Maize AMEIOTIC1 is essential for multiple early meiotic processes and likely required for the initiation of meiosis

New Frontiers in Potato Breeding: Tinkering with Reproductive Genes and Apomixis

Potato is the most important non-cereal crop worldwide, and, yet, genetic gains in potato have been traditionally delayed by the crop's biology, mostly the genetic heterozygosity of autotetraploid cultivars and the intricacies of the reproductive system. Novel site-directed genetic modification techniques provide opportunities for designing climate-smart cultivars, but they also pose new possibilities (and challenges) for breeding potato. As potato species show a remarkable reproductive diversity, and their ovules have a propensity to develop apomixis-like phenotypes, tinkering with reproductive genes in potato is opening new frontiers in potato breeding. Developing diploid varieties instead of tetraploid ones has been proposed as an alternative way to fill the gap in genetic gain, that is being achieved by using gene-edited self-compatible genotypes and inbred lines to exploit hybrid seed technology. In a similar way, modulating the formation of unreduced gametes and synthesizing apomixis in diploid or tetraploid potatoes may help to reinforce the transition to a diploid hybrid crop or enhance introgression schemes and fix highly heterozygous genotypes in tetraploid varieties. In any case, the induction of apomixis-like phenotypes will shorten the time and costs of developing new varieties by allowing the multi-generational propagation through true seeds. In this review, we summarize the current knowledge on potato reproductive phenotypes and underlying genes, discuss the advantages and disadvantages of using potato's natural variability to modulate reproductive steps during seed formation, and consider strategies to synthesize apomixis. However, before we can fully modulate the reproductive phenotypes, we need to understand the genetic basis of such diversity. Finally, we visualize an active, central role for genebanks in this endeavor by phenotyping properly genotyped genebank accessions and new introductions to provide scientists and breeders with reliable data and resources for developing innovations to exploit market opportunities.

Overcoming roadblocks for in vitro nurseries in plants: induction of meiosis

Efforts to increase genetic gains in breeding programs of flowering plants depend on making genetic crosses. Time to flowering, which can take months to decades depending on the species, can be a limiting factor in such breeding programs. It has been proposed that the rate of genetic gain can be increased by reducing the time between generations by circumventing flowering through the in vitro induction of meiosis. In this review, we assess technologies and approaches that may offer a path towards meiosis induction, the largest current bottleneck for in vitro plant breeding. Studies in non-plant, eukaryotic organisms indicate that the in vitro switch from mitotic cell division to meiosis is inefficient and occurs at very low rates. Yet, this has been achieved with mammalian cells by the manipulation of a limited number of genes. Therefore, to experimentally identify factors that switch mitosis to meiosis in plants, it is necessary to develop a high-throughput system to evaluate a large number of candidate genes and treatments, each using large numbers of cells, few of which may gain the ability to induce meiosis.

Engineering apomixis in crops

Apomixis is an asexual mode of reproduction through seeds where progeny are clones of the mother plants. Naturally apomictic modes of reproduction are found in hundreds of plant genera distributed across more than 30 plant families, but are absent in major crop plants. Apomixis has the potential to be a breakthrough technology by allowing the propagation through seed of any genotype, including F1 hybrids. Here, we have summarized the recent progress toward synthetic apomixis, where combining targeted modifications of both the meiosis and fertilization processes leads to the production of clonal seeds at high frequencies. Despite some remaining challenges, the technology has approached a level of maturity that allows its consideration for application in the field.

3D cellular morphometrics of ovule primordium development in Zea mays reveal differential division and growth dynamics specifying megaspore mother cell singleness

Introduction

Differentiation of spore mother cells marks the somatic-to-reproductive transition in higher plants. Spore mother cells are critical for fitness because they differentiate into gametes, leading to fertilization and seed formation. The female spore mother cell is called the megaspore mother cell (MMC) and is specified in the ovule primordium. The number of MMCs varies by species and genetic background, but in most cases, only a single mature MMC enters meiosis to form the embryo sac. Multiple candidate MMC precursor cells have been identified in both rice and Arabidopsis, so variability in MMC number is likely due to conserved early morphogenetic events. In Arabidopsis, the restriction of a single MMC per ovule, or MMC singleness, is determined by ovule geometry. To look for potential conservation of MMC ontogeny and specification mechanisms, we undertook a morphogenetic description of ovule primordium growth at cellular resolution in the model crop maize.

Methods

We generated a collection of 48 three-dimensional (3D) ovule primordium images for five developmental stages, annotated for 11 cell types. Quantitative analysis of ovule and cell morphological descriptors allowed the reconstruction of a plausible developmental trajectory of the MMC and its neighbors.

Results

The MMC is specified within a niche of enlarged, homogenous L2 cells, forming a pool of candidate archesporial (MMC progenitor) cells. A prevalent periclinal division of the uppermost central archesporial cell formed the apical MMC and the underlying cell, a presumptive stack cell. The MMC stopped dividing and expanded, acquiring an anisotropic, trapezoidal shape. By contrast, periclinal divisions continued in L2 neighbor cells, resulting in a single central MMC.

Discussion

We propose a model where anisotropic ovule growth in maize drives L2 divisions and MMC elongation, coupling ovule geometry with MMC fate.

Genetic Regulation of Mitosis-Meiosis Fate Decision in Plants: Is Callose an Oversighted Polysaccharide in These Processes?

Timely progression of the meiotic cell cycle and synchronized establishment of male meiosis in anthers are key to ascertaining plant fertility. With the discovery of novel regulators of the plant cell cycle, the mechanisms underlying meiosis initiation and progression appear to be more complex than previously thought, requiring the conjunctive action of cyclins, cyclin-dependent kinases, transcription factors, protein-protein interactions, and several signaling components. Broadly, cell cycle regulators can be classified into two categories in plants based on the nature of their mutational effects: (1) those that completely arrest cell cycle progression; and (2) those that affect the timing (delay or accelerate) or synchrony of cell cycle progression but somehow complete the division process. Especially the latter effects reflect evasion or obstruction of major steps in the meiosis but have sometimes been overlooked due to their subtle phenotypes. In addition to meiotic regulators, very few signaling compounds have been discovered in plants to date. In this review, we discuss the current state of knowledge about genetic mechanisms to enter the meiotic processes, referred to as the mitosis-meiosis fate decision, as well as the importance of callose (β-1,3 glucan), which has been unsung for a long time in male meiosis in plants.

Synthetic apomixis: the beginning of a new era

Apomixis is a process of asexual reproduction that enables plants to bypass meiosis and fertilization to generate clonal seeds that are identical to the maternal genotype. Apomixis has tremendous potential for breeding plants with desired characteristics, given its ability to fix any elite genotype. However, little is known about the origin and dynamics of natural apomictic plant systems. The introgression of apomixis-related genes from natural apomicts has achieved limited success. Therefore, synthetic apomixis, engineered to include apomeiosis, autonomous embryo formation, and autonomous endosperm development, has been proposed as a promising platform to effectuate apomixis in any crop. In this study, we have summarized recent advances in the understanding of synthetic apomixis and discussed the limitations of current synthetic apomixis systems and ways to overcome them.

Maize cytosolic invertase INVAN6 ensures faithful meiotic progression under heat stress

Faithful meiotic progression ensures the generation of viable gametes. Studies suggested the male meiosis of plants is sensitive to ambient temperature, but the underlying molecular mechanisms remain elusive. Here, we characterized a maize (Zea mays ssp. mays L.) dominant male sterile mutant Mei025, in which the meiotic process of pollen mother cells (PMCs) was arrested after pachytene. An Asp-to-Asn replacement at position 276 of INVERTASE ALKALINE NEUTRAL 6 (INVAN6), a cytosolic invertase (CIN) that predominantly exists in PMCs and specifically hydrolyses sucrose, was revealed to cause meiotic defects in Mei025. INVAN6 interacts with itself as well as with four other CINs and seven 14-3-3 proteins. Although INVAN6^Mei025 , the variant of INVAN6 found in Mei025, lacks hydrolytic activity entirely, its presence is deleterious to male meiosis, possibly in a dominant negative repression manner through interacting with its partner proteins. Notably, heat stress aggravated meiotic defects in invan6 null mutant. Further transcriptome data suggest INVAN6 has a fundamental role for sugar homeostasis and stress tolerance of male meiocytes. In summary, this work uncovered the function of maize CIN in male meiosis and revealed the role of CIN-mediated sugar metabolism and signalling in meiotic progression under heat stress.

Anther development-The long road to making pollen

Anthers express the most genes of any plant organ, and their development involves sequential redifferentiation of many cell types to perform distinctive roles from inception through pollen dispersal. Agricultural yield and plant breeding depend on understanding and consequently manipulating anthers, a compelling motivation for basic plant biology research to contribute. After stamen initiation, two theca form at the tip, and each forms an adaxial and abaxial lobe composed of pluripotent Layer 1-derived and Layer 2-derived cells. After signal perception or self-organization, germinal cells are specified from Layer 2-derived cells, and these secrete a protein ligand that triggers somatic differentiation of their neighbors. Historically, recovery of male-sterile mutants has been the starting point for studying anther biology. Many genes and some genetic pathways have well-defined functions in orchestrating subsequent cell fate and differentiation events. Today, new tools are providing more detailed information; for example, the developmental trajectory of germinal cells illustrates the power of single cell RNA-seq to dissect the complex journey of one cell type. We highlight ambiguities and gaps in available data to encourage attention on important unresolved issues.

Heat shock-induced failure of meiosis I to meiosis II transition leads to 2n pollen formation in a woody plant

The formation of diploid gametes through chromosome doubling is a major mechanism of polyploidization, diversification, and speciation in plants. Unfavorable climate conditions can induce or stimulate the production of diploid gametes during meiosis. Here, we demonstrated that heat shock stress (38°C for 3 or 6 h) induced 2n pollen formation, and we generated 42 triploids derived from heat shock-induced 2n pollen of Populus canescens. Meiotic analysis of treated pollen mother cells revealed that induced 2n pollen originated from the complete loss of meiosis II (MII). Among the 42 triploids, 38 triploids derived from second division restitution (SDR)-type 2n pollen and 4 triploids derived from first division restitution-type 2n pollen were verified using simple sequence repeats (SSR) molecular markers. Twenty-two differentially expressed genes related to the cell cycle were identified and characterized by expression profile analysis. Among them was POPTR_0002s08020g (PtCYCA1;2), which encodes a type A Cyclin CYCA1;2 that is required for the meiosis I (MI) to MII transition. After male flower buds were exposed to heat shock, a significant reduction was detected in PtCYCA1;2 expression. We inferred that the failure of MI-to-MII transitions might be associated with downregulated expression of PtCYCA1;2, leading to the formation of SDR-type 2n pollen. Our findings provide insights into mechanisms of heat shock-induced 2n pollen formation in a woody plant and verify that sensitivity to environmental stress has evolutionary importance in terms of polyploidization.

Exploring key developmental phases and phase-specific genes across the entirety of anther development in maize

Anther development from stamen primordium to pollen dispersal is complex and essential to sexual reproduction. How this highly dynamic and complex developmental process is controlled genetically is not well understood, especially for genes involved in specific key developmental phases. Here we generated RNA sequencing libraries spanning 10 key stages across the entirety of anther development in maize (Zea mays). Global transcriptome analyses revealed distinct phases of cell division and expansion, meiosis, pollen maturation, and mature pollen, for which we detected 50, 245, 42, and 414 phase-specific marker genes, respectively. Phase-specific transcription factor genes were significantly enriched in the phase of meiosis. The phase-specific expression of these marker genes was highly conserved among the maize lines Chang7-2 and W23, indicating they might have important roles in anther development. We explored a desiccation-related protein gene, ZmDRP1, which was exclusively expressed in the tapetum from the tetrad to the uninucleate microspore stage, by generating knockout mutants. Notably, mutants in ZmDRP1 were completely male-sterile, with abnormal Ubisch bodies and defective pollen exine. Our work provides a glimpse into the gene expression dynamics and a valuable resource for exploring the roles of key phase-specific genes that regulate anther development.

A set of sampling, preparation, and staining techniques for studying meiosis in cucumber

The development of genetic and genomic resources for biological studies in cucumber has experienced an unprecedented boom in recent years. To investigate the function of putative meiotic genes and germplasm in breeding programs, an accurate cytogenetic characterization is required. Cytological methods and reference to investigate meiosis in cucumber are limited at present. Here we provide a set of cytological techniques that have been adapted for the study of meiosis in cucumber. The meiotic stages can be identified with high precision using hierarchical criteria from developing buds, undisturbed meiocytes, and freshly stained chromosomes. A meiotic cytological atlas of all stages is presented as a reference for identifying particular stages and for comparison of meiosis between normal and mutant plants. We performed a comparative analysis of the distribution of cytoplasmic organelles between cucumber and Arabidopsis, and we described a highly nonsynchronous condensation of chromosome parts during diplotene. A simplified fluorescence in situ hybridization (FISH) protocol, using robustly spread chromosomes, were developed. In addition, we designed a single oligonucleotide probe for 5S rDNA to use in karyotyping and monitoring of homologous chromosome pairing, which will make FISH analysis of 5S rDNA easier and more economical.

Engineering Apomixis: Clonal Seeds Approaching the Fields

Apomixis is a form of reproduction leading to clonal seeds and offspring that are genetically identical to the maternal plant. While apomixis naturally occurs in hundreds of plant species distributed across diverse plant families, it is absent in major crop species. Apomixis has a revolutionary potential in plant breeding, as it could allow the instant fixation and propagation though seeds of any plant genotype, most notably F₁ hybrids. Mastering and implementing apomixis would reduce the cost of hybrid seed production, facilitate new types of hybrid breeding, and make it possible to harness hybrid vigor in crops that are not presently cultivated as hybrids. Synthetic apomixis can be engineered by combining modifications of meiosis and fertilization. Here, we review the current knowledge and recent major achievements toward the development of efficient apomictic systems usable in agriculture.

Options for Engineering Apomixis in Plants

In plants, embryogenesis and reproduction are not strictly dependent on fertilization. Several species can produce embryos in seeds asexually, a process known as apomixis. Apomixis is defined as clonal asexual reproduction through seeds, whereby the progeny is identical to the maternal genotype, and provides valuable opportunities for developing superior cultivars, as its induction in agricultural crops can facilitate the development and maintenance of elite hybrid genotypes. In this review, we summarize the current understanding of apomixis and highlight the successful introduction of apomixis methods into sexual crops. In addition, we discuss several genes whose overexpression can induce somatic embryogenesis as candidate genes to induce parthenogenesis, a unique reproductive method of gametophytic apomixis. We also summarize three schemes to achieve engineered apomixis, which will offer more opportunities for the realization of apomictic reproduction.

Reproductive cells and peripheral parietal cells collaboratively participate in meiotic fate acquisition in rice anthers

In flowering plants, the transition from mitosis to meiosis is the precondition for gametogenesis, which is the most crucial event during sexual reproduction. Here, we report an intriguing mechanism whereby germ cells and surrounding somatic cells cooperatively involve in the meiotic switch during anther development in rice (Oryza sativa). In double mutants with loss function of both leptotene chromosome establishment- and somatic cell layer differentiation-associated genes, chromosome morphology in the reproductive cells remains the same as that in somatic cells, and sporogenous cells fail to differentiate into pollen mother cells. OsSPOROCYTELESS and MICROSPORELESS1, two pivotal genes involved in meiosis entry, are prominently downregulated in anthers of plants with mutations in both MULTIPLE SPOROCYTE1 and LEPTOTENE 1. In addition, the transcription of redox-related genes is also affected. Therefore, germ cells and the surrounding somatic cells collaboratively participate in meiosis initiation in rice.

A CENH3 mutation promotes meiotic exit and restores fertility in SMG7-deficient Arabidopsis

Meiosis in angiosperm plants is followed by mitotic divisions to form multicellular haploid gametophytes. Termination of meiosis and transition to gametophytic development is, in Arabidopsis, governed by a dedicated mechanism that involves SMG7 and TDM1 proteins. Mutants carrying the smg7-6 allele are semi-fertile due to reduced pollen production. We found that instead of forming tetrads, smg7-6 pollen mother cells undergo multiple rounds of chromosome condensation and spindle assembly at the end of meiosis, resembling aberrant attempts to undergo additional meiotic divisions. A suppressor screen uncovered a mutation in centromeric histone H3 (CENH3) that increased fertility and promoted meiotic exit in smg7-6 plants. The mutation led to inefficient splicing of the CENH3 mRNA and a substantial decrease of CENH3, resulting in smaller centromeres. The reduced level of CENH3 delayed formation of the mitotic spindle but did not have an apparent effect on plant growth and development. We suggest that impaired spindle re-assembly at the end of meiosis limits aberrant divisions in smg7-6 plants and promotes formation of tetrads and viable pollen. Furthermore, the mutant with reduced level of CENH3 was very inefficient haploid inducer indicating that differences in centromere size is not the key determinant of centromere-mediated genome elimination.

Meiosis is a reductional cell division that halves number of chromosomes during two successive rounds of chromosome segregation without intervening DNA replication. Such mode of chromosome segregation requires extensive reprogramming of the cell division machinery at the entry to meiosis, and inactivation of the meiotic program upon the formation of haploid spores. Here we showed that Arabidopsis partially deficient in the RNA decay factor SMG7 fail to exit meiosis and continue with attempts to undergo additional cycles of post-meiotic chromosome segregations without genome replication. This results in a reduced number of viable pollen and diminished fertility. To find genes involved in meiotic exit, we performed a suppressor screen for the SMG7-deicient plants that re-gain fertility. We found that reducing the amount of centromeric histone partially restores pollen formation and fertility in smg7 mutants. This is likely due to inefficient formation of centromere-microtubule interactions that impairs spindle reassembly and re-entry into aberrant rounds of post-meiotic chromosome segregation.

Meiosis in crops: from genes to genomes

Meiosis is a key feature of sexual reproduction. During meiosis homologous chromosomes replicate, recombine, and randomly segregate, followed by the segregation of sister chromatids to produce haploid cells. The unique genotypes of recombinant gametes are an essential substrate for the selection of superior genotypes in natural populations and in plant breeding. In this review we summarize current knowledge on meiosis in diverse monocot and dicot crop species and provide a comprehensive resource of cloned meiotic mutants in six crop species (rice, maize, wheat, barley, tomato, and Brassica species). Generally, the functional roles of meiotic proteins are conserved between plant species, but we highlight notable differences in mutant phenotypes. The physical lengths of plant chromosomes vary greatly; for instance, wheat chromosomes are roughly one order of magnitude longer than those of rice. We explore how chromosomal distribution for crossover recombination can vary between species. We conclude that research on meiosis in crops will continue to complement that in Arabidopsis, and alongside possible applications in plant breeding will facilitate a better understanding of how the different stages of meiosis are controlled in plant species.

How to Switch from Mitosis to Meiosis: Regulation of Germline Entry in Plants

One of the major cell fate transitions in eukaryotes is entry into meiosis. While in single-celled yeast this decision is triggered by nutrient starvation, in multicellular eukaryotes, such as plants, it is under developmental control. In contrast to animals, plants have only a short germline and instruct cells to become meiocytes in reproductive organs late in development. This situation argues for a fundamentally different mechanism of how plants recruit meiocytes, and consistently, none of the regulators known to control meiotic entry in yeast and animals are present in plants. In recent years, several factors involved in meiotic entry have been identified, especially in the model plant Arabidopsis, and pieces of a regulatory network of germline control in plants are emerging. However, the corresponding studies also show that the mechanisms of meiotic entry control are diversified in flowering plants, calling for further analyses in different plant species. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Formation of Bivalents and the Control of Plant Meiotic Recombination

During the first meiotic division, the segregation of homologous chromosomes depends on the physical association of the recombined homologous DNA molecules. The physical tension due to the sites of crossing-overs (COs) is essential for the meiotic spindle to segregate the connected homologous chromosomes to the opposite poles of the cell. This equilibrated partition of homologous chromosomes allows the first meiotic reductional division. Thus, the segregation of homologous chromosomes is dependent on their recombination. In this review, we will detail the recent advances in the knowledge of the mechanisms of recombination and bivalent formation in plants. In plants, the absence of meiotic checkpoints allows observation of subsequent meiotic events in absence of meiotic recombination or defective meiotic chromosomal axis formation such as univalent formation instead of bivalents. Recent discoveries, mainly made in Arabidopsis, rice, and maize, have highlighted the link between the machinery of double-strand break (DSB) formation and elements of the chromosomal axis. We will also discuss the implications of what we know about the mechanisms regulating the number and spacing of COs (obligate CO, CO homeostasis, and interference) in model and crop plants.

Ubiquitination in Plant Meiosis: Recent Advances and High Throughput Methods

Meiosis is a specialized cell division which is essential to sexual reproduction. The success of this highly ordered process involves the timely activation, interaction, movement, and removal of many proteins. Ubiquitination is an extraordinarily diverse post-translational modification with a regulatory role in almost all cellular processes. During meiosis, ubiquitin localizes to chromatin and the expression of genes related to ubiquitination appears to be enhanced. This may be due to extensive protein turnover mediated by proteasomal degradation. However, degradation is not the only substrate fate conferred by ubiquitination which may also mediate, for example, the activation of key transcription factors. In plant meiosis, the specific roles of several components of the ubiquitination cascade-particularly SCF complex proteins, the APC/C, and HEI10-have been partially characterized indicating diverse roles in chromosome segregation, recombination, and synapsis. Nonetheless, these components remain comparatively poorly understood to their counterparts in other processes and in other eukaryotes. In this review, we present an overview of our understanding of the role of ubiquitination in plant meiosis, highlighting recent advances, remaining challenges, and high throughput methods which may be used to overcome them.

The Role of Structural Maintenance of Chromosomes Complexes in Meiosis and Genome Maintenance: Translating Biomedical and Model Plant Research Into Crop Breeding Opportunities

Cohesin is a multi-unit protein complex from the structural maintenance of chromosomes (SMC) family, required for holding sister chromatids together during mitosis and meiosis. In yeast, the cohesin complex entraps sister DNAs within tripartite rings created by pairwise interactions between the central ring units SMC1 and SMC3 and subunits such as the α-kleisin SCC1 (REC8/SYN1 in meiosis). The complex is an indispensable regulator of meiotic recombination in eukaryotes. In Arabidopsis and maize, the SMC1/SMC3 heterodimer is a key determinant of meiosis. In Arabidopsis, several kleisin proteins are also essential: SYN1/REC8 is meiosis-specific and is essential for double-strand break repair, whereas AtSCC2 is a subunit of the cohesin SCC2/SCC4 loading complex that is important for synapsis and segregation. Other important meiotic subunits are the cohesin EXTRA SPINDLE POLES (AESP1) separase, the acetylase ESTABLISHMENT OF COHESION 1/CHROMOSOME TRANSMISSION FIDELITY 7 (ECO1/CTF7), the cohesion release factor WINGS APART-LIKE PROTEIN 1 (WAPL) in Arabidopsis (AtWAPL1/AtWAPL2), and the WAPL antagonist AtSWITCH1/DYAD (AtSWI1). Other important complexes are the SMC5/SMC6 complex, which is required for homologous DNA recombination during the S-phase and for proper meiotic synapsis, and the condensin complexes, featuring SMC2/SMC4 that regulate proper clustering of rDNA arrays during interphase. Meiotic recombination is the key to enrich desirable traits in commercial plant breeding. In this review, I highlight critical advances in understanding plant chromatid cohesion in the model plant Arabidopsis and crop plants and suggest how manipulation of crossover formation during meiosis, somatic DNA repair and chromosome folding may facilitate transmission of desirable alleles, tolerance to radiation, and enhanced transcription of alleles that regulate sexual development. I hope that these findings highlight opportunities for crop breeding.

Functional analysis of a conserved domain in SWITCH1 reveals a role in commitment to female meiocyte differentiation in Arabidopsis

We have investigated the mechanism of action of SWITCH1/DYAD (SWI1), an important regulator of plant meiosis in Arabidopsis that is required for meiotic chromosome organization including maintenance of sister chromatid cohesion. The central portion of SWI1 contains a domain of unknown function that shows strong conservation between SWI1 and its orthologs in maize and rice and is also found in paralogs including MALE MEIOCYTE DEATH 1 (MMD1). In order to examine the role of this domain we performed domain swap experiments into SWI1 in a swi1 mutant background. Domain swap analysis revealed functional conservation of the central domain between SWI1 and its orthologs but not with the domain from MMD1 suggesting that the domain plays an important role in SWI1 function that has been conserved in orthologs and diverged in paralogs in plant evolution. Analysis of expression of the non-complementing MMD1 domain swap SWI1(DS_MMD1)::GFP transgenic lines revealed an altered pattern of expression that suggests a role for SWI1 in commitment to female meiocyte differentiation and meiosis. The results suggest that SWI1 may also play a developmental role as an identity determinant in the female germ cell lineage in addition to its known role in meiotic chromosome organization.

The role of the nuclear envelope in the regulation of chromatin dynamics during cell division

The nuclear envelope delineates the eukaryotic cell nucleus. The membrane system of the nuclear envelope consists of an outer nuclear membrane and an inner nuclear membrane separated by a perinuclear space. It serves as more than just a static barrier, since it regulates the communication between the nucleoplasm and the cytoplasm and provides the anchoring points where chromatin is attached. Fewer nuclear envelope proteins have been identified in plants in comparison with animals and yeasts. Here, we review the current state of knowledge of the nuclear envelope in plants, focusing on its role as a chromatin organizer and regulator of gene expression, as well as on the modifications that it undergoes to be efficiently disassembled and reassembled with each cell division. Advances in knowledge concerning the mitotic role of some nuclear envelope constituents are also presented. In addition, we summarize recent progress on the contribution of the nuclear envelope elements to telomere tethering and chromosome dynamics during the meiotic division in different plant species.

Controlling Apomixis: Shared Features and Distinct Characteristics of Gene Regulation

In higher plants, sexual and asexual reproduction through seeds (apomixis) have evolved as alternative strategies. As apomixis leads to the formation of clonal offspring, its great potential for agricultural applications has long been recognized. However, the genetic basis and the molecular control underlying apomixis and its evolutionary origin are to date not fully understood. Both in sexual and apomictic plants, reproduction is tightly controlled by versatile mechanisms regulating gene expression, translation, and protein abundance and activity. Increasing evidence suggests that interrelated pathways including epigenetic regulation, cell-cycle control, hormonal pathways, and signal transduction processes are relevant for apomixis. Additional molecular mechanisms are being identified that involve the activity of DNA- and RNA-binding proteins, such as RNA helicases which are increasingly recognized as important regulators of reproduction. Together with other factors including non-coding RNAs, their association with ribosomes is likely to be relevant for the formation and specification of the apomictic reproductive lineage. Subsequent seed formation appears to involve an interplay of transcriptional activation and repression of developmental programs by epigenetic regulatory mechanisms. In this review, insights into the genetic basis and molecular control of apomixis are presented, also taking into account potential relations to environmental stress, and considering aspects of evolution.

Following the Formation of Synaptonemal Complex Formation in Wheat and Barley by High-Resolution Microscopy

Wheat and barley have large genomes of 15 Gb and 5.1 Gb, respectively, which is much larger than the human genome (3.3 Gb). The release of their respective genomes has been a tremendous advance the understanding of the genome organization and the ability for deeper functional analysis in particular meiosis. Meiosis is the cell division required during sexual reproduction. One major event of meiosis is called recombination, or the formation of crossing over, a tight link between homologous chromosomes, ensuring gene exchange and faithful chromosome segregation. Recombination is a major driver of genetic diversity but in these large genome crops, the vast majority of these events is constrained at the end of their chromosomes. It is estimated that in barley, about 30% of the genes are located within the poor recombining centromeric regions, making important traits, such as resistance to pest and disease for example, difficult to access. Increasing recombination in these crops has the potential to speed up breeding program and requires a good understand of the meiotic mechanism. However, most research on recombination in plant has been carried in Arabidopsis thaliana which despite many of the advantages it brings for plant research, has a small genome and more spread out of recombination compare to barley or wheat. Advance in microscopy and cytological procedures have emerged in the last few years, allowing to follow meiotic events in these crops. This protocol provides the steps required for cytological preparation of barley and wheat pollen mother cells for light microscopy, highlighting some of the differences between the two cereals.

Signaling-mediated meiotic recombination in plants

Meiotic recombination provides genetic diversity in populations and ensures accurate homologous chromosome segregation for genome integrity. During meiosis, recombination processes, from DNA double strand breaks (DSBs) to crossover formation are tightly linked to higher order chromosome structure, including chromatid cohesion, axial element formation, homolog pairing and synapsis. The extensive studies on plant meiosis have revealed the important conserved roles for meiotic proteins in homologous recombination. Recent works have focused on elucidating the mechanistic basis of how meiotic proteins regulate recombination events via protein complex formation and modifications such as phosphorylation, ubiquitination, and SUMOylation. Here, we highlight recent advances on the signaling and modifications of meiotic proteins that mediate the formation of DSBs and crossovers in plants.

Defective Leptotene Chromosome 1 (DLC1) encodes a type-B response regulator and is required for rice meiosis

Meiosis is critical for sexual reproduction and the generation of new allelic variations in most eukaryotes. In this study, we report the isolation of a meiotic gene, DLC1, using a map-based cloning strategy. The dlc1 mutant is sterile in both male and female gametophytes due to an earlier defect in the leptotene chromosome and subsequent abnormalities at later stages. DLC1 is strongly expressed in the pollen mother cells (PMCs) and tapetum and encodes a nucleus-located rice type-B response regulator (RR) with transcriptional activity. Further investigations showed that DLC1 interacts with all five putative rice histidine phosphotransfer proteins (HPs) in yeast and planta cells, suggesting a possible participation of the two-component signalling systems (TCS) in rice meiosis. Our results demonstrated that DLC1 is required for rice meiosis and fertility, providing useful information for the role of TCS in rice meiosis.

Preparation of Barley Pollen Mother Cells for Confocal and Super Resolution Microscopy

Recombination (crossover) drives the release of genetic diversity in plant breeding programs. However, in barley, recombination is skewed toward the telomeric ends of its seven chromosomes, restricting the re-assortment of about 30% of the genes located in the centromeric regions of its large 5.1 Gb genome. A better understanding of meiosis and recombination could provide ways of modulating crossover distribution and frequency in barley as well as in other grasses, including wheat. While most research on recombination has been carried out in the model plant Arabidopsis thaliana, recent studies in barley (Hordeum Vulgare) have provided new insights into the control of crossing over in large genome species. A major achievement in these studies has been the use of cytological procedures to follow meiotic events. This protocol provides detailed practical steps required to perform immunostaining of barley meiocytes (pollen mother cells) for confocal or structured illumination microscopy.

SWITCH 1/DYAD is a WINGS APART-LIKE antagonist that maintains sister chromatid cohesion in meiosis

Mitosis and meiosis both rely on cohesin, which embraces the sister chromatids and plays a crucial role for the faithful distribution of chromosomes to daughter cells. Prior to the cleavage by Separase at anaphase onset, cohesin is largely removed from chromosomes by the non-proteolytic action of WINGS APART-LIKE (WAPL), a mechanism referred to as the prophase pathway. To prevent the premature loss of sister chromatid cohesion, WAPL is inhibited in early mitosis by Sororin. However, Sororin homologs have only been found to function as WAPL inhibitors during mitosis in vertebrates and Drosophila. Here we show that SWITCH 1/DYAD defines a WAPL antagonist that acts in meiosis of Arabidopsis. Crucially, SWI1 becomes dispensable for sister chromatid cohesion in the absence of WAPL. Despite the lack of any sequence similarities, we found that SWI1 is regulated and functions in a similar manner as Sororin hence likely representing a case of convergent molecular evolution across the eukaryotic kingdom.

Defining the developmental program leading to meiosis in maize

In multicellular organisms, the entry into meiosis is a complex process characterized by increasing meiotic specialization. Using single-cell RNA sequencing, we reconstructed the developmental program into maize male meiosis. A smooth continuum of expression stages before meiosis was followed by a two-step transcriptome reorganization in leptotene, during which 26.7% of transcripts changed in abundance by twofold or more. Analysis of cell-cycle gene expression indicated that nearly all pregerminal cells proliferate, eliminating a stem-cell model to generate meiotic cells. Mutants defective in somatic differentiation or meiotic commitment expressed transcripts normally present in early meiosis after a delay; thus, the germinal transcriptional program is cell autonomous and can proceed despite meiotic failure.

Pre-meiotic anther development

Most genetic and molecular analyses of anther development utilize Arabidopsis thaliana, Oryza sativa (rice), and Zea mays (maize). Especially in maize, early stages of anther development are easy to study because: (1) Maize has unisex flowers. (2) Compared to rice or A. thaliana, maize anthers are relatively large, making dissection for molecular and biochemical analyses easy. (3) Anther developmental stage is strongly correlated with maize anther length. Besides these technical advantages, understanding anther and pollen development in maize is of significant agricultural importance. Today maize is a worldwide cereal crop: approximately 25% of all consumed food contains maize. Yield stability or even increases depend on maintenance of hybrid vigor, and production of hybrid seed requires manual detasseling or genetic control of pollen development. Knowledge of pollen development can also be used to manage transgene containment. In the first section of this chapter, we will describe the current model for sequential cell fate specification in maize anther lobes, with reference to rice and A. thaliana to point out similarities and differences. In the second section of this chapter, we will review what is known about the individual cell types in anther lobes. The diversity of anther organization is addressed to a limited extent by cytological studies of anthers, often directed to clarify taxonomic relationships. In the third section, we will comment on how new lines of investigation could clarify questions remaining in our current appreciation of anther development.

Transcriptomic analysis of differentially expressed genes in flower-buds of genetic male sterile and wild type cucumber by RNA sequencing

Cucumber (Cucumis sativus L.) pollen development involves a diverse range of gene interactions between sporophytic and gametophytic tissues. Previous studies in our laboratory showed that male sterility was controlled by a single recessive nuclear gene, and occurred in pollen mother cell meiophase. To fully explore the global gene expression and identify genes related to male sterility, a RNA-seq analysis was adopted in this study. Young male flower-buds (1-2 mm in length) from genetic male sterility (GMS) mutant and homozygous fertile cucumber (WT) were collected for two sequencing libraries. Total 545 differentially expressed genes (DEGs), including 142 up-regulated DEGs and 403 down-regulated DEGs, were detected in two libraries (Fold Change ≥ 2, FDR < 0.01). These genes were involved in a variety of metabolic pathways, like ethylene-activated signaling pathway, sporopollenin biosynthetic pathway, cell cycle and DNA damage repair pathway. qRT-PCR analysis was performed and showed that the correlation between RNA-Seq and qRT-PCR was 0.876. These findings contribute to a better understanding of the mechanism that leads to GMS in cucumber.

OsSPL regulates meiotic fate acquisition in rice

In angiosperms, the key step in sexual reproduction is successful acquisition of meiotic fate. However, the molecular mechanism determining meiotic fate remains largely unknown. Here, we report that OsSPOROCYTELESS (OsSPL) is critical for meiotic entry in rice (Oryza sativa). We performed a large-scale genetic screen of rice sterile mutants aimed to identify genes regulating meiotic entry and identified OsSPL using map-based cloning. We showed that meiosis-specific callose deposition, chromatin organization, and centromere-specific histone H3 loading were altered in the cells corresponding to pollen mother cells in Osspl anthers. Global transcriptome analysis showed that the enriched differentially expressed genes in Osspl were mainly related to redox status, meiotic process, and parietal cell development. OsSPL might form homodimers and interact with TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factor OsTCP5 via the SPL dimerization and TCP interaction domain. OsSPL also interacts with TPL (TOPLESS) corepressors, OsTPL2 and OsTPL3, via the EAR motif. Our results suggest that the OsSPL-mediated signaling pathway plays a crucial role in rice meiotic entry, which appears to be a conserved regulatory mechanism for meiotic fate acquisition in angiosperms.

Stage-Specific Gene Profiling of Germinal Cells Helps Delineate the Mitosis/Meiosis Transition

In flowering plants, germ lines are induced from somatic meristems within reproductive organs. Within anthers, germinal cell initials first undergo several rounds of mitotic proliferation before synchronously entering meiosis. Our understanding of the progression and the molecular basis of this mitosis to meiosis transition is still limited. Taking advantage of the correlation between anther length and premeiotic germinal cell development in maize (Zea mays), we studied the transcriptome dynamics of germinal cells at three sequential stages, mitotic archesporial cells, enlarging pollen mother cells at the premeiosis interphase, and pollen mother cells at the early prophase of meiosis, using laser microdissection-based expression profiling. Our analysis showed that cells undergoing the mitosis-meiosis switch exhibit robust transcriptional changes. The three stages are distinguished by the expression of genes encoding transcription factor subsets, meiotic chromosome recombination proteins, and distinct E3 ubiquitin ligases, respectively. The transcription level of genes encoding protein turnover machinery was significantly higher in these three stages of germinal cells than in mature pollen, parenchyma cells, or seedlings. Our experimental results further indicate that many meiotic genes are not only transcribed, but also translated prior to meiosis. We suggest that the enlarging pollen mother cells stage represents a crucial turning point from mitosis to meiosis for developing germinal cells.

Short periods of high temperature during meiosis prevent normal meiotic progression and reduce grain number in hexaploid wheat (Triticum aestivum L.)

Exposure of wheat to high temperatures during male meiosis prevents normal meiotic progression and reduces grain number. We define a temperature-sensitive period and link heat tolerance to chromosome 5D. This study assesses the effects of heat on meiotic progression and grain number in hexaploid wheat (Triticum aestivum L. var. Chinese Spring), defines a heat-sensitive stage and evaluates the role of chromosome 5D in heat tolerance. Plants were exposed to high temperatures (30 or 35 °C) in a controlled environment room for 20-h periods during meiosis and the premeiotic interphase just prior to meiosis. Examination of pollen mother cells (PMCs) from immature anthers immediately before and after heat treatment enabled precise identification of the developmental phases being exposed to heat. A temperature-sensitive period was defined, lasting from premeiotic interphase to late leptotene, during which heat can prevent PMCs from progressing through meiosis. PMCs exposed to 35 °C were less likely to progress than those exposed to 30 °C. Grain number per spike was reduced at 30 °C, and reduced even further at 35 °C. Chinese Spring nullisomic 5D-tetrasomic 5B (N5DT5B) plants, which lack chromosome 5D, were more susceptible to heat during premeiosis-leptotene than Chinese Spring plants with the normal (euploid) chromosome complement. The proportion of plants with PMCs progressing through meiosis after heat treatment was lower for N5DT5B plants than for euploids, but the difference was not significant. However, following exposure to 30 °C, in euploid plants grain number was reduced (though not significantly), whereas in N5DT5B plants the reduction was highly significant. After exposure to 35 °C, the reduction in grain number was highly significant for both genotypes. Implications of these findings for the breeding of thermotolerant wheat are discussed.

Germline Development and Fertilization Mechanisms in Maize

Maize is the most important agricultural crop used for food, feed, and biofuel as well as a raw material for industrial products such as packaging material. To increase yield and to overcome hybridization barriers, studies of maize gamete development, the pollen tube journey, and fertilization mechanisms were initiated more than a century ago. In this review, we summarize and discuss our current understanding of the regulatory components for germline development including sporogenesis and gametogenesis, the progamic phase of pollen germination and pollen tube growth and guidance, as well as fertilization mechanisms consisting of pollen tube arrival and reception, sperm cell release, fusion with the female gametes, and egg cell activation. Mechanisms of asexual seed development are not considered here. While only a few molecular players involved in these processes have been described to date and the underlying mechanisms are far from being understood, maize now represents a spearhead of reproductive research for all grass species. Recent development of essentially improved transformation and gene-editing systems may boost research in this area in the near future.

Evolution of meiotic recombination genes in maize and teosinte

Background

Meiotic recombination is a major source of genetic variation in eukaryotes. The role of recombination in evolution is recognized but little is known about how evolutionary forces affect the recombination pathway itself. Although the recombination pathway is fundamentally conserved across different species, genetic variation in recombination components and outcomes has been observed. Theoretical predictions and empirical studies suggest that changes in the recombination pathway are likely to provide adaptive abilities to populations experiencing directional or strong selection pressures, such as those occurring during species domestication. We hypothesized that adaptive changes in recombination may be associated with adaptive evolution patterns of genes involved in meiotic recombination.

Results

To examine how maize evolution and domestication affected meiotic recombination genes, we studied patterns of sequence polymorphism and divergence in eleven genes controlling key steps in the meiotic recombination pathway in a diverse set of maize inbred lines and several accessions of teosinte, the wild ancestor of maize. We discovered that, even though the recombination genes generally exhibited high sequence conservation expected in a pathway controlling a key cellular process, they showed substantial levels and diverse patterns of sequence polymorphism. Among others, we found differences in sequence polymorphism patterns between tropical and temperate maize germplasms. Several recombination genes displayed patterns of polymorphism indicative of adaptive evolution.

Conclusions

Despite their ancient origin and overall sequence conservation, meiotic recombination genes can exhibit extensive and complex patterns of molecular evolution. Changes in these genes could affect the functioning of the recombination pathway, and may have contributed to the successful domestication of maize and its expansion to new cultivation areas.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3486-z) contains supplementary material, which is available to authorized users.

IRREGULAR POLLEN EXINE1 Is a Novel Factor in Anther Cuticle and Pollen Exine Formation

Anther cuticle and pollen exine are protective barriers for pollen development and fertilization. Despite that several regulators have been identified for anther cuticle and pollen exine development in rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana), few genes have been characterized in maize (Zea mays) and the underlying regulatory mechanism remains elusive. Here, we report a novel male-sterile mutant in maize, irregular pollen exine1 (ipe1), which exhibited a glossy outer anther surface, abnormal Ubisch bodies, and defective pollen exine. Using map-based cloning, the IPE1 gene was isolated as a putative glucose-methanol-choline oxidoreductase targeted to the endoplasmic reticulum. Transcripts of IPE1 were preferentially accumulated in the tapetum during the tetrad and early uninucleate microspore stage. A biochemical assay indicated that ipe1 anthers had altered constituents of wax and a significant reduction of cutin monomers and fatty acids. RNA sequencing data revealed that genes implicated in wax and flavonoid metabolism, fatty acid synthesis, and elongation were differentially expressed in ipe1 mutant anthers. In addition, the analysis of transfer DNA insertional lines of the orthologous gene in Arabidopsis suggested that IPE1 and their orthologs have a partially conserved function in male organ development. Our results showed that IPE1 participates in the putative oxidative pathway of C16/C18 ω-hydroxy fatty acids and controls anther cuticle and pollen exine development together with MALE STERILITY26 and MALE STERILITY45 in maize.

Apomixis: Engineering the Ability to Harness Hybrid Vigor in Crop Plants

Apomixis, commonly defined as asexual reproduction through seed, is a reproductive trait that occurs in only a few minor crops, but would be highly valuable in major crops. Apomixis results in seed-derived progenies that are genetically identical to their maternal parent. The advantage of apomixis would lie in seed propagation of elite food, feed, and biofuel crops that are heterozygous such as hybrid corn and switchgrass or self-pollinating crops for which no commercial-scale hybrid production system is available. While hybrid plants often outperform parental lines in growth and higher yields, production of hybrid seed is accomplished through carefully controlled, labor intensive crosses. Both small farmers in developing countries who produce their own seed and commercial companies that market hybrid seed could benefit from the establishment of engineered apomixis in plants. In this chapter, we review what has been learned from studying natural apomicts and mutations in sexual plants leading to apomixis-like development, plus discuss how the components of apomixis could be successfully engineered in plants.

In Favor of Establishment: Regulation of Chromatid Cohesion in Plants

In eukaryotic organisms, the correct regulation of sister chromatid cohesion, whereby sister chromatids are paired and held together, is essential for accurate segregation of the sister chromatids and homologous chromosomes into daughter cells during mitosis and meiosis, respectively. Sister chromatid cohesion requires a cohesin complex comprised of structural maintenance of chromosome adenosine triphosphatases and accessory proteins that regulate the association of the complex with chromosomes or that are involved in the establishment or release of cohesion. The cohesin complex also plays important roles in the repair of DNA double-strand breaks, regulation of gene expression and chromosome condensation. In this review, we summarize progress in understanding cohesion dynamics in plants, with the aim of uncovering differences at specific stages. We also highlight dissimilarities between plants and other eukaryotes with respect to the key players involved in the achievement of cohesion, pointing out areas that require further study.

MEIOTIC F-BOX Is Essential for Male Meiotic DNA Double-Strand Break Repair in Rice

F-box proteins constitute a large superfamily in plants and play important roles in controlling many biological processes, but the roles of F-box proteins in male meiosis in plants remain unclear. Here, we identify the rice (Oryza sativa) F-box gene MEIOTIC F-BOX (MOF), which is essential for male meiotic progression. MOF belongs to the FBX subfamily and is predominantly active during leptotene to pachytene of prophase I. mof meiocytes display disrupted telomere bouquet formation, impaired pairing and synapsis of homologous chromosomes, and arrested meiocytes at late prophase I, followed by apoptosis. Although normal, programmed double-stranded DNA breaks (DSBs) form in mof mutants, foci of the phosphorylated histone variant γH2AX, a marker for DSBs, persist in the mutant, indicating that many of the DSBs remained unrepaired. The recruitment of Completion of meiosis I (COM1) and Radiation sensitive51C (RAD51C) to DSBs is severely compromised in mutant meiocytes, indicating that MOF is crucial for DSB end-processing and repair. Further analyses showed that MOF could physically interact with the rice SKP1-like Protein1 (OSK1), indicating that MOF functions as a component of the SCF E3 ligase to regulate meiotic progression in rice. Thus, this study reveals the essential role of an F-box protein in plant meiosis and provides helpful information for elucidating the roles of the ubiquitin proteasome system in plant meiotic progression.

Long non-coding RNAs are major contributors to transcriptome changes in sunflower meiocytes with different recombination rates

BACKGROUND

Meiosis is a form of specialized cell division that marks the transition from diploid meiocyte to haploid gamete, and provides an opportunity for genetic reassortment through recombination. Experimental data indicates that, relative to their wild ancestors, cultivated sunflower varieties show a higher recombination rate during meiosis. To better understand the molecular basis for this difference, we compared gene expression in male sunflower meiocytes in prophase I isolated from a domesticated line, a wild relative, and a F1 hybrid of the two.

RESULTS

Of the genes that showed differential expression between the wild and domesticated genotypes, 63.62 % could not be identified as protein-coding genes, and of these genes, 70.98 % passed stringent filters to be classified as long non-coding RNAs (lncRNAs). Compared to the sunflower somatic transcriptome, meiocytes express a higher proportion of lncRNAs, and the majority of genes with exclusive expression in meiocytes were lncRNAs. Around 40 % of the lncRNAs showed sequence similarity with small RNAs (sRNA), while 1.53 % were predicted to be sunflower natural antisense transcripts (NATs), and 9.18 % contained transposable elements (TE). We identified 6895 lncRNAs that are exclusively expressed in meiocytes, these lncRNAs appear to have higher conservation, a greater degree of differential expression, a higher proportion of sRNA similarity, and higher TE content relative to lncRNAs that are also expressed in the somatic transcriptome.

CONCLUSIONS

lncRNAs play important roles in plant meiosis and may participate in chromatin modification processes, although other regulatory functions cannot be excluded. lncRNAs could also be related to the different recombination rates seen for domesticated and wild sunflowers.

MS5 Mediates Early Meiotic Progression and Its Natural Variants May Have Applications for Hybrid Production in Brassica napus

During meiotic prophase I, chromatin undergoes dynamic changes to establish a structural basis for essential meiotic events. However, the mechanism that coordinates chromosome structure and meiotic progression remains poorly understood in plants. Here, we characterized a spontaneous sterile mutant MS5(b)MS5(b) in oilseed rape (Brassica napus) and found its meiotic chromosomes were arrested at leptotene. MS5 is preferentially expressed in reproductive organs and encodes a Brassica-specific protein carrying conserved coiled-coil and DUF626 domains with unknown function. MS5 is essential for pairing of homologs in meiosis, but not necessary for the initiation of DNA double-strand breaks. The distribution of the axis element-associated protein ASY1 occurs independently of MS5, but localization of the meiotic cohesion subunit SYN1 requires functional MS5. Furthermore, both the central element of the synaptonemal complex and the recombination element do not properly form in MS5(b)MS5(b) mutants. Our results demonstrate that MS5 participates in progression of meiosis during early prophase I and its allelic variants lead to differences in fertility, which may provide a promising strategy for pollination control for heterosis breeding.

Genome-wide identification and expression analysis of SWI1 genes in Boechera species

As a mode of reproduction in plants, apomixis leads to the generation of clones via seeds. Apomictic plants form viable diploid female gametes without meiosis (apomeiosis) and produce embryos without fertilization (parthenogenesis). Apomeiosis, as a major component of apomixis, has recently been reported in some Arabidopsis thaliana mutants; dyad mutants of SWI1 showed developmental processes common to apomeiosis, such as producing functional diploid gametes. However, the orthologs of SWI1 genes in natural apomicts has not been previously reported. To identify the relationship between the SWI1 gene and the apomeiosis process, we isolated and sequenced SWI1 orthologs from Boechera species, including apomictic and sexual species. Boechera species are close relatives of A. thaliana and thus are advantageous model species for apomixis research. The SWI1 cDNAs were obtained by RT-PCR from apomictic and sexual Boechera young flower buds. We sequenced partial SWI1 transcripts that were 650bp for B. holboellii and 684bp for B. stricta. These SWI1-like sequences showed 86% similarity for B. holboellii and 92% for B. stricta to the A. thaliana SWI1 transcript. We also used available genome data and amplified genomic sequences for SWI1 orthologs in B. holboellii and B. stricta. The predicted proteins contain a phospholipase C domain and a nuclear localization signal. Sequence analysis did not show significant mutations related to apomixis, and phylogenetic analysis showed that SWI1-like sequences were common across plant families, regardless of the presence of a sexual or apomictic reproduction system. We also investigated the expression levels of SWI1 mRNA in the B. holboellii and B. stricta young unopened flower buds and found that relatively high levels of expression occurred in apomicts.

Recombination patterns in maize reveal limits to crossover homeostasis

During meiotic recombination, double-strand breaks (DSBs) are formed in chromosomal DNA and then repaired as either crossovers (COs) or non-crossovers (NCOs). In most taxa, the number of DSBs vastly exceeds the number of COs. COs are required for generating genetic diversity in the progeny, as well as proper chromosome segregation. Their formation is tightly controlled so that there is at least one CO per pair of homologous chromosomes whereas the maximum number of COs per chromosome pair is fairly limited. One of the main mechanisms controlling the number of recombination events per meiosis is CO homeostasis, which maintains a stable CO number even when the DSB number is dramatically altered. The existence of CO homeostasis has been reported in several species, including mouse, yeast, and Caenorhabditis elegans. However, it is not known whether homeostasis exists in the same form in all species. In addition, the studies of homeostasis have been conducted using mutants and/or transgenic lines exhibiting fairly severe meiotic phenotypes, and it is unclear how important homeostasis is under normal physiological conditions. We found that, in maize, CO control is robust only to ensure one CO per chromosome pair. However, once this limit is reached, the CO number is linearly related to the DSB number. We propose that CO control is a multifaceted process whose different aspects have a varying degree of importance in different species.

The plant-specific protein FEHLSTART controls male meiotic entry, initializing meiotic synchronization in Arabidopsis

Meiosis marks the transition from the sporophyte to the gametophyte generation in the life cycle of flowering plants, and creates genetic variations through homologous recombination. In most flowering plants, meiosis is highly synchronized within each anther, which is significant for efficient fertilization. To date, little is known about the molecular mechanisms of entry into meiosis and exit from it, and only a few genes in Arabidopsis have been characterized with a role in regulating meiotic progression. In this study, we report the functional characterization of a plant-specific basic helix-loop-helix (bHLH) protein, FEHLSTART (FST), a defect in which leads to premature meiotic entry and asynchronous meiosis, and results in decreased seed yield. Investigation of the time course of meiosis showed that the onset of leptotene, the first stage of prophase I, frequently occurred earlier in fst-1 than in the wild type. Asynchronous meiosis followed, which could manifest in the disruption of regular spindle structures and symmetric cell divisions in fst-1 mutants during the meiosis I/II transition. In accordance with frequently accelerated meiotic entry, whole-transcriptome analysis of fst-1 anthers undergoing meiosis revealed that 19 circadian rhythm genes were affected and 47 pollen-related genes were prematurely expressed at a higher level. Taken together, we propose that FST is required for normal meiotic entry and the establishment of meiotic synchrony.

Global Identification of Genes Specific for Rice Meiosis

The leptotene-zygotene transition is a major step in meiotic progression during which pairing between homologous chromosomes is initiated and double strand breaks occur. OsAM1, a homologue of maize AM1 and Arabidopsis SWI1, encodes a protein with a coiled-coil domain in its central region that is required for the leptotene-zygotene transition during rice meiosis. To gain more insight into the role of OsAM1 in rice meiosis and identify additional meiosis-specific genes, we characterized the transcriptomes of young panicles of Osam1 mutant and wild-type rice plants using RNA-Seq combined with bioinformatic and statistical analyses. As a result, a total of 25,750 and 28,455 genes were expressed in young panicles of wild-type and Osam1 mutant plants, respectively, and 4,400 differentially expressed genes (DEGs; log₂ Ratio ≥ 1, FDR ≤ 0.05) were identified. Of these DEGs, four known rice meiosis-specific genes were detected, and 22 new putative meiosis-related genes were found by mapping these DEGs to reference biological pathways in the KEGG database. We identified eight additional well-conserved OsAM1-responsive rice meiotic genes by comparing our RNA-Seq data with known meiotic genes in Arabidopsis and fission yeast.