo sac abortion in soybean. Map-based cloning revealed that the causal gene of Gmfms1 is Glycine max MutL homolog 1 (GmMLH1), and CRISPR/Cas9 knockout approach further validated that disruption of GmMLH1 confers the female-male sterility phenotype in soybean. Loss of GmMLH1 function disrupted bivalent formation, leading to univalent mis-segregation during meiosis and ultimately to female-male sterility. The Gmmlh1 mutant showed about a 78.16% decrease in meiotic crossover frequency compared to the wild type. The residual chiasmata followed a Poisson distribution, suggesting that interference-sensitive crossover formation was affected in the Gmmlh1 mutant. Furthermore, GmMLH1 could interact with GmMLH3A and GmMLH3B both in vivo and in vitro. Overall, our work demonstrates that GmMLH1 participates in interference-sensitive crossover formation in soybean, and provides additional information about the conserved functions of MLH1 across plant species.

Enhanced recombination empowers the detection and mapping of Quantitative Trait Loci

Modern plant breeding, such as genomic selection and gene editing, is based on the knowledge of the genetic architecture of desired traits. Quantitative trait loci (QTL) analysis, which combines high throughput phenotyping and genotyping of segregating populations, is a powerful tool to identify these genetic determinants and to decipher the underlying mechanisms. However, meiotic recombination, which shuffles genetic information between generations, is limited: Typically only one to two exchange points, called crossovers, occur between a pair of homologous chromosomes. Here we test the effect on QTL analysis of boosting recombination, by mutating the anti-crossover factors RECQ4 and FIGL1 in Arabidopsis thaliana full hybrids and lines in which a single chromosome is hybrid. We show that increasing recombination ~6-fold empowers the detection and resolution of QTLs, reaching the gene scale with only a few hundred plants. Further, enhanced recombination unmasks some secondary QTLs undetected under normal recombination. These results show the benefits of enhanced recombination to decipher the genetic bases of traits.

A catalogue of recombination coldspots in interspecific tomato hybrids

Increasing natural resistance and resilience in plants is key for ensuring food security within a changing climate. Breeders improve these traits by crossing cultivars with their wild relatives and introgressing specific alleles through meiotic recombination. However, some genomic regions are devoid of recombination especially in crosses between divergent genomes, limiting the combinations of desirable alleles. Here, we used pooled-pollen sequencing to build a map of recombinant and non-recombinant regions between tomato and five wild relatives commonly used for introgressive tomato breeding. We detected hybrid-specific recombination coldspots that underscore the role of structural variations in modifying recombination patterns and maintaining genetic linkage in interspecific crosses. Crossover regions and coldspots show strong association with specific TE superfamilies exhibiting differentially accessible chromatin between somatic and meiotic cells. About two-thirds of the genome are conserved coldspots, located mostly in the pericentromeres and enriched with retrotransposons. The coldspots also harbor genes associated with agronomic traits and stress resistance, revealing undesired consequences of linkage drag and possible barriers to breeding. We presented examples of linkage drag that can potentially be resolved by pairing tomato with other wild species. Overall, this catalogue will help breeders better understand crossover localization and make informed decisions on generating new tomato varieties.

SCC3 is an axial element essential for homologous chromosome pairing and synapsis

Cohesin is a multi-subunit protein that plays a pivotal role in holding sister chromatids together during cell division. Sister chromatid cohesion 3 (SCC3), constituents of cohesin complex, is highly conserved from yeast to mammals. Since the deletion of individual cohesin subunit always causes lethality, it is difficult to dissect its biological function in both mitosis and meiosis. Here, we obtained scc3 weak mutants using CRISPR-Cas9 system to explore its function during rice mitosis and meiosis. The scc3 weak mutants displayed obvious vegetative defects and complete sterility, underscoring the essential roles of SCC3 in both mitosis and meiosis. SCC3 is localized on chromatin from interphase to prometaphase in mitosis. However, in meiosis, SCC3 acts as an axial element during early prophase I and subsequently situates onto centromeric regions following the disassembly of the synaptonemal complex. The loading of SCC3 onto meiotic chromosomes depends on REC8. scc3 shows severe defects in homologous pairing and synapsis. Consequently, SCC3 functions as an axial element that is essential for maintaining homologous chromosome pairing and synapsis during meiosis.

Apomixis and the paradox of sex in plants

BACKGROUND

The predominance of sex in eukaryotes, despite the high costs of meiosis and mating, remains an evolutionary enigma. Many theories have been proposed, none of them being conclusive on its own, and they are, in part, not well applicable to land plants. Sexual reproduction is obligate in embryophytes for the great majority of species.

SCOPE

This review compares the main forms of sexual and asexual reproduction in ferns and angiosperms, based on the generation cycling of sporophyte and gametophyte (leaving vegetative propagation aside). The benefits of sexual reproduction for maintenance of genomic integrity in comparison to asexuality are discussed in the light of developmental, evolutionary, genetic and phylogenetic studies.

CONCLUSIONS

Asexual reproduction represents modifications of the sexual pathway, with various forms of facultative sexuality. For sexual land plants, meiosis provides direct DNA repair mechanisms for oxidative damage in reproductive tissues. The ploidy alternations of meiosis-syngamy cycles and prolonged multicellular stages in the haploid phase in the gametophytes provide a high efficiency of purifying selection against recessive deleterious mutations. Asexual lineages might buffer effects of such mutations via polyploidy and can purge the mutational load via facultative sexuality. The role of organelle-nuclear genome compatibility for maintenance of genome integrity is not well understood. In plants in general, the costs of mating are low because of predominant hermaphroditism. Phylogenetic patterns in the archaeplastid clade suggest that high frequencies of sexuality in land plants are concomitant with a stepwise increase of intrinsic and extrinsic stress factors. Furthermore, expansion of genome size in land plants would increase the potential mutational load. Sexual reproduction appears to be essential for keeping long-term genomic integrity, and only rare combinations of extrinsic and intrinsic factors allow for shifts to asexuality.

A word of caution: T-DNA-associated mutagenesis in plant reproduction research

T-DNA transformation is prevalent in Arabidopsis research and has expanded to a broad range of crops and model plants. While major progress has been made in optimizing the Agrobacterium-mediated transformation process for various species, a variety of pitfalls associated with the T-DNA insertion may lead to the misinterpretation of T-DNA mutant analysis. Indeed, secondary mutagenesis either on the integration site or elsewhere in the genome, together with epigenetic interactions between T-DNA inserts or frequent genomic rearrangements, can be tricky to differentiate from the effect of the knockout of the gene of interest. These are mainly the case for genomic rearrangements that become balanced in filial generations without consequential phenotypical defects, which may be confusing particularly for studies that aim to investigate fertility and gametogenesis. As a cautionary note to the plant research community studying gametogenesis, we here report an overview of the consequences of T-DNA-induced secondary mutagenesis with emphasis on the genomic imbalance on gametogenesis. Additionally, we present a simple guideline to evaluate the T-DNA-mutagenized transgenic lines to decrease the risk of faulty analysis with minimal experimental effort.

Meiosis through three centuries

Meiosis is the specialized cellular program that underlies gamete formation for sexual reproduction. It is therefore not only interesting but also a fundamentally important subject for investigation. An especially attractive feature of this program is that many of the processes of special interest involve organized chromosomes, thus providing the possibility to see chromosomes "in action". Analysis of meiosis has also proven to be useful in discovering and understanding processes that are universal to all chromosomal programs. Here we provide an overview of the different historical moments when the gap between observation and understanding of mechanisms and/or roles for the new discovered molecules was bridged. This review reflects also the synergy of thinking and discussion among our three laboratories during the past several decades.

Molecular mechanisms and regulation of recombination frequency and distribution in plants

KEY MESSAGE

Recent developments in understanding the distribution and distinctive features of recombination hotspots are reviewed and approaches are proposed to increase recombination frequency in coldspot regions. Recombination events during meiosis provide the foundation and premise for creating new varieties of crops. The frequency of recombination in different genomic regions differs across eukaryote species, with recombination generally occurring more frequently at the ends of chromosomes. In most crop species, recombination is rare in centromeric regions. If a desired gene variant is linked in repulsion with an undesired variant of a second gene in a region with a low recombination rate, obtaining a recombinant plant combining two favorable alleles will be challenging. Traditional crop breeding involves combining desirable genes from parental plants into offspring. Therefore, understanding the mechanisms of recombination and factors affecting the occurrence of meiotic recombination is important for crop breeding. Here, we review chromosome recombination types, recombination mechanisms, genes and proteins involved in the meiotic recombination process, recombination hotspots and their regulation systems and discuss how to increase recombination frequency in recombination coldspot regions.

Absence of chromosome axis protein recruitment prevents meiotic recombination chromosome-wide in the budding yeast Lachancea kluyveri

Meiotic recombination shows broad variations across species and along chromosomes and is often suppressed at and around genomic regions determining sexual compatibility such as mating type loci in fungi. Here, we show that the absence of Spo11-DSBs and meiotic recombination on Lakl0C-left, the chromosome arm containing the sex locus of the Lachancea kluyveri budding yeast, results from the absence of recruitment of the two chromosome axis proteins Red1 and Hop1, essential for proper Spo11-DSBs formation. Furthermore, cytological observation of spread pachytene meiotic chromosomes reveals that Lakl0C-left does not undergo synapsis. However, we show that the behavior of Lakl0C-left is independent of its particularly early replication timing and is not accompanied by any peculiar chromosome structure as detectable by Hi-C in this yet poorly studied yeast. Finally, we observed an accumulation of heterozygous mutations on Lakl0C-left and a sexual dimorphism of the haploid meiotic offspring, supporting a direct effect of this absence of meiotic recombination on L. kluyveri genome evolution and fitness. Because suppression of meiotic recombination on sex chromosomes is widely observed across eukaryotes, the mechanism for recombination suppression described here may apply to other species, with the potential to impact sex chromosome evolution.

Meiosis: The silk moth and the elephant

In most eukaryotes, balanced chromosome segregation at meiosis requires crossovers, but female Bombyx mori lack these structures. Instead, the synaptonemal complex is repurposed to compensate for this absence of crossovers, a remarkable example of exaptation.

How plants with hundreds of centromeres crossover

Beaksedges harbour multiple centromeres in each chromosome, yet crossover distribution is distally biased, like in monocentric species, but with no correlation with (epi)genomic features. This study suggests that synapsis dynamics starting from chromosomal ends is key to the recombination pattern.

ZmTDM1 encodes a tetratricopeptide repeat domain protein and is required for meiotic exit in maize

Elaborate cell-cycle control must be adopted to ensure the continuity of the meiotic second division and termination after that. Despite its importance, however, the genetic controls underlying the meiotic cell cycle have not been reported in maize. Here, we characterized a meiotic cell-cycle controller ZmTDM1, which is a homolog of Arabidopsis TDM1 and encodes a canonical tetratricopeptide repeat domain protein in maize. The Zmtdm1 homozygous plants exhibited complete male sterility and severe female abortion. In Zmtdm1 mutants, cell-cycle progression was almost identical to that of wild type from leptotene to anaphase II. However, chromosomes in the tetrad failed meiotic termination at the end of the second division and underwent additional divisions in succession without DNA replication, reducing the ploidy to less than haploid in the product. In addition, two ZmTDM1-like homologs (ZmTDML1 and ZmTDML2) were not functional in meiotic cell-cycle control. Moreover, ZmTDM1 interacted with RING-type E3 ubiquitin ligase, revealing that it acts as a subunit of the APC/C E3 ubiquitin ligase complex. Overall, our results identified a regulator of meiotic cell cycle in maize and demonstrated that ZmTDM1 is essential for meiotic exit after meiosis II.

Control of meiotic crossover interference by a proteolytic chaperone network

Meiosis is a specialized eukaryotic division that produces genetically diverse gametes for sexual reproduction. During meiosis, homologous chromosomes pair and undergo reciprocal exchanges, called crossovers, which recombine genetic variation. Meiotic crossovers are stringently controlled with at least one obligate exchange forming per chromosome pair, while closely spaced crossovers are inhibited by interference. In Arabidopsis, crossover positions can be explained by a diffusion-mediated coarsening model, in which large, approximately evenly spaced foci of the pro-crossover E3 ligase HEI10 grow at the expense of smaller, closely spaced clusters. However, the mechanisms that control HEI10 dynamics during meiosis remain unclear. Here, through a forward genetic screen in Arabidopsis, we identified high crossover rate3 (hcr3), a dominant-negative mutant that reduces crossover interference and increases crossovers genome-wide. HCR3 encodes J3, a co-chaperone related to HSP40, which acts to target protein aggregates and biomolecular condensates to the disassembly chaperone HSP70, thereby promoting proteasomal degradation. Consistently, we show that a network of HCR3 and HSP70 chaperones facilitates proteolysis of HEI10, thereby regulating interference and the recombination landscape. These results reveal a new role for the HSP40/J3-HSP70 chaperones in regulating chromosome-wide dynamics of recombination via control of HEI10 proteolysis.

Meiotic recombination dynamics in plants with repeat-based holocentromeres shed light on the primary drivers of crossover patterning

Centromeres strongly affect (epi)genomic architecture and meiotic recombination dynamics, influencing the overall distribution and frequency of crossovers. Here we show how recombination is regulated and distributed in the holocentric plant Rhynchospora breviuscula, a species with diffused centromeres. Combining immunocytochemistry, chromatin analysis and high-throughput single-pollen sequencing, we discovered that crossover frequency is distally biased, in sharp contrast to the diffused distribution of hundreds of centromeric units and (epi)genomic features. Remarkably, we found that crossovers were abolished inside centromeric units but not in their proximity, indicating the absence of a canonical centromere effect. We further propose that telomere-led synapsis of homologues is the feature that best explains the observed recombination landscape. Our results hint at the primary influence of mechanistic features of meiotic pairing and synapsis rather than (epi)genomic features and centromere organization in determining the distally biased crossover distribution in R. breviuscula, whereas centromeres and (epi)genetic properties only affect crossover positioning locally.

Incorrect recombination partner associations contribute to meiotic instability of neo-allopolyploid Arabidopsis suecica

Combining two or more related homoeologous genomes in a single nucleus, newly formed allopolyploids must rapidly adapt meiosis to restore balanced chromosome segregation, production of euploid gametes and fertility. The poor fertility of such neo-allopolyploids thus strongly selects for the limitation or avoidance of genetic crossover formation between homoeologous chromosomes. In this study, we have reproduced the interspecific hybridization between Arabidopsis thaliana and Arabidopsis arenosa leading to the allotetraploid Arabidopsis suecica and have characterized the first allopolyploid meioses. First-generation neo-allopolyploid siblings vary considerably in fertility, meiotic behavior and levels of homoeologous recombination. We show that centromere dynamics at early meiosis is altered in synthetic neo-allopolyploids compared with evolved A. suecica, with a significant increase in homoeologous centromere interactions at zygotene. At metaphase I, the presence of multivalents involving homoeologous chromosomes confirms that homoeologous recombination occurs in the first-generation synthetic allopolyploid plants and this is associated with a significant reduction in homologous recombination, compared to evolved A. suecica. Together, these data strongly suggest that the fidelity of recombination partner choice, likely during the DNA invasion step, is strongly impaired during the first meiosis of neo-allopolyploids and requires subsequent adaptation.

TaRECQ4 contributes to maintain both homologous and homoeologous recombination during wheat meiosis

Introduction

Meiotic recombination (or crossover, CO) is essential for gamete fertility as well as for alleles and genes reshuffling that is at the heart of plant breeding. However, CO remains a limited event, which strongly hampers the rapid production of original and improved cultivars. RecQ4 is a gene encoding a helicase protein that, when mutated, contributes to improve recombination rate in all species where it has been evaluated so far.

Methods

In this study, we developed wheat (Triticum aestivum L.) triple mutant (TM) for the three homoeologous copies of TaRecQ4 as well as mutants for two copies and heterozygous for the last one (Htz-A, Htz-B, Htz-D).

Results

Phenotypic observation revealed a significant reduction of fertility and pollen viability in TM and Htz-B plants compared to wild type plants suggesting major defects during meiosis. Cytogenetic analyses of these plants showed that complete absence of TaRecQ4 as observed in TM plants, leads to chromosome fragmentation during the pachytene stage, resulting in problems in the segregation of chromosomes during meiosis. Htz-A and Htz-D mutants had an almost normal meiotic progression indicating that both TaRecQ4-A and TaRecQ4-D copies are functional and that there is no dosage effect for TaRecQ4 in bread wheat. On the contrary, the TaRecQ4-B copy seems knocked-out, probably because of a SNP leading to a Threonine>Alanine change at position 539 (T539A) of the protein, that occurs in the crucial helicase ATP bind/DEAD/ResIII domain which unwinds nucleic acids. Occurrence of numerous multivalents in TM plants suggests that TaRecQ4 could also play a role in the control of homoeologous recombination.

Discussion

These findings provide a foundation for further molecular investigations into wheat meiosis regulation to fully understand the underlying mechanisms of how TaRecQ4 affects chiasma formation, as well as to identify ways to mitigate these defects and enhance both homologous and homoeologous recombination efficiency in wheat.

Structural variation and DNA methylation shape the centromere-proximal meiotic crossover landscape in Arabidopsis

BACKGROUND

Centromeres load kinetochore complexes onto chromosomes, which mediate spindle attachment and allow segregation during cell division. Although centromeres perform a conserved cellular function, their underlying DNA sequences are highly divergent within and between species. Despite variability in DNA sequence, centromeres are also universally suppressed for meiotic crossover recombination, across eukaryotes. However, the genetic and epigenetic factors responsible for suppression of centromeric crossovers remain to be completely defined.

RESULTS

To explore the centromere-proximal meiotic recombination landscape, we map 14,397 crossovers against fully assembled Arabidopsis thaliana (A. thaliana) genomes. A. thaliana centromeres comprise megabase satellite repeat arrays that load nucleosomes containing the CENH3 histone variant. Each chromosome contains a structurally polymorphic region of ~3-4 megabases, which lack crossovers and include the satellite arrays. This polymorphic region is flanked by ~1-2 megabase low-recombination zones. These recombination-suppressed regions are enriched for Gypsy/Ty3 retrotransposons, and additionally contain expressed genes with high genetic diversity that initiate meiotic recombination, yet do not crossover. We map crossovers at high-resolution in proximity to CEN3, which resolves punctate centromere-proximal hotspots that overlap gene islands embedded in heterochromatin. Centromeres are densely DNA methylated and the recombination landscape is remodelled in DNA methylation mutants. We observe that the centromeric low-recombining zones decrease and increase crossovers in CG (met1) and non-CG (cmt3) mutants, respectively, whereas the core non-recombining zones remain suppressed.

CONCLUSION

Our work relates the genetic and epigenetic organization of A. thaliana centromeres and flanking pericentromeric heterochromatin to the zones of crossover suppression that surround the CENH3-occupied satellite repeat arrays.

Temporal and Functional Relationship between Synaptonemal Complex Morphogenesis and Recombination during Meiosis

During prophase of meiosis I, programmed double strand breaks (DSBs) are processed into crossovers, a critical requirement for segregation of homologous chromosomes (homologs) and genome haploidization in sexually reproducing organisms. Crossovers form via homologous recombination in close temporospatial association with morphogenesis of the synaptonemal complex (SC), a proteinaceous structure that connects paired homologs along their length during the pachytene stage. Synapsis and recombination are a paradigm for the interplay between higher order chromosome structure and DNA metabolism, yet their temporal and functional relationship remains poorly understood. Probing linkage between these processes in budding yeast, we show that SC assembly is associated with a distinct threshold number of unstable D-loops. The transition from bona fide paranemic D-loops to plectonemic DSB single end invasions (SEIs) is completed during midpachynema, when the SC is fully assembled. Double Holliday junctions (dHJs) form at the time of desynapsis and are resolved into crossovers during diplonema. The SC central element component Zip1 shepherds recombination through three transitions, including DSB first end strand exchange and second end capture, as well as dHJ resolution. Zip1 mediates SEI formation independent of its polymerization whereas precocious Zip1 assembly interferes with double Holliday junction resolution. Together, our findings indicate that the synaptonemal complex controls recombination while assembled but also beyond its disassembly, possibly by establishing spatial constraints at recombination sites.

The RING Domain of Rice HEI10 is Essential for Male, But Not Female Fertility

HEI10 is a conserved E3 ubiquitin ligase involved in crossover formation during meiosis, and is thus essential for both male and female gamete development. Here, we have discovered a novel allele of HEI10 in rice that produces a truncated HEI10 protein missing its N-terminal RING domain, namely sh1 (shorter hei10 1). Unlike previously reported hei10 null alleles that are completely sterile, sh1 exhibits complete male sterility but retains partial female fertility. The causative sh1 mutation is a 76 kb inversion between OsFYVE4 and HEI10, which breaks the integrity of both genes. Allelic tests and complementation assays revealed that the gamete developmental defects of sh1 were caused by disruption of HEI10. Further studies demonstrated that short HEI10 can correctly localise to the nucleus, where it could interact with other proteins that direct meiosis; expressing short HEI10 in hei10 null lines partially restores female fertility. Our data reveal an intriguing mutant allele of HEI10 with differential effects on male and female fertility, providing a new tool to explore similarities and differences between male and female meiosis.

Crossover shortage in potato is caused by StMSH4 mutant alleles and leads to either highly uniform unreduced pollen or sterility

The balanced segregation of homologous chromosomes during meiosis is essential for fertility and is mediated by crossovers (COs). A strong reduction of CO number leads to the unpairing of homologous chromosomes after the withdrawal of the synaptonemal complex. This results in the random segregation of univalents during meiosis I and ultimately to the production of unbalanced and sterile gametes. However, if CO shortage is combined with another meiotic alteration that restitutes the first meiotic division, then uniform and balanced unreduced male gametes, essentially composed of nonrecombinant homologs, are produced. This mitosis-like division is of interest to breeders because it transmits most of the parental heterozygosity to the gametes. In potato, CO shortage, a recessive trait previously referred to as desynapsis, was tentatively mapped to chromosome 8. In this article, we have fine-mapped the position of the CO shortage locus and identified StMSH4, an essential component of the class I CO pathway, as the most likely candidate gene. A 7 base-pair insertion in the second exon of StMSH4 was found to be associated with CO shortage in our mapping population. We also identified a second allele with a 3,820 base-pair insertion and confirmed that both alleles cannot complement each other. Such nonfunctional alleles appear to be common in potato cultivars. More than half of the varieties we tested are carriers of mutational load at the StMSH4 locus. With this new information, breeders can choose to remove alleles associated with CO shortage from their germplasm to improve fertility or to use them to produce highly uniform unreduced male gametes in alternative breeding schemes.

Meiotic instability and irregular chromosome pairing underpin heat-induced infertility in bread wheat carrying the Rht-B1b or Rht-D1b Green Revolution genes

Mutations in the Rht-B1a and Rht-D1a genes of wheat (Triticum aestivum; resulting in Rht-B1b and Rht-D1b alleles) cause gibberellin-insensitive dwarfism and are one of the most important elements of increased yield introduced during the 'Green Revolution'. We measured the effects of a short period of heat imposed during the early reproductive stage on near-isogenic lines carrying Rht-B1b or Rht-D1b alleles, with respect to the wild-type (WT). The temperature shift caused a significant fertility loss within the ears of Rht-B1b and Rht-D1b wheats, greater than that observed for the WT. Defects in chromosome synapsis, reduced homologous recombination and a high frequency of chromosome mis-segregation were associated with reduced fertility. The transcription of TaGA3ox gene involved in the final stage of gibberellic acid (GA) biosynthesis was activated and ultra-performance liquid chromatography-tandem mass spectrometry identified GA1 as the dominant bioactive GA in developing ears, but levels were unaffected by the elevated temperature. Rht-B1b and Rht-D1b mutants were inclined to meiotic errors under optimal temperatures and showed a higher susceptibility to heat than their tall counterparts. Identification and introduction of new dwarfing alleles into modern breeding programmes is invaluable in the development of climate-resilient wheat varieties.

Synthetically induced Arabidopsis thaliana autotetraploids provide insights into the analysis of meiotic mutants with altered crossover frequency

Mutations affecting crossover (CO) frequency and distribution lead to the presence of univalents during meiosis, giving rise to aneuploid gametes and sterility. These mutations may have a different effect after chromosome doubling. The combination of altered ploidy and mutations could be potentially useful to gain new insights into the mechanisms and regulation of meiotic recombination; however, studies using autopolyploid meiotic mutants are scarce. Here, we have analyzed the cytogenetic consequences in colchicine-induced autotetraploids (colchiploids) from different Arabidopsis mutants with an altered CO frequency. We have found that there are three types of mutants: mutants in which chiasma frequency is doubled after chromosome duplication (zip4, mus81), as in the control; mutants in which polyploidy leads to a higher-than-expected increase in chiasma frequency (asy1, mer3, hei10, and mlh3); and mutants in which the rise in chiasma frequency produced by the presence of two extrachromosomal sets is less than doubled (msh5, fancm). In addition, the proportion of class I/class II COs varies after chromosome duplication in the control. The results obtained reveal the potential of colchiploid meiotic mutants for better understanding of the function of key proteins during plant meiosis. This is especially relevant considering that most crops are polyploids.

Single-Cell Transcriptome Atlas and Regulatory Dynamics in Developing Cotton Anthers

Plant anthers are composed of different specialized cell types with distinct roles in plant reproduction. High temperature (HT) stress causes male sterility, resulting in crop yield reduction. However, the spatial expression atlas and regulatory dynamics during anther development and in response to HT remain largely unknown. Here, the first single-cell transcriptome atlas and chromatin accessibility survey in cotton anther are established, depicting the specific expression and epigenetic landscape of each type of cell in anthers. The reconstruction of meiotic cells, tapetal cells, and middle layer cell developmental trajectories not only identifies novel expressed genes, but also elucidates the precise degradation period of middle layer and reveals a rapid function transition of tapetal cells during the tetrad stage. By applying HT, heterogeneity in HT response is shown among cells of anthers, with tapetal cells responsible for pollen wall synthesis are most sensitive to HT. Specifically, HT shuts down the chromatin accessibility of genes specifically expressed in the tapetal cells responsible for pollen wall synthesis, such as QUARTET 3 (QRT3) and CYTOCHROME P450 703A2 (CYP703A2), resulting in a silent expression of these genes, ultimately leading to abnormal pollen wall and male sterility. Collectively, this study provides substantial information on anthers and provides clues for heat-tolerant crop creation.

Automated Quantification of Meiotic Recombination Foci Position and Intensity

During meiosis, homologous chromosomes reciprocally exchange segments of DNA via the formation of crossovers. However, the frequency and position of crossover events along chromosomes are not random. Each chromosome must receive at least one crossover, and the formation of a crossover at one location inhibits the formation of additional crossovers nearby. These crossover patterning phenomena are referred to as "crossover assurance" and "crossover interference," respectively. One key method for quantifying meiotic crossover patterning is to immunocytologically measure the position and intensity of crossover-associated protein foci along the length of meiotic prophase I chromosomes. This approach was recently used to map the position of a conserved E3 ligase, HEI10, along Arabidopsis pachytene chromosomes, providing experimental support for a novel mechanistic "coarsening model" for crossover patterning. Here we describe a user-friendly method for automatically measuring the position and intensity of recombination-associated foci along meiotic prophase I chromosomes that is broadly applicable to studies in different eukaryotic species.

HEI10 is subject to phase separation and mediates RPA1a degradation during meiotic interference-sensitive crossover formation

Reciprocal exchanges of DNA between homologous chromosomes during meiosis, or crossovers (COs), shuffle genetic information in gametes and progeny. In many eukaryotes, the majority of COs (class I COs) are sensitive to a phenomenon called interference, which influences the occurrence of closely spaced double COs. Class I COs depend on a group of factors called ZMM (Zip, Msh, Mer) proteins including HEI10 (Human Enhancer of Invasion-10). However, how these proteins are recruited to class I CO sites is unclear. Here, we show that HEI10 forms foci on chromatin via a liquid-liquid phase separation (LLPS) mechanism that relies on residue Ser70. A HEI10S70F allele results in LLPS failure and a defect in class I CO formation. We further used immunoprecipitation-mass spectrometry to identify RPA1a (Replication Protein A 1) as a HEI10 interacting protein. Surprisingly, we find that RPA1a also undergoes phase separation and its ubiquitination and degradation are directly regulated by HEI10. We also show that HEI10 is required for the condensation of other class I CO factors. Thus, our results provide mechanistic insight into how meiotic class I CO formation is controlled by HEI10 coupling LLPS and ubiquitination.

Epigenetic regulation during meiosis and crossover

Meiosis is a distinctive type of cell division that reorganizes genetic material between generations. The initial stages of meiosis consist of several crucial steps which include double strand break, homologous chromosome pairing, break repair and crossover. Crossover frequency varies depending on the position on the chromosome, higher at euchromatin region and rare at heterochromatin, centromeres, telomeres and ribosomal DNA. Crossover positioning is dependent on various factors, especially epigenetic modifications. DNA methylation, histone post-translational modifications, histone variants and non-coding RNAs are most probably playing an important role in positioning of crossovers on a chromosomal level as well as hotspot level. DNA methylation negatively regulates crossover frequency and its effect is visible in centromeres, pericentromeres and heterochromatin regions. Pericentromeric chromatin and heterochromatin mark studies have been a centre of attraction in meiosis. Crossover hotspots are associated with euchromatin regions having specific chromatin modifications such as H3K4me3, H2A.Z. and H3 acetylation. This review will provide the current understanding of the epigenetic role in plants during meiotic recombination, chromosome synapsis, double strand break and hotspots with special attention to euchromatin and heterochromatin marks. Further, the role of epigenetic modifications in regulating meiosis and crossover in other organisms is also discussed.

Meiosis: Dances Between Homologs

The raison d'être of meiosis is shuffling of genetic information via Mendelian segregation and, within individual chromosomes, by DNA crossing-over. These outcomes are enabled by a complex cellular program in which interactions between homologous chromosomes play a central role. We first provide a background regarding the basic principles of this program. We then summarize the current understanding of the DNA events of recombination and of three processes that involve whole chromosomes: homolog pairing, crossover interference, and chiasma maturation. All of these processes are implemented by direct physical interaction of recombination complexes with underlying chromosome structures. Finally, we present convergent lines of evidence that the meiotic program may have evolved by coupling of this interaction to late-stage mitotic chromosome morphogenesis.

Dynamic gene regulatory networks improving spike fertility through regulation of floret primordia fate in wheat

The developmental process of spike is critical for spike fertility through affecting floret primordia fate in wheat; however, the genetic regulation of this dynamic and complex developmental process remains unclear. Here, we conducted a high temporal-resolution analysis of spike transcriptomes and monitored the number and morphology of floret primordia within spike. The development of all floret primordia in a spike was clearly separated into three distinct phases: differentiation, pre-dimorphism and dimorphism. Notably, we identified that floret primordia with meiosis ability at the pre-dimorphism phase usually develop into fertile floret primordia in the next dimorphism phase. Compared to control, increasing plant space treatment achieved the maximum increasement range (i.e., 50%) in number of fertile florets by accelerating spike development. The process of spike fertility improvement was directed by a continuous and dynamic regulatory network involved in transcription factor and genes interaction. This was based on the coordination of genes related to heat shock protein and jasmonic acid biosynthesis during differentiation phase, and genes related to lignin, anthocyanin and chlorophyll biosynthesis during dimorphism phase. The multi-dimensional association with high temporal-resolution approach reported here allows rapid identification of genetic resource for future breeding studies to realise the maximum spike fertility potential in more cereal crops.

MSH2 stimulates interfering and inhibits non-interfering crossovers in response to genetic polymorphism

Meiotic crossovers can be formed through the interfering pathway, in which one crossover prevents another from forming nearby, or by an independent non-interfering pathway. In Arabidopsis, local sequence polymorphism between homologs can stimulate interfering crossovers in a MSH2-dependent manner. To understand how MSH2 regulates crossovers formed by the two pathways, we combined Arabidopsis mutants that elevate non-interfering crossovers with msh2 mutants. We demonstrate that MSH2 blocks non-interfering crossovers at polymorphic loci, which is the opposite effect to interfering crossovers. We also observe MSH2-independent crossover inhibition at highly polymorphic sites. We measure recombination along the chromosome arms in lines differing in patterns of heterozygosity and observe a MSH2-dependent crossover increase at the boundaries between heterozygous and homozygous regions. Here, we show that MSH2 is a master regulator of meiotic DSB repair in Arabidopsis, with antagonistic effects on interfering and non-interfering crossovers, which shapes the crossover landscape in relation to interhomolog polymorphism.

Meiosis in Plasmodium: how does it work?

Meiosis is sexual cell division, a process in eukaryotes whereby haploid gametes are produced. Compared to canonical model eukaryotes, meiosis in apicomplexan parasites appears to diverge from the process with respect to the molecular mechanisms involved; the biology of Plasmodium meiosis, and its regulation by means of post-translational modification, are largely unexplored. Here, we discuss the impact of technological advances in cell biology, evolutionary bioinformatics, and genome-wide functional studies on our understanding of meiosis in the Apicomplexa. These parasites, including Plasmodium falciparum, Toxoplasma gondii, and Eimeria spp., have significant socioeconomic impact on human and animal health. Understanding this key stage during the parasite's life cycle may well reveal attractive targets for therapeutic intervention.

Exploring Plant Meiosis: Insights from the Kinetochore Perspective

The central player for chromosome segregation in both mitosis and meiosis is the macromolecular kinetochore structure, which is assembled by >100 structural and regulatory proteins on centromere DNA. Kinetochores play a crucial role in cell division by connecting chromosomal DNA and microtubule polymers. This connection helps in the proper segregation and alignment of chromosomes. Additionally, kinetochores can act as a signaling hub, regulating the start of anaphase through the spindle assembly checkpoint, and controlling the movement of chromosomes during anaphase. However, the role of various kinetochore proteins in plant meiosis has only been recently elucidated, and these proteins differ in their functionality from those found in animals. In this review, our current knowledge of the functioning of plant kinetochore proteins in meiosis will be summarized. In addition, the functional similarities and differences of core kinetochore proteins in meiosis between plants and other species are discussed, and the potential applications of manipulating certain kinetochore genes in meiosis for breeding purposes are explored.