What is Reproduction | Definition | types of reproduction | Importance

Reproduction | Definition It's types | Importance

Definition : Reproduction is the production of young ones like parents.
Reproduction is  defined as a biological  process in which an organism gives lise to  young ones (offspling) similar to itself.

Reproduction is an essential process as it leads to continuation of species as well as to maintain the continuity of life. Each organism has its own particular method of reproduction. The offspling grow, and  in tum produce new offspling. Thus, there is  a cycle of birth, growth  and death. Reproduction  enables the continuity of the species, generation after generation.

There is a large diversity  in the biological world  and each organism has  evolved its own  mechanism to multiply  and produce offspring. The organisms hobitat its internal physiology and several other factors are collectively responsible for how it reproduces. Based on whether there is participation of one  organism or two in the  process of reproduction,  it is of

Two types of Reproduction
i. Asexual reproduction
ii. Sexual reproduction

i. Asexual reproduction :
Asexual reproduction is defined as the mode of reproduction which does not involve in fusion of two compatible gametes or sex cells (Male & Female gametes) . It is the process resulting in the production of genetically identical progeny from a single organism and inherits the genes of the parent. Such morphologically and genetically identical individuals are called clones. Asexual  reproduction is common  among single-celled  organisms, and in plants and animals With relatively simple organisations. Organisms choose to reproduce asexually by different modes or ways like fragmentation, budding, binary fission, spore formation, Conidia formation, Gemmules formation etc.

1. Fragmentation : Fragmentation in multicellular organisms is a form of asexual reproduction in which an organism is split into fragments. Each of these fragments develop into matured, full grown individuals that are identical to their parents.

Many sea stars reproduce asexually by fragmentation. For example, if the arm of an individual sea star is broken off it will regenerate a new sea star. Fragmentation also occurs in annelid worms, turbellarians, and poriferans.

2. Budding : It is the most common method of asexual reproduction in unicellular Yeast. Usually it takes place during favourable conditions by producing one or more outgrowths (buds). These buds on seperation develop into new individual. Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult, which breaks away from the main body; whereas in coral budding, the bud does not detach and multiplies as part of a new colony.

Fig : Budding in Hydra.

3. Spore formation : In Chlamydomonas asexual reproduction occurs by flagellated, motile zoospores which can grow independently into new individuals. The method of spore formation occurs in both unicellular and multi-cellular organisms. This process takes place in plants. In spore formation, the parent plant produces hundreds of reproductive units called spores in its spore case. When this spore case of the plant bursts, these spores travel in air and land on food or soil. Here they germinate and produce new plants.

Fig : Spore formation

Fungi like Rhizopus, Mucor, etc. are examples of spore formation.

4. Regeneration :
Regeneration is an asexual method of reproduction. In this process, if the body of a parent organism gets cut, then each cut part can regenerate and form a whole new organism from its body parts. This happens because when the body of an organism that can undergo regeneration gets cut then the cells of cut body part divide rapidly and form a ball of cells. These cells then move to their proper places to form organs and body parts.

Regeneration occurs in both plants and animals. Hydra and planaria undergo regeneration.

5. Parthenogenesis
Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, some ants, wasps, and bees. Some vertebrate animals, such as certain reptiles, amphibians, and fish, also reproduce through parthenogenesis.

6. Binary Fission : Fission, also called binary fission, occurs in prokaryotic microorganisms and in some invertebrate, multi-celled organisms. After a period of growth, an organism splits into two separate organisms. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates, forming a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps also reproduce through fission.

Vegetative Reproduction :
Plants reproduce asexually through their vegetative parts. Hence, the new plants formed are genetically identical to their parents. There are also few methods which would not occur naturally in the plants. Agriculture and horticulture exploit vegetative reproduction in order to multiply fresh stocks of plants. Artificial methods are used to propagate desired varieties according to human requirements.

The various methods are as follows :
a. Cutting :
The small piece of any vegetative part of a plant having one or more buds is used for propagation viz. Stem cutting - e.g. Rose, Bougainvillea; leaf cutting - e.g. Sansvieria; root cutting e.g. Blackberry.

b. Grafting :
Here parts of two plants are joined in such a way that they grow as one plant. In this method, part of the stem containing more than one bud (Scion) is joined onto a rooted plant called stock, is called grafting. Whereas budding is also called bud grafting in which only one bud is joined on the stock, e.g. Apple, Pear, Rose, etc.

Fig : grafting in rose 

c. Tissue culture : It is a method by which a small amount of plant tissue is carefully grown to give many plant lets. Micropropagation method is also used now a days.

Advantages of Asexual Reproduction :
1. The energy requirements for reproduction are minimal.
2. It allows for species survival.
3. Multiple forms of asexual reproduction are available.
4. Only one organism is required to establish a colony.
5. In plant organisms, asexual reproduction eliminates the need for seeds
6. Crop losses can be balanced with this reproduction method.

ii. Sexual Reproduction :
Sexual reproduction is defined as the mode of reproduction which involves in fusion of two compatible gametes or sex cells ( male and female gametes). All organisms reach to the maturity in their life before they can reproduce sexually. In plants, the end of juvenile or vegetative phase marks the begining of the reproductive phase and can be seen easily in the higher plants at the time of flowering.  Sexual reproduction involves two major events viz. meiosis and fusion of gametes to form diploid zygote and the production of genetically dissimilar offsprings. Sexual reproduction is characterised by fusion of the male and female gametes (fertilization), the formation of zygote and embryogenesis.

All orgmisms have to  reach a certain stage of growth and maturity in their life. before they con  reproduce sexually. That  period of growth is called  tho Juvenile phase. It is known as vegetative phase in plants. This phase is variable duration in different organisms.

There are concomitant  changes in the body (like  slowing of metabolism.  etc.) during this last phase of life span. Old  age ultimately leads to death. In both plants and  animals hormones are  responsible for the transitions between the  three phases. Interaction between hormones and certain environmental factors regulate the  reproductive processes and the associated behavioural expressions of organisms.

Events in sexual reproduction : After attainment of maturity. all sexually reproducing organisms exhibit events and processes that have remarkable fundamental similarity.  even though the structures associated with sexual reproduction are indeed very different. The  events of sexual  reproduction though  elaborate and complex.  follow a regular  sequence. Sexual reproduction is characterised by the fusion (or fertilisation)  of the male and female gametes. the formation of zygote  and embryogenesis.

Sequential events that occur in sexual reproduction are grouped into three distinct stages are
1. Pre-fertilization,
2. Fertilization and the
3. Post-fertilization.

1. Pre-fertilisation  Events : These include  all the events of sexual reproduction prior to the fusion of gametes. The two main pre-fertilisation events are gametogenesis and gamete transfer.

2. Fertilisation :
The most vital event of sexual reproduction is perhaps the fusion of  gametes. This process  called syngamy results  in the formation of a  diploid zygote. Th  term fertilisation is also  often  used for this process.  The terms syngamy and  fertilisation are  frequently used though  ,  interchangeably.

3. Post-fertilisation  Events : Elents in sexual  reproduction after the  formation of zygote are  called post-fertilization  events.  Like zygote and embryogenesis.

Hermaphroditism :
Hermaphroditism occurs in animals where one individual has both male and female reproductive parts. Invertebrates, such as earthworms, slugs, tapeworms and snails, are often hermaphroditic. Hermaphrodites may self-fertilize or may mate with another of their species, fertilizing each other and both producing offspring. Self fertilization is common in animals that have limited mobility or are not motile, such as barnacles and clams.

The advantages of sexual reproduction:
1. It produces genetic variation in the offspring.
2. The species can adapt to new environments due to variation, which gives them a survival advantage.
3. A disease is less likely to affect all the individuals in a population.
4. One of the most important advantages of sexual reproduction is that it results in genetic variation among offsprings.
5. Genetic recombination is another advantage of sexual reproduction.