Describe the Development of Female Gametophyte

Development of Female Gametophyte:

The process of development of female gametophyte is called megagametogenesis. Megaspore(n) is the first female gametophyte. The functional megaspore becomes enlarged at the expense of the nucellus and thus forms the female gametophyte. Its single nucleus undergoes three successive mitotic divisions to produce 8 nuclei. Nuclei get organized into three groups – 3-celled egg apparatus towards the micropylar end, two middle polar nuclei, and three antipodal cells towards the chalazal end. The two polar nuclei fuse together and form the 2n nucleus, the definitive nucleus or polar fusion nucleus.

In the egg apparatus, each nucleus is surrounded by a viscous mass of cytoplasm without any wall of which the middle one is the largest and called the egg, ovum, or oosphere and the rest two are the synergids. This type of embryo sac development is very common in angiosperms and is known as the normal type or Polygonum type. This type is also known as the monosporic type. In 1950, P. Maheshwari classified the female gametophyte into monosporic, bisporic, and tetrasporic embryo sacs depending upon the number of megaspore nuclei taking part in the development.

Characteristics of Development of Female Gametophyte:

1. Female gametophyte development occurs inside the megasporangium within each ovule. Development begins when one diploid cell in the megasporangium of the ovule is known as a megaspore mother cell, enlarges and divides by meiosis to form four haploid megaspore cells.

2. Integuments are the protective layers of sporophytic tissue that develop into the seed coat. Each megasporangium is surrounded by the two integuments, except at a gap, micropyle.

3. The embryo sac or multicellular female gametophyte is formed following three mitotic division of the megaspore without cytokinesis.

4. Cell fate of the female gametophyte nuclei is determined by an auxin gradient at the microspyle.

5. Synergids are 2 cells at the micropylar end that flank the egg and are involved in attracting and guiding the pollen tube towards the embryo sac.