A defect in synapsis causes male sterility in a T-DNA-tagged Arabidopsis thaliana mutant

Fluorescence microscopy was used to study meiosis in microsporocytes from wild-type Arabidopsis thaliana and a T-DNA-tagged meiotic mutant. Techniques for visualizing chromosomes and β-tubulin in other plant species were evaluated and modified in order to develop a method for analyzing meiosis in A. thaliana anthers. Like most dicots, A. thaliana microsporocytes undergo simultaneous cytokinesis in which both meiotic divisions are completed prior to cytokinesis. However, two unique events were observed in wild-type A. thaliana that have not been reported in other angiosperms: (1) polarization of the microsporocyte cytoskeleton during prophase I prior to nuclear envelope breakdown, and (2) extensive depolymerization of microtubules just prior to metaphase II. The first observation could have implications regarding a previously uncharacterized mechanism for determining the axis of the metaphase I spindle during microsporogenesis. The second observation is peculiar since microtubules are known to be involved in chromosome alignment in other species; possible explanations will be discussed. A T-DNA-tagged meiotic mutant of A. thaliana ( syn1 ), which had previously been shown to produce abnormal microspores with variable DNA content, was also cytologically characterized. The first observable defect occurs in microsporocytes at telophase I, where some chromosomes are scattered throughout the cytoplasm, usually attached to stray microtubules. Subsequent developmental stages are affected, leading to complete male sterility. Based on similarities to synaptic mutants that have been described in other species, it is suggested that this mutant is defective in synaptonemal complex formation and/or cohesion between sister chromatids.     

Female meiosis in wild-type Arabidopsis thaliana and in two meiotic mutants

Female meiosis in Arabidopsis has been analysed cytogenetically using an adaptation of a technique previously applied to male meiosis. Meiotic progression was closely correlated with stages of floral development, including the length and morphology of the gynoecium. Meiosis in embryo sac mother cells (EMCs) occurs later in development than male meiosis, in gynoecia that range in size between 0.3 and 0.8 mm. The earliest stages in EMCs coincide with the second division to tetrad stages in pollen mother cells. However, the details of meiotic chromosome behaviour in EMCs correspond closely to the observations we have previously made in male meiosis. In addition, BrdU labelling coupled with an immunolocalisation detection system was used to mark the S phase in cells preceding their entry into prophase I. These techniques allow female meiotic stages of Arabidopsis to be analysed in detail, from the S-phase through to the tetrad stage, and are shown to be equally applicable to the analysis of female meiosis in meiotic mutants. Received: 3 April 2000 / Revision accepted: 2 August 2000     

Confocal microscopy of whole ovules for analysis of reproductive development: the elongate1 mutant affects meiosis II

Analysis of female meiosis (megasporogenesis) and embryo sac development (megagametogenesis) in angiosperms is technically challenging because the cells are enclosed within the nucellus and ovule tissues of the female flower. This is in contrast to male sporogenesis and gametogenesis where development can readily be observed through the easily dissectable developing anthers. Observation of embryo sac development is a particular problem in crassinucellate ovules such as those of maize. To overcome the problems in observing reproductive development, we developed a simple Feulgen staining procedure optimized for use with confocal microscopy to observe reproductive progression in the crassinucellate ovules of maize. The procedure greatly facilitates the observation of nuclei and cell structures of all stages of megasporogenesis and embryo sac development. The high resolution obtained using the technique enabled us to readily visualize chromosomes from individual cells within ovule tissue samples of maize. A propidium iodide staining technique was also used and compared with the Feulgen-based technique. Static cytometry of relative DNA content of individual nuclei was possible using Imaris software on both Feulgen and propidium iodide-stained samples. The techniques also proved successful for the observation of Arabidopsis and Hieracium aurantiacum female gametophyte and seed development, demonstrating the general applicability of the techniques. Using both staining methods, we analysed the maize meiotic mutant elongate1, which produces functional diploid instead of haploid embryo sacs. The precise defect in meiosis from which diploid embryo sacs arise in elongate1 has not previously been reported. We used confocal microscopy followed by static cytometry using Imaris software to show that the defect by which diploid embryo sacs arise in the maize mutant elongate1 is the absence of meiosis II with one of the dyad cells directly initiating megagametogenesis.     

Comparative analysis of female and male meiosis in three meiotic mutants of tomato.

Female meiosis was analysed in squash preparations of ovules from three meiotic mutants and wild-type plants of tomato. In the completely asynaptic mutant as6, chromosome pairing and chiasma formation were virtually absent in both sexes. In the partially asynaptic mutant asb, with intermediate levels of chromosome pairing at pachytene, there were a higher number of chiasmate chromosome arms in female meiosis than in male meiosis, whereas in the desynaptic mutant as5 there were normal levels of chromosome pairing at pachytene and a similar reduction in chiasma frequency in the two sexes. In wild-type tomato, we found slightly higher numbers of chiasmate chromosome arms in female meiosis than in male meiosis. We propose that the higher female chiasma frequencies in mutant asb and wild-type tomato result from a longer duration of female meiotic prophase. This would allow chromosomes more time to pair and recombine. It is possible that a longer duration of prophase I does not affect mutants as5 and as6, either because the meiotic defect acts before the pairing process begins (in as6) or because it acts at a later stage and involves chiasma maintenance (in as5).     

The DUET gene is necessary for chromosome organization and progression during male meiosis in Arabidopsis and encodes a PHD finger protein

Progression through the meiotic cell cycle is an essential part of the developmental program of sporogenesis in plants. The duet mutant of Arabidopsis was identified as a male sterile mutant that lacked pollen and underwent an aberrant male meiosis. Male meiocyte division resulted in the formation of two cells instead of a normal tetrad. In wild type, male meiosis extends across two successive bud positions in an inflorescence whereas in duet, meiotic stages covered three to five bud positions indicating defective progression. Normal microspores were absent in the mutant and the products of the aberrant meiosis were uni- to tri-nucleate cells that later degenerated, resulting in anthers containing largely empty locules. Defects in male meiotic chromosome organization were observed starting from diplotene and extending to subsequent stages of meiosis. There was an accumulation of meiotic structures at metaphase 1, suggesting an arrest in cell cycle progression. Double mutant analysis revealed interaction with dyad, a mutation causing chromosome cohesion during female meiosis. Cloning and molecular analysis of DUET indicated that it potentially encodes a PHD-finger protein and shows specific expression in male meiocytes. Taken together these data suggest that DUET is required for male meiotic chromosome organization and progression.     

Absence of Positive Selection on Centromeric Histones in Tetrahymena Suggests Unsuppressed Centromere-Drive in Lineages Lacking Male Meiosis

Centromere-drive is a process where centromeres compete for transmission through asymmetric "female" meiosis for inclusion into the oocyte. In symmetric "male" meiosis, all meiotic products form viable germ cells. Therefore, the primary incentive for centromere-drive, a potential transmission bias, is believed to be missing from male meiosis. In this article, we consider whether male meiosis also bears the primary cost of centromere-drive. Because different taxa carry out different combinations of meiotic programs (symmetric?+?asymmetric, symmetric only, asymmetric only), it is possible to consider the evolutionary consequences of centromere-drive in the context of these differing systems. Groups with both types of meiosis have large, rapidly evolving centromeric regions, and their centromeric histones (CenH3s) have been shown to evolve under positive selection, suggesting roles as suppressors of centromere-drive. In contrast, taxa with only symmetric male meiosis have shown no evidence of positive selection in their centromeric histones. In this article, we present the first evolutionary analysis of centromeric histones in ciliated protozoans, a group that only undergoes asymmetric "female" meiosis. We find no evidence of positive selection acting on CNA1, the CenH3 of Tetrahymena species. Cytological observations of a panel of Tetrahymena species are consistent with dynamic karyotype evolution in this lineage. Our findings suggest that defects in male meiosis, and not mitosis or female meiosis, are the primary selective force behind centromere-drive suppression. Our study raises the possibility that taxa like ciliates, with only female meiosis, may therefore undergo unsuppressed centromere drive.     

Genome-wide crossover distribution in Arabidopsis thaliana meiosis reveals sex-specific patterns along chromosomes

In most species, crossovers (COs) are essential for the accurate segregation of homologous chromosomes at the first meiotic division. Their number and location are tightly regulated. Here, we report a detailed, genome-wide characterization of the rate and localization of COs in Arabidopsis thaliana, in male and female meiosis. We observed dramatic differences between male and female meiosis which included: (i) genetic map length; 575 cM versus 332 cM respectively; (ii) CO distribution patterns: male CO rates were very high at both ends of each chromosome, whereas female CO rates were very low; (iii) correlations between CO rates and various chromosome features: female CO rates correlated strongly and negatively with GC content and gene density but positively with transposable elements (TEs) density, whereas male CO rates correlated positively with the CpG ratio. However, except for CpG, the correlations could be explained by the unequal repartition of these sequences along the Arabidopsis chromosome. For both male and female meiosis, the number of COs per chromosome correlates with chromosome size expressed either in base pairs or as synaptonemal complex length. Finally, we show that interference modulates the CO distribution both in male and female meiosis.     

Recombination correlates with synaptonemal complex length and chromatin loop size in bovids—insights into mammalian meiotic chromosomal organization

Homologous chromosomes exchange genetic information through recombination during meiosis, a process that increases genetic diversity, and is fundamental to sexual reproduction. In an attempt to shed light on the dynamics of mammalian recombination and its implications for genome organization, we have studied the recombination characteristics of 112 individuals belonging to 28 different species in the family Bovidae. In particular, we analyzed the distribution of RAD51 and MLH1 foci during the meiotic prophase I that serve, respectively, as proxies for double-strand breaks (DSBs) which form in early stages of meiosis and for crossovers. In addition, synaptonemal complex length and meiotic DNA loop size were estimated to explore how genome organization determines DSBs and crossover patterns. We show that although the number of meiotic DSBs per cell and recombination rates observed vary between individuals of the same species, these are correlated with diploid number as well as with synaptonemal complex and DNA loop sizes. Our results illustrate that genome packaging, DSB frequencies, and crossover rates tend to be correlated, while meiotic chromosomal axis length and DNA loop size are inversely correlated in mammals. Moreover, axis length, DSB frequency, and crossover frequencies all covary, suggesting that these correlations are established in the early stages of meiosis.     

Effects of several meiotic mutations on female meiosis in maize

A modified enzyme digestion technique of ovary isolation followed by staining and squash preparation has allowed us to observe female meiosis in normal maize meiotically dividing megaspore mother cells (MMCs). The first meiotic division in megasporogenesis of maize is not distinguishable from that in mi-crosporogenesis. The second female meiotic division is characterized as follows: (1) the two products of the first meiotic division do not simultaneously enter into the second meiotic division; as a rule, the chalazal-most cell enters division earlier than the micropylar one, (2) often the second of the two products does not proceed with meiosis, but degenerates, and (3) only a single haploid meiotic product of the tetrad remains alive, and this cell proceeds with three rounds of mitoses without any intervening cell wall formation to produce the eight-nucleate embryo sac. This technique has allowed us to study the effects of five meiotic mutations (aml, aml-pral, afdl, dsy *-9101, and dvl) on female meiosis in maize. The effects of the two alleles of the aml gene (aml and aml-pral) and of the afdl and dsy *-9101mutations are the same in both male and female meiosis. The aml allele prevents the entrance of MMCs into meiosis and meiosis is replaced by mitosis; the aml-pral permits MMCs to enter into meiosis, but their progress is stopped at early prophase I stages. The afdl gene is responsible for substitution of the first meiotic (reductional) division by an equational division including the segregation of sister chromatid centromeres at anaphase I. The dsy * -9101 gene exhibits abnormal chromosome pairing; paired homologous chromosomes are visible at pachytene, but only univalents are observed at diakinesis and metaphase I stages. These mutation specific patterns of abnormal meiosis are responsible for the bisexual sterility of these meiotic mutants. The abnormal divergent shape of the spindle apparatus and the resulting abnormal segregation of homologous chromosomes observed in micro-sporogenesis in plants homozygous for the dv1 mutation have not been found in meiosis of megasporogenesis. Only male sterility is induced by the dv1 gene in the homozygous condition. © 1993 Wiley-Liss, Inc.     

Genetic dissection of heterochromatin in Drosophila: The role of basal X heterochromatin in meiotic sex chromosome behaviour

We have examined the female meiotic behaviour of three X chromosomes which have large deletions of the basal heterochromatin in Drosophila melanogaster. We find that most of this heterochromatin can be removed without substantially altering pairing and segregation of the two Xs. To compare the role of heterochromatin in male meiosis we have constructed individuals which carry two extra identical heterochromatic mini X chromosomes. These minis behave as univalents even though their heterochromatin is known to contain satellite DNA. We conclude therefore that this satellite DNA is not sufficient to allow effectively normal meiotic behaviour. In all other respects our results in the male extend and confirm Cooper's postulate that there exist specific pairing sites in the X heterochromatin. Thus we find no support in either female or male meiosis for the concept that satellite DNA is involved in meiotic chromosome pairing of either a chiasmate or an achiasmate kind.     

Smc1β is required for activation of SAC during mouse oocyte meiosis

Smc1β is a meiosis-specific cohesin subunit that is essential for sister chromatid cohesion and DNA recombination. Previous studies have shown that Smc1β-deficient mice in both sexes are sterile. Ablation of Smc1β during male meiosis leads to the blockage of spermatogenesis in pachytene stage, and ablation of Smc1β during female meiosis generates a highly error-prone oocyte although it could develop to metaphase II stage. However, the underlying mechanisms regarding how Smc1β maintains the correct meiotic progression in mouse oocytes have not been clearly defined. Here, we find that GFP-fused Smc1β is expressed and localized to the chromosomes from GV to MII stages during mouse oocyte meiotic maturation. Knockdown of Smc1β by microinjection of gene-specific morpholino causes the impaired spindle apparatus and chromosome alignment which are highly correlated with the defective kinetochore-microtubule attachments, consequently resulting in a prominently higher incidence of aneuploid eggs. In addition, the premature extrusion of polar bodies and escape of metaphase I arrest induced by low dose of nocodazole treatment in Smc1β-depleted oocytes indicates that Smc1β is essential for activation of spindle assembly checkpoint (SAC) activity. Collectively, we identify a novel function of Smc1β as a SAC participant beyond its role in chromosome cohesion during mouse oocyte meiosis.     

The dynamics of homologous chromosome pairing during male Drosophila meiosis

BACKGROUND: Meiotic pairing is essential for the proper orientation of chromosomes at the metaphase plate and their subsequent disjunction during anaphase I. In male Drosophila melanogaster, meiosis occurs in the absence of recombination or a recognizable synaptonemal complex (SC). Due to limitations in available cytological techniques, the early stages of homologous chromosome pairing in male Drosophila have not been observed, and the mechanisms involved are poorly understood.RESULTS: Chromosome tagging with GFP-Lac repressor protein allowed us to track, for the first time, the behavior of meiotic chromosomes at high resolution, live, at all stages of male Drosophila meiosis. Homologous chromosomes pair throughout the euchromatic regions in spermatogonia and during the early phases of spermatocyte development. Extensive separation of homologs and sister chromatids along the chromosome arms occurs in mid-G2, several hours before the first meiotic division, and before the G2/M transition. Centromeres, on the other hand, show complex association patterns, with specific homolog pairing taking place in mid-G2. These changes in chromosome pairing parallel changes in large-scale chromosome organization.CONCLUSIONS: Our results suggest that widespread interactions along the euchromatin are required for the initiation, but not the maintenance, of meiotic pairing of autosomes in male Drosophila. We propose that heterochromatic associations, or chromatid entanglement, may be responsible for the maintenance of homolog association during late G2. Our data also suggest that the formation of chromosome territories in the spermatocyte nucleus may play an active role in ensuring the specificity of meiotic pairing in late prophase by disrupting interactions between nonhomologous chromosomes.     

Genetic control of meiosis in Drozophila

By the beginning of 2000, more than 80 genes specifically controlling meiosis and meiotic recombination in Drosophila melanogaster have been described. Meiosis in Drosophila is different from the classical model. In females, these differences concern cytological features of prophase I, which have no principal genetic significance. Drosophila males lack lateral synapsis of chromosomes, recombination and chiasmata, and their chromosomes segregate in meiosis I following the "touch-and-go" principle. Meiotic genes in Drosophila can be classified according to their functions as affecting prerequisites for recombination and crossing over, controlling chromosome segregation in meiosis I separately in males and females and controlling sister-chromatid segregation in meiosis II in both sexes. Some meiotic genes are pleiotropic. There are meiotic genes controlling mitosis, and vice versa. Some genes for DNA repair in somatic cells are also involved in meiosis. Meiotic genes in Drosophila are compared with their counterparts in other organisms.     

Effects of sperm on female longevity in the bumble-bee Bombus terrestris L

The male ejaculate, particularly the accessory gland products, has been shown to affect female survival (as is best understood in Drosophila melanogaster). So far, these findings have primarily been discussed in the context of a sexual conflict and multiple mating. Here, we show that in the bumble-bee Bombus terrestris, male genotype influences female longevity even though B. terrestris generally is a singly mated species and male and female interests may thus be more convergent. In addition, the effect could not be owing to accessory gland products, as we artificially inseminated the queens with the content of the accessory testes only.     

The Arabidopsis MEI1 gene encodes a protein with five BRCT domains that is involved in meiosis-specific DNA repair events independent of SPO11-induced DSBs

Arabidopsis thaliana MEI1 was first described as a gene involved in male meiosis, encoding a short protein showing homology with a human acrosin-trypsin inhibitor. We have isolated a new allele of mei1, and shown that in both mutants male and female meiosis are affected. In both reproductive pathways, meiosis proceeds while chromosomes become fragmented, resulting in aberrant meiotic products and in a strongly reduced fertility. We have shown that the gene mutated in mei1 mutants actually encodes a protein of 972 amino acids that contains five BRCA1 C-terminus (BRCT) domains and is similar to proteins involved in the response to DNA damage and replication blocks in eukaryotes. During meiosis, recombination is initiated by the formation of DNA double strand breaks (DSBs) induced by the protein SPO11. We analysed meiotic chromosome behaviour of the mei1 mutant in a spo11 mutant background and proved that the meiotic fragmentation observed in mei1 mutants was not the consequence of defects in the repair of meiotic DSBs induced by SPO11. We also analysed the effect of mei1 on the mitotic cell cycle but could not detect any sensitivity of mei1 seedlings to DNA-damaging agents like gamma-rays or UV. Therefore, MEI1 is a BRCT-domain-containing protein that could be specific to the meiotic cell cycle and that plays a crucial role in some DNA repair events independent of SPO11 DSB recombination repair.     

Spontaneous Chromosome Breakage at Male Meiosis Associated with Male Recombination in DROSOPHILA MELANOGASTER

An inbred line (OK1) of Drosophila melanogaster , recently derived from a natural population in Oklahoma, has been found by Woodruff and Thompson to exhibit a low frequency of spontaneous male recombination when outcrossed to marker stocks. There is also a reciprocal-cross effect, such that recombination is found only if OK1 males are used in the initial cross. When OK1 females are used, however, male recombination is again found if their male progeny are used for a subsequent cross.-In the present cytological analysis, chromosome behavior at male meiosis was studied in reciprocal crosses between the OK1 line and both a marker gene stock and an inversion stock. If the recombination events were "conventional" and premeiotic (gonial) in origin, no chromosome aberrations would be expected during meiosis. If they were "conventional" and meiotic, some dicentric bridges with free fragments would be expected in the inversion heterozygote, but none should be present in the marker gene cross.-The results demonstrated that the occurrence of recombination in males is most likely a meiotic event, though the occurrence of some limited premeiotic recombination can not be disproven. Meiosis was found to be perfectly normal in all crosses lacking male recombination. In all of the inversion stock and noninversion marker stock crosses that showed male recombination, however, anaphase bridges were found at both first and second meiotic divisions. These were often accompanied by more than the single fragment expected from a conventional inversion bridge and fragment situation. In extreme cases, almost complete pulverization of one or more autosomes was found.-All metaphase I stages were perfectly normal, suggesting that no comparable breakage occurs in premeiotic gonial mitoses. The form of chromosome damage is similar in many ways to that produced by some DNA synthesis inhibitors, or by some viral or mycoplasma infections. This possibility is discussed, and some of the evolutionary implications of the system are briefly considered.     

A meiotic time-course for<Emphasis Type="Italic"> Arabidopsis thaliana</Emphasis>

A meiotic time-course for Arabidopsis pollen mother cells has been established based on BrdU pulse-labelling of nuclear DNA in the meiotic S-phase. Labelled flower buds were sampled at intervals and the progress of labelled cells through meiosis assessed by anti-BrdU antibody detection. The overall duration of meiosis from the end of meiotic S-phase to the tetrad stage, at 18.5°C, was 33 h, which is about three times longer than the mitotic cell cycle in seedlings. The onset of leptotene was defined by reference to the loading of the axis-associated protein Asy1, and this permitted the detection of a definite G2 stage, having a maximum duration of 9 h. It is likely, from two independent sources of evidence, that the meiotic S-phase has a duration similar to that of G2. The durations of leptotene and zygotene/pachytene are 6 h and 15.3 h, respectively, but the remaining meiotic division stages are completed very rapidly, within 3 h. The establishment of a meiotic time-course provides a framework for determining the relative timing and durations of key molecular events of meiosis in Arabidopsis in relation to cytologically defined landmarks. In addition, it will be important in a broader developmental context for determining the timing of epigenetic mechanisms that are known or suspected to occur during meiosis.