Before diving deep, let`s first understand the definition of megasporogenesis. Megasporogenesis is defined as the process of haploid megaspore formation by meiotic division of diploid megaspore (MMC) mother cells into the megasporangium (egg). The haploid megaspore undergoes repeated mitotic divisions to form the embryo sac. Figure 11.10. Examples of embryological development. (Note: micropyle below in total.) A–D. Monosporic megasporogenesis. A. Oocyte with simple megasporocytes. B.
After the first meiotic division. C. Second Meiotic Division. D. After second meiotic division with four megaspores; F.M. = functional (proximal) megaspore. E. Female gametophus of polygon type (embryonic sac), with 3 antipodes, 2 polar nuclei and 2 of the 3 cells of the egg apparatus. F,G.
Megagametogenesis of Lilium sp., with tetrasporic development, female gametophyte of the Fritillaria type. F. Whole (anatropic) ovum, with mature female gametophyte (e.g. G. Mature female gametophorus in close-up showing 2 of the 3 triploid antipodes, 2 polar nuclei (one haploid, the other triploid) and haploid cells of the ovoid apparatus. A flower has two different parts for reproduction: its pollen sacs and its gynoecium. These largely refer to the “male” and “female” parts of the flower. The gynoecium is where megaspoorogenesis takes place.
A cell called a megasporocyte divides into four parts per meiosis, and these four parts become four megaspores. In general, only one of these megaspores continues to live while the others collapse. The surviving megaspore becomes the embryonic sac of the plant in which the egg is located. This megaspore is eventually fertilized by pollen from the same plant or another. The development of female gametophytes occurs in two phases, called megasporogenesis and megagametogenesis. The type of Polygonum development is observed in more than 70% of angiosperms such as Brassicaceae, Gramineae, Malvaceae, Leguminosae and Solanaceae (Huang and Russell, 1992; Maheshwari, 1950). The mother cell of megaspore (2n) undergoes meiosis during megasporogenesis and leads to four haploid nuclei. Of these four haploid nuclei, three degenerate and one functional megaspore are divided mitotically, resulting in an embryonic sac (mature female gametophyte). The first mitotic division of the megaspore resulted in a structure with two nuclei called the binuclear megaspore. These nuclei move to opposite poles of the developing embryo sac and again undergo two mitotic cell divisions to form four haploid state nuclei at each polar end.
A nucleus at each end approaches the center, followed by fusion to form a primary endosperm nucleus. The three nuclei, both at the micropylier and chalazalene, each form two synergistic cells, an ovum and three antipodian cells. At maturity, the female gametophyte is therefore an “eight-cell seven-cell structure or embryonic sac” (Fig. 11.2A). NEW ONLINE: Arabidopsis nucleoporin NUP1 is essential for megasporogenesis and early stages of pollen development t.co/q0IUgAOwo1 In this article, we learn more about what megasporogenesis is and how it is constructed with the diagram. RNAs are key players in development. The transition from #megasporogenesis to #megagametogenesis is regulated by #sRNAs pathways t.co/IvWDtNO9 After megasporogenesis, the megaspore develops into a female gametophyte (the embryo sac) in a process called megagametogenesis. The process of megagametogenesis varies depending on the pattern of megaspoorogenesis that has occurred. Some species, such as Tridax trilobata, Ehretia laevis and Alectra thomsoni, can go through different models of megasporogenesis and therefore different models of megagametogenesis. When the monosporic pattern has occurred, the single nucleus becomes mitosis three times, creating an eight-nucleated cell. These eight nuclei are arranged in two groups of four.
These groups both send a nucleus to the center of the cell; These become the polar nuclei. Depending on the species, these nuclei fuse before or during fertilization of the central cell. The three nuclei at the end of the cell near the micropyle become the egg apparatus, with an egg in the middle and two synergids. At the other end of the cell, a cell wall forms around the nuclei and forms the antipodes. Therefore, the resulting embryo sac is a seven-cell structure consisting of a central cell, an egg, two synergistic cells and three antipodian cells. [2] [3] The process of developing megaspores from megaspore mother cells is called megasporogenesis. Megaspores develop in female reproductive organs. In gymnosperms and flowering plants, megaspore is produced in the nucelle of the egg. During megasporogenesis, a diploid progenitor cell, the megasporocyte or megaspore mother cell, undergoes meiosis to initially produce four haploid cells (the megaspores). [1] Angiosperms exhibit three patterns of megasporogenesis: monosporic, biporic and tetrasporic, also known as polygonum type, alisma type, and Drusa type. The monosporic pattern is the most common (>70% of angiosperms) and is found in many economically and biologically important groups such as Brassicaceae (eg. Arabidopsis, Capsella, Brassica), Gramineae (e.g.
maize, rice, wheat), Malvaceae (e.g. cotton), legumes (e.g. beans, soybeans) and Solanaceae (e.g. peppers, tobacco, tomatoes, potatoes, petunia). [2] Megasporogenesis refers to the development of megaspores from the megasporocyte, the cell that undergoes meiosis. Meiosis of the megasporocytic nucleus leads to the formation of four haploid megaspore nuclei. In most taxa, meiosis is followed by cytokinesis, resulting in four megaspore cells. This pattern is called monosporic megasporogenesis; because of the four megaspores produced, only one of them contributes to the female gametophyte (Figures 11.9, 11.10A–D). However, in some angiosperm taxa, cytokinesis occurs after the first meiotic division, but not after the second, resulting in two cells, each containing two haploid nuclei. This pattern of development is called biporic megasporogenesis because one of the binuclear cells containing two megaspore nuclei contributes to the female gametophyte (Figure 11.9).
Finally, in other taxa, cytokinesis does not occur at all after meiosis, resulting in a single cell with four haploid nuclei. Since the four haploid nuclei of megaspores contribute to the female gametophyte, this pattern is called tetrasporic megasporogenesis (Figure 11.9). In 2n diploid plants, female meiosis (megasporogenesis) in the megaspore mother cell (MMC) results in four haploid megaspores (n), three of which are degenerated (Fig. 1). From the remaining functional megaspore, the megagametophyte (also called female gametophyte or embryo sac) develops through three mitotic divisions, at the mature stage consisting of 8 nuclei and 7 cells (one egg, two synergids, one central cell and three antipodes). The two polar nuclei of the central cell fuse to form a 2n nucleus. Male meiosis (microsporogenesis) produces four haploid microspores, collectively called tetrads. The nucleus of the microspore divides mitotically into a vegetative cell and a generative cell, the latter dividing into two sperm nuclei.
The microgametophyte (pollen) is released at the dinucleate or trinucleate stage. Saffron is triploid; Therefore, abnormal chromosomal pairing occurs in prophase and leads to infertile gametes. The abnormal microspores produce abnormal pollen found in the triploid. Pollen abnormalities are based on shape and size. During meiotic division abnormalities found in triploids, microspores exhibit cytoplasmic degeneration leading to incomplete meiosis and abnormal microspores. The number of abnormal microspores is much lower in C. sativus than in C. cartrightianus (Karasawa 1933). The scars have three independent ducts, each of which extends to the celled ovaries. Ovum orientation in the ovary, megasporogenesis and embryo sac development are performed as in other iridus-like plants with seven cells in the embryo sac (Chichiricco, 1989a, b, 1987). Pollination incompatibility exists in saffron and cross-fertilization between C.
sativus and other species is also limited (Grilli Caiola 1999; Grilli Caiola et al., 2001). Infertility in pollen is much higher than in oocytes (Chichiricco, 1984). In triploids, abnormalities are often observed during meiotic division. An irregular chromosomal assortment occurs and the production of megaspores is not the same and will be genetically unbalanced. Often, the egg does not reach the fertilized stage due to the unsuccessful development of megaspores (Grilli Caiola and Canini 2010). However, according to one report, if we have 20% (Grilli Caiola and Zanler, 2005) or 10% (Karasawa, 1933) of normal pollination, of which 4% produce pollen tubes, and all are able to fertilize an egg (which may not be the case), the number of seeds produced will be well below 1%. Therefore, we can consider tubers as the only source of spread of saffron. Viable saffron seeds have larger dimensions than diploid crocus species due to their triploidy.
The seed produces a small tuber and a leaf in the first year. In the second year, it produces a larger tuber, which has a small elliptical shape with a net-shaped tunic. In the third year, the tuber can produce cormlets.