Information

How do asexually reproductive organisms mutate?


In accordance to my limited knowledge of biology, asexual organisms exactly copy their DNA as there is no "contaminant", which in sexual reproduction is the other organism's DNA.


It is not necessary that no mutation occurs in the system where no gene recombination or "contaminant" takes place. There are mutations (which are sudden abrupt inheritable changes)

There can be errors in genome which can cause mutations.

Take example of replication of DNA-:

There are many enzymes linked with it : DNA polymerases ( I, II,III) in prokaryotes where asexual reproduction occurs which add nucleotides to make replicas of DNA of parent ( you can check the complete process at wikipedia)

Come to the point : DNA polymerase III, after synthesising dna chains has ability of proof reading. It can correct the mismatches ( hope you know that adenine bonds with thymine and cytosine with guanine, they are nitrogen bases of dna)

DNA polymerase III can correct the mismatches but it is not error proof- it can't distinguish between thymine and uracil (another nitrogen base ; thymine is methylated uracil)

This is only one possible error, there can be many such hence asexual reproduction isn't error proof


Organisms That Reproduce Asexually

Asexual reproduction encompasses the forms of reproduction that involve a single parent, and lead to the creation of a genetically identical progeny. Although, such reproductive processes are observed primarily in unicellular organisms, a wide array of multicellular organisms also exhibit asexual reproduction. The current article provides an elaborate account of the same.

Asexual reproduction encompasses the forms of reproduction that involve a single parent, and lead to the creation of a genetically identical progeny. Although, such reproductive processes are observed primarily in unicellular organisms, a wide array of multicellular organisms also exhibit asexual reproduction. The current article provides an elaborate account of the same.

Reproduction is one of the distinguishing features of living organisms, and is essential for the survival of any species. Such perpetuation of species is achieved through two fundamental modes – sexual and asexual. Sexual reproduction involves the formation of a zygote, through the fusion of gametes from two parents. The zygote develops into an embryo, and grows into an offspring. On the other hand, asexual reproduction involves the formation of a genetically identical offspring from a single individual.

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The primitive life forms predominantly exhibit asexual reproduction. The ability to exist in two genders evolved with the changing environment. With the increase in complexity of the newly evolved life forms, the dominance of asexual reproduction decreased. Nevertheless, each kingdom encompasses a set of strictly asexual organisms, as well as a set of organisms characterized by the presence of both sexual and asexual phases.

What is Asexual Reproduction?

Asexual reproduction is a reproductive process devoid of gamete formation and fertilization, and mostly proceeds via mitosis. The resultant offspring is a clone of the parent organism due to the absence of genetic exchange. Budding, fission, fragmentation, vegetative propagation, etc. are the various mechanisms of asexual reproduction. Such reproductive cycles complete in less time, as compared to sexual reproduction, which serves as an evolutionary advantage. However, owing to the lack of genetic variation, the ability to adapt to changing environment is limited. In asexual organisms, genetic variations can occur only through random mutations. Being so, they are dependent on favorable mutations to cope with environmental changes.

List of Organisms that Reproduce Asexually

Asexual reproduction is the primary mode of reproduction in archaea, bacteria, and protists. Fungi and plants show a combination of sexual and asexual modes of reproduction. The primary mode of reproduction in animals is sexual reproduction, but certain animals have retained the ability to reproduce asexually. Some organisms where asexual reproduction is the only means for perpetuation, as well as those with a dominant asexual phase have been enlisted below.

Kingdom Archaebacteria

All archaebacteria reproduce asexually through budding, fission, or fragmentation processes. The commonly known ones include:

» Methanogens: Methanococcus jannaschii, Methanocalculus pumilus
» Thermophiles: Pyrolobus fumarii, Sulfolobus solfataricus
» Halophiles: Halococcus thailandensis, Halobacterium salinarum
» Psychrophiles (Cryophiles): Arthrobacter chlorophenolicus, A. crystallopoietes

Kingdom Eubacteria

Although most of the bacteria reproduce asexually, in certain bacteria, genetic exchange occurs through bacterial conjugation and transformation processes.

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► Strictly Asexual

»Bacilli: Bacillus licheniformis, Staphylococcus aureus
» Nitrifying bacteria: Nitrosomonas europaea, Nitrococcus mobilis
» Purple sulfur bacteria: Thiocapsa purpurea, Alkalilimnicola ehrlichii
» Purple non-sulfur bacteria: Rhodospirillum rubrum, Rhodocyclus tenuis
»Cyanobacteria: Oscillatoria princeps, Nostoc commune
»Spirochaetes: Treponema pallidum, Leptospira interrogans
» Others: Prosthecochloris aestuarii, Clostridium tetani, Rickettsia rickettsii, Zymomonas mobilis

Asexual and Sexual

Escherichia coli, Shigella dysenteriae, Chromatium okenii, Chlorobium tepidum, Vibrio cholerae

Kingdom Protista

This is the kingdom that first showed a distinct presence of gametes, and an alternation in the sexual and asexual forms. In protists, reproduction occurs through budding, fragmentation or sexual and asexual spores. However, the asexual phase dominates the life cycle of such protists.

Strictly Asexual

»Fungus-like protists: Dictyostelium discoideum, Labyrinthula terrestris
» Plant-like protists: Euglena sanguinea
» Animal-like protists: Amoeba proteus

Asexual and Sexual

»Fungus-like protists: Phytophthora infestans, Pythium insidiosum, Physarum polycephalum, Didymium iridis
» Plant-like protists: Pfiesteria piscicida, Navicula lanceolata, Rhoicosphenia curvata, Dinobryon divergens
» Animal-like protists: Leishmania donovani, Toxoplasma gondii, Cyclospora cayetanensis, Paramecium aurelia

Kingdom Fungi

Fungal reproduction occurs through sexual and asexual spores. The asexual spores germinate and give rise to hyphae in the presence of moisture. It is the deuteromycetes (Fungi imperfecti) that reproduce solely through asexual mechanisms, and belong to the phylum Ascomycota or Basidiomycota.

Strictly Asexual
» Ascomycetes: Aspergillus niger, Penicillium roqueforti
» Basidiomycetes: Tilachlidiopsis racemosa

Asexual and Sexual

» Chytridiomycetes: Batrachochytrium dendrobatidis, Allomyces macrogynus
»Ascomycetes: Saccharomyces cerevisiae (Budding yeast), Schizosaccharomyces pombe (Fission yeast)
»Basidiomycetes: Puccinia graminis, Heterobasidion annosum (Spiniger meineckellus)
» Zygomycetes: Rhizopus oryzae, Mucor mucedo

Kingdom Plantae

Asexual reproduction in plants occurs through vegetative propagation and apomixis. Very few plant species reproduce strictly through asexual modes. A majority of them exhibit an alternation of both sexual and asexual reproductive phases.

Strictly Asexual

Vittaria appalachiana (Appalachian shoestring fern), Dahlia imperialis (Bell tree dahlia)

Asexual and Sexual

»Algae: Sargassum muticum, Chlamydomonas reinhardtii
» Mosses: Campylopus introflexus, Acrophyllum dentatum, Philonotis tenuis
» Ferns: Dicksonia squarrosa, Asplenium flabellifolium, Hypolepis ambigua
» Gymnosperms: Cycas revoluta, Pinus oocarpa, Juniperus horizontalis, Sequoia sempervirens
» Angiosperms: Solanum tuberosum (Potato), Fragaria virginiana (Strawberry), Taraxacum officinale (Dandelion)

Kingdom Animalia

Animals, the most evolved organisms, perpetuate primarily through sexual reproduction. However, a few of the species, mostly aquatic invertebrates, exhibit the presence of asexual reproduction as well.

Strictly Asexual

Hydra

Asexual and Sexual

» Sponges: Racekiela ryderi, Spongilla lacustris
»Corals: Zoopilus echinatus, Diaseris fragilis
»Sea anemones: Epiactis prolifera, Anthopleura elegantissima
» Starfish (Sea stars): Coscinasterias tenuispina, Stephanasterias albula
» Ringed worms: Sabellastarte spectabilis, Lumbriculus variegatus
»Flat worms: Dugesia dorotocephala, Pseudoceros pardalis
» Insects: Dahlica fennicella (Bag worm moth), Timema morongensis
» Reptiles: Cnemidophorus neomexicanus (New Mexico whiptail lizard)
» Sharks: Instances of asexual reproduction in hammerhead and bonnethead sharks have been identified recently.

Types of Asexual Reproduction

Budding: This process involves the formation of small outgrowths, called buds, on the parent organism. The bud enlarges, and then separates from the parent after a particular stage of maturity is attained. The separated bud develops into a new organism.
Commonly exhibited in yeasts, bacteria and hydra.

Fission: In this mode of reproduction, the parent organism grows in size and divides into two or more organisms. Binary fission implies the splitting of parent organism into two new organisms, whereas multiple fission implies a division into more than two daughter organisms.
Commonly exhibited in archaea, bacteria, yeasts and protists.

Gemmulation: It is the process of formation of an internal bud comprising a specialized mass of cells. The internal buds then germinate under favorable conditions, and develop into a mature organism.
Commonly exhibited in sponges.

Fragmentation: This mode is characterized by the development of an entire organism from a fragment of the parent organism. The parent body breaks into two or more parts which develop into new organisms.
Commonly exhibited in flatworms, sea anemones, mosses and ferns.

Sporulation: This form refers to the formation of asexual spores, followed by their release and dispersal. These spores germinate and develop into a new organism.
Commonly exhibited in algae and fungi.

Parthenogenesis: It is the unique process wherein a female gamete develops into an embryo without fertilization with the male gamete. However, an exceptional fact about this process is that the formation of female gametes can occur through mitosis or meiosis, and hence the resultant offspring may or may not be genetically identical to the parent organism.
Commonly exhibited in aquatic invertebrates, arthropods and reptiles.

Apomixis: It is a process specific to plants, and refers to the development of seeds without fertilization. Such seed development may be through parthenogenesis or through embryo development from ovarian cells.
Only exhibited in plants.

Vegetative propagation: The various forms of asexual reproduction in plants is collectively termed vegetative propagation/reproduction. It involves the formation of specialized structures like bulbils, stolons (rhizomes), bulbs, suckers, etc. In certain plant species vegetative reproduction occurs through the formation of buds on leaves, stems or roots.
Only exhibited in plants.

Asexual reproduction is the primary mode of reproduction in lower life forms like bacteria, algae, protists, etc. On the other hand, higher organisms either show only sexual reproduction or a combination of both sexual and asexual reproductive processes. Each mode of reproduction has its own set of unique features that provide an evolutionary advantage to the organism.

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Multiple Choice

What is a likely evolutionary advantage of sexual reproduction over asexual reproduction?

  1. sexual reproduction involves fewer steps
  2. less chance of using up the resources in a given environment
  3. sexual reproduction results in greater variation in the offspring
  4. sexual reproduction is more cost-effective

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[hidden-answer a=�″]3[/hidden-answer]

Which type of life cycle has both a haploid and diploid multicellular stage?

  1. an asexual life cycle
  2. diploid-dominant
  3. haploid-dominant
  4. alternation of generations

[reveal-answer q=�″]Show Answer[/reveal-answer]
[hidden-answer a=�″]4[/hidden-answer]

Which event leads to a diploid cell in a life cycle?

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[hidden-answer a=�″]2[/hidden-answer]


Meiosis – Sexual Reproduction

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves Monterey pine trees produce seeds from which Monterey pine seedlings emerge and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same. While many single-celled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method.

Figure 1: Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)

Sexual reproduction is the production by parents of sex cells and the fusion of two sex cells to form a single, unique cell. In multicellular organisms, this new cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and fertilization to reproduce.

Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. The fact that most eukaryotes reproduce sexually is evidence of its evolutionary success. In many animals, it is the only mode of reproduction. And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to be more advantageous. If the parent organism is successfully occupying a habitat, offspring with the same traits would be similarly successful. There is also the obvious benefit to an organism that can produce offspring by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. There is no need to expend energy finding or attracting a mate. That energy can be spent on producing more offspring. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, asexual populations only have female individuals, so every individual is capable of reproduction. In contrast, the males in sexual populations (half the population) are not producing offspring themselves. Because of this, an asexual population can grow twice as fast as a sexual population in theory. This means that in competition, the asexual population would have the advantage. All of these advantages to asexual reproduction, which are also disadvantages to sexual reproduction, should mean that the number of species with asexual reproduction should be more common.

However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so common? This is one of the important questions in biology and has been the focus of much research from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms. In addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes, and the genes are mixed into different combinations by the process of meiosis. Meiosis is the division of the contents of the nucleus that divides the chromosomes among gametes. Variation is introduced during meiosis, as well as when the gametes combine in fertilization.


What is asexual reproduction: nature’s virgin birth

Not every living thing has two parents. Some animals and plants, oddly enough for us humans, are made from just one parent. That’s because these organisms reproduce asexually, unlike sexual reproduction which requires two parents.

Asexual reproduction does not involve gametes (sex cells like sperm and eggs) or fertilisation. As a result, there is no mixing of genetic information and every offspring is genetically identical to the parent. They are all basically clones.

Rather than sex cells, asexual reproduction involves somatic cells that divide mitotically, giving rise to new sets of cells with the same genetic composition.

All bacteria, and all prokaryotic organisms in general, reproduce asexually. This is easily verifiable since bacteria have just one chromosome, rather than X and Y chromosomes that humans and other organisms that reproduce sexually have.

Main differences between sexual and asexual reproduction

Advantages and disadvantages of asexual reproduction

Obviously not being forced to gain access to a potential mate or compete with others in order to reproduce is one of the most important advantages of asexual reproduction.

Asexual organisms are the ultimate single parents, reproducing on their own and on their own terms. They literally don’t need no man (or woman) in their lives. A single asexual individual can replenish an entire population whereas if there is only a single individual left that sexually reproduces, the entire species is doomed.

As a result, an asexual organism can reproduce phenomenally fast and in massive numbers. Bacteria, for instance, may divide several times per hour. In just a few hours, 100 bacteria can spawn millions of other individuals — but that’s under ideal conditions. In the real world, the vast majority of individuals do not live long enough to reproduce because they are limited by resources, predators, and other factors.

Secondly, plants that bypass the sexual process can reproduce during severe droughts since motile sperm require water to fertilize the egg.

From an economic standpoint, agricultural plants with certain desirable characteristics can be produced faster and cheaper through cloning compared to plants that require sexual reproduction. The problem with this particular approach is that if a single fatal mutation occurs, the entire society of clones can collapse. Likewise, monoculture plants are vulnerable to disease outbreaks.

The banana, which is the world’s most popular crop with over 100 million metric tons produced annually, is illustrative in this sense. Virtually all bananas sold in developed countries are of the Cavendish variety, all of which are clones and vulnerable to fungal outbreaks that may one day wipe out the entire family. This has happened before in the 1960s with a different subgroup of bananas, which is now all but extinct.

Organisms that employ asexual reproduction are often only suited to one habitat, which is another drawback. But the most important downside of asexual reproduction is probably that it does not lead to genetic variation in a population. That’s not to say that asexual species can’t evolve. Asexual reproduction will occasionally introduce genetic variation into the population if a random mutation in the organism’s DNA is passed on to the offspring. But compared to sexual reproduction, the variation remains near to zero.

Neither sexual nor asexual reproduction is better than the other. Both modes of reproduction are adaptations to temporally or spatially variable environments. For instance, perennial plants that grow in environments with rich soil nutrients or water content tend to promote clonal growth over sexual reproduction. In contrast, under high light-quantity conditions, plants tend to allocate more biomass to sexual reproduction and less to clonal propagation.

Types of asexual reproduction

There are six main types of asexual reproduction, which lead to distinct reproductive strategies. These are:

  • binary fission,
  • fragmentation,
  • budding,
  • vegetative reproduction,
  • spore formation and,
  • agamogenesis.

Binary fission

In binary fission, or just fission, a parent organism splits into two or more identical individuals. This is the most common mode of reproduction among single-celled organisms such as bacteria, archaea, and some fungi.

This process is extremely stable because prokaryotes tend to have a very simple genome, so there are relatively few mutations as compared to eukaryotes. Eukaryotes must undergo many cell divisions before the gametes can be produced for sexual reproduction, and in the process many more mutations can be introduced before offspring are even created.

Fragmentation

Fragmentation in a starfish. One of the arms of the starfish splits off and after some time it starts developing into a new starfish. Credit: OpenStax.

Fragmentation is a form of asexual reproduction that mostly occurs in multicellular organisms (i.e. organisms made up of more than a single cell). During fragmentation, a living organism splits into parts that later grow identical to the original parent with respect to shape, size, and other characteristics.

Fungi, lichens, molds, worms, sea stars, and sponges are some of the common examples for which fragmentation asexual reproduction occurs. It is also common in plants. Planting parts of the tuber of a potato can result in a new potato plant with the same genetic makeup. When broken apart, weeds can regrow from each fragmented underground stem, which is partly why they’re so pesky.

Fragmentation is different from regeneration. The process of fragmentation leads to the growth of two separate identical organisms whereas regeneration allows certain organisms to regrow certain body parts that are lost. Lizards and octopi are some examples of creatures that can regrow lost limbs or tentacles, respectively. However, a new lizard won’t grow out of its lost tail.

Budding involves the formation of an outgrowth, or bud, from the body of an organism. This bud tends to be much smaller than the parent body and can later develop into a new individual, which is a clone of the parent.

In the yeast Saccharomyces cerevisiae, a smaller daughter cell grows on the larger mother cell. The bud forms and stays for a while, and then detaches to grow fully as a new individual. Credit: Masur/BiologyOnline.

Budding is common among fungi, such as yeast, as well as plants, such as the spider plant. Some invertebrates also employ budding to reproduce, including hydra, corals, echinoderm larvae, and some acoel flatworms.

Vegetative reproduction

When budding in plants is induced artificially, such as during the horticulture practice of grafting wherein the bud of one plant is inserted onto another plant so that both plants grow together, the process is considered a form of vegetative reproduction. Roses are just one example of a plant that is commonly bud grafted.

Vegetative reproduction is an umbrella term that refers to any type of asexual reproduction observed in plants, not just budding. Anytime a plant reproduces without the help of seeds or spores, that is an instance of vegetative reproduction. Instead, plants reproduce using this type of reproduction through bulbs, tubers, shoots, suckers, stolons, and rhizomes.

Spore formation

Spores are haploid reproductive cells, typically found in plants, algae, and some protists, that can fully develop without fusing with another cell, unlike gametes. Plants such as ferns, moss, and fungi reproduce by this method.

Spores are stored in a sac called sporangia. When this knob-like structure bursts, countless minute single-celled spores disperse into the air, where they are scattered by rain, wind, or insects.

Agamogenesis

Komodo drago females can reproduce the old-fashioned way, by mating with a male and laying eggs. Or she can lay eggs without having mated, through a sort of virgin birth process called parthenogenesis.Credit: PublicDomainPictures.

Agamogenesis refers to reproduction that does not involve a male gamete (sperm). There are two subtypes: parthenogenesis and apomixis.

Parthenogenesis refers to reproduction where the growth and development of embryos occur without fertilization. Examples of organisms falling into this category of asexual reproduction include aphids, rotifers, nematodes, as well as many plants and, believe it or not, the fearsome Komodo dragon.


How Evolution Works

Bacteria reproduce asexually. This means that, when a bacteria cell splits, both halves of the split are identical -- they contain exactly the same DNA. The offspring is a clone of the parent.

As explained in How Human Reproduction Works, higher organisms like plants, insects and animals reproduce sexually, and this process makes the actions of evolution more interesting. Sexual reproduction can create a tremendous amount of variation within a species. For example, if two parents have multiple children, all of the children can be remarkably different. Two brothers can have different hair color, different heights, different blood types and so on. Here's why that happens:

  • Instead of a long loop of DNA like a bacterium, cells of plants and animals have chromosomes that hold the DNA strands. Humans have 23 pairs of chromosomes, for a total of 46 chromosomes. Fruit flies have five pairs. Dogs have 39 pairs, and some plants have as many as 100.
  • Chromosomes come in pairs. Each chromosome is a tightly packed strand of DNA. There are two strands of DNA joined together at the centromere to form an X-shaped structure. One strand comes from the mother and one from the father.
  • Because there are two strands of DNA, it means that animals have two copies of every gene, rather than one copy as in an E. coli cell.
  • When a female creates an egg or a male creates a sperm, the two strands of DNA must combine into a single strand. The sperm and egg from the mother and father each contribute one copy of each chromosome. They meet to give the new child two copies of each gene.
  • To form the single strand in the sperm or egg, one or the other copy of each gene is randomly chosen. One or the other gene from the pair of genes in each chromosome gets passed on to the child.

Because of the random nature of gene selection, each child gets a different mix of genes from the DNA of the mother and father. This is why children from the same parents can have so many differences.

A gene is nothing but a template for creating an enzyme. This means that, in any plant or animal, there are actually two templates for every enzyme. In some cases, the two templates are the same (homozygous), but in many cases the two templates are different (heterozygous).

Here is a well-known example from pea plants that helps understand how pairs of genes can interact. Peas can be tall or short. The difference comes, according to Carol Deppe in the book "Breed your own Vegetable Varieties":

One thing to notice in Deppe's quote is that a mutation in a single gene may have no effect on an organism, or its offspring, or its offspring's offspring. For example, imagine an animal that has two identical copies of a gene in one allele. A mutation changes one of the two genes in a harmful way. Assume that a child receives this mutant gene from the father. The mother contributes a normal gene, so it may have no effect on the child (as in the case of the "short" pea gene). The mutant gene might persist through many generations and never be noticed until, at some point, both parents of a child contribute a copy of the mutant gene. At that point, taking the example from Deppe's quote, you might get a short pea plant because the plant does not form the normal amount of gibberellin.

Another thing to notice is that many different forms of a gene can be floating around in a species. The combination of all of the versions of all of the genes in a species is called the gene pool of the species. The gene pool increases when a mutation changes a gene and the mutation survives. The gene pool decreases when a gene dies out.

One of the simplest examples of evolution can be witnessed in an E. coli cell. To get a better grip on the process, we'll take a look at what happens in this cell.


How do asexually reproductive organisms mutate? - Biology

Any reproductive process that does not involve meiosis or syngamy is said to be asexual, or vegetative. The absence of syngamy means that such an event can occur in the sporophyte generation or the gametophyte stage. Because of the lack of new genetic material, an organism clones itself through this process and makes genetically identical organisms. This can be advantageous in some circumstances, but deleterious in others, depending on how the makeup of the plant suits its ecosystem. There are a few major ways in which plants asexually reproduce in their life cycles to secure future generations.

New plants can grow by the separation of parts of the original plant. When fragmentation, or division, occurs, an offspring is created by the breakup of a single part of the plant. By planting parts of the tuber of a potato, one can create new organisms with the same genetic makeup. When weeds are broken apart, they can regrow from each fragmented underground stem. In Marchantia, fragmentaion of the thallus gives rise to vegitative reproduction. When rain drops hit the plants, these structures are splashed out and may germinate into completely new plants. With these vegetative structures, many clones can be formed from one original parent. Bulbs and Rhizomes are also examples of asexual reproduction.

Special dispersal structures such as a capsule and a hood, and mini adults may also aid a plant when it reproduces asexually. Gemma cups are an example of a dispersal structure that leads to a genetically identical organism. With mini-adults, the morphology of the reproductive unit is similar to that of the parent. A plant may produce plantlets (mini plants) on its stems or leaves that will later germinate into clones of the original.

Finally, some plants have developed a way to produce seeds without their flowers being fertilized. In apomixis, an embryo is created from a diploid cell in the ovule. Then the ovules mature into seeds. The dandelion is one plant that uses this form of vegitative reproduction.

Asexual reproduction can be advantageous and/or disadvatageous. One positive aspect is that it can create individuals rapidly and in large quantities. Secondly, bypassing the sexual process can help a plant in times of dryness since motile sperm require water to fertilize the egg. Another advantage lies in the fact that plants with the desired characteristics can be cloned for economic reasons (agriculture). However, if something goes wrong, such as as the occurance of a fatal mutation, the whole society of clones can be terminated. For this reason, farmers are careful in determining how to propagate their vegetation. In conclusion, the asexual process of reproduction is an important one to plants.

Raspberries, among other fruits, have succeeded from cloning techniques.


Asexual Reproduction in Animals

Sexual and asexual reproduction are the two means of producing offspring. Read this article to gain more information about asexual reproduction in the animal kingdom.

Sexual and asexual reproduction are the two means of producing offspring. Read this article to gain more information about asexual reproduction in the animal kingdom.

The different modes of asexual reproduction in animals are parthenogenesis, budding, polyembryony, fragmentation, etc. Some examples are provided in the article below, regarding this type of reproduction. It helps in understanding the concept in a better manner.

Facts

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A form of reproduction that takes place without gamete fusion is termed as asexual reproduction. The general definition is as follows: the method, in which fertilization, ploidy reduction, or meiosis doesn’t take place is known as asexual reproduction.

Description

There are different modes, by which animals reproduce asexually, and their explanations are provided below.

Budding

It is the process, in which cells of an organism split into parent and daughter cells. For example, organisms like sponges and hydra reproduce by means of budding. In hydra, this process is similar to that of reproduction in yeasts. Initially, a small bud is formed on the side of the body, which enlarges and develops tentacles, which help in feeding the daughter bud that breaks off from the parent body. In organisms like echinoderms and jellyfish, the buds break off and form their independent bases. The mechanism is different in corals, and their buds do not detach from their parent body, leading to formation of large colonies.

Parthenogenesis

Eggs produced by females develop into adult individuals without getting fertilized this process is known as parthenogenesis. Some species of fish, frogs, and insects reproduce by means of this method. In few organisms, this process occurs under specific conditions. For example, when aphids get enough food to eat in the spring season, they resort to asexual reproduction it is a quicker means of producing offspring. However, these creatures can also undergo sexual reproduction.

Gemmules

Animals that reproduce by means of gemmules (internal buds) release cell masses from their body. Sponges reproduce by means of budding, along with gemmule reproduction, and produce external buds. Later, these cells develop into independent offspring.

Polyembryony

It is a condition, in which a single egg leads to the development of two or more embryos. By this method, a large number of propagules are produced in the organism’s body. As they are released, they directly develop and enter the next phase of their life cycle. The flukes belonging to class Trematoda reproduce by means of polyembryony.

Asexual reproduction in animals doesn’t take place on a large scale in nature, and it gives rise to homozygosity. The harmful mutations of the organisms (reproducing by asexual reproduction) get exposed to natural selection. Thus, weeding out of such animals (by natural selection) happens easily. In sexual reproduction, owing to heterozygosity, harmful mutations may not get exposed, and hence, animals stay unaffected from natural selection.

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Asexual Reproduction in Living Organisms

The type of reproduction that takes place without the process of gamete (sex cell) formation is called asexual reproduction.

This type of reproduction takes place commonly in lower plants and animals, where the body is not very complex.

There are different forms of asexual reproduction.

Binary fission:

Binary fission occurs under favourable environmental conditions. Binary fission is the division of one cell into two similar cells. This is the simplest method of asexual reproduction. It occurs in unicellular organisms like bacteria, yeast, Euglena, Amoeba and Paramecium. In some organisms (e.g., Leishmania, which causes kala-azar) binary fission takes place in definite orientation due to their specific body structure.

Take a permanent slide of Amoeba showing binary fission. Observe it under a microscope. You will see that the nucleus first divides amitotically into two, followed by the division of the cytoplasm. The Amoeba finally splits into two daughter cells.

Multiple fission:

Under unfavourable circumstances some unicellular organisms develop a hard protective covering over the cell, called cyst. The nucleus of the cell divides repeatedly, producing many nuclei.

Each nucleus is surrounded by a small amount of cytoplasm and many daughter cells are produced within the cyst. When favourable conditions return, the offspring are released. Multiple fission is seen in many algae and the malarial parasite (Plasmodium).

Budding:

Sometimes new individuals develop from the body wall of the parent as bulblike projections called buds. The buds may be unicellular or multicellular depending upon the type of parent organism. The buds finally separate to form new individuals. Budding occurs in yeast. Hydra and sponges.

Put some yeast in 10% sugar solution kept in a glass. Cover the glass and keep it in a warm place for a day. The yeast cells grow and reproduce in the sugar solution. These cells are known as a culture of yeast cells. Take a drop of the yeast culture solution on a slide and cover it with a coverslip. Examine it under the microscope. You will see buds on the yeast cells.

Fragmentation:

Obtain some pond water. You may see green filamentous structures floating in it. Take some of these structures on a slide. Put a drop of glycerine on them and cover them with a coverslip. Observe under a microscope.

The green filamentous structures you see are an alga named Spirogyra, which grows in ponds, ditches and springs. Each filament has a single row of cylindrical cells. Each cell has spiral bands of chloroplasts.

When a Spirogyra filament breaks into pieces, each piece grows into a new filament by cell division. This process is fragmentation. During this process the body of an individual breaks up into two or more parts and each part develops into a complete organism.

Some animals like sponges. Hydra and flatworms (Planaria) also reproduce by a similar method known as regeneration. If they are cut into pieces, each piece can regenerate into an entire individual.

In complex organisms all cells are not similar. The cells are organized into tissues and tissues into organs. The different organs are placed at definite positions. If such an organism breaks off at any point, the broken part cannot grow into a complete organism with all organs.

Spore formation:

Spores are asexual reproductive bodies enclosed in a thick-walled structure called sporangium, which can tide over unfavourable conditions such as extreme heat, dryness, acidity, and so on. Spore formation is a common method of asexual reproduction in many lower forms of life such as algae, bacteria and fungi.

Under favourable conditions, the spores are released by the breaking of the thick wall of the sporangium. The spores then germinate into new individuals. In fungi, sporangia burst and release spores. By this method of asexual reproduction, organisms can overcome unfavourable conditions. Some fungi, e.g., Rhizopus and Mucor reproduce by producing spores.

Take a piece of moistened bread and keep it inside a polythene bag for 2-3 days in a warm, humid place. You will observe yellow and black patches on the bread slice. After 4-5 days you will see a powdery substance with spores. The spores are of bread mould (Rhizopus). The moulds also have threadlike structures called hyphae through which they draw nutrients from the bread.

Vegetative Propagation in Plants:

The vegetative parts of a plant such as the root, stem, leaf, etc., can produce new plants. You must have seen gardeners taking cuttings from the stem of a rose plant and planting them in the soil. Under suitable conditions, the cuttings grow into new rose plants.

Vegetative propagation is common in plants like orchids, ornamental plants and grasses. Plants such as banana, rose, jasmine, etc., which do not produce seeds, can be grown by vegetative methods. The new plants are genetically similar to and bear the characteristics of the parent plant.

In some plants like Dahlia, sweet potato, etc., the adventitious roots become swollen due to storage of food. Adventitious buds are also present on them. When roots bearing such buds are planted in the soil, new plants are produced as a result of vegetative propagation.

Observe a potato closely. You will see ‘eyes’ on its surface. These eyes are actually buds. You know that the stem has buds from which leaves and smaller branches arise. Cut a potato into small pieces. Plant some of the pieces with eyes and some without eyes in moist sand.

Observe the changes taking place in these pieces over the next few days. You will find stems, leaves and roots growing from the potato pieces that had eyes.

Some plants produce sub aerial stems which develop as lateral branches from the mother plant and give rise to a new plant after getting detached from the mother plant. For example, in runners such as grasses the stem grows along the surface of the soil and produces roots where it touches the ground to give rise to a new plant.

In some plants the underground stem gets modified for storage of food, and under favourable conditions it produces shoots and gives rise to a new plant. Such stems include rhizomes, tubers, bulbs and corms.

The fleshy leaves of Bryophyllum bear adventitious buds in the notches along the leaf margin. These buds develop into small plants (plantlets) under favourable conditions. These plantlets can be easily separated to grow as independent plants.

Artificial modes of vegetative propagation:

Farmers, gardeners and horticulturists have developed various artificial methods of vegetative propagation, like grafting, layering, cutting and tissue culture for growing plants in gardens and nurseries.

Cutting is a very simple method of propagation in which a piece of the parent plant’s stem with nodes and internodes is placed in moist soil. This grows into a new plant. In grafting the cutting of a plant is attached to the stem of a rooted plant.

The attached cutting becomes a part of the rooted plant, draws nutrition from it and grows roots at the joint. Now if it is separated, it grows into a new plant. In layering, one or more branches of the parent plant are bent close to the ground and covered with moist soil. The covered portions grow roots and develop into new plants.

Cut two pieces from a money plant—one with leaves (i.e., a portion with nodes) and the other without leaves (i.e., a portion of an internodes). Place these with one end immersed in water kept in a transparent bottle. Leave them like this for a week.

You will see that roots and new leaves grow on the piece with leaves, while the other piece gradually withers. This is because a plant can grow new leaves and branches only if it has nodes. (New leaves and branches arise at the nodes.) The piece of money plant which does not have nodes cannot grow because it cannot produce new leaves.

Tissue culture:

In this technique some tissue from a desired plant is placed in a suitable nutrient medium under proper conditions. The tissue grows into an unorganized mass, known as callus. A small part of this is put in another medium, which contains growth hormones that induce the formation of plantlets from the callus.

When plantlets grow, they can be transplanted in the soil or in pots for developing to maturity. Tissue culture allows us to grow a whole plant from cells taken from any part of the plant body. Many plants can be grown from one parent plant in the laboratory under controlled, disease-free conditions.


1- Quick expansion

This form of reproduction produces large numbers of offspring by simply placing a particular organism in a suitable habitat.

2- No mobility required

With asexual reproduction, organisms can be reproduced in a single area, without the need for transfer.

3- The couple is not needed

Asexual reproduction does not need the pair to reproduce. This feature is favorable when new areas are colonized since only one parent is needed.

4- It is favorable for the environment

This form of reproduction has no negative impact on the environment. On the other hand, asexual reproduction would cause some organisms not to survive in aggressive environments due to their susceptibility, sensitive stages during the process and their fragile organs.

5- It is practical in case of emergency

In difficult situations, asexual plants and animals are still able to stay alive and continue producing offspring without other reproductive sources. Basically, there are no major drawbacks regarding adverse environmental situations when it comes to asexual reproduction.

6- No investment needed

Organisms that reproduce asexually do not have to carry the offspring over a long period, unlike those reproduced through sexual reproduction, which, on the other hand, are generally limited to a single offspring.

As can be seen, there is no wear of energy or time to produce offspring. In addition, certain asexual plants and animals can produce countless clones without having to consider any investment. In short, reproduction process without complexity and requiring less energy.

Disadvantages

7- Impedes diversity

As the characteristics and traits of a single parent are transmitted to their offspring, asexual reproduction hinders the genetic diversity of all their generations. This makes the procreated population exactly identical.

With sexual reproduction, the great advantage is the possibility of mixing groups of genes to guarantee a diverse ecosystem.

8- Possesses some inheritance problems

Most of the time, a single asexual progenitor is required to copy chromosomes and genes, which means that genetic defects or mutations that occur in asexual reproduction will continue to exist in offspring, with no exception.

This disadvantage may even lead to more unfavorable mutations, which result in organisms being produced asexually susceptible to disease, which also means that a large number of offspring would be destroyed.

9- The organisms tend to extinction

The identical traits and characteristics imply the same shortcomings and weaknesses. Therefore, an evolved predatory organism that attacks them, can kill an entire population not prepared to fight for its existence.

10- Can not control quantity

This form of reproduction can not control population growth. Each organism is able to reproduce itself, which means that its own population will double in each reproductive cycle. However, the process stops automatically when the number is excessive.

11- Agencies fail to adapt to changes in the environment

The organisms transmit characteristics to their descendants. But in the absence of variations, the adaptive capacity and survival of the changes in the environment are not developed.

12- Adverse environmental conditions

The process of asexual reproduction can occur in unfavorable conditions, such as extreme temperatures or other variations, which means that whole communities can be extinguished.


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