Reproduction in Organisms: Reproduction is the biological process in which an organism produces young ones (offspring) of its own kind.
Reproduction is essential for the continuation of the line of succession and maintenance of a species in the biosphere. The methods of reproduction are broadly categorized into two types–asexual reproduction and sexual reproduction.
All the organisms grow to a certain maturity in their life before they start reproducing sexually. The period of growth between their birth up to their reproductive maturity is called the juvenile phase.
In plants, the period of growth between seed germination up to initiation of flowering is called the vegetative phase.
The juvenile phase and reproductive phase are of variable durations in different organisms.
On the basis of breeding time, animals may be either seasonal breeders (reproduce in particular period of the year) or continuous breeders (continue to breed throughout their sexual maturity).
The females of placental mammals exhibit cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase.
In non-primate mammals like cows, sheep, rats, deer, dogs, tigers, etc., such cyclical changes during reproduction are called oestrus cycle whereas in primates (monkeys, apes, and humans) it is called the menstrual cycle.
Events in Sexual Reproduction
These events of sexual reproduction take place before the fusion of gametes. These include gametogenesis and gamete transfer.
It is the process of the formation of gametes. The reproductive units in sexual reproduction are specialized cells called gametes which are of two kinds: male and female.
In some lower plants (e.g., in some algae), the two gametes are morphologically similar, such gametes are called isogametes (or homogametic).
When male and female gametes are morphologically distinct, they are called an isogametes (or heterogametes). In such organisms, male gametes are called microgametes, or spermatozoa, and female gametes are termed macrogametes or ova.
In plants when both male and female reproductive structures are present on the same plant then these plants are called as bisexual or monoecious e.g., cucurbits and coconut and when these reproductive structures are present on di_erent plants then these plants are referred to as unisexual or dioecious e.g., papaya and date palm.
In some lower animals, both male and female sex organs are present in the same individual.
These animals are called hermaphrodite or monoecious or bisexual, e.g., tapeworm, earthworm, and leech. Most of the animals are unisexual or dioecious with distinct male and female individuals e.g., Ascaris, cockroach, frog, lizards, birds, mammals.
Gamete transfer: In most organisms, male gamete is motile and the female gamete is nonmotile. A medium is needed through which male gametes can move. In algae, bryophytes, and pteridophytes, water serves as the medium through which gamete transfer takes place. In flowering plants, pollen grains carry the male gametes which are transferred from the anther to the stigma by pollination. Many animals have copulatory organs to transfer male gametes.
It is the complete and permanent fusion of two gametes from different parents or from the same parent to form a diploid zygote (syngamy).
Syngamy is of two types with regard to the source of fusing gametes:
(i) Endogamy (Self-fertilisation): It involves the fusion of male and female gametes of the same parent. _us it is uniparental, e.g., Taenia (tapeworm).
(ii) Exogamy (Cross-fertilisation): It involves the fusion of two gametes produced by different parents. Thus it is biparental, e.g., Rabbit.
When fertilization occurs outside the body of the organism, it is called external fertilization or external syngamy.
The external medium such as water is required for this type of fertilization e.g., fish and frog.
When the egg is retained inside the female body where it fuses with the male gamete, the process is called internal fertilization or internal syngamy e.g., reptiles, birds, mammals, etc.
Post-fertilization events include:
Formation of the zygote: After fertilization, a diploid zygote is formed in almost all sexually reproducing organisms. Further development of the zygote depends on the type of life cycle of the organism and environmental conditions.
Embryogenesis: Embryo develops from zygote by a process called embryogenesis.
During embryogenesis, the 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 specialized tissues and organs to form an organism.
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. In viviparous animals such as the majority of mammals including human beings, the zygote develops into a young one inside the body of the female individual.
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 individual gives birth to the young ones e.g., sharks and rattlesnakes.
In flowering plants, the zygote is formed inside the ovule of the female sex organs.
The fertilized ovule matures and gets converted into seeds. The wall of the ovary produces the pericarp (fruit wall).
The ripened ovary enclosing the seeds forms fruits. The pericarp protects the young seeds.
After dispersal, the seeds germinate to form new plants.