Megasporogenesis refers to the formation and development of haploid megaspores from the megaspore mother cell through meiosis. It occurs within the megasporangia or ovules present within the ovary of the carpel.
The carpel or Megasporophyll
The gynoecium is the female reproductive part of the flower. The unit of gynoecium is called carpel which represents the megasporophyll. Each carpel has three parts, stigma, style, and ovary.
Stigma is the terminal receptive disc and serves as the landing platform for pollen grains. The slender stalk-like structure present below the stigma is known as style. The lower swollen part of the carpel is called the ovary. The ovary contains ovules in its chamber or locules.
The megasporangium (Ovule)
The integumented megasporangium is called an ovule. The ovule has a stalk and a body.
Parts of ovule
Funicle– The ovule remains attached to the placenta by a stalk called funicle or funiculus.
Hilum– The point of attachment of the funicle to the body of the ovule is called the hilum.
Nucellus- The main body of the ovule is made up of a mass of thin-walled parenchymatous cells called the nucellus.
Integuments– The one or two multicellular layers that enclose the nucellus except at the apex are called the integuments.
Micropyle– The small opening at the apex is called a micropyle.
Chalaza– The basal part of the ovule from where the integuments arise is known as chalaza. It is the seat for various biochemical reactions.
Raphe– It is the ridge-like extension of the funicle along the side of the anatropous ovule is called raphe.
At the centre lies the embryo sac and the mature embryo sac contains the egg, synergids, polar nuclei, and three antipodal cells which are formed after megagametogenesis.
Types of ovule based on integuments
Ategmic ovule– In this type, ovules contains no integument. This type of ovule can be seen in Santalum, Loranthus etc.
Unitegmic ovule– This type of ovule contains one integument. Examples are Cycas and Pinus.
Bitegmic ovule– This type of ovule contains two integuments. Examples are Hibiscus, and some monocots.
Tritegmic ovule– This type of ovule contains three integuments. Example- Asphodelus.
Types of ovule based on nucellus
Teninucellate ovule– It is a type of ovule with a thin and underdeveloped nucellus. Example – Members of the family Compositae.
Crassinucellate ovule– It is a type of ovule with a thick and well-developed nucellus. Example- It is mainly found in gymnosperms.
Types of ovule based on position
Orthotropus ovule– In this type, the body of the ovule is straight, and the micropyle, chalaza, and funicle lie on the same axis. Example Cycas, Polygonum, etc.
Anatropus ovule– It is the most common type of ovule in angiosperms in which the body of the ovule is inverted due to the curvature of the funicle at its apex. The micropyle lies near the hilum and the chalazal end lies upwards.
Example- Family Fabaceae
Hemitropus/ hemianatropus ovule- This is the type of ovule in which the body is twisted halfway thus it becomes the right angle to the funicle. In this type, the micropyle and chalaza lie perpendicular to that of the funicle. Example- Ranunculus, Primula etc.
Amphitropus ovule- In this type, the ovule is horse-shoe-shaped. Example– Family-Butomaceae.
Campylotropus ovule– In this type, the ovule is bent more or less right angle to the funicle. However, the micropylar end shows more curvature so that the ovule looks bean-shaped. Example – Family Cruciferae
Circinotropus ovule– In this type, the funicle is very long surrounds the body of the ovule, and looks like a coiled structure. Example- Opuntia.
Stages of megasporogenesis in angiosperms
One of the nucellar cells in the micropylar region is differentiated into primary archesporial cells or archesporial initial. The cell is characterized by dense cytoplasm and a prominent nucleus.
The archesporial cell divides periclinally to form an outer primary parietal cell and an inner primary sporogenous cell.
The primary sporogenous cell acts as a megaspore mother cell (MMC). It divides meiotically to form four haploid megaspores.
The four haploid megaspores are generally arranged in a linear tetrad.
Generally, the lowermost or chalazal megaspore remains functional out of the tetrad of megaspores, and the other three lie towards the micropyle degenerates.
The functional megaspore enlarges and prepares for further development into the female gametophyte.
Types of embryo sac development
Generally, the functional megaspore can follow any of three patterns to develop into an embryo sac depending upon the number of megaspore nuclei contributing to its formation-
Monosporic development– The development of the embryo sac takes part from only one megaspore nucleus. This kind of embryo sac development can be seen in Polygonum sp.
Bisporic development– In this type of embryo sac development, the two megaspore nuclei remain functional and contribute to developing the embryo sac. This kind of embryo sac development can be seen in Allium sp.
Tetrasporic development– In this case, all four megaspore nuclei remain functional and contribute to the formation of the embryo sac without wall formation after meiosis. Example Peperomia sp.
Role of MMC in ovule development
Megaspore mother cell is responsible for megasporogenesis and ovule development. It ensures genetic variation by recombination during meiosis which enhances the genetic diversity of the offspring. The cells produced by MMC are the precursors of the embryo sac which includes the egg cell, synergids, antipodal cells, and polar nuclei. All of these are required during fertilization. The megaspore mother cell undergoes meiosis and this helps in maintaining the ploidy of the offspring when the two gametes fuse.
Common abnormalities affecting megasporogenesis
Failure of meiosis– Meiotic division may fail or precede incorrectly which fails the formation of haploid megaspores. This may lead to infertility or abnormal embryo sac development.
Degeneration of Megaspore mother cell– Degeneration of MMC caused by genetic defects, environmental stress, and nutrient deficiency prevents megaspore formation.
Abnormal tetrad formation– Errors during meiosis lead to abnormal tetrad formation or fused megaspores which affect the selection of functional megaspores and embryo sac development.
Aneuploidy or polyploidy– Aneuploid megaspore is not viable and polyploidy leads to aberrant embryo sac formation.
Micropylar-chalazal polarity defects– Anomalies in the establishment of polarity can cause the functional megaspore to be misplaced or the embryo sac to be oriented improperly.
Incomplete Development of the Megaspore– Functional megaspores may not expand or undergo subsequent mitotic divisions necessary for gametophyte development. Sterile ovules or misshapen embryo sacs can occur.
Mutations in Regulatory Genes– Mutations in genes that regulate meiosis or megagametogenesis can cause defective megaspores. Examples include disruptions to genes such as AGO9 and WUSCHEL, which are important for development.
Environmental Stress-Induced Abnormalities– Extreme temperatures, drought, or salinity may cause abnormal meiotic divisions or megaspore degeneration in response to the stress factor.
Disruption in Tapetal Support– The developing megaspores are nurtured by the tapetum. When this component fails or deteriorates earlier than it ought to have, the resulting megaspores are often poorly developed or non-viable.
Advancements in research on megasporogenesis
- Advancements in molecular biology have revealed some of the regulatory genes such as WUSCHEL, and SPOROCYTELESS determine the differentiation of MMC. The epigenetic mechanism helps in fine-tuning gene expression through RNA and chromatin remodeling.
- Regulation of megasporogenesis through hormones such as auxins, cytokinins, and gibberellins helps in the growth and development of MMC and surrounding tissues to ensure proper development of the embryo sac.
- High-resolution imaging technologies such as electron microscopy are used for observation of megasporogenesis at the cellular and sub-cellular level which provides detailed information in the development of megaspores.
- The gene-editing technologies, such as CRISPR/Cas9, have made it possible to precisely manipulate genes involved in megasporogenesis. Such advances help in studying gene functions and developing genetically modified plants with improved fertility, stress tolerance, and hybrid vigor.
- Transcriptomics, proteomics, and metabolomics have exposed the intricate networks of genes, proteins, and metabolites that are active during megasporogenesis. The new markers and regulatory pathways have been identified through such comprehensive studies, which help to explain this complex process better.
- Environmental conditions such as temperature, light, and nutrient availability play a significant role in megasporogenesis. New studies have recently shown how plants activate stress-responsive pathways to protect ovule development under adverse conditions, ensuring reproductive success.
- The development of artificial ovule cultures in controlled environments (in vitro) enables advanced research on developing stress-resistant and high-yielding species.
- Epigenetic changes, including DNA methylation and histone acetylation, play a significant role in controlling gene expression during megasporogenesis. Such changes lead to proper cell differentiation and developmental progression and have consequences for transgenerational inheritance.
- Non-coding RNAs, such as microRNAs and long non-coding RNAs have recently emerged as key regulators of megasporogenesis. They play a role in regulating gene expression to coordinate the development of the MMC and surrounding tissues.
Key applications of megasporogenesis studies in biotechnology
- The developmental basis of megasporogenesis helps in hybrid seed production by ensuring controlled gametophyte development. Techniques such as Cytoplasmic male sterility CMS can be used in hybrid seed production.
- Biotechnological studies help in the identification and correction of genetic defects leading to improved fertility. It also helps in restoring fertility in plants with abnormal megaspore development.
- The application of biotechnology helped in understanding a phenomenon called apomixes and implementing apomixes into crops allows the production of genetically identical high-yielding plants without the need for hybrid production.
- Gene editing tools help in developing transgenic plants with reproduction efficiency, stress tolerance, and fertility. For example, CRISPR/Cas9 and other gene editing tools have been used in order to introduce changes to the genes, which would enhance the possibility of regulation over reproductive traits.
- Genetic engineering of stress-tolerant pathways helps in the production of ovules during stressful environmental conditions.
- Biotechnological strategies such as ovule culture help in the conservation of threatened plant species and also promote species with poor reproductive ability.
Megasporogenesis across different plant families
Variations in embryo sac development- The most common type of embryo sac development is the monosporic or polygonum type. It is found in almost 70% of angiosperms. However, we can find bisporic or allium types in families like Liliaceae. Tetrasporic type can be found in families Piperaceae.
Tetrad arrangement– Linear tetrad arrangement is common in families such as Poaceae, whereas decussate tetrad arrangements can be seen in families like Lamiaceae.
Ovule structure and orientation– The structure of ovules and their orientation vary amongst families, e.g. Anatropus ovules are dominant in families like Fabaceae, Orthotropus in Piperaceae, and Campylotropus in Chenopodiaceae, representing adaptations to their reproduction.
Polyploidy- It is a condition common mostly in Brassicaceae, and Poaceae, which increases variation and flexibility in the gametophyte.
Megagametogenesis
Megaspore is the first cell of the female gametophyte which grows in size and obtains nutrition from the nucellus.
The nucleus of the megaspore divides mitotically to form two nuclei. Each nucleus moves towards the opposite pole and both the nuclei at the poles divide twice mitotically to form 4 nuclei at each pole.
Out of four nuclei, one nucleus migrates from each of the poles toward the center and is known as a polar nuclei. The remaining three nuclei at each pole are surrounded by cytoplasm to form cells due to cytokinesis. Out of the three cells towards the micropylar end, one cell is large and more distinct which is called an egg cell, and the remaining two smaller cells are called synergids.
These three cells are collectively known as egg apparatus. The three cells formed at the chalaza are called antipodal cells. Both the polar nuclei lie at the centre and fuse just before fertilization to form a secondary nucleus (diploid). Therefore a seven-celled, eight-nucleated structure is formed inside the embryo sac.
This type of embryosac is known as polygonum type. Fingers-like processes are formed from the outer wall of synergids which are known as filiform apparatus. With the help of the filiform apparatus, the synergids absorb food from the nucellus and transfer it to the embryo. Filiform apparatus also secrete chemicals to attract pollen tubes.
References
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