The following points highlight the three events that take place during the sexual reproduction. The events are: 1. Pre-Fertilization Events 2. Fertilisation 3. Post-Fertilisation Events.
1. Pre-Fertilization Events:
All the events of sexual reproduction that take place before the fusion of gametes are included in this category. Two main pre-fertilisation events are gametogenesis (formation of gametes) and gamete transfer.
The process of formation of male and female gametes is called gametogenesis. Gametes are haploid cells. Organisms such as monerans, fungi, algae and bryophytes have haploid parental body. Such type of organisms produce gametes by mitotic division and if the parent body is diploid; gametes are formed by meiosis.
When male and female gametes are similar appearance and it is not possible to differentiate them into male and female gametes, they are called homogametic or isogametes. If the male and female gametes are morphologically dissimilar they are called heterogametes. Among heterogametes, the male reproductive unit is called antherozoid or sperm and female reproductive unit is known as egg or ovum.
Cell Division during Gamete Formation:
Plants belonging to the pteridophyte, gymnosperm, angiosperm and most of the animals including human beings have diploid parental body. In such organisms, specialised cells are present, which take part in the production of gametes. These cells are called meiocytes.
At the time of gamete formation, meiocytes undergo meiotic division. As a result of meiotic division, the number of chromosomes in the daughter cells (i.e., in the gametes) reduce to half and thus from the diploid meiocytes thus, haploid gametes are formed.
(ii) Gamete Transfer:
In some algae and fungi, both male and female gametes are motile. In majority of organisms, male gametes are motile while, the female gametes are stationary. After gametes formation, male and female gametes should come in physical association, so that they can be fused with each other.
In algae, bryophytes and pteridophytes, for the transfer of male gametes water acts as the medium. During this transfer a large number of gametes fail to reach the female gametes. To fulfil this loss, the number of male gametes produced is several thousands times the number of female gametes.
In flowering planes, pollen grains carry male gametes. When anther bursts, a large number of pollen grains are released. These pollen grains with the help of an agent (e.g., wind, water, insects) are transferred to the stigma of the pistil.
The transfer of pollen grains from anther to the stigma is called pollination. On reaching the stigma, pollen grain germinates and a tube-like structure called pollen tube comes out of it. Pollen tube carries male gametes and grows downwards through the style until it reaches to ovule situated in the ovary. On reaching the ovule, pollen tube enters into it and then releases male gametes near the egg.
The process of fusion of male gametes with the female gametes is called fertilisation or syngamy. It results in the formation of a diploid zygote.
It is mainly of two types:
(i) External Fertilisation:
In majority of the aquatic organisms, e.g., algae, fish and in amphibians, the process of fertilisation takes place outside the body of the organism, i.e., in the water. This type of gametic fusion is called external fertilisation. To enhance the chances of fertilisation, the organisms exhibiting external fertilisation discharge a large number of gametes into the water, e.g., bony fishes, amphibians, etc.
(ii) Internal Fertilisation:
In most of the terrestrial organisms, e.g., fungi, higher animals and majority of plants such as bryophytes, pteridophytes, gymnosperms and angiosperms, the process of fertilisation takes place inside the body of the organism. This type of gametic fusion is called internal fertilisation.
In the organisms exhibiting internal fertilisation, non-motile egg is formed inside the female body and male gametes are motile. The number of ova produced are less, but a large number of male gametes are formed, as many of them fail to reach the ova.
3. Post-Fertilisation Events:
During the sexual reproduction, the events which take place after the formation of zygote are called post-fertilisation events.
Fertilisation leads to the formation of a diploid cell called zygote. It is the vital link that ensures the continuity of species between the organisms of one generation and of the next.
Development of zygote depends on:
(a) Type of life cycle of the organism.
(b) Environment it is exposed.
In the fungi and algae, the zygote before germination undergoes a resting period. A thick wall is developed around the zygote, which prevents it from desiccation and damage.
In organisms with haplontic life cycle, zygote divides by meiosis to form haploid spores, which grow into haploid individual.
The process of development of embryo from the zygote is called embryogenesis,
Embryogenesis involves the following process:
(a) Cell division to increase in number of cells.
(b) Cell enlargement or growth to increase in mass/volume of living matter.
Embryogenesis in Animals:
Based on whether the development of the zygote takes place outside or inside the body of the female parent, animals are divided into two categories
Those animals, in which development of zygote takes place outside the female parent, are called oviparous. They lay fertilised eggs covered with hard calcareous shell in a safe place in the environment, e.g., reptiles and birds. After a period of incubation, the young ones hatch out from the egg.
The animals in which the development of zygote takes place into a young one inside the body of the female parent, arc called viviparous. After attaining a certain stage of growth, the young ones are delivered out of the body of the female organism.
The chances of survival of young ones is greater in these because of proper embryonic care and protection.
Embryogenesis in Plants:
In all flowering plants, the zygote is formed inside the ovule. In most of the plants, with the formation of zygote, all the parts of the flower except the pistil wither and fall off. In some plants, such as tomato and brinjal, the sepals are persistent and remain attached to the developing fruit. In ovule, the zygote divides several times to form an embryo.
Meanwhile the wall of the ovule becomes hard and it develops into seed. With these developments, the wall of the ovary also starts to swell. As a result, the ovary develops into fruit. A thick wall that covers fruit is called pericarp. It is protective in function. When seeds mature, they are dispersed. Under favourable conditions, these seeds germinate to produce new plants.