Fact-checked by Grok 2 weeks ago

Megagametogenesis

Megagametogenesis is the developmental process in angiosperms by which a single functional haploid megaspore, produced via megasporogenesis in the ovule, undergoes three successive mitotic divisions to form the mature female gametophyte, known as the embryo sac.^[1]^[2] This eight-nucleate, seven-celled structure contains the egg cell, two synergids, two polar nuclei in the central cell, and three antipodal cells, serving as the site for double fertilization where sperm nuclei fuse with the egg and central cell to initiate embryo and endosperm development.^[1]^[3] The process begins immediately after megasporogenesis, where a diploid megaspore mother cell undergoes meiosis to yield four haploid megaspores, with three degenerating and the chalazal-most one surviving as the functional megaspore.^[1]^[2] In the most common form, the Polygonum-type megagametogenesis (monosporic development), the functional megaspore first divides mitotically to produce a two-nucleate stage, followed by a four-nucleate stage where nuclei migrate to the micropylar and chalazal poles, separated by a central vacuole.^[3] A third mitosis then generates eight nuclei, which cellularize to form the characteristic seven cells: the egg apparatus (egg cell and two synergids) at the micropylar end, the central cell with fused polar nuclei, and the three antipodals at the chalazal end.^[2]^[3] This developmental pathway, observed in approximately 70% of angiosperm species including model organisms like Arabidopsis thaliana, is highly regulated and synchronous within the pistil, ensuring precise timing for pollination and fertilization.^[3] Variations exist, such as bisporic or tetrasporic types where multiple megaspores contribute nuclei, leading to different nuclear configurations, but the Polygonum type predominates and exemplifies the alternation of generations in seed plants.^[3] The embryo sac's cellular organization is critical for pollen tube guidance, via synergid-secreted signals, and post-fertilization events, with antipodals often degenerating before seed maturation.^[2]^[3]

Overview and Prerequisites

Definition and Biological Role

Megagametogenesis is the developmental process in angiosperms by which a haploid megaspore, produced through meiosis, undergoes mitotic divisions to form a mature female gametophyte, also known as the embryo sac, embedded within the ovule's nucellus.^[4] This process typically involves three sequential rounds of mitosis, resulting in the formation of eight nuclei that organize into seven cells with specific structures, including the egg cell, two synergids, three antipodals, and a central cell containing two polar nuclei.^[2] It follows megasporogenesis, the meiotic phase that generates the functional haploid megaspore from a diploid megasporocyte.^[4] The biological role of megagametogenesis is pivotal in angiosperm reproduction, as the resulting female gametophyte houses the egg cell and central cell essential for double fertilization. Upon pollination, one sperm nucleus fertilizes the egg to form the diploid zygote, which develops into the embryo, while the second sperm fuses with the central cell's polar nuclei to produce the triploid endosperm, providing nourishment for the developing seed.^[5] The synergids facilitate pollen tube guidance and rupture, while the antipodals may contribute to nutrient transfer, ensuring the coordinated progression of fertilization and early seed development.^[5] This process is central to sexual reproduction in flowering plants, promoting genetic diversity through the recombination of parental genomes and enabling the production of viable seeds that propagate the species.^[6] In contrast, apomixis represents an asexual alternative in some angiosperms, where seeds form without meiosis or fertilization, bypassing megagametogenesis to produce clonal offspring and maintain ploidy levels.^[7] Megagametogenesis has been extensively studied in model organisms such as Arabidopsis thaliana, where it exemplifies the typical Polygonum-type development, ensuring haploid (1n) ploidy in the gametes to support balanced fertilization outcomes.^[3]

Relation to Megasporogenesis and Microgametogenesis

Megagametogenesis directly follows megasporogenesis, the meiotic process that initiates female gametophyte development in angiosperms. During megasporogenesis, a diploid megasporocyte (or megaspore mother cell) within the ovule undergoes meiosis to produce four haploid megaspores arranged in a tetrad.^[8] In the majority of angiosperms, this tetrad forms linearly, with the chalazal-most megaspore surviving while the other three degenerate through programmed cell death, ensuring a single functional megaspore enters megagametogenesis.^[8] This degeneration mechanism promotes uniparental (maternal) inheritance by selecting one megaspore lineage for further development, preventing contributions from multiple megaspores.^[8] Although linear tetrad formation predominates in most angiosperms, variations exist, such as T-shaped or isobilateral arrangements, and in some cases, coenomeiotic processes where cytokinesis fails during meiosis, leading to bisporic or tetrasporic development without full megaspore separation.^[8] These patterns highlight the flexibility in megasporogenesis but consistently culminate in the initiation of megagametogenesis from one or more viable megaspore nuclei. Microgametogenesis serves as the male counterpart, occurring in the pollen sac (anther) where diploid microsporocytes undergo meiosis to yield four haploid microspores, all of which typically survive and develop into pollen grains containing a generative cell that divides to form two sperm cells.^[9] Unlike megagametogenesis, which results in a seven-celled female gametophyte (embryo sac), microgametogenesis produces a reduced three-celled male gametophyte, reflecting the highly streamlined gametophytes in angiosperms compared to the more elaborate versions in gymnosperms.^[9] These processes occur within the context of the angiosperm life cycle, characterized by alternation of generations where the dominant diploid sporophyte phase produces haploid spores via meiosis.^[9] Megagametogenesis thus represents the post-meiotic phase in the female lineage, embedded in the ovule of the sporophyte, paralleling the male lineage's progression to ensure gamete formation for double fertilization.^[9]

Core Developmental Process

Initial Mitotic Divisions

Following megasporogenesis, in which a single functional megaspore is selected from the tetrad produced by meiosis in the ovule, megagametogenesis commences in the Polygonum-type pattern, the most prevalent form observed in over 70% of angiosperm species.^[8] The functional megaspore, a haploid cell containing one nucleus, initiates the process by undergoing the first mitotic division, resulting in a binucleate stage where both nuclei remain within the same cytoplasm without cytokinesis.^[10] This division is free-nuclear, meaning no cell walls form, preserving a coenocytic structure that allows for subsequent nuclear proliferations. The second mitosis follows synchronously in each of the two nuclei, producing a tetranucleate stage with four haploid nuclei distributed within the elongated megagametophyte.^[8] Again, this division is mitotic and free-nuclear, with no accompanying cytokinesis, which maintains the syncytial organization and facilitates the spatial arrangement of nuclei along the micropylar-chalazal axis.^[10] The absence of cell plate formation during these early divisions is a hallmark of the Polygonum type, enabling efficient nuclear multiplication before later cellularization events. The third and final mitosis involves the synchronous division of all four nuclei, yielding an eight-nucleate stage that represents the completion of nuclear proliferation in this developmental phase.^[8] All three divisions are strictly mitotic, ensuring the maintenance of haploid nuclei throughout, and remain free-nuclear with delayed cytokinesis until the post-proliferation stages.^[10] This eight-nucleate coenocyte sets the foundation for the mature embryo sac in the standard Polygonum-type megagametogenesis.

Nuclear Migration and Cellular Differentiation

Following the three rounds of mitotic divisions that produce eight free nuclei within the coenocytic megagametophyte, these nuclei undergo directed migration along the micropylar-chalazal axis to establish positional cues for cellular organization. Three nuclei migrate toward the micropylar end, positioning themselves to form the egg apparatus, while three move to the chalazal end to become the antipodal cells; the remaining two polar nuclei relocate to the central region.^[11] This migration is primarily mediated by the actin cytoskeleton, which facilitates rapid and precise nuclear movement over distances within the elongated embryo sac, ensuring accurate spatial patterning.^[12]^[13] Subsequent cellularization involves the formation of cell walls around the positioned nuclei, resulting in a mature embryo sac comprising seven cells and eight nuclei. At the micropylar end, the egg apparatus differentiates into a single uninucleate egg cell and two synergid cells; the central region develops into a large binucleate central cell containing the polar nuclei, which may fuse to form a diploid nucleus prior to fertilization in some species or remain separate until double fertilization; and the chalazal end yields three uninucleate antipodal cells.^[14]^[15] Cell fate specification during differentiation is largely position-dependent, with transcriptional regulators and hormonal gradients, such as auxin, reinforcing the identity of each domain to prevent ectopic development.^[11] The differentiated cells acquire specialized functions critical for reproduction. Synergid cells develop a prominent filiform apparatus at their micropylar extremities—a thickened, labyrinthine cell wall structure that secretes chemotropic signals to attract and guide the pollen tube toward the embryo sac.^[16] The egg cell, positioned adjacent to the synergids, serves as the target for sperm fertilization to form the zygote and initiate embryogenesis. The central cell provides the site for endosperm formation following fusion with the second sperm nucleus, supporting nutrient storage for the developing seed. Antipodal cells, though variable across taxa, typically facilitate nutrient uptake or transfer early in development and often degenerate post-fertilization, although they may proliferate persistently in certain lineages to enhance resource allocation.^[14]^[15] This organized nuclear migration and cellular differentiation culminate in the canonical 7-celled, 8-nucleate embryo sac of the Polygonum-type development, predominant in over 70% of angiosperm species, priming the female gametophyte for double fertilization.^[11]

Types of Megagametogenesis

Monosporic Development

Monosporic development represents the most prevalent mode of megagametogenesis in angiosperms, wherein a single functional megaspore survives meiosis while the other three degenerate, leading to the formation of an embryo sac composed entirely of genetically identical (clonal) cells derived from that haploid megaspore.^[5] This process ensures haploid uniformity across all nuclei in the mature embryo sac, maintaining genetic consistency from the meiotic product and distinguishing it from apomixis, which circumvents meiosis to produce unreduced gametophytes.^[17] In this adaptation of the standard mitotic process, the isolated functional megaspore—typically the chalazal-most one in a linear tetrad—undergoes three sequential mitoses without initial cytokinesis, yielding a coenocytic eight-nucleate stage.^[5] A large central vacuole forms progressively, pushing the nuclei toward the poles and micropylar end to organize them into distinct domains: three at the micropylar pole (for the egg apparatus) and five at the chalazal pole (for antipodals and central cell).^[5] Cellularization follows the final mitosis, resulting in a seven-celled embryo sac with three antipodal cells, two synergids, one egg cell, and a binucleate central cell.^[17] This type dominates in approximately 70% of angiosperm species and is particularly characteristic of eudicots, where the Polygonum-type exemplifies the process in model organisms such as Arabidopsis thaliana.^[18]

Bisporic Development

Bisporic development in megagametogenesis is characterized by the contribution of nuclei from two adjacent megaspores within a dyad, formed due to the absence of a cell wall following meiosis II in the functional chalazal cell of the dyad. After meiosis I, cytokinesis produces a dyad of two cells, with the micropylar cell typically degenerating and the chalazal cell undergoing meiosis II without cytokinesis, resulting in a binucleate megaspore containing one nucleus derived from the micropylar megaspore (M3) and one from the chalazal megaspore (M4). This binucleate structure then serves as the progenitor for the embryo sac, introducing a chimeric composition where nuclei originate from genetically distinct sister megaspores with an average relatedness of 1/3.^[17] The developmental process proceeds through two successive mitotic divisions (bimitotic pattern) in this coenocytic binucleate cell. The first mitosis yields four nuclei: two derived from M3 at the micropylar end and two from M4 at the chalazal end. The second mitosis doubles this to eight nuclei, maintaining the separation into a micropylar quartet (from M3) and a chalazal quartet (from M4). Cellularization subsequently organizes these into the typical seven-celled, eight-nucleate embryo sac, with the three micropylar nuclei from M3 forming the egg apparatus (egg cell and two synergids), one M3 nucleus and one M4 nucleus fusing to form the diploid central cell, and the three M4 nuclei developing into antipodals, which often persist or degenerate depending on the species. This results in a genetically heterogeneous gametophyte, where the egg is derived from the micropylar megaspore lineage and the central cell incorporates contributions from both lineages, potentially enhancing intragametophytic genetic variation compared to the clonal monosporic type.^[17]^[5] Bisporic development is relatively rare among angiosperms, occurring in approximately 30 families (less than 20% of species), primarily in monocots and basal angiosperms such as the Podostemaceae, Liliaceae (e.g., Lilium-type with persistent antipodals), and Amaryllidaceae (e.g., Allium-type). In the Lilium-type, for instance, the embryo sac features prominent antipodals that may proliferate, while the Allium-type shows a more standard arrangement with transient antipodals. This pattern contrasts with the more prevalent monosporic development by allowing nuclear mixing across megaspore boundaries, though it remains less common overall.^[17]^[4]

Tetrasporic Development

Tetrasporic development represents a rare variant of megagametogenesis in angiosperms, where cytokinesis fails to occur after the meiotic divisions of the megaspore mother cell, resulting in a coenomegaspore comprising all four haploid megaspore nuclei without intervening cell walls.^[19] This syncytial structure then undergoes a series of mitotic divisions, typically one to three, to produce a mature embryo sac with 8 to 16 nuclei that differentiate into egg cell, synergids, antipodals, and a central cell, often featuring polyploid nuclei due to fusion events.^[20] Unlike the more common monosporic pattern, this type maximizes contributions from the entire meiotic tetrad, leading to chimeric nuclear genotypes within the embryo sac and potentially higher genetic diversity among gametophytic cells.^[19] The process exhibits significant variability across subtypes, classified based on the number of mitotic divisions and nuclear organization. In the Peperomia-type, prevalent in the Piperaceae family, the four megaspore nuclei each undergo two additional mitoses to yield 16 free nuclei that organize into the mature embryo sac, with the central cell containing multiple polar nuclei that confer polyploidy (up to 8n).^[21] The Adoxa-type, observed in Adoxaceae and some Liliaceae, involves only one mitotic division of the four nuclei to form an 8-nucleate embryo sac, where the chalazal nuclei may function as additional eggs or fuse into a polyploid central cell without dedicated antipodals.^[19] Other subtypes, such as the Fritillaria-type, feature spindle overlaps during mitosis leading to triploid chalazal nuclei, further enhancing ploidy variation.^[20] These patterns arise from modifications in cytokinesis and nuclear migration, regulated by genes like WUSCHEL and ARGONAUTE9 that influence cell fate specification.^[22] Tetrasporic embryo sacs occur in fewer than 30 angiosperm families (less than 10% of species), including basal groups like Piperaceae and Gunneraceae, as well as derived lineages such as Liliaceae and Plumbaginaceae, often resembling gymnosperm-like development in their polyploid central cells. This type promotes the highest level of genetic chimerism in the female gametophyte, with nuclei sharing only partial relatedness (e.g., sisters at 1/2, cousins at 1/4), potentially fostering intragametophytic competition.^[19] Recent studies in model plants, such as Arabidopsis mutants, underscore tetrasporic development's role in elucidating cell identity mechanisms, including epigenetic regulation via the RNA-directed DNA methylation pathway that ensures germline specification without degeneration.^[22]

Taxonomic Variations

Patterns in Eudicots

In eudicots, the largest clade of angiosperms comprising approximately 75% of all flowering plant species, megagametogenesis follows a predominantly monosporic pattern of the Polygonum type, in which a single functional megaspore undergoes three rounds of mitosis to form the seven-celled, eight-nucleate embryo sac.^[23] This developmental mode is conserved across highly diversified eudicot lineages, including rosids and asterids, and accounts for the majority of cases within the clade.^[24] Eudicot ovules are typically bitegmic, featuring two integuments that surround the nucellus and contribute to micropyle formation, and tenuinucellate, with a single-layered nucellus lacking substantial hypodermal tissue above the megasporocyte.^[25] A distinctive feature of the Polygonum-type embryo sac in eudicots is the transient nature of the antipodal cells, which often undergo rapid degeneration shortly after maturation or following fertilization, thereby limiting their role to potential short-term nutrient support.^[26] Prior to double fertilization, the two polar nuclei within the central cell fuse to form a diploid secondary nucleus, establishing a homodiploid (2n) state that is receptive to sperm fusion for endosperm formation.^[27] In the model eudicot Arabidopsis thaliana, the TSO1 gene plays a key regulatory role in cell specification and division during female gametophyte development, ensuring proper cytokinesis and oriented cell expansion in the ovule primordia and early embryo sac stages; mutations in TSO1 lead to disorganized nuclear divisions and incomplete cellularization.^[6] While the Polygonum type dominates, representing over 70% of angiosperm female gametophytes and an even higher proportion in eudicots, rare variations occur, including bisporic and tetrasporic modes in select families.^[23] For instance, members of the Plumbaginaceae exhibit tetrasporic development of the Plumbago type, where all four megaspore nuclei contribute to the embryo sac without cytokinesis after meiosis, resulting in a four-nucleate structure.^[28] Recent research has revisited the molecular mechanisms of female megaspore (FM) specification in eudicots, highlighting conserved transcriptional networks that integrate positional cues and hormonal signals for germline establishment. These patterns in eudicots are further linked to broader floral evolution through MADS-box genes, such as SHATTERPROOF1/2, which regulate ovule integument identity and indirectly support megagametophyte enclosure and development.^[29]

Patterns in Monocots and Basal Angiosperms

In monocots, megagametogenesis frequently exhibits bisporic or tetrasporic patterns, contrasting with the monosporic dominance observed in eudicots. A notable proportion of monocot species, particularly in families like Liliaceae and Amaryllidaceae, display non-monosporic development, contributing to greater variability in embryo sac structure and function.^[17] A representative bisporic example is the Lilium type, seen in lilies, where meiosis I in the megaspore mother cell forms a cell plate, the micropylar nucleus degenerates, and the chalazal binucleate cell undergoes three successive mitoses without cytokinesis, yielding eight free nuclei that later cellularize into a seven-celled embryo sac.^[30] The Allium type, a tetrasporic pattern in onions and related Allium species, involves all four meiotic products contributing nuclei; these undergo three mitoses to form a 16-nucleate embryo sac, with cellularization producing varied configurations including multiple polar nuclei.^[31] Persistent and often proliferating antipodals are a common feature in monocot embryo sacs, remaining viable post-fertilization to support endosperm development through nutrient transfer and cellular expansion.^[32] Basal angiosperms show even greater diversity in megagametogenesis, reflecting transitional forms between gymnosperm-like multicellular gametophytes and derived angiosperm patterns, with some retaining features suggestive of fused archegonia from gymnosperm ancestors. In Nymphaeales, such as water lilies (Nymphaea thermarum), development is monosporic, but the embryo sac is reduced to a four-nucleate, four-celled structure (one egg, two synergids, and a large central cell), achieved through early cellularization after two mitoses in the functional chalazal megaspore; this configuration lacks distinct archegonia and directly abuts the nucellus for nutrient exchange.^[33] In Amborellales, Amborella exhibits a unique tetrasporic, nine-nucleate embryo sac derived from all four megaspores. Tetrasporic patterns occur in other basal lineages, such as certain Piperales (e.g., Peperomia), where all four megaspores contribute to a 16-nucleate embryo sac, emphasizing plesiomorphic traits like nuclear multiplicity akin to gymnosperm megagametophytes with multiple archegonia.^[34] Overall, these variations in basal groups highlight an evolutionary bridge, with embryo sacs often exhibiting reduced cellularization and persistent nucellar tissues that echo gymnosperm organization.^[18] A 2024 investigation into Actinidia arguta detailed monosporic Polygonum-type morphogenesis, with sequential stages of ovule priming and embryo sac maturation providing insights into conserved patterns in eudicots.^[35]

Post-Developmental Events

Integration with Double Fertilization

Following the maturation of the embryo sac, integration with double fertilization occurs as the pollen tube, generated from a germinated pollen grain on the stigma, navigates through the style toward the ovule. The pollen tube enters the ovule via the micropyle, an opening in the integuments, where it is precisely guided by chemical signals from the synergid cells flanking the egg cell in the mature embryo sac.^[36]^[37] Upon reaching the synergids, the pollen tube tip ruptures, releasing two immobile sperm cells into the embryo sac.^[38] Double fertilization, a hallmark process unique to angiosperms, ensues as one sperm cell fuses with the haploid egg cell to form the diploid zygote, which will develop into the embryo, while the second sperm cell fuses with the diploid central cell to produce the triploid primary endosperm cell in monosporic embryo sacs.^[39]^[40] This coordinated fusion ensures the formation of both embryonic and nutritive tissues, with the endosperm providing heterotrophic nutrition to the developing embryo.^[41] The receptive synergid cell, which attracts and receives the pollen tube, undergoes programmed cell death upon its arrival, enabling perception of the pollen tube contents and facilitating sperm release while helping to prevent additional pollen tube attractions.^[42]^[43]^[44] The persistent synergid, the non-receptive counterpart, typically degenerates post-fertilization to avoid polytubey (multiple pollen tube entries), a process regulated by ethylene signaling and reactive oxygen species (ROS) homeostasis.^[45]^[46] In mutants exhibiting persistent synergid syndrome, such as those defective in the ARABINOGALACTAN PROTEIN 4 (AGP4)/JAGGER gene, the persistent synergid fails to degenerate promptly, leading to supernumerary pollen tube attractions and disrupted fertilization efficiency.^[47] Recent studies in model plants like Arabidopsis thaliana have elucidated cell communication mechanisms during this integration, including paternal signals from sperm cells that trigger central cell proliferation upon fusion and small secreted peptides like EGG CELL 1 (EC1) that promote sperm activation and preferential fertilization.^[41]^[48] These findings highlight intercellular signaling networks, involving RALF peptides and ROS-mediated apoptosis, that ensure precise timing and fidelity of double fertilization.^[45]

Contribution to Seed and Endosperm Formation

Following double fertilization, the central cell of the embryo sac fuses with a sperm cell to initiate endosperm development, forming a triploid primary endosperm cell with a 2:1 maternal-to-paternal genome ratio that serves as the primary nutritive tissue for the developing embryo.^[49] Endosperm development proceeds through distinct modes: nuclear (syncytial) endosperm, characterized by free nuclear divisions without cell walls, allowing rapid proliferation; or cellular endosperm, where synchronous cytokineses establish cell boundaries early on, progressing from the periphery inward to accumulate storage reserves like starch, proteins, and lipids that nourish the embryo.^[49] These modes vary across angiosperms, with nuclear types predominant in many eudicots and monocots for efficient nutrient mobilization.^[50] In parallel, the fertilized egg cell (zygote) undergoes asymmetric division to form the embryo proper and suspensor, with subsequent mitotic divisions establishing the embryonic axis and organ primordia, while the surrounding integuments differentiate into the protective seed coat, enclosing and shielding the embryo and endosperm.^[51] In cereals such as maize and wheat, the endosperm further specializes, developing an outer aleurone layer—a single to few-celled periphery rich in enzymes and storage compounds—that persists post-germination to hydrolyze reserves for seedling growth.^[52] Variations in megagametogenesis influence endosperm ploidy, with monosporic development typically yielding triploid (3n) endosperm from the fusion of two maternal polar nuclei and one paternal genome, whereas bisporic or tetrasporic types can produce higher ploidy levels, such as pentaploid (5n) in certain tetrasporic patterns where the central cell contributes four maternal genomes.^[50] These ploidy differences affect nutrient allocation, seed size, and developmental timing, playing key roles in seed dormancy and germination success by modulating storage capacity and hormonal balance.^[17] Post-fertilization, the embryo sac's accessory cells degenerate as the endosperm and embryo expand, reallocating resources to seed maturation. Additionally, epigenetic mechanisms, including small RNA-directed imprinting in the central cell, regulate endosperm development post-fertilization, influencing gene expression and nutrient allocation.^[53] Recent studies highlight that the pre-fertilization state of the female gametophyte correlates with post-developmental seed viability, with disruptions in embryo sac integrity linked to reduced longevity and germination rates in crops like rice.^[54]

Evolutionary and Applied Aspects

Evolutionary Origins and Diversity

Megagametogenesis in angiosperms evolved from the more complex, multi-archegonial female gametophytes of gymnosperms, where multiple egg cells form within a multicellular structure, to a highly reduced, typically seven-celled embryo sac that supports double fertilization.^[55] This transition coincided with the origin of angiosperms during the Early Cretaceous, approximately 140-145 million years ago, though recent fossil evidence suggests possible earlier origins in the Jurassic around 174 million years ago, marking a key innovation in seed plant reproduction.^[56]^[57] Double fertilization, in which one sperm fertilizes the egg to form the embryo and another fuses with the central cell to produce triploid endosperm, arose as a defining feature of early angiosperms, likely adapting the ancestral gymnosperm-like process to enhance nutrient provisioning for the embryo.^[58] The precise evolutionary mechanism remains debated, but fossil and phylogenetic evidence indicates it emerged with the diversification of basal angiosperm lineages.^[59] Diversity in megagametogenesis across angiosperms stems from variations in megasporogenesis patterns—monosporic, bisporic, and tetrasporic—leading to differences in embryo sac ploidy and cellular organization. Monosporic development, where only the chalazal megaspore survives to form a haploid gametophyte, is considered ancestral and predominant in derived eudicots, while bisporic and tetrasporic modes, involving shared cytoplasm from multiple megaspores, are derived and have arisen multiple times independently.^[60] These ploidy shifts influence endosperm formation, with tetrasporic types yielding higher ploidy levels (e.g., 4n to 8n central cells) that may optimize resource allocation in diverse ecological niches.^[61] Gene duplications, particularly in MADS-box transcription factors, have driven this diversification; for instance, type I MADS-box genes like AGL23 regulate megagametophyte initiation, while type II genes such as SHP1/2 coordinate ovule identity and cell divisions essential for gametophyte maturation.^[62]^[29] Basal angiosperms exhibit diverse embryo sac types, including the monosporic Nuphar-type in Nymphaeales like Nuphar and tetrasporic types in lineages such as Amborella, reflecting greater ontogenetic lability in early-diverging groups.^[11]^[63] Recent studies highlight how polyploidy correlates with variable gametophyte development and apomixis in basal and monocot lineages, potentially enhancing reproductive flexibility. In polyploid Asteraceae species, megagametogenesis remains conserved despite ploidy variation, suggesting stabilizing selection on core processes while allowing peripheral adaptations.^[64]^[65] Apomixis, an asexual deviation bypassing meiosis in megagametogenesis, represents a derived trait evolving from sexual pathways, primarily in polyploid contexts to maintain hybrid vigor without genetic recombination. Monocots and basal angiosperms exhibit greater variability in these patterns compared to the streamlined monosporic efficiency in eudicots, underscoring adaptive transitions tied to angiosperm radiation.

Implications for Plant Breeding and Biotechnology

Manipulation of megagametogenesis variations plays a pivotal role in plant breeding by enabling the induction of apomixis, which fixes hybrid vigor and simplifies seed production in crops. In maize, interspecific hybridization with Tripsacum dactyloides has yielded apomictic 56-chromosome lines with restored fertility, allowing the clonal propagation of high-yielding hybrids without the need for repeated parental crosses. This breakthrough addresses limitations in traditional breeding, where heterosis is lost in subsequent generations, and has potential applications in staple crops to enhance yield stability. Biotechnological interventions, particularly CRISPR/Cas9 genome editing, target genes regulating female germline development to alter megagametogenesis pathways. Recent advances include editing regulators of germline specification, as highlighted in comprehensive reviews of female germline mechanisms, enabling precise control over gamete formation and fertility. In sunflower (Helianthus annuus), the identification of facultative parthenogenesis—termed "virgin births"—facilitates clonal propagation via haploid or diploid embryo formation, accelerating the development of disease-resistant and high-oil-content varieties without sexual recombination. Monocot cereals, which predominantly exhibit the Polygonum-type monosporic megagametogenesis, supply over 50% of global human caloric intake through crops like rice, wheat, and maize. Breeding efforts in these polyploid species face challenges from genomic redundancy and meiotic irregularities, complicating fertility restoration and trait introgression. As of 2025, advances in CRISPR applications for germline engineering in monocot cereals offer strategies to overcome these barriers and improve crop resilience in hybrid systems.^[66] Emerging synthetic biology tools hold promise for designing artificial gametophytes that incorporate climate-resilience traits, such as drought tolerance, directly into reproductive lineages of major crops. These approaches could integrate with endosperm formation to boost seed yield under abiotic stress, building on recent progress in haploid induction and genome editing.

References

[1]
14.1 Gametogenesis – The Science of Plants
The lone surviving megaspore subsequently undergoes three mitotic divisions to form eight haploid cells that are held within the ovule in an embryo sac. These ...
[2]
Megasporogenesis, Megagametogenesis and Ontogeny of the Aril ...
The functional megaspore undergoes three successive mitotic divisions to produce an 8-nucleate megagametophyte. The first two nuclei migrate to the poles of the ...
[3]
[PDF] Megagametogenesis in Arabidopsis wild type and the Gf mutant
Megagametogenesis in Arabidopsis is divided into eight stages, with syn-ergid cell degeneration triggered by pollination. The Gf mutant affects an early step.
[4]
Megagametogenesis - an overview | ScienceDirect Topics
Megagametogenesis is the development of the female gametophyte from the haploid product(s) of meiosis. The particular type of megagametogenesis is a function of ...
[5]
The Female Gametophyte - PMC - PubMed Central
Dec 26, 2011 · The angiosperm female gametophyte is critical for plant reproduction. It contains the egg cell and central cell that become fertilized and give rise to the ...
[6]
The Female Gametophyte - BioOne
Dec 1, 2011 · The angiosperm female gametophyte is critical for plant reproduction. It contains the egg cell and central cell that become fertilized and give rise to the ...
[7]
Apomictic and Sexual Germline Development Differ with Respect to ...
In contrast to sexual reproduction, in apomicts the first cell of the female reproductive lineage omits or aborts meiosis (apomeiosis) to initiate gamete ...
[8]
https://doi.org/10.1199/tab.0155
[9]
https://doi.org/10.3389/fpls.2014.00452
[10]
https://doi.org/10.1105/tpc.018192
[11]
Pattern formation in miniature: the female gametophyte of flowering ...
Jan 15, 2010 · (B) In the Polygonum-type embryo sac (Arabidopsis, maize), the functional megaspore undergoes three sequential mitotic divisions that generate ...
[12]
Plant egg cell fate determination depends on its exact position in ...
Feb 17, 2021 · Migration of the nuclei is dependent on actin filaments, because a high concentration of LATB inhibits nuclear migration (Fig. 2C).
[13]
Gamete Nuclear Migration in Animals and Plants - Frontiers
Among land plants, ferns use microtubule, whereas angiosperms use F-actin for sperm nuclear migration. The arrow shows the overall direction of the nuclear ...<|control11|><|separator|>
[14]
The female gametophyte: an emerging model for cell type-specific ...
During megagametogenesis, the mature female gametophyte is formed through mitotic divisions, nuclear migration, and cellularization. For the mature embryo sac, ...
[15]
Patterning the Female Gametophyte of Flowering Plants
E and F, Nuclear migration (E) and cellularization (F) compartmentalize the FG. G, Mature FG with fused polar nuclei. H to K, Schematic representation of FG ...
[16]
Developmental, ultrastructural and cytochemical investigations of ...
Nov 14, 2020 · New embryological information about the processes of megasporogenesis and megagametogenesis provided in this paper expand the current knowledge ...<|separator|>
[17]
Poles Apart: Monosporic, Bisporic, and Tetrasporic Embryo Sacs ...
Sep 14, 2020 · In most angiosperms with monosporic development, the megaspore nearest the chalaza (M4) is the germinal megaspore and the somatic megaspores (M ...
[18]
Comparative Ovule and Megagametophyte Development in ...
Occasional development of more than one functional megaspore from a multicellular archesporium is common in angiosperms (Maheshwari, 1950), but multiple ...Ovule Development In... · Megasporogenesis And... · Nucellus And Integument...<|control11|><|separator|>
[19]
https://doi.org/10.3389/fevo.2020.516640
[20]
[PDF] A CRITICAL REVIEW OF THE TYPES OF EMBRYO SACS IN ...
In tetrasporic embryo sacs no permanent walls are laid down during the reduction divisions, and all four megaspore nuclei take part in the formation of the ...
[21]
https://doi.org/10.3732/ajb.95.3.272
[22]
[PDF] Understanding megasporogenesis through model plants
Apr 2, 2024 · The role of genes and transcription fac- tors in the development of the MMC and in the regulation of various processes studied in selected model.
[23]
https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.00128/full
[24]
Identifying and Engineering Genes for Parthenogenesis in Plants
Circa 70% of the angiosperm species produce female gametophytes of the Polygonum-type, which consists of seven cells only: two gametes, the egg cell and central ...
[25]
[PDF] Endosperm formation in aposporous Crataegus (Rosaceae ...
Sep 13, 2006 · highly diversified clades in both monocots and eudicots have the Polygonum type of embryo sac, where the endosperm. Blackwell Publishing Ltd.
[26]
Angiosperm ovules: diversity, development, evolution - PMC
Although ovules have their own developmental dynamics, some structural properties of ovules, such as curvature and symmetry, are dependent on their position in ...
[27]
Developmental origins of the conjoined twin mature embryo sacs in ...
Dec 1, 2013 · A conspicuous common feature for both single mature embryo sacs and conjoined twins is the rapid degeneration of the antipodals, which is ...
[28]
The gametic central cell of Arabidopsis determines the lifespan of ...
The central cell initially comprises two haploid polar nuclei, which, in many flowering plant species, fuse before fertilization, generating a diploid secondary ...
[29]
Summary APG II
PLUMBAGINACEAE Jussieu - A opposite C, ovule circinotropous, funicle long and curved, embryo sac tetrasporic, 4-nucleate [Plumbago-type]. - 27/836.
[30]
The MADS Box Genes ABS, SHP1, and SHP2 Are Essential for the ...
Two closely related MADS-box genes, SHATTERPROOF 1 and 2 (SHP1 and SHP2) are involved in specifying ovule integument identity in Arabidopsis thaliana.
[31]
Megasporogenesis - an overview | ScienceDirect Topics
In angiosperms, male and female gametes are produced during microsporogenesis and megasporogenesis, respectively. ... The alternation of generation. The sexual ...
[32]
Megasporogenesis: Process, Types, Stages, Significance
Feb 21, 2025 · Degeneration of Megaspore mother cell– Degeneration of MMC caused by genetic defects, environmental stress, and nutrient deficiency prevents ...
[33]
Central Cell in Flowering Plants: Specification, Signaling, and ...
Oct 20, 2020 · Hence, the central cell is homodiploid (2n) in most angiosperms and polyploidy in some other species due to fusion of more than two nuclei ...Missing: eudicots | Show results with:eudicots
[34]
Floral biology and ovule and seed ontogeny of Nymphaea ...
Floral biology and ovule and seed ontogeny of Nymphaea thermarum, a water lily at the brink of extinction with potential as a model system for basal angiosperms.
[35]
Female gametophyte & early seed development in Peperomia
Jan 1, 2010 · All female gametophytes in Peperomia contain one egg cell; however, synergid number may range from zero (52) to four (52), and antipodal cell ...Missing: non- source
[36]
https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.15420
[37]
The role of the integuments in pollen tube guidance in flowering plants
Aug 31, 2018 · In angiosperms, pollen tube entry into the ovule generally takes place through the micropyle, but the exact role of the micropyle in pollen ...
[38]
Chemical signaling for pollen tube guidance at a glance
Jan 29, 2018 · The pollen tube grows through the pistil and reaches the ovule, where it invades the synergid cell and subsequently ruptures; this leads to the ...Introduction · Emergence Of Pollen Tubes... · Ovular Guidance<|separator|>
[39]
Synergid Cell Death in Arabidopsis Is Triggered following Direct ...
These results suggest that synergid cell death occurs after entry of the pollen tube into the ovule micropyle. To validate these observations, we scored pollen ...
[40]
Fertilization Mechanisms in Flowering Plants - PMC - PubMed Central
Following release of the two sperm cells they interact and fuse with two dimorphic female gametes (egg and central cell) forming the major seed components ...
[41]
The beginning of a seed: regulatory mechanisms of double fertilization
In angiosperms, the haploid gametophytic generations produce the male and female gametes required to execute double fertilization. Both gametophytes are reduced ...Missing: comparison | Show results with:comparison
[42]
A paternal signal induces endosperm proliferation upon fertilization ...
Feb 8, 2024 · Using Arabidopsis, we show that a sperm-derived signal induces the proliferation of a female gamete, the central cell, precisely upon fertilization.Missing: studies | Show results with:studies
[43]
Synergid Cell Death in Arabidopsis Is Triggered following Direct ...
We demonstrate that synergid cell death initiates after the pollen tube arrives at the female gametophyte but before pollen tube discharge.
[44]
Ethylene Signaling Is Required for Synergid Degeneration and the ...
We show that the process of fertilization activates an ethylene-response cascade, which is required for synergid cell death and concomitant formation of a ...
[45]
Fertilization-induced synergid cell death by RALF12-triggered ROS ...
Mar 29, 2025 · We observed that before pollination, synergid mitochondria constitutively accumulate alongside a compartment that we name filiform apparatus ...<|control11|><|separator|>
[46]
ROS homeostasis mediated by MPK4 and SUMM2 determines ...
Apr 1, 2022 · Mutations in the highly glycosylated ARABINOGALACTAN PROTEIN 4 (AGP4)/ JAGGER affects the persistent synergid's cell death after fertilization ...Missing: syndrome | Show results with:syndrome
[47]
JAGGER, an AGP essential for persistent synergid degeneration ...
Aug 2, 2016 · This persistent synergid, is, as we suggested, the cell responsible for attracting an extra pollen tube into the embryo sacs.Missing: syndrome | Show results with:syndrome
[48]
EGG CELL 1 contributes to egg-cell-dependent preferential ...
We also found that sperm-activating small secreted EGG CELL 1 proteins are involved in the regulation of egg-cell-dependent preferential fertilization.
[49]
Mechanisms of endosperm initiation | Plant Reproduction
Jul 23, 2016 · Here, we review recent discoveries concerning the developmental events and signalling of FG maturation, fertilization, and endosperm development.Fertilization · Endosperm Initiation And... · Autonomous Endosperm...Missing: paper | Show results with:paper<|control11|><|separator|>
[50]
Endosperm of Angiosperms and Genomic Imprinting - PMC - NIH
Jul 3, 2020 · The Polygonum-type embryo sac has been found in 80% of the examined angiosperm species in 239 families out of about 295 recognized ones. The ...
[51]
The beginning of a seed: regulatory mechanisms of double fertilization
The launch of seed development in flowering plants (angiosperms) is initiated by the process of double fertilization.
[52]
Endosperm: food for humankind and fodder for scientific discoveries
May 29, 2012 · The endosperm nurtures embryo development and, in some species including cereals, stores the seed reserves and represents a major source of food for humankind.
[53]
Female gametophyte development, pollen‒pistil interactions and ...
Jun 25, 2025 · During megagametogenesis, the surviving megaspore undergoes three mitotic divisions to form the mature female gametophyte (FG), which is ...
[54]
[PDF] Insights into the Molecular Evolution of Fertilization Mechanism in ...
Jan 6, 2021 · ploidy from an evolutionary perspective. 21. In gymnosperms, different forms of double fertilization have been observed (Figure 2). In the.
[55]
The Angiosperm Terrestrial Revolution and the origins of modern ...
Oct 26, 2021 · Flowering plants first appeared in the fossil record during the Early Cretaceous and came to dominate plant diversity by the Late Cretaceous ( ...
[56]
Full article: The origin of double fertilization in flowering plants
In flowering plants (angiosperms), embryogenesis starts with two parallel events: the egg cell fuses with a sperm cell to form the embryo and the two polar ...Missing: time | Show results with:time
[57]
Evolutionary origins of the endosperm in flowering plants - PMC
Triploidy of the endosperm Apart from those exceptions producing endosperm with high ploidy, the widespread occurrence of triploid endosperm among angiosperms ...
[58]
Comparative Ovule and Megagametophyte Development in ...
Current evidence suggests that bisporic and tetrasporic embryo sacs are derived conditions that have evolved multiple times in angiosperms, especially in ...Comparative Ovule And... · Ovule Development In... · Nucellus And Integument...<|control11|><|separator|>
[59]
[PDF] Ploidy and the Evolution of Endosperm of Flowering Plants
In some angiosperms, endosperms derived from poly- sporic gametophytes are alternatives to 1m:1p or 2m:1p endosperms. In a tetrasporic or a bisporic gametophyte ...
[60]
AGL23, a type I MADS‐box gene that controls female gametophyte ...
Mar 13, 2008 · Our data show that AGL23 is important in the initiation of megagametogenesis and plays a key role during embryogenesis. The agl23 mutant shows, ...Results · Discussion · Plasmid Construction And...
[61]
https://www.zoology.ubc.ca/let/pdfs/Cailleau-etal_2010.pdf
[62]
The Rise of Apomixis in Natural Plant Populations - Frontiers
Apomixis, the asexual reproduction via seed, has many potential applications for plant breeding by maintaining desirable genotypes over generations.Abstract · Introduction · The Foundational Phase: the... · Natural VS. Cultivated...