Sexual reproduction:
Sexual reproduction involves the formation and fusion of gametes to form the zygote which develops to form the new organism.
After attainment of maturity, all sexually reproducing organisms exhibit events and processes that have remarkable fundamental similarity, even though the structures associated with sexual reproduction are indeed very different. These sequential events may be grouped into three distinct stages, namely, the pre-fertilization, fertilization and post-fertilization events. 1. Pre-fertilization events: These are events in sexual reproduction which occur prior to the process of fertilization. The two main pre fertilization events are gametogenesis and gamete transfer.
Gametogenesis: It refers to the process of formation of gametes-male and female. Gametes are always haploid, i.e., they possess only one set of chromosomes or genome through the parent body producing gametes that may be either haploid so surely in the haploid parent, gametes are produced by mitotic division and for diploid parents, the gametes will be formed by the process of meiosis.
Gamete transfer: After the formation of male and female gametes, compatible gametes must be physically brought together to facilitate fusion (fertilization or syngamy). In a few fungi and algae, both types of gametes are motile. But in the majority of organisms, male gamete is motile and the female gamete is non-motile. So there is a need for a medium through which the male gametes move. In algae, Bryophytes and Pteridophytes, water is the medium for gamete transfer. Many male gametes, however, fail to reach the female gametes. To compensate for this loss of gametes, the number of male gametes produced is several thousand times the number of female gametes produced. In seed plants, both male and female gametes are non-motile. Here pollen grains are the carrier of male gametes and ovule has the egg. As the male gamete is non-motile so it cannot swim through water medium to reach female gamete rather pollen tubes serve this purpose. For this pollen grain produced in anther are transferred to the stigma of female organs, i.e., carpel through the process of pollination.
2. Fertilization: The most vital event of sexual reproduction is the fusion of gametes. This process is called syngamy or fertilization, which results in the formation of a diploid zygote.
Syngamy can occur in the external medium as well as inside the body of an organism. On this basis, syngamy can be distinguished into two types - External fertilization (fertilization occurs outside the body) and internal fertilization (fertilization occurs inside the body). 3. Post-fertilization events: Events in sexual reproduction after the formation of the zygote are called 'post-fertilization events'. The fusion of male and female gametes results in the formation of a diploid zygote. This step is universal in all sexually reproducing organisms. The formation of zygote occurs outside (usually water) in case of external fertilization and inside the body of the organism in case of internal fertilization. This process will be followed by embryogenesis. The process of development of the embryo from the zygote is called 'embryogenesis'. During this process, the zygote undergoes cell division (mitosis) and followed by differentiation or specialization of the cells (cell differentiation). Cell division is necessary to increase the number of cells in the developing embryo, cell differentiation allows a group of cells to undergo certain modifications to form specialized tissues and organs to form an organism.