Pre-fertilization changes within the Ovule-digieduco

Development of Megaspore Mother Cell, Megasporogenesis and Megagametogenesis i.e. Pre-fertilization changes within the Ovule :
A. FORMATION OF MEGASPORE MOTHER CELL-One hypodermal cell of the nucellus of the ovule becomes more prominent owing to its larger size and denser cytoplasm, and forms primary archesporial cell. Archesporial cell may divide to form an outer primary parietal cell and an inner primary sporogenous cell. Primary sporogenous cell functions as megaspore mother cell, while primary parietal cell develops into wall ; sometimes primary archesporial cell directly functions as megaspore mother cell (2n) without being divided into primary parietal cell and primary sporogenous cell.

B. MEGASPOROGENESIS-The megaspore mother cell (diploid) undergoes reduction division i.e. meiosis and divides at first transversely into two cell (diad). These two cells again divide transversely ; as a result, a linear row of four haploid megaspores (so-called ‘linear tetrad’) develops. Of these four megaspores, only one megaspore, lying towards the chalaza, behaves as functional megaspore, while the remaining upper three megaspores, towards the micropylar end, degenerate. The functional megaspore gives rise to the female gametophyte i.e. the embryo sac.


C. MEGAGAMETOGENESIS i.e. THE FORMATION OF FEMALE GAMETOPHYTE (EMBRYO SAC)-
(1) Megaspore is the first cell of the female gametophyte.
(2) The megaspore increases in size and its nucleus divides mitotically into two nuclei which move apart to opposite pole
(3) Two daugher nuclei undergo another division giving rise to 4-nucleate stage.
(4) Second division is followed by another i.e. third division which gives 8-nucleate structure comprising a micropylar and chalazal quartet.
(5) One nucleus from each quartet proceeds towards the centre of the embryo sac and then these two nuclei fuse forming what is called the fusion or secondary nucleus. The three nuclei remaining at the micropylar region form the egg apparatus, each being surrounded by viscid mass of cytoplasm without any cellulose wall. In the egg apparatus, the middle one is the largest and is known as oosphere, egg or ovum ; the other two naked cells on its either side are called synergids. The three remaining nuclei at the chalazal end are surrounded by viscid mass of cytoplasm and cellulose wall-these walled-cells are known as antipodal cells.

The above mentioned method of the development of female gemetophyte is known as the normal 8-nucleate type, because this type is the most common among the angiosperms and ultimately the 8 nuclei contribute in the formation of the gametophyte. It is also known 38 monosporic, because only one (functional ) of the 4 megaspores takes part in the development of the gametophyte.


Depending on the number of megaspores taking part in the development, the female gametophyte of angiosperms may be monosporic, bisporic and tetrasporic. When only one of the four megaspores takes part in the development of the gametophyte, it is called monosporic. When two of the four megaspores take part in the formation, it is called bisporic and when all four of the megaspores take part in the development, it is called tetrasporic.

The monosporic female gametophyte may be 8nucleate type or 4 nucleate type. The 8 nucleate type of female gametophyte is the most common and is therefore designated as the ‘normal type’. It is also called Polygonum type as this type was first illustrated by Strasburger (1879) in Polygonum divaricatum (Polygonaceae). The 4-nucleate type is also called ‘Oenothera type’ as it is found in Oenothera sp. (Onagraceae) and some other members of the family Onagraceae-in this type the functional megaspore nucleus (one) divides only twice, as a result a micropylar quartet alone is formed. This quartet gives rise to egg apparatus (consisting of 3 nuclei, i.e. centrally placed oosphere or ovum and 2 lateral ones are the synergids) and a single polar nucleus The lower polar nucleus and the antipodal cells are absent.

The bisporic female gametophytes are typically 8-nucleate type-this type is also known as ‘Allium type’ as bisporic type of gametophyte was first noted in the Allium sp. of Liliaceae. This type of gametophyte develops from one of the two diad cells (haploid) formed after the first division of the meiosis (meiosis I) of the megaspore mother cell. The functional dyad cell after second division of the meiosis (meiosis II) gives rise to two free nuclei (as no wall is laid down after meiosis II) which are the megaspore nuclei-these two megaspore nuclei by two successive divisions take part in the development of the 8-nucleate female gametophyte.

In the tetrasporic female gametophytes, all the four megaspore-nuclei take part in the fomation of the gametophyte. Here all the four megaspore-nuclei lie free within the single cell (young embryo sac) after the meiosis of megaspore mother cell. Depending on the polarity and organisation of the nuclei in the sac, tetrasporic female gametophytes are of the following types:

(a) Peperomia type-Noted in Peperomia pellucida (Piperaceae). Each of the 4 megaspore nuclei divides twice, as a result 16 nuclei are formed which are uniformly distributed in the thick cytoplasmic layer of the gametophyte. Of the 16 nuclei, 2 nuclei at the micropylar end are organised to form the oosphere and a synergid, 8 fuse to form the secondary nucleus and the remaining 6 are cut off at the periphery of the gametophyte as antipodal cells.


(b) Penaea type-Found in three genera e.g. Penaea, Brachysiphon, Sarcocolla, etc. of the family Penaeaceae. Here 16 nuclei lie in four distinct quartets in a crosswise manner i.e. one at each end of the gametophyte and two at the sides. Next 3 nuclei of each quartet are cut off as cells while the fourth one remains free and moves to the centre of the gametophyte. Therefore, 4 ‘triads’ and 4 polar nuclei are formed. But the egg cell of the micropylar ‘triad’ only is functinal and therefore forms the oosphere.

(c) Drusa type-Observed in Drusa oppositifolia (Umbelliferae). Here 3 of the 4 megaspore nuclei move to the basal end of the embryo sac i.e. gametophyte and only one remains towards the micropylar end ( arrangement of megaspore nuclei in the sac). This arrangement is followed by next stage. The 4 micropylar nuclei constitute the egg apparatus and upper polar nucleus and 12 chalazal nuclei to a lower polar nucleus and 11 antipodal cells.

(d) Fritillaria type-Noted in Fritillaria sp. (Liliaceae), Lilium sp. etc. Here arrangement of megaspre nuclei takes place at first in the gametophyte and the 3 chalazal nuclei come to he very close to each other. During the next stage the single micropylar nucleus divides normally, but the spindles of 3 chalazal nuclei fuse to form a single common spindIe-hence at the close of the division there are 2 haploid nuclei at the micropylar end and 2 triploid nuclei at the chalazal region. Another division takes place resulting in 8 nuclei ; of these 8 nuclei, 4 micropylar are haploid and 4 chalazal nuclei are triploid. The mature female gametophyte thus consists of three haploid cells (the oosphere and two synergids), 3 triploid cells (the antipodals) and a tetraploid secondary nucleus formed by the fusion of one haploid polar nucleus and the other triploid polar nucleus,

(e) Plumbagella type-Noted in Plumbagella micrantha (Plumbaginaceae). In this type also the 4 megaspore nuclei take up an arrangement. The 3 chalazal nuclei move towards each other and ultimately fuse to give rise to single triploid nucleus. Both the haploid micropylar and triploid chalazal nuclei divides-as a result 2 haploid micropylar nuclei and 2 triploid chalazal nuclei are formed.The nucleus nearest the micropylar end organises into the oosphere, the triploid nucleus nearest the chalazal end constitute the single antipodal cell, while the 2 remaining nuclei, one haploid from micropylar end and one triploid from chalazal end, fuse to form a tetraploid single polar nucleus.

(f) Plumbago type-Found in Plumbago capensis (Plumbaginaceae). Here 4 megaspore nuclei are arranged within the sac in crosswise manner. These 4 megaspore nuclei divide into 8 free nuclei which are arranged in four pairs. One nucleus of the micropylar pair cut off to form the lenticular egg cell. Of the 7 remaining nuclei, 4 nuclei approach one another and constitute polar nuclei. The remaining 3 nuclei degenerate at their original place, but often 1, 2 or all 3 of them are cut off at the periphery to form cells which may persists and assume an egg-like appearance, here synergids are absent.

(g) Adoxa type-Found in Adam moschatellina (Adoxaceae). In this type, the 4 megaspore nuclei undergo one more division forming 8 nuclei. All the 8 nuclei part in the formation of 8 nucleate gametophyte i.e. gametophyte having a normal egg apparatus, 2 polar nuclei and 3 antipodal cells.