Post Fertilization Events:

Events in sexual reproduction after the fertilization (formation of zygote) are called post fertilization events. These events may be described under two headings: zygote and embryo- genesis.

(i) Zygote:

After fertilization a diploid zygote is formed in all sexually reproducing organisms. In external fertilization, zygote is formed in the external medium (usually water) whereas in internal fertilization, zygote is formed inside the body of the organism. Further development of the zygote depends on the type of life cycle of the organism and environmental conditions.

(a) In many fungi and algae, the zygote develops a thick wall and forms spore called zygospore. Zygospore undergoes a period of rest. It germinates during next growing season. The zygospore undergoes meiosis to produce haploid individuals. It leads haploidic life cycle.

(b) In most animals, zygote does not take rest. It divides by mitosis first forming a diploid embryo and then the individual which is also diploid. It leads a diplontic life cycle.

(c) In most plants the zygote first forms an embryo and then the diploid sporophyte. The sporophyte has sporangia where meiosis takes place to form haploid spores. The latter produce haploid gametophytes. Gametes are produced in the gametophytes. It leads a diplohaplontic life cycle.

 

(ii) Embryogenesis:

The process of development of embryo from the zygote is called embryogenesis. During embryogenesis zygote undergoes mitotic cell division and cell differentiation. Cell division increases the number of cells in the developing embryo while cell differentiation helps to form specialised tissues and organs to form an organism.

(i) On the basis of the development of the zygote, animals are grouped into oviparous, viviparous and ovoviviparous. The oviparous animals such as reptiles and birds lay eggs. Their fertilized eggs are covered by hard calcareous shell and are laid in a safe place in the environment. After incubation period, young ones hatch out. In viviparous animals such as majority of mammals including human beings, the zygote develops into a young one inside the body of the female individual.

After a certain growth, the young ones are delivered by the female individual. Due to proper care and protection, the chances of survival of young ones are more in viviparous individuals. In ovoviviparous animals, the female retains the eggs inside its body after fertilization and allows the development of the embryo inside the body without providing extra nourishment to the developing embryo as the placenta is absent. However, the female animals give birth to the young ones. Examples of ovoviviparous animals are sharks and rattle snakes.

(ii) In flowering plants, the zygote is formed inside the ovule of the female sex organs. After fertilization, the sepals, petals and stamens of the flower become faded and fall off. The sepals remain attached in Hibiscus. However, the pistal remains attached to the plant.

Seed and Fruit Formation:

In angiosperms double fertilization produces two structures − a diploid zygote (= oospore) and a triploid primary endosperm cell. Zygote forms the embryo. The triploid primary endosperm cell gives rise to a nutritive tissue called en­dosperm. Endosperm provides food to the growing embryo. The fertilized ovules mature and convert into seeds. The wall of the ovary forms the pericarp (fruit wall). The ripened ovary with pericarp and seeds is called fruit. The pericarp protects the young seeds. After dispersal the seeds germinate to form new plants.

Maintenance of Chromosome Number:

The reproductive units in sexual reproduction are the male and female gametes that are produced by testes and ovaries respectively. The gametes are haploid with only N chromosomes. Consequently the zygote resulting from fusion of two such haploid gametes becomes diploid with 2N chromosomes. The offspring that develops from the zygote is also diploid.