What is reproduction ?
Reproduction is a process in which an organism gives rise to new organism (offspring ) similar to itself. Reproduction enables the continuity of species, generation after generation. Internal physiology and other factors are responsible for how reproduction happen. Reproduction can happen in two main ways Asexual reproduction and Sexual reproduction.
What is Asexual Reproduction?
In this method a single parent is capable of producing offspring and gamete formation does not take place. The offspring that are produced are morphologically and genetically similar to each other and its parent (Clone). Asexual reproduction is common among single-celled organisms, and in animals and plants with relatively simple organisations.
Cell division : In Protista and Monera, the parent cells divides into two to give rise to new individuals. Thus, in these organisms cell division is itself the mode of reproduction.
Binary fission : A cell divides into two halves and each rapidly grow into an adult. (e.g., Amoeba, paramecium). Many single-celled organisms produced by binary fission.
Budding : When cell division is unequal small buds are produced that remain attached initially to the parent cell which eventually gets detached and mature into new organisms. (e.g., yeast)
Special Asexual Reproductive Structures
Members of kingdom fungi and algae are reproduce through special asexual reproductive structures by spore formation.
Zoospores : A zoospores are microscopic motile structure and flagellated asexual spores produced by some fungi and algae species, It is haploid or diploid in nature.
Conidia : In fungi these are the asexual spores produced by mitosis. (e.g.,Penicillium)
Buds : Cell division unequal in yeast as a result little outgrowth produced on parent cell in hydra these outgrowth are called buds. Buds are appended to parent which gets separated into daughter cell and become a new independent yeast organism. (e.g.,Hydra)
Gemmules : Gemmules are internal buds which are formed by little organism called fresh water sponge each gemmule gives rise to new organism. (e.g., Sponge)
It is a form of asexual reproduction seen in plants. Plants can grow by a part of their body without seeds this method of reproduction is called vegetative propagation. Vegetative reproduction grouped into natural and artificial.
The structures of vegetative propagation are runner, rhizome, sucker, tuber, offset and bulb these structures are called vegetative propagules.
WATER HYACINTH (Terror of Bengal) is a aquatic plant which is one of the most invasive weeds found growing wherever there is standing water. It drains oxygen from the water which leads to death of fishes. This plant introduced in India because of its pretty flowers & shape of leaves. Vegetative propagation occurs at a phenomenal rate. It spread all over the water body in a short period of time. It is very difficult to get rid off them.
Sexual reproduction involves formation of male and female gametes either by the same individual or by two different individual of the opposite sex. These gametes fuse to form the zygote which develops to form the new organism (offspring). It is slow process because of the fusion of male and female gametes. Offspring are not identical to the parents.
Phases of sexual reproduction
All organisms have to reach a certain stage of growth and maturity in their life, before they can reproduce sexually.
- Juvenile phase : The period from birth to sexual maturity is called juvenile phase. In plants it is known as vegetative phase.
- Reproductive phase : The end of juvenile/vegetative phase marks the beginning of the reproductive phase can be seen easily in the higher plants when they come to flower. Some plants show seasonal flowering and some other flowers in all seasons. A few plants exhibit unusual flowering phenomenon : some of them such as bamboo species flower only once in their lifetime generally after 50-100 years, produce large number of fruits and die. Another plant, Strobilanthus kunthiana (neelakuranji),flowers once in 12 years.
- Oestrus cycle: The females of placental mammals exhibit cyclical changes in the activities of ovaries and accessory ducts as well as hormones during the reproductive phase. Such cyclic changes during reproduction are called oestrus cycle. It is occur in non-primate mammals cows, ship, rats, dogs, deers, and tigers etc.
- Menstrual cycle : Cyclic change in the activities of the ovaries and the oviduct in primates (monkeys, apes and humans) it is called menstrual cycle.
- Seasonal breeders : Mammals that can reproduce only in the favourable seasons in their reproductive phase.
- Continuous breeders : Mammals which are reproductively active throughout their reproductive phase are called continuous breeders.
- Senescent phase : End of reproductive phase is called senescent phase. The rate of metabolism slows down. It ultimately leads to death.
Hormones are responsible for the transitions between the three phases.
Events in sexual reproduction : Pre-fertilisation, fertilisation and post fertilisation.
All the events of sexual reproduction before the fusion of gametes are included in it. Two main evaents are gametogenesis and gamete transfer.
Gametogenesis – Refers to the process of formation of the two types of gametes – male and female. Gametes are haploid cells which may be similar or dissimilar in structure.
Homogametes – In algae, the two gametes are similar in appearance that is not possible to categorise them into male and female gametes called homogametes (isogametes).
Heterogametes – The gametes produced are of two morphologically distinct types called heterogametes, male gametes are called antherozoid or sperm and female gametes are called ovum or egg.
Sexuality in organisms
Bisexual : Organisms may have both male and female reproductive structures in the same plant called bisexual. In several fungi and plants homothallic and monoecious terms are used to describe the bisexual condition.
Unisexual : Organisms with either male or female reproductive structures called unisexual. In several fungi and plants heterothallic and dioecious are the terms used to describe unisexual conditions.
Staminate: In flowering plants the unisexual male flower bearing stamen is staminate.
Pistillate: Unisexual female flower bearing pistil.
Hermaphrodites: Animals that possess both male and female reproductive organs in same individual called bisexual or hermaphrodites. These are examples of hermaphrodites (Earthworms, sponge, tapeworm and leech).
Cell division during gamete formation
All gametes are haploid (having half set of chromosome). The parent plant body can be either haploid or diploid. All animals are diploid. A haploid parent produces gametes by mitotic division, whereas a diploid parent produces gametes by meiotic division. Several organisms belonging to monera, fungi, algae and bryophytes have haploid plant body. But in organisms belonging to pteridophytes, gymnosperms, angiosperms and most of the animals including human, the parental body is diploid (having double set of chromosome).
Meiocytes – In diploid organisms, gamete mother cell (meiocytes) undergoes meiosis in which one set of chromosome get incorporated into each gamete.
Gamete Transfer – After gamete formation male and female gametes must be physically brought together to facilitate fusion (fertilization). The male gametes need a medium to be transferred for their fusion
- The male gamete is motile and female gametes are non-motile (stationary). In some fungi and algae both gametes are motile.
- A large number of male gametes fail to reach the female gametes. To compensate this lose of male gamete during transfer, several thousands times of male gametes are produced than the female gametes.
- In seed plants (angiosperms), pollen grain are the carriers of male gametes and ovule (egg) carries the female gamete.
- Pollen grains are produced in anther and need to be transferred to the stigma to facilitate fertilisation.
- Transfer of pollen grains to the stigma is easy in self fertilizing (monocious) plants as both the anther and the stigma are located close to each other. In dioecious plants it is done by pollination.
The fusion of male and female gamete is called fertilization or syngamy. It results in the formation of diploid zygote.
Parthenogenesis – It is a process of development of new organism without fertilisation of female. E.g., honey bees, turkey, and lizards
External fertilisation – Fertilisation occurs outside the body of the organism called external fertilisation. It release large numbers of gametes in the surrounding that produce large number of offspring. E.g., Bony fishes and frogs.
Internal fertilisation – Fertilisation occurs inside the body of the organism called internal fertilisation. This happen in the majority of plants and terrestrial organism
- Events in sexual reproduction after the formation of zygote are called post-fertilisation events.
- A diploid zygote is formed as a result of the fusion of gametes in all organisms.
- In external fertilisation, the zygote is formed in the external medium (usually water), and in internal fertilisation zygote is formed inside the body of the organism.
- The development of the zygote depends on the type of a life cycle of an organism and its environment.
- In organisms belonging to fungi and algae, the zygote develops a thick wall around itself that is resistant to damage and desiccation.
- Zygote is the vital link that ensures continuity of species between organisms of one generation and the next.
Embryogenesis – It is the process of development of embryo from the zygote.
- During embryogenesis zygote undergoes mitotic cell division and cellular differentiation.
- Cell division increase the number of cells and cell differentiation help group of cells to form specialised tissues and organs to form new organism.
- Animals are grouped into oviparous and viviparous based on the development of the zygote takes place outside the female body or inside.
Oviparous: Egg laying animals called oviparous like reptiles and birds.
- The fertilised eggs coverd by calcareous shell.
- The young ones hatch out after a period of incubation.
Viviparous: The zygote develops into the young one inside the body of the female organism. After attaining a certain stage of growth the young ones are delivered out of the body of the female organism. In flowering plants zygote is formed inside the ovule.
Post fertilisation events in flowering plants:
- Pistil remain intact to the plant
- sepals, petals and stamens wither and fall off
- Zygote develop into embryo.
- Ovules develop into seed.
- Ovary develop into fruit which develops a thick wall called pericarp.
- Seeds germinate under favourable conditions to produce new plants.