Non-sister Chromatids Research Articles

Connections between sister and non-sister telomeres of segregating chromatids maintain euploidy

The complete separation of sister chromatids during anaphase is a fundamental requirement for successful mitosis. Therefore, divisions with either persistent DNA-based connections or lagging chromosome fragments threaten aneuploidy if unresolved. Here, we demonstrate the existence of an anaphase mechanism in normally dividing cells in which pervasive connections between telomeres of segregating chromosomes aid in rescuing lagging chromosome fragments. We observe that in a large proportion of Drosophila melanogaster neuronal stem cell divisions, early anaphase sister and non-sister chromatids remain connected by thin telomeric DNA threads. Normally, these threads are resolved in mid-to-late anaphase via a spatial mechanism. However, we find that the presence of a nearby unrepaired DNA break recruits histones, BubR1 kinase, Polo kinase, Aurora B kinase, and BAF to the telomeric thread of the broken chromosome, stabilizing it. Stabilized connections then aid lagging chromosome rescue. These results suggest a model in which pervasive anaphase telomere-telomere connections that are normally resolved quickly can instead be stabilized to retain wayward chromosome fragments. Thus, the liability of persistent anaphase inter-chromosomal connections in normal divisions may be offset by their ability to maintain euploidy in the face of chromosome damage and genome loss.

Evaluation of Crossover Number, Distribution, and Interference Using Cytological Assays in Arabidopsis.

Meiosis involves the replication of nuclear chromosomes in a parent cell, followed by two successive nuclear divisions to produce haploid spores, which differentiate into the gametophyte generations that produce the egg and sperm in plants. Meiotic recombination or crossover (CO) is a hallmark of meiosis that allows shuffling of genetic information between homologous chromosomes (homologs), thereby giving rise to genetically diverse progeny cells and, ultimately, individuals in the progeny; this opens vast opportunities for genetic differentiation and hence speciation. Meiotic COs also ensure the formation of bivalents and fidelity of their equal segregation. Therefore, mechanisms that regulate meiotic recombination have been extensively studied in multiple species. Several approaches have been developed to observe or estimate the frequency of CO, in which CO can be visualized and analyzed cytologically by estimating the number of chiasma (plural chiasmata), a position where non-sister chromatids exchange genetic material between homologs. Furthermore, one CO event might influence the occurrence of another one nearby, along a chromosome; this is known as CO interference. Over the past decades, visualizing CO events and measuring CO interference have contributed greatly to our understanding of the regulatory mechanisms of meiotic recombination. Here, we describe protocols to estimate the number of chiasmata and CO interference in Arabidopsis using cytological methods involving chromosome spreads and immunostaining. Specifically, we describe how chromosome spreads can be used to estimate the number of chiasmata based on the conformations of metaphase I bivalents and provide a revised acid-based quick immunostaining assay that permits high-throughput and quantitative digital estimation of the relative distance between adjacent interference-sensitive CO foci at diakinesis. These methods can be easily established or modified, if necessary, for studying meiotic recombination in other plants and crops. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Estimation of the number of chiasmata per nucleus based on metaphase I bivalent conformations Basic Protocol 2: A chromosome spread-based immunostaining method for relative distance analysis of adjacent interference-sensitive CO foci at diakinesis in Arabidopsis meiocytes.

Sources of genomic diversity in the self-fertile plant pathogen, Sclerotinia sclerotiorum, and consequences for resistance breeding.

The ascomycete, Sclerotinia sclerotiorum, has a broad host range and causes yield loss in dicotyledonous crops world wide. Genomic diversity was determined in a population of 127 isolates obtained from individual canola (Brassica napus) fields in western Canada. Genotyping with 39 simple sequence repeat (SSR) markers revealed each isolate was a unique haplotype. Analysis of molecular variance showed 97% was due to isolate and 3% due to geographical location. Testing of mycelium compatibility among 133 isolates identified clones of mutually compatible isolates with 86–95% similar SSR haplotype, whereas incompatible isolates were highly diverse. In the Province of Manitoba, 61% of isolates were compatible forming clones and stings of pairwise compatible isolates not described before. In contrast, only 35% of isolates were compatible in Alberta without forming clones and strings, while 39% were compatible in Saskatchewan with a single clone, but no strings. These difference can be explained by wetter growing seasons and more susceptible crop species in Manitoba favouring frequent mycelium interaction and more life cycles over time, which might also explain similar differences observed in other geographical areas and host crops. Analysis of linkage disequilibrium rejected random recombination, consistent with a self-fertile fungus, restricted outcrossing due to mycelium incompatibility, and only a single annual opportunity for genomic recombination during meiosis in the ascospore stage between non-sister chromatids in the rare event nuclei from different isolates come together. More probable sources of genomic diversity is slippage during DNA replication and point mutation affecting single nucleotides that accumulate and likely increase mycelium incompatibility in a population over time. A phylogenetic tree based on SSR haplotype grouped isolates into 17 sub-populations. Aggressiveness was tested by inoculating one isolate from each sub-population onto B. napus lines with quantitative resistance. Analysis of variance was significant for isolate, line, and isolate by line interaction. These isolates represent the genomic and pathogenic diversity in western Canada, and are suitable for resistance screening in canola breeding programs.

Mutagenicity in haploid yeast meiosis resulting from repair of DSBs by the sister chromatid.

Mutations in diploid budding yeast occur in meiosis at higher frequencies than in cells grown vegetatively. Such meiotic mutations are thought to result from the repair of double-strand breaks (DSBs) in meiosis, during the process of recombination. Here, we report studies of mutagenicity in haploid strains that may undergo meiosis due to the expression of both mating-type alleles, MATa and MATα. We measure the rate of mutagenicity in the reporter gene CAN1, and find it to be fivefold higher than in mitotic cells, as determined by fluctuation analysis. This enhanced meiotic mutagenicity is shown to depend on the presence of SPO11, the gene responsible for meiotic DSBs. Mutations in haploid meiosis must result from repair of the DSBs through interaction with the sister chromatid, rather than with non-sister chromatids as in diploids. Thus, mutations in diploid meiosis that are not ostensibly associated with recombination events can be explained by sister-chromatid repair. The spectrum of meiotic mutations revealed by Sanger sequencing is similar in haploid and in diploid meiosis. Compared to mitotic mutations in CAN1, long Indels are more frequent among meiotic mutations. Both, meiotic and mitotic mutations are more common at G/C sites than at A/T, in spite of an opposite bias in the target reporter gene. We conclude that sister-chromatid repair of DSBs is a major source of mutagenicity in meiosis.

Paternal gender specificity and mild phenotypes in Charcot-Marie-Tooth type 1A patients with de novo 17p12 rearrangements.

BackgroundCharcot–Marie–Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP) are developed by duplication and deletion of the 17p12 (PMP22) region, respectively.MethodsDe novo rates were determined in 211 CMT1A or HNPP trio families, and then, analyzed gender‐specific genetic features and clinical phenotypes of the de novo cases.ResultsThis study identified 40 de novo cases (19.0%). Paternal origin was highly frequent compared to maternal origin (p = .005). Most de novo CMT1A rearrangements occurred between non‐sister chromatids (p = .003), but it was interesting that three of the four sister chromatids exchange cases were observed in the less frequent maternal origin. Paternal ages at the affected child births were slightly higher in the de novo CMT1A group than in the non‐de novo CMT1A control group (p = .0004). For the disability score of CMTNS, the de novo CMT1A group had a slightly lower value compared to the control group (p = .005). Electrophysiological studies showed no significant differences between the two groups.ConclusionThis study suggests that de novo CMT1A patients tend to have milder symptoms and that the paternal ages at child births in the de novo group are higher than those of the non‐de novo group.

Drosophila ananassae: a species characterized by spontaneous male recombination in appreciable frequency

Mutation and recombination are primarily responsible for generating the genetic variability in natural populations of microorganisms, plant and animal species including humans. Upon such genetic variations, elemental forces of evolution such as natural selection, random genetic drift and migration operate to bring about micro-evolutionary changes. Recombination or crossing-over produces new combinations of genes due to interchange of corresponding segments between nonsister chromatids of homologous chromosomes, thus, it is an important evolutionary factor. Since the time of T. H. Morgan, Drosophila has been subjected to extensive investigations on crossing over while employing a number of markers, which were used for gene mapping. Interestingly, recombination occurs in females of D. melanogaster but not in males. Later on, male crossing over was investigated in various species and its occurrence was reported in D. melanogaster, D. ananassae, D. simulans, D. willistoni, D. littoralis and D. bipectinata. Recombination occurs at very low rate in all these species except for D. ananassae, which shows spontaneous male crossing over in appreciable frequency, which is meiotic in origin. This unusual phenomenon in D. ananassae is influenced by various genetic factors as well as it shows strain variation as far as frequency of male recombination is concerned. Further, the presence of chiasmata during meiosis in males at a frequency capable of accounting for the observed recombination frequency extends evidence for meiotic origin of recombination in males of D. ananassae.

How to deduce the seven basic class asci of ordered tetrads in Neurospora

Genetic analysis is an important part of undergraduate genetics teaching and tetrad analysis is unique and integral for genetic analysis of fungi. The ordered tetrad in Neurospora is an important material for genetic analysis, which can not only be used to study recombination between genes and centromeres, but also between genes themselves, as well as study the fine cross patterns between non-sister chromatids of homologous chromosomes. However, in textbooks and related professional journals, there is a lack of specific introduction to the induction methods of the seven basic class asci used in two genes analysis. In the present paper, we designed a table presenting the correlation between the three tetrad types (PD, NPD, T) and the four segregation pattern groups (Ⅰ Ⅰ, Ⅱ Ⅱ, Ⅰ Ⅱ, Ⅱ Ⅰ) to visually show the 12 possible combinations (3×4=12). Then five of them were excluded through the "×" symbol and in addition with three comments attached with the table, thus finally we obtained seven basic ascus types. We hope that this analytical method can assist the teaching of ordered tetrad analysis in Neurospora.

Interchromosomal template-switching as a novel molecular mechanism for imprinting perturbations associated with Temple syndrome

BackgroundIntrachromosomal triplications (TRP) can contribute to disease etiology via gene dosage effects, gene disruption, position effects, or fusion gene formation. Recently, post-zygotic de novo triplications adjacent to copy-number neutral genomic intervals with runs of homozygosity (ROH) have been shown to result in uniparental isodisomy (UPD). The genomic structure of these complex genomic rearrangements (CGRs) shows a consistent pattern of an inverted triplication flanked by duplications (DUP-TRP/INV-DUP) formed by an iterative DNA replisome template-switching mechanism during replicative repair of a single-ended, double-stranded DNA (seDNA), the ROH results from an interhomolog or nonsister chromatid template switch. It has been postulated that these CGRs may lead to genetic abnormalities in carriers due to dosage-sensitive genes mapping within the copy-number variant regions, homozygosity for alleles at a locus causing an autosomal recessive (AR) disease trait within the ROH region, or imprinting-associated diseases.MethodsHere, we report a family wherein the affected subject carries a de novo 2.2-Mb TRP followed by 42.2 Mb of ROH and manifests clinical features overlapping with those observed in association with chromosome 14 maternal UPD (UPD(14)mat). UPD(14)mat can cause clinical phenotypic features enabling a diagnosis of Temple syndrome. This CGR was then molecularly characterized by high-density custom aCGH, genome-wide single-nucleotide polymorphism (SNP) and methylation arrays, exome sequencing (ES), and the Oxford Nanopore long-read sequencing technology.ResultsWe confirmed the postulated DUP-TRP/INV-DUP structure by multiple orthogonal genomic technologies in the proband. The methylation status of known differentially methylated regions (DMRs) on chromosome 14 revealed that the subject shows the typical methylation pattern of UPD(14)mat. Consistent with these molecular findings, the clinical features overlap with those observed in Temple syndrome, including speech delay.ConclusionsThese data provide experimental evidence that, in humans, triplication can lead to segmental UPD and imprinting disease. Importantly, genotype/phenotype analyses further reveal how a post-zygotically generated complex structural variant, resulting from a replication-based mutational mechanism, contributes to expanding the clinical phenotype of known genetic syndromes. Mechanistically, such events can distort transmission genetics resulting in homozygosity at a locus for which only one parent is a carrier as well as cause imprinting diseases.

Biphasic recruitment of TRF2 to DNA damage sites promotes non-sister chromatid homologous recombination repair.

TRF2 (TERF2) binds to telomeric repeats and is critical for telomere integrity. Evidence suggests that it also localizes to non-telomeric DNA damage sites. However, this recruitment appears to be precarious and functionally controversial. We find that TRF2 recruitment to damage sites occurs by a two-step mechanism: the initial rapid recruitment (phase I), and stable and prolonged association with damage sites (phase II). Phase I is poly(ADP-ribose) polymerase (PARP)-dependent and requires the N-terminal basic domain. The phase II recruitment requires the C-terminal MYB/SANT domain and the iDDR region in the hinge domain, which is mediated by the MRE11 complex and is stimulated by TERT. PARP-dependent recruitment of intrinsically disordered proteins contributes to transient displacement of TRF2 that separates two phases. TRF2 binds to I-PpoI-induced DNA double-strand break sites, which is enhanced by the presence of complex damage and is dependent on PARP and the MRE11 complex. TRF2 depletion affects non-sister chromatid homologous recombination repair, but not homologous recombination between sister chromatids or non-homologous end-joining pathways. Our results demonstrate a unique recruitment mechanism and function of TRF2 at non-telomeric DNA damage sites.

Current status and prospects of the meiosis-specific function of recombinase in plants

Meiosis is a specialized cell division, essential in most reproducing organisms to halve the number of chromosomes, thereby enabling the restoration of ploidy levels during fertilization. A key step in meiosis is homologous recombination, which promotes homologous pairing and generates crossovers (COs) to connect homologous chromosomes until their separation at anaphase I. These CO sites, seen cytologically as chiasmata, represent a reciprocal exchange of genetic information between two homologous non-sister chromatids. RAD51, the eukaryotic homolog of the bacterial RecA recombinase, plays a central role in homologous recombination (HR) in yeast and animals. Loss of RAD51 function causes lethality in the flowering plant, Arabidopsis thaliana, suggesting that RAD51 has a meiotic stage-specific function that is different from homologous pairing activity.

Analysis of the impact of the absence of RAD51 strand exchange activity in Arabidopsis meiosis.

The ploidy of eukaryote gametes must be halved to avoid doubling of numbers of chromosomes with each generation and this is carried out by meiosis, a specialized cell division in which a single chromosomal replication phase is followed by two successive nuclear divisions. With some exceptions, programmed recombination ensures the proper pairing and distribution of homologous pairs of chromosomes in meiosis and recombination defects thus lead to sterility. Two highly related recombinases are required to catalyse the key strand-invasion step of meiotic recombination and it is the meiosis-specific DMC1 which is generally believed to catalyse the essential non-sister chromatid crossing-over, with RAD51 catalysing sister-chromatid and non-cross-over events. Recent work in yeast and plants has however shown that in the absence of RAD51 strand-exchange activity, DMC1 is able to repair all meiotic DNA breaks and surprisingly, that this does not appear to affect numbers of meiotic cross-overs. In this work we confirm and extend this conclusion. Given that more than 95% of meiotic homologous recombination in Arabidopsis does not result in inter-homologue crossovers, Arabidopsis is a particularly sensitive model for testing the relative importance of the two proteins—even minor effects on the non-crossover event population should produce detectable effects on crossing-over. Although the presence of RAD51 protein provides essential support for the action of DMC1, our results show no significant effect of the absence of RAD51 strand-exchange activity on meiotic crossing-over rates or patterns in different chromosomal regions or across the whole genome of Arabidopsis, strongly supporting the argument that DMC1 catalyses repair of all meiotic DNA breaks, not only non-sister cross-overs.

Superresolution expansion microscopy reveals the three-dimensional organization of the Drosophila synaptonemal complex

The synaptonemal complex (SC), a structure highly conserved from yeast to mammals, assembles between homologous chromosomes and is essential for accurate chromosome segregation at the first meiotic division. In Drosophila melanogaster, many SC components and their general positions within the complex have been dissected through a combination of genetic analyses, superresolution microscopy, and electron microscopy. Although these studies provide a 2D understanding of SC structure in Drosophila, the inability to optically resolve the minute distances between proteins in the complex has precluded its 3D characterization. A recently described technology termed expansion microscopy (ExM) uniformly increases the size of a biological sample, thereby circumventing the limits of optical resolution. By adapting the ExM protocol to render it compatible with structured illumination microscopy, we can examine the 3D organization of several known Drosophila SC components. These data provide evidence that two layers of SC are assembled. We further speculate that each SC layer may connect two nonsister chromatids, and present a 3D model of the Drosophila SC based on these findings.

Understanding and Manipulating Meiotic Recombination in Plants.

Meiosis is a specialized cell division, essential in most reproducing organisms to halve the number of chromosomes, thereby enabling the restoration of ploidy levels during fertilization. A key step of meiosis is homologous recombination, which promotes homologous pairing and generates crossovers (COs) to connect homologous chromosomes until their separation at anaphase I. These CO sites, seen cytologically as chiasmata, represent a reciprocal exchange of genetic information between two homologous nonsister chromatids. This gene reshuffling during meiosis has a significant influence on evolution and also plays an essential role in plant breeding, because a successful breeding program depends on the ability to bring the desired combinations of alleles on chromosomes. However, the number and distribution of COs during meiosis is highly constrained. There is at least one CO per chromosome pair to ensure accurate segregation of homologs, but in most organisms, the CO number rarely exceeds three regardless of chromosome size. Moreover, their positions are not random on chromosomes but exhibit regional preference. Thus, genes in recombination-poor regions tend to be inherited together, hindering the generation of novel allelic combinations that could be exploited by breeding programs. Recently, much progress has been made in understanding meiotic recombination. In particular, many genes involved in the process in Arabidopsis (Arabidopsis thaliana) have been identified and analyzed. With the coming challenges of food security and climate change, and our enhanced knowledge of how COs are formed, the interest and needs in manipulating CO formation are greater than ever before. In this review, we focus on advances in understanding meiotic recombination and then summarize the attempts to manipulate CO formation. Last, we pay special attention to the meiotic recombination in polyploidy, which is a common genomic feature for many crop plants.

Inverted meiosis and its place in the evolution of sexual reproduction pathways

Inverted meiosis is observed in plants (Cyperaceae and Juncaceae) and insects (Coccoidea, Aphididae) with holocentric chromosomes, the centromeres of which occupy from 70 to 90% of the metaphase chromosome length. In the first meiotic division (meiosis I), chiasmata are formed and rodlike bivalents orient equationally, and in anaphase I, sister chromatids segregate to the poles; the diploid chromosome number is maintained. Non-sister chromatids of homologous chromosomes remain in contact during interkinesis and prophase II and segregate in anaphase II, forming haploid chromosome sets. The segregation of sister chromatids in meiosis I was demonstrated by example of three plant species that were heterozygous for chromosomal rearrangements. In these species, sister chromatids, marked with rearrangement, segregated in anaphase I. Using fluorescent antibodies, it was demonstrated that meiotic recombination enzymes Spo11 and Rad5l, typical of canonical meiosis, functioned at the meiotic prophase I of pollen mother cells of Luzula elegance and Rhynchospora pubera. Moreover, antibodies to synaptonemal complexes proteins ASY1 and ZYP1 were visualized as filamentous structures, pointing to probable formation of synaptonemal complexes. In L. elegance, chiasmata are formed by means of chromatin threads containing satellite DNA. According to the hypothesis of the author of this review, equational division of sister chromatids at meiosis I in the organisms with inverted meiosis can be explained by the absence of specific meiotic proteins (shugoshins). These proteins are able to protect cohesins of holocentric centromeres from hydrolysis by separases at meiosis I, as occurs in the organisms with monocentric chromosomes and canonical meiosis. The basic type of inverted meiosis was described in Coccoidea and Aphididae males. In their females, the variants of parthenogenesis were also observed. Until now, the methods of molecular cytogenetics were not applied for the analysis of inverted meiosis in Coccoidea and Aphididae. Evolutionary, inverted meiosis is thought to have appeared secondarily as an adaptation of the molecular mechanisms of canonical meiosis to chromosome holocentrism.

Mek1/Mre4 is a master regulator of meiotic recombination in budding yeast.

Sexually reproducing organisms create gametes with half the somatic cell chromosome number so that fusion of gametes at fertilization does not change the ploidy of the cell. This reduction in chromosome number occurs by the specialized cell division of meiosis in which two rounds of chromosome segregation follow a single round of chromosome duplication. Meiotic crossovers formed between the non-sister chromatids of homologous chromosomes, combined with sister chromatid cohesion, physically connect homologs, thereby allowing proper segregation at the first meiotic division. Meiotic recombination is initiated by programmed double strand breaks (DSBs) whose repair is highly regulated such that (1) there is a bias for recombination with homologs rather than sister chromatids, (2) crossovers are distributed throughout the genome by a process called interference, (3) crossover homeostasis regulates the balance between crossover and non-crossover repair to maintain a critical number of crossovers and (4) each pair of homologs receives at least one crossover. It was previously known that the imposition of interhomolog bias in budding yeast requires meiosis-specific modifications to the DNA damage response and the local activation of the meiosis-specific Mek1/Mre4 (hereafter Mek1) kinase at DSBs. However, because inactivation of Mek1 results in intersister, rather than interhomolog DSB repair, whether Mek1 had a role in interhomolog pathway choice was unknown. A recent study by Chen et al. (2015) reveals that Mek1 indirectly regulates the crossover/non-crossover decision between homologs as well as genetic interference. It does this by enabling phosphorylation of Zip1, the meiosis-specific transverse filament protein of the synaptonemal complex (SC), by the conserved cell cycle kinase, Cdc7-Dbf4 (DDK). These results suggest that Mek1 is a “master regulator” of meiotic recombination in budding yeast.

Centromere Locations in Brassica A and C Genomes Revealed Through Half-Tetrad Analysis.

Locating centromeres on genome sequences can be challenging. The high density of repetitive elements in these regions makes sequence assembly problematic, especially when using short-read sequencing technologies. It can also be difficult to distinguish between active and recently extinct centromeres through sequence analysis. An effective solution is to identify genetically active centromeres (functional in meiosis) by half-tetrad analysis. This genetic approach involves detecting heterozygosity along chromosomes in segregating populations derived from gametes (half-tetrads). Unreduced gametes produced by first division restitution mechanisms comprise complete sets of nonsister chromatids. Along these chromatids, heterozygosity is maximal at the centromeres, and homologous recombination events result in homozygosity toward the telomeres. We genotyped populations of half-tetrad-derived individuals (from Brassica interspecific hybrids) using a high-density array of physically anchored SNP markers (Illumina Brassica 60K Infinium array). Mapping the distribution of heterozygosity in these half-tetrad individuals allowed the genetic mapping of all 19 centromeres of the Brassica A and C genomes to the reference Brassica napus genome. Gene and transposable element density across the B. napus genome were also assessed and corresponded well to previously reported genetic map positions. Known centromere-specific sequences were located in the reference genome, but mostly matched unanchored sequences, suggesting that the core centromeric regions may not yet be assembled into the pseudochromosomes of the reference genome. The increasing availability of genetic markers physically anchored to reference genomes greatly simplifies the genetic and physical mapping of centromeres using half-tetrad analysis. We discuss possible applications of this approach, including in species where half-tetrads are currently difficult to isolate.

Genome-Wide Maps of Recombination and Chromosome Segregation in Human Oocytes and Embryos Show Selection for Maternal Recombination Rates

Conservative estimates have been made that 10% to 30% of natural conceptions are aneuploid. The underlying causes are still unclear. Maternal age is one well-known cause of aneuploidy. Another important factor that predisposes to aneuplody is recombinant segregation errors in meiosis during fetal development in females. Crossover segregation errors occurring in oocytes during fetal development affect the risk of women having an aneuploid conception decades later in adult life. Recombinant chromosomes in offspring result from crossovers—the reciprocal exchange of DNA between homologous chromosomes. Crossover recombination is critical for the fidelity of meiosis. Reshuffling of genes prevents errors in segregation that lead to aneuploidy. Crossovers keep chromosome homologs physically linked during meiosis. Maintenance of the linkage is also helped by cohesion between sister chromatids. To understand the origin of human aneuploidies, it is important to assess all 3 meiotic products in unselected oocytes and embryos. The 3 products of female meiosis include the first and second polar bodies (PB1 and PB2) and the corresponding activated oocytes or fertilized embryos. Extensive population-based studies have examined crossover segregation errors, but the oocyte is the only product of female meiosis that has been analyzed. The inability to study the other products of female meiosis prevents direct identification of the origin of chromosome segregation errors and provides only partial information on crossovers in meiosis I. The investigators generated a more complete view of recombination by analyzing the genome of the oocyte or embryo and both polar bodies—the first polar body being a product of meiosis I and the second a product of meiosis II. Single-nucleotide polymorphisms were examined from DNA obtained from polar bodies, oocytes, and embryos from women at fertility clinics, allowing a view of 23 complete meioses. It was originally hypothesized that trisomies occurring at a high rate in the natural population of women of advanced maternal age was the result of meiosis I nondysjunction, where both homologs segregate to the oocyte at meiosis I, followed by a normal second division. However, direct cytological examination of human oocytes that failed to fertilize in in vitro fertilization clinics suggested that the major cause of human age-related trisomies is separation of many sister chromatids during meiosis I, staying paired with their nonsister chromatid—a new chromosome segregation pattern the investigators called “reverse segregation.” Another finding of the investigators suggested that higher genome-wide recombination rates were selected for because they were more likely to give rise to a euploid oocyte. Collectively, these data imply that recombination not only is critical for the accurate segregation of homologs at meiosis I, but also affects the fate of sister chromatid segregation at meiosis II. Events attributed to mistakes in chromosome segregation in meiosis II may have their origin at meiosis I in human female meiosis.

Transcriptomic changes following synthesis of a Populus full-sib diploid and allotriploid population with different heterozygosities driven by three types of 2n female gamete.

Diploid gametes are usually applied to produce triploids of Populus [originating from first-division restitution (FDR), second-division restitution (SDR), and postmeiotic restitution (PMR) 2n eggs]. Three types of 2n gametes transmitted different parental heterozygosities in Populus. Failed spindle formation and no chromosomal separation to opposite poles during meiosis I mean that FDR 2n gametes carry nonsister chromatids that are potentially heterozygous. By contrast, SDR 2n gametes result from failed sister chromatid separation in meiosis II, and therefore, they carry sister chromatid that are potentially homozygous. Completely homozygous 2n gametes can arise from the PMR mechanism. The alteration of gene expression resulting from allopolyploidization is a prominent feature in plants. We compared gene expression in the full-sib progeny of three allotriploid Populus populations (triploid-F, triploid-S, and triploid-P) with that in its parent species, and their full-sib diploid F1 hybrid. Genome-wide expression level dominance was biased toward the maternal in the diploid F1 hybrid and three allotriploid populations, whereas our data indicated important, but different, effects of the transmission of different heterozygosity by 2n female gametes in the expression patterns of allopolyploids. Because of the higher level of heterozygosity, the triploids had higher rates of non-additive and transgressive expression patterns in the triploid-F than in triploid-S and triploid-P. Compared with diploid F1, about 30-fold more genes (251) were differently expressed in the triploid-F than in the triploid-S (9) and triploid-P (8), respectively. These findings indicate that hybridization and polyploidization have immediate and distinct effects on the large-scale patterns of gene expression, and different effects on the transmission of heterozygosity by three 2n female gametes.

Alternative meiotic chromatid segregation in the holocentric plant Luzula elegans.

Holocentric chromosomes occur in a number of independent eukaryotic lineages. They form holokinetic kinetochores along the entire poleward chromatid surfaces, and owing to this alternative chromosome structure, species with holocentric chromosomes cannot use the two-step loss of cohesion during meiosis typical for monocentric chromosomes. Here we show that the plant Luzula elegans maintains a holocentric chromosome architecture and behaviour throughout meiosis, and in contrast to monopolar sister centromere orientation, the unfused holokinetic sister centromeres behave as two distinct functional units during meiosis I, resulting in sister chromatid separation. Homologous non-sister chromatids remain terminally linked after metaphase I, by satellite DNA-enriched chromatin threads, until metaphase II. They then separate at anaphase II. Thus, an inverted sequence of meiotic sister chromatid segregation occurs. This alternative meiotic process is most likely one possible adaptation to handle a holocentric chromosome architecture and behaviour during meiosis.

Chiasmatic and achiasmatic inverted meiosis of plants with holocentric chromosomes

Meiosis is a specialized cell division in sexually reproducing organisms before gamete formation. Following DNA replication, the canonical sequence in species with monocentric chromosomes is characterized by reductional segregation of homologous chromosomes during the first and equational segregation of sister chromatids during the second meiotic division. Species with holocentric chromosomes employ specific adaptations to ensure regular disjunction during meiosis. Here we present the analysis of two closely related plant species with holocentric chromosomes that display an inversion of the canonical meiotic sequence, with the equational division preceding the reductional. In-depth analysis of the meiotic divisions of Rhynchospora pubera and R. tenuis reveals that during meiosis I sister chromatids are bi-oriented, display amphitelic attachment to the spindle and are subsequently separated. During prophase II, chromatids are connected by thin chromatin threads that appear instrumental for the regular disjunction of homologous non-sister chromatids in meiosis II.