Mega sporangium (ovule) – types and evolution, mega sporogenesis, embryo sac types, structure of egg, synergids, and antipodal cells
Types of ovule:- On the basis of the position of the micropyle with respect to the funiculus, mature ovule can be classified into six main types. These are:
1. Orthotropous ovule:- Orthotropous ovule is also known as atropous. It is upright. In this type the micropyle, chalaza and the funiculus lie in one straight line as in Polygonaceae and Piperaceae.
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2. Anatropous ovule:- In this type, the funiculus is long; the body of the ovule becomes completely inverted so that micropyle comes to lie close to the base of the funiculus. This happens due to unilateral growth of the ovule. The nucellus remains straight so micropyle and chalaza lie in one line and funiculus lie parallel to it. It is the most common type of ovule inAngiosperms.
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3. Campylotropous ovule:- In campylotropous ovules body of the ovule is not completely inverted, the curvature is less than that in anatropous ovules. The micropyle and chalaza do not lie in the straight line and the funiculus lies at right angle to the chalaza as in Chenopodiaceae and Capparaceae.
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4. Amphitropous ovule:- It is similar to campylotropous, but in this case the curvature of the ovule also affects the nucellus/embryo-sac so that it bent like "horse shoe" as in Alismaceae and Butomaceae.
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5. Hemianatropous ovule:- Also known as hemitropous. In this type of ovule the funiculus is at right angle to the nucellus and the integuments. Micropyle and chalaza, lie in the same plane as in Ranunculus, Nothoscordum, and Tulbaghia.
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6. Circinotropous ovule:- A very peculiar type of ovule is seen in some members of the Plumbaginaceae. Here the nucellar protuberance is at first in the same line as the axis, but the rapid growth on one side causes it to become anatropous. The curvature does not stop but continues until the ovule has turned over completely so that the micropylar end again points upwards. It has been suggested that this kind of ovule, also seen in Opuntia, is distinctive enough to merit a separate name, Circinotropous.
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Megasporogenesis:-
> It is the formation of megaspores in the megasporangium.
> Megasporangium, or macrosporangium, is the sporangium in which megaspores, (macrospores or female spores) are produced by the division of sporogenous cells. Together with its protective integuments, megasporangium forms the ovule. Thus, ovule is an integumented megasporangium.
> Ovule is a small oval structure. It is attached to the placenta on the inner wall of the ovary by means of a short and slender stalk, called funiculus or funicle. The point of attachment of the funicle to the body proper of the ovule is called hilum. In some cases, the funicle continues beyond the hilum along the surface of the ovule as a ridge, called raphe.
> A mature ovule consists of a central mass of tissue, called Nucellus. It is almost completely enclosed by one or two protective integuments, leaving a small opening at the apical end, called micropyle. Micropyle is the major passage for the entry of pollen tube into the ovule.
> The basal part of the ovule, where nucellus, integuments and funicle meet and merge together, is called chalaza.
> Lying embedded in the nucellus at the micropylar end is a large oval cell, called embryo sac. It represents the female gametophyte. Typically, an embryo sac contains eight haploid nuclei, three at the micropylar end, and three at the chalazal end, and two in the centre. The chalazal nuclei organize to form antipodal cells, the micropylar nuclei organize to form the egg apparatus, and the central ones become the polar nuclei. The egg apparatus consists of an egg, or female gamete, and two synergids. In some cases, before fertilization, the polar nuclei fuse together forming diploid secondary nucleus.
> Megasporogenesis begins with the differentiation and development of a single cell in the hypodermal region. This cell becomes distinct by its large size, prominent nucleus, and dense cytoplasm. It is called the archesporial cell or archesporial initial. Its further development varies with the type of the ovule.
> Three types of development can be recognized:
i. Crassinucellate type:- In crassinucellate type of megasporogenesis, the hypodermal archesporeal undergoes periclinal division, forming an outer primary parietal cell (PPC) and a inner primary sporogenous cell (PSC). The PPC may remain undivided, or it may undergo periclinal and anticlinal divisions forming a mass of cells which will push the PSC deep into the nucellar tissue. Thus, the PSC becomes sub-hypodermal and it behaves as the megaspore mother cell or megasporophyte. E.g., Myriophyllum intermedium.
ii. Pseudo-crassinucellate type:- This is similar to the crassinucellate type. In it, the archesporal cell divid periclinally, forming PPC and PSC. PSC becomes sub-hypodermal due to cello sion in nucellar epidermis, and not by the divisions of PPC. In this case also, PSC behaves as the megaspore mother cell. E.g., Nigella damascena.
iii. Tenuinucellate type:- In this type, the archesporial cell directly functions as the megaspore mother without undergoing division. E.g., Elytraria acaulis.
Development of megaspore:- The diploid megaspore mother cell is the last cell of the sporophytic generation. It undergoes meiosis, forming four haploid megaspores. These form a megaspore tetrad. In most cases, this tetrad is linear. In others, it can be T-shaped, 1-shaped, decussate, or rarely tetrahedral of the four megaspores of a tetrad, the one lying towards the chalazal end survives as the functional megaspore and it represents the first cell of the female gametophyte or embryo sac. The remaining ones degenerate and are consumed by the functional megaspore. Callose deposition occurs around the functional megaspore isolating it from other cells.
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Embryo sac types:- Depending on how many megaspore nuclei are involved in embryosac formation, the embryosac may be:
1. Monosporic embryo sac:- The chief characteristic of a monosporic embryo sac is that it is derived from only one of the four megaspores. All the nuclei in such an embryo sac are genetically identical because they are derived through mitosis of a single nucleus. There are two types of monosporic embryo sacs:
i. Polygonum Type:- It is the most common type (81% families). It was first time described in Polygonum divaricatum by Strasburger (1879). The embryo sac is formed by the chalazal megaspore of the tetrad and is eight nucleate. The mature embryo sac comprises a 3- celled egg apparatus, three antipodal cells, and a binucleate central cell.
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ii. Oneothera Type:- This type of embryo sac is derived from the micropylar megaspore of the tetrad and is four nucleate. The organization of the mature embryo sac is_ an egg apparatus and a uninucleate central cell. It does not have antipodals. This type of embryo sac is characteristic of the family Onagraceae. Schisandra chinensis, however, is the only example outside this family where such a type of embryo sac occurs.
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2. Bisporic embryo sac:- In plants bearing bisporic embryo sacs the first meiotic division is accomplished by wall formation, so that a dyad is formed. Only one of the dyad cells undergoes the second meiotic division whereas the other one degenerates. In the functional dyad cell wall formation does not occur after the second division, and both the megaspore nuclei contribute to the formation of the embryo sac. Each megaspore nucleus undergoes two mitotic divisions forming eight nuclei.
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On the basis of the position of functional dyad bisporic embryo sacs are of two types:
i. Allium Type:- The embryo sac is derived from the chalazal dyad cell.
ii. Endymion Type:- The embryo sac is formed by the micropylar dyad cell.
3. Tetrasporic embryo sac:- In this type of embryo sac, meiotic division of the megaspore mother cell is not accompanied by cytokinesis and hence all the four haploid nuclei lie in a single cell called Coenomegaspore. All four nuclei of coenomegaspore participate in the formation of embryo sac. Genetically, it is more heterogeneous than bisporic type of embryo sac because the four products of meiosis involved in its formation are genetically different. The tetrasporic embryo sacs are further divided into many types:
i. Adoxa Type:- It has 8 nuclei which are formed by the mitotic division of the four haploid nuclei of the coenomegaspore. The arrangement of the 8 nuclei in the embryo sac is the same as in Polygonum type. Example –Adoxa, Sambucus, Ulmus, Tulipa, Erythronium etc.
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ii. Plumbago Type:- This type of embryo sac is characterized by the absence of synergids and antipodals. Out of four haploid coenomegaspore one migrates to the micropylar end, one at chalazal end and two at the lateral sides. Each nuclei divides again and formed four groups of two nuclei. One of the nucleus from each group moves to the center of the cell and form four polar nuclei. The remaining nucleus at the micropylar is cut off by a membrane and form the egg. There are no synergids. The other three nuclei usually disappear but occasionally they too may be cut off by membranes and appear as accessory egg cells. Example –Plumbaginaceae family.
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iii. Penaea Type:- The four haploid nuclei of the coenomegaspore undergo two successive mitotic divisions forming 16 nuclei. These nuclei arrange themselves in four groups of four each, one at the micropylar end, one at chalazal end and one each on the two lateral sides. Now one nucleus from each groups migrates to the centre, and these four nuclei in the centre form polar nuclei. The three nuclei at the micropylar end are cut off by membranes and form the egg apparatus. The remaining three groups of nuclei (one chalazal and two lateral) degenerate at maturity. Example –Family Penaeaceae, Malpighiaceae and Euphorbiaceae.
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iv. Peperomia Type:- The egg apparatus of Peperomia type is characterized by a single synergid. The four haploid nuclei of coenomegaspore undergo two successive mitotic divisions forming 16 nuclei. Two nuclei at the micropylar end form egg and a synergid, eight fuse in the centre of the cell to form a polar nucleus and the remaining six at the chalazal end formed antipodals. Example-Peperomia and Gunnera.
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v. Drusa Type:- 16 nucleate embryosac. This type of embryo sac is characterized by large number of antipodals. In the mature embryo sac three nuclei form egg apparatus. Two act as polar nuclei and the remaining 11 nuclei are cut off by membrane and form antipodal cells. The number and organization of nuclei may vary due to irregularity in the divisions. Example– Drusa, Rubia, Chrysanthemum,Ulmus etc.
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Structure of Egg, Synergids, and Antipodal cells:-
> The mature embryo sac contains 7 cells ( 3 antipodal cells, 2 synergid cells, 1 egg cell and 1 central cell) and 8 nuclei ( 3 of antipodal cell, 2 of synergid cells, 1 of egg cell and 2 of central cell).
> One nucleus is located in the egg cell near the opening of embryo sac. Two nuclei are located in two synergid cells which flank the egg cell. The synergids and egg cell together constitute the egg apparatus. Two nuclei are located in a single cell called central cell present in the middle of the embryo sac and are called polar nuclei. The other three nuclei reside in the cells called antipodals which are located at the end of embryo sac, opposite to egg cell. Thus, embryo sac has one egg cell, two synergids, three antipodal cells and two polar nuclei.
