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Megagametogenesis

Megagametogenesis is the process of maturation of the female gametophyte, or megagametophyte, in plants During the process of megagametogenesis, the megaspore, which arises from megasporogenesis, develops into the embryo sac, which is where the female gamete is housed. These megaspores then develop into the haploid female gametophytes. This occurs within the ovule, which is housed inside the ovary

The Process

Refer to the figure above to observe the process of megagametogenesis, which is detailed below.

Prior to megagametogenesis, a developing embryo undergoes meiosis during a process called megasporogenesis. Next, three out of four megaspores disintegrate, leaving only the megaspore that will undergo the megagametogenesis. The following steps are shown in Figure 1, and detailed below.

1. The remaining megaspore undergoes a round of mitosis. This results in a structure with two nuclei, also called a binucleate embryo sac

2. The two nuclei migrate to opposite sides of the embryo sac

3. Each haploid nucleus then undergoes two rounds of mitosis which creates 4 haploid nuclei on each end of the embryo sac

4. One nucleus from each set of 4 migrates to the center of the embryo sac. These form the binucleate endosperm mother cell. This leaves three remaining nuclei on the micropylar end and three remaining nuclei on the antipodal end. The nuclei on the micropylar end is composed of an egg cell, two synergid cells, and the micropyle, an opening that allows the pollen tube to enter the structure. The nuclei on the antipodal end are simply known as the antipodal cells. These cells are involved with nourishing the embryo, but often undergo programmed cell death before fertilization occurs.

5. Cell plates form around the antipodal nuclei, egg ell, and synergid cells

Variations

Plants exhibit three main types of megagametogenesis. The number of haploid nuclei in the functional megaspore that is involved in megagametogenesis is the main difference between these three types.

Monosporic

Bisporic megagaetogenesis is shown in the figure and outlined below.

The most common type of megagametogenesis, monosporic megagametogenesis, is outlined above. This type of megagemetogenesis only allows one megaspore to undergo megagametogenesis, while the other three undergo programmed cell death.

Bisporic

As the name implies, bisporic megagametogenesis involves two genetically different haploid nuclei.

1. These two nuclei undergo a round of mitosis
2. Then, the nuclei on the micropylar end of the structure undergo a second round of mitosis
3. Next, the nuclei rearrange to form a trinucleate endosperm mother cell and the characteristic arrangement of the micropylar end, with an egg cell and two synergid cells
4. Cell plates form around the egg cell and synergid cells

Post Megagametogenesis

Megagametogenesis creates the female gametophyte, which is an integral part of pollination, a very prominent process in plants. The male counterpart to megagametogenesis is called microgametogenesis. Microgametogenesis is the process of the formation of the male gametophyte. During pollination, the female gametophyte communicates with the pollen tube to ensure that it comes in contact with the ovule. When contact is made, the pollen tube grows through the micropyle opening into a synergid cell, that dies when this occurs. The death of the synergid cell signals to the pollen tube to release the sperm. This process creates the embryo, seed coat, and endosperm which, after pollination, will become crucial parts of the seed.

Implications

Pollination is an essential process of global crop production. Its success is economically crucial for farmers. Additionally, pollination success is required for global food security. Cereals, or the seeds of grain crops, are most important staple food to humans around the world. They make up 48% of the calories consumed by humans.