Gametogenesis

• The production of male and female gametes from the micro and megaspores respectively after mitosis is known as gametogenesis.

Important events involved during the process of seed formation are:

1. Microsporogenesis and micro gametogenesis
2. Megasporogenesis and megagametogenesis
3. Pollination and fertilization

a. Microsporogenesis

• Each anther has 4 pollen sacs (microsporangium) that contain pollen mother cells.
• Each pollen mother cells undergo 2 steps reduction division (meiosis) to produce four haploid cells or microspores. This process is known as microsporogenesis.
• The microspores mature into pollen grains mainly by thickening their cell walls (epidermis and endothecium). It is the first cell of the male gametophytes.

b. Micro gametogenesis

• This refers to the production of male gamete or sperm.
• During the maturation of pollen (microspore), the microspore nucleus divides mitotically to produce a generative and a vegetative or tube nucleus.
• The pollen is generally released in the binucleate stage.
• When the pollen reaches the stigma of the flower, it is known as pollination, shortly after pollination, the pollen germinates.
• The pollen tube enters the stigma and grows through the style.
• The generative nucleus now undergoes a mitotic division to produce two male gametes or sperms.
• The pollen along the pollen tube is known as microgametophyte and the process of development of microgametophyte is known as micro gametogenesis.
• The pollen tube finally enters the ovule through a small pore micropyle and discharges the two sperms into the embryo.

c. Megasporogenesis

• Megasporogenesis occurs in the ovules, that are present inside the ovary.
• In a simple ovary, the ovule primordia are usually located near the suture of the ovary wall where the pistils are fused.
• From within the specialized tissues of the pistil (i.e. nucellus) one cell called the archesporial cell develops special characteristics.
• It grows in size, the nucleus becomes larger and the cytoplasm grows dense in preparation for cell division.
• The first cell division results in a megaspore mother cell (MMC) and a parietal cell.
• Usually, the parietal cell remains undivided and soon degenerates.
• The MMC undergoes meiosis to produce four haploid megaspores, three of which degenerate leaving one functional megaspore per ovule and this process is known as megasporogenesis.

d. Megagametogenesis

• The nucleus of the functional megaspore divides three successive nuclear mitotic divisions to produce 8 haploid nuclei. Three of these nuclei move to one pole and produce a central egg cell and two synergid cells. One synergid is situated on either side of the egg cell.
• The two nuclei remaining in the center, the polar nuclei fuse to form a secondary nucleus. Soon these 8 nuclei arrange themselves within the enlarging embryo sac. Cell wall formation occurs in each nucleus except the two polar nuclei, that is located at the center of the sac.
• The megaspore thus develops into a mature megagametophyte or embryo sac.
• The development of an embryo sac from a megaspore is known as megagametogenesis.
• The developed megagametophyte is now ready to receive the male gametophyte.
• Variations may occur with species although the above-mentioned process occurs in most of the species.
• The egg cell is a complete cell containing a haploid nucleus (n) with surrounding cytoplasm enclosed in a thin wall called a follicle. It is positioned near the small opening (micropyle)of the ovule formed by the surrounding integuments.