A frond of Adiantum mairisi showing marginal sori that contain sporangia. The terminal cones of Cupressus sp. The stamens and carpels of a Magnolia sp. In both chlorophyte and charophyte algal sister groups to the land plants meiosis occurs immediately following fertilization Becker and Marin, In the single-celled chlorophyte alga, Chlamydomonas reinhardtii two haploid mating types, plus and minus , differentiate into gametes which fuse during fertilization to form a single-celled zygote Fig.
Following fertilization GSP1 and GSM1 heterodimerize, translocate to the nucleus, and initiate zygotic gene expression patterns Lee et al.
Constitutive expression of either GSP1 or GSM1 in the opposite gamete type is sufficient to trigger zygote development in the absence of fertilization Zhao et al. Stable C. In contrast to chlorophytes, charophytes have a multicellular haploid body that generates free-swimming sperm in antheridia and egg cells that are retained within an oogonium on the parent plant Fig. Egg retention oogamy is an innovation shared with land plants thought to have been a key adaptation in their evolution McCourt et al.
In contrast to their algal sisters, all land plants have a period of multicellular diploid growth, the extent of which varies by plant group Lewis and McCourt, ; McCourt et al.
The bryophyte sister groups to the vascular plants exhibit limited post-embryonic development with no indeterminate apical growth Mishler and Churchill, ; Shaw and Renzaglia, ; Donoghue, Sporophytes comprise a small single stem with a terminal sporangium that represents the simplest basal land plant body plan Kenrick, ; Donoghue, ; Qiu et al.
In liverworts and mosses sporangium development arrests diploid growth, whereas hornwort sporophytes contribute to their own nutrition and have sporangia that grow indeterminately from a basal meristem Boyce, ; Kato and Akiyama, A sub-epidermal archesporial cell layer is specified during sporangium development and divides either by meiosis to generate spores mosses or spore mother cells and interspersed elater cells that perform nutritive or dispersal functions liverworts and hornworts.
The tissues surrounding the archesporial cell layer perform dispersal functions specific to each bryophyte group Bower, The genetic and developmental mechanisms that regulate bryophyte sporophyte development are currently poorly understood, but interest has recently accelerated due to the establishment of moss Physcomitrella patens and liverwort Marchantia polymorpha models Ishizaki et al.
Two gene classes that affect sporangium development in P. A pair of LFY homologues redundantly control the first zygotic division in P.
In mutants that do not arrest, sporangium number, initiation, and development are perturbed. These defects may arise as a consequence of abnormal sporophytic development, although spore number and germination are also highly variable in the mutants, suggesting meiotic defects Tanahashi et al.
Key features that distinguish vascular plants from bryophytes are the elaboration of an indeterminately growing and branching diploid body Mishler and Churchill, ; Donoghue, ; Langdale and Harrison, Fossil plants whose form is not represented in living plants, such as Cooksonia , have low orders of branching and may have amplified spore numbers by increasing numbers of terminal sporangia Fig. These fossils raise interesting questions about the developmental nature of the association between axis development, sporangium development, and branching and, intriguingly, rare bryophyte branching mutants strikingly resemble Cooksonia sporophytes Fig.
Alternative lateral sporangial placements appear independent of branching and may have served a similar purpose in other fossil groups Fig. This arrangement is exhibited in modern lycophytes, and sporangia arise either at the base of leaves or from the stem via one or two sub-epidermal archesporial cell layers. These archesporial cells give rise to sporogenous tissue Lycopodium or sporogenous and tapetal tissues Selaginella , Isoetes Bower, Monilophyte sporangia are diverse in terms of their size, the number of spores produced per sporangium, and their number and position on the plant Fig.
The eusporangiate basal monilophyte grade comprising marattioid ferns, horsetails, ophioglossoid ferns, and whisk ferns possess sporangia that develop from several cells and produce thousands of spores Bower, ; Wagner, ; Parkinson, ; Pryer et al.
By contrast, the leptosporangiate ferns develop numerous, small sporangia from single cells, which typically contain tens of spores Bower, ; Pryer et al. Sporangia may show terminal, adaxial, abaxial, or marginal locations on leaves Fig.
With the exception of the leptosporangiate and whisk ferns, nutritive tapetal tissues arise from non-sporogenous tissue Bower, ; Parkinson, As in bryophytes, the genetic basis of diploid development is poorly characterized in lycophytes and monilophytes. Notably sporophytic KNOX expression is conserved, and meristematic expression domains suggest likely roles in indeterminate growth Bharathan et al.
Thus the structure and dispersal functions of sporangia vary broadly across the land plants and the developmental context for the initiation of meiosis is lineage specific.
An evolutionary trend towards the amplification of spore numbers by alterations in body plan, sporangium size, and the number of sporangia is apparent Bower, In seed plants gymnosperms and angiosperms a prolonged period of vegetative growth is followed by the reproductive transition. This transition involves a change in meristem identity and leads to the development of cones or flowers Steeves and Sussex, Seeds develop in the context of the ovule following fertilization of the female egg cell by a male sperm cell transferred in pollen, thus dispersal functions are provided both by haploid pollen and diploid seed.
Ovules are the site of megasporangium nucellus development, which precedes meiosis. Whilst in gymnosperms one to several nucellar cells enter meiosis, in angiosperms a single megaspore mother cell undergoes meiosis to form a tetrad, three members of which degenerate to form a single functional megaspore, which divides mitotically to form the embryo sac Campbell, ; Colombo et al.
Pollen sac microsporangium development occurs from a microsporophyll or in the anther in gymnosperms and angiosperms respectively. In both, sub-epidermal cells are specified as archesporial cells that divide periclinally to form a layer of parietal cells surrounding the sporogenous cells Campbell, ; Feng and Dickinson, Sporogenous cells may then either directly enter meiosis or continue to proliferate. Parietal cells divide further to form a variable number of concentrically arranged cell layers, the innermost of which differentiates into the nutritive tapetum Campbell, ; Feng and Dickinson, Callose appears to play an important role in sporogenesis, as tapetal expression of callase causes male sterility in tobacco Worrall et al.
Following meiosis, the resulting haploid microspores undergo mitosis and differentiate into pollen grains. The genetic control of vegetative development, the reproductive transition, and sporangium formation are well studied in the angiosperm A. Thus, in A. Flower development follows conversion of indeterminate, vegetative shoot meristems to reproductive fates. This switch is controlled by a large network of genes that ensure reproduction is co-ordinated with environmental and developmental conditions Baurle and Dean, Floral organ identity genes encode three functional classes of MADS-box transcription factors A, B, and C that are expressed in overlapping domains to specify the four floral organ types Coen and Meyerowitz, Thus genes involved in meristem identity also play roles in the specification of reproductive fate.
In both micro- and megasporangia archesporial cell specification precedes sporangium formation Gifford and Foster, Ectopic activation of SPL in agamous mutants is sufficient to induce staminoid development and pollen formation Ito et al.
Male sporocyte identity in A. The first archesporial cell division normally separates reproductive sporocyte fate from non-reproductive wall and tapetal fates.
Interestingly, additional LRR receptor kinases have also been implicated in proper differentiation of the anther cell layers Albrecht et al. How these signalling processes are organized between the cell types within the developing anther is not yet clear. Developmental genetic studies in A. Future goals will be to tie together our understanding of the context, initiation, and progress of meiosis in diverse plant groups so that potential variation can be released to breeding.
During plant diversification, genes that may have originally been involved in reproductive development have been co-opted to vegetative development pathways.
The mechanisms for archesporial cell development are not yet fully characterized in flowering plants and are unknown in non-flowering plants. The derivation of nutritive tapetal tissues in different plant groups may or may not be independent of archesporial lineages.
A detailed picture is emerging of the mechanisms that control plant meiotic chromosome pairing, synapsis, recombination, and segregation in A. Understanding how these mechanisms integrate during progression of the meiotic cell cycle will be a major challenge. Equally, the pattern of CO hot- and cold-spots is complex and the mechanisms that determine plant CO frequency remain to be determined.
Knowledge of these mechanisms may allow CO to be targeted during crop breeding and facilitate the generation of novel high-yielding agricultural strains. Google Scholar. Google Preview. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide.
Sign In or Create an Account. Sign In. Advanced Search. Search Menu. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents Abstract. Meiotic cell divisions. Programmed DNA breakage and repair. Control of meiotic crossover frequency. Control of meiotic cell cycle progression.
The developmental context for meiosis. Meiosis in chlorophyte and charophyte algae. Sporophyte development and meiosis in bryophytes. Protracheophytes and seed plant sister groups. Sporophyte development in seed plants. Future perspectives.
Meiosis in flowering plants and other green organisms. Jill Harrison , C. Oxford Academic. Elizabeth Alvey. Ian R. Revision received:. Cite Cite C. Select Format Select format. Permissions Icon Permissions. Abstract Sexual eukaryotes generate gametes using a specialized cell division called meiosis that serves both to halve the number of chromosomes and to reshuffle genetic variation present in the parent.
Meiosis , organogenesis , recombination , sex , sporangia. Open in new tab Download slide. Identification and analysis of DYAD : a gene required for meiotic chromosome organization and female meiotic progression in Arabidopsi s.
Google Scholar PubMed. Detailed dissection of the chromosomal region containing the Ph1 locus in wheat Triticum asestivum : with deletion mutants and expression profiling. Google Scholar Crossref. Search ADS. Meiotic chromosomes: Integrating structure and function.
Annual Review of Genetics 33 , — Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities.
Prenatal Screen Detects Fetal Abnormalities. Synteny: Inferring Ancestral Genomes. Telomeres of Human Chromosomes. Chromosomal Abnormalities: Aneuploidies. Chromosome Abnormalities and Cancer Cytogenetics. Copy Number Variation and Human Disease. Genetic Recombination. Human Chromosome Number. Trisomy 21 Causes Down Syndrome. X Chromosome: X Inactivation. Chromosome Theory and the Castle and Morgan Debate.
Developing the Chromosome Theory. Meiosis, Genetic Recombination, and Sexual Reproduction. Mitosis and Cell Division. Genetic Mechanisms of Sex Determination. Sex Chromosomes and Sex Determination. Sex Chromosomes in Mammals: X Inactivation. Sex Determination in Honeybees. Citation: O'Connor, C. Nature Education 1 1 How is the same process responsible for genetic recombination and diversity also the cause of aneuploidy? Understanding the steps of meiosis is essential to learning how errors occur.
Aa Aa Aa. Figure 1. Figure Detail. Meiosis Is a Highly Regulated Process. Figure 2. Meiosis I. Figure 3. Meiosis II. Figure 4. Figure 5. Figure 6: Visualization of chromosomal bridges in Allium fistulosum and Allium cepa plant meiocytes.
The sites of double-stranded break DSB dependent homologue interaction can be seen as approximately nm bridges between chromosome axes.
These bridges, which probably contain a DSB that is already engaged in a nascent interaction with its partner DNA, occur in large numbers. Their formation depends on the RecA recombination protein homologues that are expressed in this species. In the next phase of homologue interaction, these nascent interactions are converted to stable strand-invasion events.
This nucleates the formation of the synaptonemal complex SC. Homologous chromosome interactions in meiosis: diversity amidst conservation. Nature Reviews Genetics 6, All rights reserved. References and Recommended Reading Gerton, J. Science , — Petes, T. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction.
Explore This Subject. Chromosome Analysis. Chromosome Structure. Mutations and Alterations in Chromosomes. Recently, protein S-palmitoylation, a lipid modification was also found to regulate the entry into meiosis Zhang et al.
In mammals, meiosis is initiated at different stages of development in females and males Bowles and Koopman, Mouse studies have revealed that retinoic acid RA produced during embryonic development can induce meiosis in both sexes.
Stimulated by RA 8 Stra8 , a vertebrate-specific gene, is then induced by RA and is required for the transition to meiosis Anderson et al. Stra8 plays no role in the mitotic phases of embryonic germ-cell development, but in females it is required for pre-meiotic DNA replication and the subsequent events of meiotic prophase.
On the other hand, Dmrt1 represses Stra8 transcription in the mitotic phase, thereby preventing meiosis Matson et al. From these studies, the mechanisms that initiate meiosis are very different, and more importantly, the genes involved share no similarity. No doubt different strategies evolved because of the different reproductive requirements of diverse organisms.
In plants, meiosis is initiated in sporogenous cells that are differentiated in ovules and anthers Bhatt et al. In each ovule, only a single megaspore mother cell MMC surrounded by the somatic nucellar cells is differentiated and then undergoes meiosis Figure 1.
During anther development, after primary sporogenous cells i. Thus, the decision of mitosis—meiosis transition must coordinate with the developmental stages of anthers and ovules. For example, the signal that starts meiosis in an anther must be generated after complete development of the somatic layers of anthers Kelliher and Walbot, Interestingly, the signal can also establish the synchronization of the meiotic cell cycle in an anther.
On the other hand, only a single MMC in each ovule is specified to enter meiosis, which accompanies the development of ovule in parallel. Thus, the regulatory mechanism of meiosis initiation may be different between female and male in plants because of distinct development of sporogenesis. Structure of plant reproductive organs in maize and sequence of events leading to spore or gametophyte formation in anthers and ovules.
A Longitudinal section of an anther with numerous pollen mother cells PMCs, shown in gray that are proliferated from primary sporogenous cells by mitosis, which accompanies the development of surrounding 4 layers of somatic cells.
By the time when the development of surrounding somatic cells shown in A is complete, unknown meiosis initiation siganl is generated to start meisois synchronously in all PMCs of an anther. Each PMC enters meiosis to produce four haploid spore cells.
C Longitudinal section of an ovule with a single megaspore mother cell MMC, shown in gray. D Schematic illustration showing the sequential development of embryo sac through sexual reproduction or apomictic pathways.
In sexual reproduction, the single MMC shown in gray is differentiated and then enters meiosis to produce a haploid functional megaspore FMS , and then develops into an embryo sac.
In apospory apomixis, somatic nucellar cells develop into embryo sac without meiosis. The first discovery about meiosis initiation was the isolation of the maize ameiotic1 am1 mutant by Rhoades The original am1 mutant allele does not undergo meiosis; instead mitosis-like divisions take place in well-developed meiocytes in both female and male organs Golubovskaya et al.
Am1 encodes a plant specific coiled-coil protein with unknown functions Pawlowski et al. All five null mutant alleles display identical phenotypes in male meiocytes in which mitosis replaces meiosis. However, female MMCs in the mutant may either undergo mitosis, or arrest at interphase.
These results suggest that AM1 is required for meiosis initiation and may also regulate meiotic progression. These differences among species may indicate that the AM1-related genes have undergone species-specific diversification. Using Agilent 44K microarrays, the authors compared transcriptomes in 1-mm and 1. In 1-mm anthers when meiosis is about to start in the wild-type, genes were missing and genes were ectopically expressed in am anthers. These genes are considered to contribute to the initiation of meiosis or the suppression of mitosis.
These results redefine the role of AM1 in the modulation of transcript accumulation for many meiotic genes rather than simply switching them on or off Nan et al. Recently, microarray analyses on laser-captured germinal and somatic initials from maize 0.
Surprisingly, more than meiotic genes are expressed in the mitotic amplification period that is long before the onset of meiosis initiation. This finding raises a possibility that precocious expression of meiotic genes permits gradual dilution of mitotic chromatin components, a hypothesis recently proposed for the mouse germ-line Hackett et al.
Another possibility is that those PMC precursors are preparing for meiosis at the transcriptional level, and may store some meiotic transcripts for translation at later developmental stages Zhang et al.
Regardless, this finding suggests that the decision to start meiosis is a series of consecutive steps rather than a single switch. Perhaps, the expression of meiotic genes may be one of the earliest actions, and the following regulatory cascade finally governs the initiation and progression of meiosis.
Thus, which transcription factors are responsible for early meiotic gene expression and whether meiotic genes are under translational control are interesting questions for further study.
In addition, identification of components in the regulatory cascade will provide better understanding of this process. A small proportion of PMCs can escape from the defects and undergo meiosis with a significant delay or continued mitotic cycles. How an RRM protein affects the initiation of meiosis is unclear at the molecular level, but this result implied a possible link between mRNA processing, transport or stability, and entry into meiosis in plants.
Studies in yeast have shown that the final trigger to start meiosis is the activation of specific cyclin—CDK complexes to initiate the meiotic S phase. Arabidopsis has at least 50 cyclins and only a few of them are specifically expressed in the inflorescence Bulankova et al.
Mutant analyses revealed that some of these cyclins contribute to distinct meiosis-related processes, but none of cyclin mutants showed meiosis initiation defects, which was attributed to gene redundancy. Thus, it will be interesting to know which, if any, cyclin—CDK complex is responsible for the transition. Besides cyclin—CDK complexes, some meiosis-specific regulators, such as replication factor MUM2 and cohesion protein REC8, are involved at the meiotic S phase although much of the basic replication apparatus is employed Strich, Therefore, what is special about the pre-meiotic S phase and which are the specific genes that differ from the mitotic S phase in plants?
Understanding of these meiosis-specific components at meiotic S phase will help us to illustrate the molecular mechanisms of meiosis initiation.
A proteomics study may offer valuable information on this aspect. To date, mutants directly affecting meiosis initiation showed similar phenotypes in that some of reproductive cells fail to enter meiosis in either female, male, or both sexes.
Although some of these mutants produce unreduced daughter cells by mitosis-like division, there is no evidence that these resulting diploid cells in ovules would undergo the apomictic pathway without fertilization. Most of the progeny were triploid, suggesting that unreduced female daughter cells after mitosis-like division are able to develop further and be fertilized by haploid male gametes Ravi et al.
Apomixis is a type of asexual reproduction through seeds that avoid both meiosis and fertilization. In the apomictic pathway, differentiated MMCs or other somatic cells in ovules that gain germinal cell fate are able to bypass meiosis or undergo an abnormal meiosis to produce unreduced spores that further divide mitotically to form an embryo sac Figure 1 ; Koltunow, ; Carman, Although apomixis is genetically regulated and occurs naturally in more than species of flowering plants, its implementation at the molecular level is still unclear.
Over the past few years, there has been increasing evidence to show that epigenetic control may regulate apomixis. In Arabidopsis, argonaute 9 ago9 mutants exhibit multiple MMCs compared to a single MMC in the wild-type ovule, and additional MMCs in the mutant are able to initiate gametogenesis without undergoing meiosis, resembling apospory Figure 1 ; Olmedo-Monfil et al.
Similarly, maize AGO, the homolog of Arabidopsis AGO9, is found to regulate reproductive fate despite some differences between maize ago and Arabidopsis ago9 phenotypes Singh et al. The maize ago mutant has a single MMC; however, defective female meiosis with aberrant condensation results in functional female gametes with an unreduced chromosome set, resembling diplospory Figure 1. Thus, loss of RdDM seems to direct somatic cells to distinct reproductive cells with an apomictic fate seen in Arabidopsis ago9 mutant or lead to apomixis in correctly specified MMCs seen in maize ago mutant.
Interestingly, both AGO9 in Arabidopsis and AGO in maize are specifically expressed in surrounding somatic nucellar cells, and not in the reproductive cells, implying that both genes control the apomictic pathway in a non-cell-autonomous manner. These results suggest a link between siRNA-dependent chromatin remodeling and the apomictic pathway Garcia-Aguilar et al.
It encodes an AGO5 protein that is required for maintaining germ cell identity and normal meiosis progression. Interestingly, the mel1 mutant also shows defective chromosome condensation with abnormal pericentromere histone modification Nonomura et al.
Further investigation is needed to understand the epigenetic regulation of plant reproduction. Over the past few years, the identification of mutants has shed light on genetic control of epigenetic mechanisms involved in apomixis.
However, it is still not clear how the RdDM-dependent process affects cell fate specification, meiosis, and gametophyte development?
Why is there a need for transposes-derived siRNA in the germ line? Perhaps, identifying the targets of the RdDM pathway at different stages will be essential for further definition of their roles.
It is worth noting that alterations in histone modification were observed in the swi1 mutant Boateng et al. Many exciting questions are awaiting further investigation. Understanding the initiation of meiosis and apomixis in plants will be enlightening, and may have many potential applications for plant breeding and in agriculture including developing a strategy for acquiring apomixis in crops, and allowing manipulation of the meiotic cell cycle.
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