Special issue on “Recent advances in meiosis from DNA replication to chromosome segregation”

Abstract

Meiosis is the special division that produces haploid gametes, such as sperm and eggs. It involves a complex series of events that integrate large structural changes at the chromosome scale with fine regulation of recombination events in localized regions. To evaluate the complexity of these processes, the meiosis field covers a variety of disciplines and model organisms, making it an exciting and rapidly changing area of research. The field as a whole highlights both the conserved aspects of meiosis, as well as the marked diversity of the means taken to ensure that, ultimately, gametes will contain a balanced number of chromosomes and genetic diversity will have been produced. Studying meiosis is also critically important for the improvement of our human condition as errors of meiosis are a leading cause of infertility, miscarriage, and developmental disabilities. Finally, the complex chromosome behavior of meiosis is a genetically tractable paradigm, the study of which improves our understanding of many fundamental cellular processes including DNA repair, genome stability, cancer etiology, chromatin structure, and chromosome dynamics.

This special issue on meiosis contains twenty-two papers, of which five are in-depth reviews that complement and put in context the experimental data presented in the seventeen original research articles. The content of this issue illustrates the diversity of topics covered by researchers in the field, ranging from the effects of environment and external factors on the success of meiosis, the cell cycle actors that control the meiotic divisions, the mechanism of chromosome segregation, and the mechanisms that ensure proper homologous chromosome pairing, recombination, and synapsis. Multiple organisms are covered. Also evident is the fact that more and more studies use multicellular organisms as a model system, in large part due to the increased availability of tools that were previously restricted to studies in budding and fission yeasts.

The issue begins with a review by Klutstein and Wasserzug-Pash that describes the environmental factors that affect a gamete throughout development, beginning during embryonic development, and then continuing after birth until puberty and throughout adult life. A particular emphasis is placed on the “epigenetic” changes, consisting of DNA modifications (methylation), chromatin changes through specific histone modifications, the effect of the small RNAs such as the piRNAs, and the impact these internal as well as environmental conditions may have on fertility and perhaps even on the well-being on the progeny, through acting on several processes such as proper chromatin organization (Wasserzug-Pash and Klutstein, 2019).

Meiotic recombination during the first meiotic prophase is required to accurately segregate parental chromosomes (homologs) and obtain gametes with a balanced chromosome content. Meiotic recombination initiates with programmed double-strand break (DSB) formation, which poses a risk for genome integrity if improperly repaired. Particularly at risk are repeated regions such as transposable elements (TEs), which may generate chromosome rearrangements if repaired with non-allelic copies in the genome. To avoid such events, meiotic recombination is usually repressed at TEs. Underwood and Choi review how meiotic recombination is controlled within these repeated elements, but also, surprisingly, how TEs can influence nearby meiotic recombination, in data gathered from yeast, fungi and plants (Underwood and Choi, 2019).

Homologous chromosomes experience many structural changes during the meiotic prophase I, while they undergo pairing, homologous recombination, and synapsis — the formation of a specialized structure between homologs, the synaptonemal complex. In many organisms, these processes are accompanied by rapid and dynamic chromosome movements. The means and purpose of these movements can vary between organisms. Jantsch and Link compare the nature of meiotic chromosome movements in two organisms: the mouse, which relies on meiotic recombination to pair homologs and Caenorhabditis elegans, which does not. Recent advances in our knowledge of the machinery that attaches meiotic chromosomes to the nuclear envelope and to the cytoskeletal components that direct movement are described. Further, the consequences due to loss of chromosome movement in both organisms suggest important roles for chromosome movement in homolog pairing and the resolution of aberrant chromosome interactions, efficient and complete synapsis between homologs, and even recombination pathway choice (Link and Jantsch, 2019).

Homolog pairing, synapsis and crossing over — the exchange of homolog arms — are essential for homolog segregation and fertility, but what happens when syntenic regions are carried on different chromosomes? Baudat and colleagues asked how the African pygmy mouse manages to segregate their sex chromosomes, which are fused to different autosomes. This species has an unusual sex determination system with a canonical X and Y and a feminizing X variant, X*. Thus, there are three separate female genotypes: XX, XX*, and X*Y and the authors investigate how meiosis deals with the unusual chromosome configurations in XX* and X*Y oocytes. They find that these chromosomes adopt a quadrivalent structure with evidence suggesting transcriptional silencing of unsynapsed chromosomes. Intriguingly, despite similar levels of fertility between XX, XX*, and X*Y oocytes, the X*Y quadrivalents have too few crossing over events to account for the accurate segregation observed, suggesting alternate achiasmate segregation mechanisms in this species (Baudat et al., 2019). On a similar topic, Page and colleagues studied wild mice populations with different Robertsonian translocations, which allow them to establish a direct link between a failure to synapse in the translocated regions and a defect in DSB repair in these regions. The authors find that delays in synapsis and DNA repair with frequent hexavalent configurations containing heterologous synapsis and meiotic silencing of unsynapsed chromosomes. Interestingly, evidence for extensive desynapsis and the finding that crossing over was frequently observed at the boundaries of synapsed and unsynapsed regions lead the authors to suggest that crossover-associated structures may prevent de-synapsis (Ribagorda et al., 2019).

The repair of a subset of programmed DSBs generates crossovers and chiasmata, which establish a physical link between homologs essential for their segregation to opposite poles at meiosis I. At least one crossover is required per homolog pair, and the balance between crossover and noncrossover DSB repair is tightly regulated, by pro- and anti-crossover activites. Helicases are very important in this process. Two papers by Goldman et al and Shinohara et al investigate the function of the Srs2 helicase in DSB repair in budding yeast meiotic cells. Using constitutive or conditional mutants of SRS2, they show that Srs2 limits the accumulation of the Rad51 protein on DNA, and prevents the formation of aberrant recombination intermediates. In the absence of Srs2, large aggregates of Rad51 are observed in late meiotic prophase, which depend on DSB formation and thus likely represent unrepaired DNA intermediates (Hunt et al., 2019; Sasanuma et al., 2019).

Scoring the efficiency of meiotic recombination in budding or fission yeasts by measuring genetic distances between chromosomal loci can be a fastidious task involving the manual dissection of thousands of tetrads containing the spores. Already developed for the plant Arabidopsis thaliana and the budding yeast, Lorenz and colleagues now develop a spore-autonomous fluorescent assay that allows assessing thousands of meioses for recombination using microscopy of whole tetrads, without requiring the spores to be viable. This new tool will be very useful to researchers studying meiotic recombination in fission yeast Schizosaccharomyces pombe, allowing the rapid assessment of meiotic recombination in mutant conditions (Li et al., 2019).

Among the factors known to be involved in meiotic crossover formation, the importance of proteosomal degradation by the ubiquitin-relay has been recently uncovered, in budding yeast and in the mouse. Along these lines, Pendas and colleagues investigated if the USP26 protease, expressed in testes at the stage where recombination occurs, is involved in meiotic recombination. Despite alleles of USP26 being implicated in human male infertility, the authors found deletion of USP26 had no effect on recombination, fertility or gametogenesis in either male or female mice (Felipe-Medina et al., 2019).

ATR is a checkpoint protein activated by the presence of unrepaired DSBs. The specific roles of ATR in meiosis have been complicated to determine because its deletion is embryonic lethal. Roig and colleagues used a Seckel mouse model with a hypomorphic mutation in ATR to investigate why female mice are sterile. Unexpectedly, they found that the mice had no defect in DSB repair or recombination during meiosis, but showed wild type levels of ATR are required for follicular cell proliferation. These findings may have implications for the use of ATR inhibitors as chemotherapeutics (Pacheco et al., 2019). NAD+-dependent deacetylase SIRT7 works as a transcriptional regulator and mediates DSB repair, making it a candidate regulator of meiotic recombination. Schindler and colleagues found that SIRT7 KO female mice are subfertile and have premature decline in fertility with age. While oocytes lacking SIRT7 appear to mature normally in vitro, fewer oocytes are produced. Oocytes lacking SIRT7 have mild synaptic defects with retained gamma-H2AX suggesting delays in break repair. The authors find that a target of SIRT7, histone H3 lysine 18 acetylation is also elevated at regions of asynapsis, suggesting a role of this mark in synapsis. Ultimately these oocytes produce fewer crossovers indicated by reduced MLH1 foci, chiasmata at metaphase, and chromosome mis-segregation (Vazquez et al., 2019).

The proper execution of homolog synapsis and meiotic recombination is subject to a checkpoint called the “pachytene checkpoint” which in budding yeast requires the AAA+ ATPase Pch2. The checkpoint function has so far been correlated with its nucleolar localization. However, using either an inactivating mutant of ORC1, which is required for Pch2 nucleolar localization or a nucleolar localization-deficient mutant of PCH2, San–Segundo et colleagues observed that, surprisingly, the pachytene checkpoint was still active. This work reveals that the checkpoint functions of Pch2 are independent of its localization to the nucleolus, which reveals a puzzling aspect of the pachytene checkpoint (Herruzo et al., 2019).

Libuda and Cahoon explore how the very different environment and timing of meiotic events between sexes can result in profound differences in meiotic processes in mouse, C. elegans, and A. thaliana. In particular, the differences between chromosome axis structure, synaptonemal complex formation, and crossing over are explored (Cahoon and Libuda, 2019). Meiotic recombination and homolog synapsis are tightly interconnected. De Muyt and colleagues describe the crosstalk between homolog synapsis and the distribution and completion of the interhomolog recombination events that ultimately lead to crossovers and chiasmata. The authors focus particularly on a family of proteins, the ZMMs, originally discovered in budding yeast, but that appear conserved, entirely or a subset of them, in many other organisms, including mammals and plants. They summarize recent findings about their molecular action both for recombination and homolog synapsis, and propose models for the coordination between these two characteristic features of meiotic prophase I (Pyatnitskaya et al., 2019). The synaptonemal complex is therefore thought to have functions in regulating key features of meiotic recombination. Elucidating the molecular structure of its components will help to understand both how they could interact together but also with other proteins such as those involved in recombination. Davies and Dunne used small-angle X-ray scattering to determine the solution structure of one of the five known components of the central element of the mouse synaptonemal complex, SYCE1. They show that SYCE1 form antiparallel dimers that may act to tether other components, and may therefore promote the structural stability of the synaptonemal complex (Dunne and Davies, 2019). The resolution of recombination into crossovers is accompanied by local, then global, disassembly of the synaptonemal complex (desynapsis). Benavente et al used super resolution microscopy in chicken oocytes to study this process in detail. The authors find that in chicken oocytes, the crossover-associated MLH1 foci persist long after synaptonemal complex disassembly allowing unequivocal analysis of the dynamics of desynapsis at crossover sites. They show that two components of the synaptonemal complex, SYCP1, a transverse filament protein, and SYCE3, a central element component, stay associated with both parts of the lateral element upon separation in early disassembly (Sciurano et al., 2019).

Chiasmata arising from crossovers allow accurate homolog segregation, as they provide tension to the spindle during the first meiotic division. However, it has been suggested, in budding yeast and Drosophila, that a backup mechanism is able to segregate achiasmate chromosomes, involving the pairing of homologous centromeres, through their heterochromatin. Dawson et al used FISH probes and immunocytology to centromere sequences and heterochromatin proteins to show that in mouse spermatocytes, pericentromeric heterochromatin establishes connections during homologous centromere synapsis that persist after centromeres have desynapsed. These connections require heterochromatin, and support the existence of a backup segregation system in mammalian meiosis (Eyster et al., 2019).

In female meiosis, interhomolog connections are maintained for long period of times, and during aging, a loss of these associations has been thought partly responsible for the higher level of aneuploidy (maternal age effect). This seems mainly due to a progressive loss of sister chromatin cohesion. Here Bickel and colleagues use Drosophila oocytes and a genetic approach to establish a link between reactive oxygen species and the maternal age effect: the authors are able to detect a decrease in the rate of age-related nondisjunction when superoxide dismutase, an enzyme that reduces the levels of reactive oxygen species (ROS), is overexpressed, specifically in aged oocytes. This suggests that cohesin is damaged during aging by ROS (Perkins et al., 2019). On a similar topic, but in the mouse oocyte, Hunter and colleagues studied how obesity may affect the quality of meiosis in oocytes. The authors used a high fat diet to induce obesity in mice and found reduced quality of sister chromatid cohesion leading to aneuploidy and precocious separation of sister chromatids. Evidence for weakened cohesion was also seen by terminalization of chiasma and increased inter-kinetochore distances in metaphase II (Yun et al., 2019).

In addition to sister chromatid cohesion and crossovers, the monopolar orientation of sister kinetochores is essential to allow successful homolog segregation at meiosis I. This is achieved by a conserved complex, monopolin, that interacts with components of the kinetochore to fuse sister kinetochores during meiosis I. Marston and colleagues combined budding yeast genetics, structural biology, and comparative genomics to explore how monopolin recognizes the kinetochore. They present the structure of the Dsn1 kinetochore component with the Csm1 monopolin component, and find that this critical interaction occurs via two regions on Dsn1 and propose a model for how this Dsn1-Csm1 promotes the crosslinking of sister centromeres via the monopolin complex (Plowman et al., 2019). In several species, meiosis I division occurs with an acentrosomal spindle. Fernandez-Alvarez and Pineda-Santaella use the fission yeast S. pombe to explore how this is achieved. Using information that they obtained previously, they used a bqt1 sad1–2 double mutant where the spindle pole body (the yeast equivalent of the centrosome) is no longer inserted in the nuclear envelope, and therefore no longer functional for nucleating a spindle. In this situation, they show using live imaging that an acentrosomal spindle is formed, similar to what is seen in mammalian oocytes, and that it is able to promote homolog segregation to some extent. This model will be a valuable tool to investigate the mechanisms of acentrosomal division (Pineda-Santaella and Fernandez-Alvarez, 2019). Finally, meiotic progression and divisions are controlled by cyclins. Among them, cyclin B3 is exclusively expressed in mouse meiosis during the leptotene to zygotene stages and has recently been shown to be important for the metaphase to anaphase transition in meiosis I in oocytes. Surprisingly, Keeney and Karasu found that cyclin B3 is dispensable for male meiotic divisions. This phenotype could not be explained by compensatory upregulation of expression of other cyclins. Nor does loss of cyclin B3 exacerbate phenotypes associated with loss of cyclin E2, which has a similar expression pattern (Karasu and Keeney, 2019).

This issue features the work of many meiosis luminaries, as well as up and coming future leaders. The issue accurately represents the depth of the field of meiosis research and we hope the reader can appreciate the diversity of organisms and modernity of approaches that make meiosis such a vibrant and illuminating research field.