The events that take place in sexual reproduction of an organism can be described as follows:
1. Pre-fertilization events
3. Post-fertilization events
1. Pre-fertilization events:
It includes all those events that take place before the fusion of gametes. These are:
Events in sexual reproduction
(b) Gamete transfer
It is the process of formation of gametes by sexually mature (parent) individual. There are two individuals (parents) of opposite sex involved in sexual reproduction. Therefore, two types of gametes are produced by individuals, i.e. male gamete and female gamete.
Gamete-> Male gamete- These are often called sperms or anthropoids in (plants).
Female gametes-> these are often called egg/ovum.
It happens in heterogametes.
In homogametes. The two gametes are so similar in appearance that it is not easily possible to identify them as male/female gamete. Hence, they are called homogametes/isogametes e.g. cladophora (alga).
The production of gametes is done by mitotic cell division in a haploid (n) parent and by meiotic cell division in diploid (2n) parent. Gametes produced by either ways of cell division have a single set (n) of chromosomes.
(b) Gamete transfer:
After the formation of gametes the next step is their transfer. To ensure it, plants and animals have developed and are adopted to different types of methods.
In plants like algae, broyophytes and pteridophytes, the plant need water medium for gamete transfer.
Also the male gametes are usually motile and female gametes stationery; male gametes may have flagella to help the mobility (e.g. isogamete of cladophora).
In higher plants (seed plants), gamete transfer is done by pollination process by various pollinating agencies.
In case of dioecious animals, animals mate or copulate. During this process in frogs the sperms are released outside the female body but in dog/cattle it is released inside the female reproductive track.
2. Fertilization (Syngamy) event.
The process of fusion of gametes (male & female) is called syngamy/fertilization. During this male and female nuclei fuse to form restore 2n status. This act can be of 2 types.
In case of external fertilization the organisms show great synchrony between the sexes and release large number of gametes (male and female) in order to increase the chances of syngamy.
In second case, however, the number of eggs produced is significantly reduced but the number of sperms released is more.
In plants pollen grains germinate and give out pollen tube that carries male gametes to the egg inside the ovary in seed plants.
Their fusion results in triple fusion and double fertilization. It results in formation of zygote (2n).
3. Post-fertilization events
The changes/events that take place after the formation of zygote are termed as post- fertilization events/changes.
The development of the zygote takes place outside or inside the body of female parent depending upon the type of syngamy taken place.
All zygotes are diploid (2n) universally in all sexually reproducing individuals. However the future development totally depends on the type of life-cycle that an individual follows.
Such as in fungi and algae. It forms a protective thick wall that is resistant to dessication and damage by decay and it rests before its germination.
In haploid individuals—zygotic meiosis takes place to form haploid spores that germinate as haploid individuals. In animals the zygote develops into an embryo by mitosis cell division.
Embryogenesis involves cell division (mitosis) and cell differentiation. This results in formation of germinal layers (3 layers) in higher animals, human to form various organs of the body in an embryo.
It is called morphogenesis. In viviparous embryo forms foetus and then adult like individual that develops inside the body of the female parent (mother) e.g. human, cow, etc.
In reptiles and birds the embryo develops inside an egg that has a protective hard calcareous shell. Such animals are called oviparous/ egg layers.
In flowering plants, following changes are seen.
The zygote is formed inside the ovule.
1. Sepals (calyx) — shed off in most cases.
2. Petals (corolla) — dry and shed off.
3. Stamens — dry and shed off.
4. Pistil — ovary develops into the fruit and the ovules develop into seeds.