Pre-fertilisation is the process that occurs before the actual fertilization, i.e. the process before the fusion of gametes. There are two events that occur before fertilization:
- Gametogenesis: Formation of male and female gametes
- Gamete transfer: Bringing the male and female gametes together.
It is the process of formation of gametes. Male gametes are formed inside the anther. The steps involve microsporogenesis and microgametogenesis. The female gametes are formed inside the ovules and involve two steps- megasporogenesis and megagametogenesis. The structures and the process is explained below.
Let us have a look at the details of the structures involved in the process of gametogenesis.
- Microsporangium (Pollen sac)- It is surrounded by four walls- epidermis, endothecium, middle layers, and tapetum. The pollen grains are formed in this. There are four pollen sacs. The centre of each sac comprises a group of compactly arranged homogenous tissues called sporogenous tissues.
- Ovules- These are the place for the formation of female gametes. These are present at the bottom of a vase-like structure called carpel, which has a neck called style and an opening called stigma at the top.
- Pollen Grains– It has variable sizes. It is double-walled, the inner wall is called endospore and composed of cellulose, the outer wall is composed of sporopollenin.
- Anther- It is a bi-lobed structure. The two lobes are attached to the strip of the sterile part.
- Stamen- It consists of a stalk called the filament. The proximal end of the filament is connected to the petal of the flower known as the thalamus.
Formation of Male Gametes
- Microsporogenesis the initial process and marks the development of microspores or pollen grains in the pollen sac of anther known as microsporangium.
- The diploid pollen mother cell gives rise to four haploid microspores by meiosis.
- Each microspore develops into a pollen grain.
- Formation of male gametophyte from the pollen grain is known as microgametogenesis.
- Pollen grain undergoes mitosis and gives rise to two cells- small generative cells and large vegetative cells. Pollination takes place here.
- The generative cells give rise to two male gametes while the vegetative cells give rise to pollen tube.
Formation of Female Gametes
- The megaspore develops inside the megasporangium. This is known as megasporogenesis.
- The process takes place inside the ovule within a structure called nucellus.
- The diploid megasporocyte inside the ovule undergoes meiosis to form four haploid megaspores.
- Only one megaspore develops into the megagametophyte, while the other three disintegrate.
- The female gamete is formed from the functional megaspore. This is known as megagametogenesis.
- The megaspore undergoes mitosis thrice and gives rise to an embryo sac with 8 nuclei.
- Each micropyle and chalaza end contains 4 nuclei initially. At the time of fertilisation, one of them from each pole fuse at the centre and form a diploid nucleus.
- The three cells at the chalaza end disintegrate later.
- At the micropyle, one of the three cells develops to form the egg cell. The remaining two secrete chemicals to guide the pollen tube.
- Formation of seeds requires pollination. In this process, the pollen grains from the anther are transferred to the stigma.
- Pollen grains germinate on the stigma and reach the ovule through the pollen tube.
- The male gametes are then released near the egg.