1. What is Asexual Reproduction?
Reproduction is the production of a new organism by an existing member or member of the same species. No living organism is immortal, so reproduction is essential for the renewal and survival of a species. This is the primary function of reproduction and it is also the basis of population growth and spread. Reproduction takes place in 2 ways, asexual and sexual. Some organisms use both ways and others use only one.
Asexual reproduction is the production of new individuals from a single parent without the production of gametes.
Asexual reproduction is particularly common in plants, simple animals and microorganisms. All the individuals produced by one parent are referred to as a clone and are genetically identical. Clone can only differ from one another by means of random mutation.
If mitosis occurred in an organism without the production of a new organism, the it will constitute growth of the organism, but if a new organism is formed, then asexual reproduction has occurred.
1.1 Overview of the Different Types of Asexual Reproduction
1.1.1 Fission – Binary or Multiple
In binary fission, one cell divides to give 2 identical daughter cells. In multiple fission, one cell divides to give many identical daughter cells.
1.1.2 Sporulation
It involves the formation of spores which are unicellular. Each spore has one or more nuclei and a small amount of cytoplasm. When environmental conditions are suitable, each spore can develop into a new individual.
1.1.3 Budding
Yeast is unicellular. A small bud emerges from one side of the yeast cell. The nucleus in the parent cell divides by mitosis to give 2 nuclei. One daughter nucleus remains in the parent cell and the other moves into the bud. Eventually, the bud detaches from the parent cell and becomes an independent cell.
1.1.4 Fragmentation
In spirogyra, an unbranched filamentous green alga, reproduces asexually by fragmentation. The filament of cells breaks up into 2 or more smaller fragments, each of which is able to grow into a longer chain of cells.
1.1.5 Vegetative Propagation
In vegetative propagation, a whole new plant is produced from an axillary bud which is formed between a leaf stalk and a stem. Axillary are buds occurring in the axil of the leaf. Axil is the angle between leaf’s upper side of the stem on which it is borne.
1.1.6 Parthenogenesis
This involves the formation of a new individual from an unfertilized egg cell. In diploid parthenogenesis, egg cells are formed by mitosis while in haploid parthenogenesis, the egg cells are formed by meiosis.
1.1.7 Cloning
It is a means of artificial propagation.
1.2 Comparing Asexual and Sexual Reproduction
In AR, only one parent is involved. In SR, it usually involves 2 parents.
In AR, it involves mitosis but not meiosis (except haploid parthenogenesis). In SR, meiosis occurs during the formation of haploid gametes.
In AR, it does not involve the production of haploid gametes (except in haploid parthenogenesis)). In SR, it involves the fusion of 2 haploid gametes to form a zygote which develops into a new organism.
In AR, the offspring are genetically identical to each other and to the parent. Genetic variation can only occur through random mutation. In SR, the offspring are genetically different from each other and from the parents. Genetic variation is introduced via meiosis, random fusion of gametes and random mutation.
In AR, a relatively large number of offspring is formed. In SR, a relatively smaller number of offspring is formed.
In AR, it does not occur in higher forms of animals and is not widespread. In SR, almost all species including bacteria reproduce by this method.
In AR, it results in a genetically uniform population that can be wiped out in the face of drastic environment changes. In SR, it generates genetic variation which can facilitate adaptation of a species to environmental changes.
In AR, it can maintain favourable genotypes over generations because of genetic uniformity. In SR, it cannot maintain favourable genotypes over generations.
In AR, it occurs at a fast rate giving rise to large numbers of offspring. In SR, it is a relatively slow process and results in a less rapid increase in numbers of organism.
In AR, it is associated with colonization. In SR, it is associated with adaptation.
2. Natural Asexual Reproduction in Organisms
2.1 Classification of Organisms into 5 Kingdoms
2.1.1 Kingdom Prokaryote or Monera
2.1.1.1 Bacteria
Bacteria reproduce asexually by the process called binary fission. Fission means division and binary refers to the fact that the cell divides into two. The 2 new cells are usually of equal size. Before the cell divides, the DNA replicated itself so that the 2 new cells have identical sets of genes.
2.1.2 Kingdom Protoctista
Most protoctists are multi-cellular. They include the algae, a group of unicellular animal like organisms called protozoa and simple relatives of the fungi such as slime moulds.
2.1.2.1 Amoeba and Paramecium
Amoeba changes it shape as it moves around in the same way as the white blood cells. Paramecium is covered in fine hairs called cilia which bring about locomotion by beating. Both organisms grow into their mature size before undergoing binary fission. As in bacteria, the DNA replicates first, followed by cell division. In the case of Amoeba, the cell divides transversely (across the middle) whereas Paramecium divides longitudinally. The daughter cells are genetically identical to the parent.
2.1.2.2 Spirogyra
It is an unbranched filamentous green alga consisting of a chain of cylindrical cells. Each cell contains chloroplasts that are spiral in form. It grows in length by cell division involving mitosis and reproduces asexually by fragmentation. The fiiament of cells breaks up into 2 or more smaller fragments, each of which is able to grow into a longer chain of cells.
2.1.2.3 Plasmodium
This is the parasite that causes malaria. It reproduces asexually by multiple fission where the nucleus divides repeatedly. Each daughter nucleus then breaks away together with a small portion of the cytoplasm. This allows the parasite to quickly invade the host, as it undergoes multiple fission immediately after infection when the parasite enters the human liver.
2.1.3 Kingdom Fungi
Examples include yeasts, moulds, mushrooms and toadstools. Apart from yeasts, which are unicellular, fungi have a characteristic body structure which is made up of a network of fine tube-like threads called hyphae. Hyphae may pack into tissue-like structures as in mushrooms and toadstools. A mass of hyphae is called mycelium. The moulds seen on bread are mycelium.
2.1.3.1 Yeast
It is unicellular and is easy to grow in the lab. It is also of great commercial importance in breweries and bakeries. It undergoes a form of asexual reproduction known as budding in which a new individual, identical to its parent, grows form the body of the parent. It starts as a bud and eventually breaks off.
2.1.3.2 Mushrooms and Moulds
These are made up of a network of fine tube-like threads called hyphae. These may pack together into tissue-like structures. A mass of hyphae is called mycelium, which can be seen as a patch of mould on stale bread. Moulds reproduce asexually by sporulation. Spores are small unicellular structures containing a nucleus each and they are kept in a sporangia before release. Some spores have thick resistant walls which enable them t survive unfavourable conditions.
2.1.3.2.1 Penicillium
It is a fungi / mould that grows on a wide range of substrates from which it obtains nutrients (one species is the source of antibiotic penicillin). It reproduces asexually by means of spores known as conidia. They form at the tips of special, vertically growing hyphae called conidiophores. The spores are coloured, light and small and they are easily dispersed by air currents and germinate to produce new hyphae when they land on a suitable medium. Their large numbers compensate for the high wastage of those that do not find a suitable medium.
2.1.3.2.2 Process of Sporulation
The structure of Rhizopus and Mucor consists of thread like hyphae which are collectively called mycelium. After 2 or 3 days in culture, they begin to produce vertically growing hyphae called sporangiophores.
The tip of each sporangiophore swells into a sporangium, which becomes separated from the sporangiophore by a dome shaped wall called the columella. Inside the sporangium, the protoplasm divides into many portions, each of which develops a wall and becomes a spore containing several nuclei.
As the sporangium matures, it becomes black and dry and the wall cracks unevenly to expose a mass of spores. The columella collapses and provides a platform from which spores are easily blown away and dispersed. The spores germinate if they land on a suitable substrate and produce new mycelia.
2.1.4 Kingdom Plantae
Asexual reproduction occurs naturally on a widespread scale in Kingdom Plantae. The most common form is vegetative propagation and usually involves the growth and development of one or more buds on part of a stem to form a new plant which becomes eventually detached from the parent and lives independently.
Each bud contains a shoot apical meristem; buds are found only on stems, so the organs of propagation must include at least a small piece of stem. The organs of propagation varu greatly in structure but they all have in common the fact that they contain a stem with buds. Even root tubers have a small piece of stem with buds at their top ends.
Some common organs of propagation are not described here. In many cases, these structures serve the equally important function of storing food surviving adverse conditions such as winter or drought, to be used for growth when conditions become more suitable for again.
2.1.4.1 Vegetative Propagation
In this, an axillary bud is used to produce a whole new plant as the axillary bud contains totipotent cells which have the ability to differentiate into any of the many types of cells.
In certain plants, the axillary bud obtains nutrients for development from food stored in the leaf, stem or root.
The structure that contains food reserves and bears the bud is called a perennating organ and it can remain dormant in the soil from one growing season to the next. During periods of unfavourable conditions, the aerial parts of the plants die off, leaving only the dormant perennating organs which contain food reserves and are usually underground and therefore able to survive adverse conditions. Upon the next growing season when favourable conditions return, the buds on these perennating organs develop into new shoots.
Plants can reproduce by vegetative propagation through various organs such as stem tuber, rhizome, bulb, corm runner and stolon. Stem tuber, rhizome, bulb and corm serve as organs of vegetative propagation as well as organs of perennation. Runner and stolon are organs of vegetative propagation only.
2.1.4.1.1 Stem Tuber e.g. potato
Potato is a stem tuber which is formed from a stem which grows underground and swells with food at its tip. The tuber possesses a bud at its tip called the terminal bud which can eventually grow into a new shoot and then new plant in the following growing season.
The tuber is also covered with axillary buds that lie in the axils of the leaves and can develop into new plants. As the structure has been underground, the leaves are not developed yet.
Stem tubers are effective in reproduction and multiplication because a potato plant can produce more than one underground stem and each stem or tuber can produce more than one shoot from the various buds.
2.1.4.1.2 Root Tuber e.g. dahlia
Dahlia forms root tubers which develop from roots which swell with food and have a bud where the root joins the stem. This bud would be the terminal bud that sprouts in the following growing season.
2.1.4.1.3 Rhizome e.g. ginger
A rhizome is a horizontally growing underground stem swollen with stored food. It bears an apical bud, axillary buds, leaves and adventitious roots. The apical bud enables the rhizome to lengthen while the axillary buds enable lateral rhizomes to form at the sides of the parental rhizomes.
When environmental conditions are favourable, the apical bud and axillary bud will develop into a shoot (flowers and leaves). From a single rhizome, a few daughter plants can be produced. Rhizomes are different from stem tubers in that rhizomes have adventitious roots whereas stem tubers do not.
2.1.4.1.4 Bulb e.g. onion, tulip, daffodil
Bulbs are modified shoot consisting of a flattened disk-like stem and numerous fleshy storage leaves. At the base of each fleshly leaf is a bud. The bud is surrounded by brown scale leaves which are the remains of the previous year’s leaves after their food reserves have been used up. Adventitious roots are also present.
When favourable conditions return, each bulb develops and gives rise to leaves and flowers. The food made in the leaves of the new plant is sent to the leaf bases and stored and the leaf bases swell and form a new bulb ready for growth in the following year. Each bud will give rise to a new bulb.
2.1.4.1.5 Corm e.g. water chestnut
It is a thick, short, swollen, vertical underground stem with food reserves. It is surrounded by dry scale leaves and one or more buds may be found in the axils of scale leaves. Adventitious roots are also present.
During periods of favourable environmental conditions, buds make use of food reserves in the corm and develop into aerial shoots bearing leaves and flowers. Leaves manufacture food and excess food is passed down and stored in the base of the new stem which gradually swells to form a new corm on top of the old corm. At the end of the growing season, contractile roots pull the new corm down into the soil.
2.1.4.1.6 Runner e.g. strawberry
A runner is a horizontal stem which grows along the surface of the ground. It grows out from one of the lower axillary buds of the parent plant; along the length f the runner are scale leaves in the axils of which are axillary buds. The axillary bud at the apex of the runner produces adventitious roots, which grow into the soil; and stem and leaves, which grow into the air. When the new plant matures, it may produce a new runner.
2.1.4.1.7 Stolon e.g. blackcurrent, gooseberry
Like a runner, a stolon is a horizontal stem growing along the surface of the ground. There is one major difference between a runner and a stolon. That is: only the axillary bud at the end of a runner will give rise to a new plant. In contrast, several axillary buds occur along the length of a stolon and all of them can develop into new plants.
2.1.4.2 Parthenogenesis and Fragmentation
Parthenogenesis is the formation of a new individual from an unfertilized egg cell. There are two types of parthenogenesis, namely, diploid and haploid parthenogenesis. In diploid parthenogenesis, the egg cells are formed by mitosis rather than by meiosis. Therefore, the resultant egg cells are diploid. In certain flowering plants, an embryo may develop from a diploid cell in the ovule, which has not undergone meiosis and fertilization. As the embryo develops, the surrounding tissues form the seed and fruit in the usual way. This occurs in a number of citrus fruits.
2.1.5 Kingdom Animalia
Kingdom animalia consists of multi-cellular organisms including sponges, worms, corals, mollusks, insects, fish, amphibians, reptiles, birds and mammals. Natural asexual reproduction is not widespread, restricted to animals having a relatively simple structure like cnidarians and the mode is via budding.
Fragmentation and parthenogenesis can also occur. Sponges and hydroid coelenterates have powers of regeneration. If a sponge is macerated by passing it through a fine gauze, the separated cells come together in groups and grow into free new individuals. This constitutes asexual reproduction via fragmentation.
Bean aphids can undergo diploid parthenogenesis in which the eggs are formed by mitosis instead of meiosis. Hence the eggs are diploid and they divide by mitosis to give rise to a diploid adult.
Bees, ants and wasps undergo haploid parthenogenesis which the eggs are produced the usual way but they develop without being fertilized into new individuals whose cells are haploid.
2.1.5.1 Hydra
Hydra is a freshwater organism unlike other marine cnidarians. It has a slender, hollow, cylindrical body up to 20 mm long, with long, thick waving tentacles surrounding the single opening to the body, the mouth, at its tip. It is usually anchored to something solid like a stone or a plant. Its epithelium consists of just 2 layers of cells so budding is possible.
Hydra reproduces asexually by budding and buds usually start to grow towards the base of the body. Cells in both layers in this region (of the bud) multiply by mitosis, forming a hollow bulge (bud) which increases in size, becomes cylindrical and eventually developing a mouth and tentacles. The new bud pinches off as a separate, genetically identical individual.
2.1.5.2 Ribbon Worms
Ribbon worms reproduce asexually and naturally by fragmentation. The body breaks into 2 or more parts, each of which will regenerate into a new individual. Ribbon worms are simple, unsegmented marine worms with long, flattened bodies up to 20 m. in length which can break up naturally into pieces.
2.1.5.3 Why do few animals undergo asexual reproduction while in is more common in plants and fungi?
Most animals show a high level of tissue differentiation and many have specialized body organs and asexual reproduction tends to be confined to animals with a less complex body system. Plants and fungi are usually in fixed positions (sedentary) in certain habitats while animals are mobile. Thus for the plants, there would be less changes in the environment and less vulnerable to new or sudden diseases. But animals need more variation in their populations so that entire populations will not be eradicated when there are natural disasters, sudden climatic changes or a disease epidemic.
2.2 Advantages and Disadvantages of Natural Asexual Reproduction and Evolutionary Consequences
Both sexual and asexual reproductions are essential and each have their own advantages and disadvantages. We should not conclude that one is better that the other should not occur. Both are suitable for different organisms and some can survive best with both or only one.
2.2.1 Advantages
Asexual reproduction involves mitosis that results in a progeny that ate genetically identical as the parent. This has important evolutionary consequences because it can be an advantage if the parent is already well-adapted to its environment and successfully competing with other organisms. Darwin’s theory of natural selection states that the fitter members of a species have a greater chance of survival. Asexual reproduction preserves successful combination of alleles and genes if environmental conditions are stable.
Since only one parent is required, time and energy is not wasted in seeking a mate. For sessile organisms, the problem of gamete transfer in sexual reproduction from one organism can be avoided.
There is no wasting of gametes since they are not required in asexual reproduction.
Dispersal of species is effective and this is important as suitable habitats with new resources some distance away from the parent can be exploited. E.g. non-motile fungi can reach new sources of food by producing spores asexually that are dispersed by air currents.
Colonization of habitat is effective. Once an organism is established in a habitat and conditions are favourable, it is more effective to spread over the habitat by asexual than by sexual means. E.g. grasses can spread rapidly over the habitat by means of rhizomes which are sent through the soil and develop into new plants while still receiving nutrients from the established parent plant.
Asexual reproduction can allow the rapid reproduction of large numbers of offspring. New habitats can therefore be exploited rapidly. Microorganisms such as bacteria and fungi provide good examples of this.
2.2.2 Disadvantages
The major disadvantage is that there is no genetic variation amongst the offspring. Although this can be an advantage, natural selection cannot occur as there is lack of genetic variation, so when there is a sudden adverse condition that occurs, the whole population may be wiped out. Darwin’s theory also states that evolution proceeds by natural selection. This in turn depends on variation existing among the members of a species since the fitter variants are the ones that are more likely to survive when there are some changes in the environment. Therefore a population that reproduces asexually may not be able to ensure its long term survival.
The production of spores may be a waste of energy because a high proportion of spores do not end up on suitable substrates to grow.
Asexual reproduction and consequent spread of an organism in one area can lead to overcrowding and exhaustion of resources such as nutrients.
3. Artificial Asexual Reproduction in Organisms
3.1 Artificial Asexual Reproduction in Plants
Agriculture is one of the world’s largest industry in economic terms and current progress is rapid. Revolutionary new methods of propagating plants artificially have been developed. New food crops and crops grown for expensive products like drugs and perfumes are being developed and there are continuing efforts to genetically improve existing crops and the efficiency of their production. This branch of agriculture that studies this is called horticulture, which is usually associated with intensive production of high-value crops such as flowers, vegetables, shrubs and fruit trees.
There are several ways of artificial plant propagation. These will be listed in the sub-headings below.
3.1.1 Cutting
This is a very common and traditional method which involves cutting a part of a plant and placing this plant in suitable conditions such that it can develop new roots and grow into a new plant.
The cutting can be a stem, leaf or root. As soon as the cuttings are made, they are usually placed in sterile, moist sand or vermiculite. Rooting hormone (auxin) may be added to stimulate rooting.
Many ornamental plants are reproduced artificially in this way. Chrysanthemum shoot is used as a cutting. African violet leaves are used as cuttings and stem cuttings are used for woody ornamentals.
3.1.2 Grafting and Budding
Grafting is the transfer of the upper part of one plant (scion) onto the lower part of another (stock) and the 2 are allowed to grow together. This is possible because the cambium tissues of the 2 plants will merge and give rise to normal conducting tissues.
The new plant usually has the root system of the stock and develops shoot system of the scion.
The stock is chosen for its vigour and the scion is chosen for it superior flowers or fruits.
Sometimes the stock is selected for its dwarfing effect, which generally makes trees bear fruits earlier.
Fruit trees like apple, pear, peach and plum, which cannot be grown from cuttings easily are commonly propagated this way.
Rose bushes are also reproduced likewise because sexual reproduction give rise to too much variation. Buds of roses are grafted onto vigorous woody stocks like the wild dog rose, When the scion is a single bud (and not the whole shoot system) on a short portion of stem, the technique is known as budding.
The quality of fruit is determined by the scion. 2 examples are as follows.
Scions from French varieties of vines that produce superior wine grapes are grafted onto root stock of American varieties, which are more resistant to certain diseases. The quality of the fruit, determined by the genes of the scion, is not diminished by the genetic makeup of the stock.
Grafting normal twigs onto dwarf stock varieties retard the vegetative growth of the shoot system. Since seeds are produced by the part of the plant derived from the scion, they would give rise to plants of the scion species if planted.
3.1.3 Layering
This is another traditional method that relies on the natural ability of many stems to produce roots when covered with soil. The portion of the stem with the bark removed is bent to the ground and covered with moist soil while still attached to the parent plant, often by pegging a convenient section of stem just beneath the surface of the soil.
Once new roots are well established, the stem can be cut and separated from the parent plant thus creating a new plant which can be transplanted to a new location. Examples of such plants are the line and bougainvillea.
3.1.4 Marcotting
This is based on the same principle as layering but instead of bending the stem down to the ground, a layer of moist soil is wrapped around the portion of the branch that has the bark removed. The soil is kept in place by a coconut husk, polythene wrapper and string. It is kept moist by watering the plant every day. Roots will soon appear and the branch can be cut and planted.
3.1.5 Tissue Culture / Micropropagation
Tissue culture refers to the aseptic growing of excised plant parts in vitro. This method is used for propagation and genotype modification, biomass production of biochemical products , preservation and storage and scientific investigations.
Micropropagation refers to the use of tissue culture technique to propagate plants, starting from very small plant parts grown aseptically in a vessel containing nutrients. The general procedures are as follows.
Surface of explants are sterilized using dilute sodium hypochlorite and are transferred aseptically to culture vessels containing nutrients and plant growth substances. The culture medium us usually solidified using agar.
The cells of an explants divide by mitosis to form a callus, which is undifferentiated, actively dividing mass of cells.
The cells of a callus can be induced to either differentiate or proliferate into particular tissues by varying the combination of various plant growth substances.
The number of calli can be increased by subculture i.e. cutting the callus into several pieces and placing them in separate culture vessels containing culture medium with a certain combination of plant growth substances. The cells in the calli proliferate to form bigger calli. This process is repeated every few weeks so that a few explants can give rise to millions of calli within a year.
The cells of callus can now be induced to differentiate into particular tissues. An organized tissue can develop into a plantlet and when I is sufficiently large, it is transferred from the lab to the soil and grown into a whole plant.
3.2 Artificial Asexual Reproduction in Animals
Artificial cloning of animals is a form of asexual reproduction. Differentiated cells from animals will often fail to divide in culture, much less develop into a new organism. Therefore, animal researchers have approached the genomic-equivalence question by replacing the nucleus of an unfertilized egg cell of zygote with the nucleus of a differentiated cell.
The pioneering experiments in nuclear transplantation were carried out by embryologists. These investigators removed and destroyed the nuclei of frog egg cells, then transplanted nuclei from embryonic and tadpole cells of the same species into the enucleated eggs.
The ability of transplanted nuclei to support animal development turned out to be inversely related to the age of the donor embryo. If the nuclei came from the relatively undifferentiated cells of an early embryo, most of the recipient eggs developed into tadpoles. But with nuclei from differentiated intestinal cells of tadpole, fewer than 2% of the eggs developed into normal tadpoles and most of the embryos make it through the earliest stages of embryonic development.
It was concluded, from the above experiments, that nuclei do change in some ways as cells differentiate. Although the base sequence of DNA usually does not change, chromatin structure and function alters in specific ways, Biologists believe that the cells of the body differ in structure and function not because they contain different genes, but because they express different portions of common gene.
The cloning of mammals has applications in both agriculture and medicine. Many copies of desirable animals may be produced in future along with animals that have had human genes introduced into their genomes so that they produce otherwise expensive medical products like hormones.
One form of animal cloning that is becoming important in animal breeding is the splitting of embryos at a very young age when they are still at the blastula stage. Splitting the embryo once would result in the formation of identical twins but when repeated splitting, this can create multiple clones. The clones are then grown in surrogate mothers. This has been done for cattle, sheep and goats.
However, the main application of animal cloning at present is not to produce many identical whole animals, but to maintain identical cells in culture. With these cells, the effect of new drugs, antibiotics and other pharmaceuticals, on animal cells can be tested without using whole animals. Some medically useful proteins that are short in supply, like the growth hormone, can also be obtained from cloned mammalian cells. The cloning of human stem cells can provide a source of replacement tissues and organs too.
3.3 Advantages and Disadvantages of Cloning
3.3.1 Advantages
The major advantage is that the clond plants are genetically identical to the parent plant. Many copies of plants of desirable characteristics can be produced. Selective breeding of plants and animals rely on sexual reproduction and it is difficult to produce true breeding lines. This is especially so if the plant is adapted for cross-pollination as in the apple tree.
Cloning is a convenient method of propagation, rather than sowing seeds for plants. The need to nature delicate seedlings is eliminated.
The problems of having sterile parents or species with poor germination or fertilization rates can be avoided. The plants like bananas or navel oranges do not produce seeds and therefore must be propagated asexually. Orchids do not germinate from seeds easily, thus cloning is a practical alternative. For animals, cloning can also overcome the problems of mating in the wild or a difference in breeding seasons.
Certain cloning techniques such as grafting may reduce or avoid juvenility and allow plants to flower and bear fruits faster than seedlings would. Grafting onto root stocks that are resistant to disease increase the chance of survival if the scion is susceptible to such diseases.
Tissue culture has various benefits associated with it and are explained below.
Rapid production of large numbers of plants from just one or few stock plants can be achieved.
Plant disease can be avoided e.g. virus-free plants can be obtained by selecting only meristematic tissue. Viruses tend to be distributed throughout the plant body by the vascular system but the meristem lack vascular tissue and are therefore free of most viruses. Micropropagation is important in the work of virus-free potatoes.
Micropropagated plants can be produced at any time of the year and can also be put in cold storage, taking up relatively little space. Combined with rapid production, this gives great flexibility in supplying consumer demand.
Exotic plants that are to produce in large quantities from seeds can be cloned in large numbers and sold at affordable prices.
Micropropagation can be linked to genetic engineering. If a gene of interest is introduced into a plant cell by genetic engineering, the modified cell can be grown into a whole plant and then cloned to produce many new plants, all containing the gene of interest. To some extent, plants can be designed to order.
Their light weight and small size mean that micropropagated plants can be air-freighted easily and cheaply, thus increasing international trade.
Standardizing the growing conditions produces batch after batch of standard plants. Such reliability and quality control is an important sales advantage.
3.3.2 Disadvantages
The major disadvantage is that it is not as convenient as sowing seeds when very large numbers of plants are required as it is labour intensive.
Being genetically identical is disadvantages when there is a sudden change in environmental conditions that may have devastating consequences if the plants cannot adapt.
Micropropagation has certain disadvantages as follows.
The processes are labour intensive and therefore high cost. Individual plants must therefore have a high market value and this is why tissue culture is mainly confined to ornamental rather than crop plants.
The work must be carried out in sterile conditions. This requires highly trained staff and imposes severe constraints on working practice.
Since the techniques are relatively new, some unforeseen problems have arisen. It was decided in the 1970s to replace oil palms in Malaysian plantation with new micropropagated varieties. 5 years later, when the first fruit should have been produced, the plants were discovered to be sterile. The problem was traced to genetic changes that occurred in the tissue culture. Such genetic changes seem to be a risk in tissue culture and strict quality controls are needed to avoid such problems.
