Introduction:

What is asexual reproduction?
Asexual reproduction is a form of reproduction which requires only one parent. Unlike sexual reproduction, there is no exchange of genetic material or fertilization. Therefore, organisms that result from asexual reproduction will have the exact same DNA as their parent. Asexual reproduction is the simplest form of reproduction, since organisms are only making genetically identical copies of themselves. These offspring are known as clones. Asexual reproduction is also quite energy efficient, since organisms don’t need to spend the energy on finding a mate.

Asexual production allows a species to produce a large population very quickly. However, the main disadvantage of asexual reproduction is that offspring are identical to the parent. Any factor that negatively affects the parent, such as disease, will also affect the offspring. Because of this, large populations can be wiped out if environmental conditions become unfavorable.

Asexual reproduction is seen as a natural method of reproduction among many different organisms in the world. It is a very common method of reproduction among less complex organisms and many higher organisms. Among the examples of asexual reproduction that we can see in the living world, there are many different methods of asexual reproduction. If we compare the reproduction of bacteria to the reproduction of yeast, for instance, we will be able to notice large differences.




Binary fission:
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The first method of asexual reproduction that we will explore is called binary fission. In binary fission, a single celled organism (unicellular) replicates its DNA and then divides itself into two separate cells. Each new cell will have the same DNA as the original cell. The cells divide to produce 2 identical daughter cells that are identical to each other and are clones of the parent cell.

Binary fission is seen among organisms such as bacteria. Bacteria do not have a nucleus, and their DNA only has one chromosome. When bacteria reproduce by binary fission, the parent cell duplicates it's DNA then the cell membrane grows inward and divides the parent cell. Binary fission is a very efficient method of reproduction. E. coli, for instance, can divide as often as every 20 minutes.

In ameoba, and other unicellular organisms that do have a nucleus, the DNA is duplicated in the nucleus and the cell undergoes mitosis.

Why is binary fission important?
Since binary fission is an efficient method of asexual reproduction, the genetic information of one bacteria can quickly be multiplied. That means that if one bacteria has a genetic mutation that gives it an advantage in an environment, it will flourish and reproduce. The daughter cells will have this same genetic mutation, and they will both grow and then reproduce as well. The four cells that result from those two cells splitting will then also grow and reproduce. In other words, since binary fission is an energy efficient method of reproduction, in a short period of time there could be a large number of identical offspring from a single bacteria.
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Here, the rapid accumulation of bacterial cells in a short period of time is seen
Here, the rapid accumulation of bacterial cells in a short period of time is seen

Examples of organisms that undergo binary fission include ameobas, bacteria and some algae.


Budding

Organisms such as hydra and yeast reproduce by a process called budding. This process is not to be confused with plant growth or germination—this natural method of reproduction is seen in a different kingdom altogether!

Budding in yeast cells is similar to the binary fission seen in bacteria, but there is an important difference. In budding, the daughter organism begins its life much smaller than the mother organism. Just like binary fission, budding begins with a replication of the DNA. However, unlike how bacteria split into two equal portions, yeast cells begin to reproduce by producing pushing their DNA outward to into a small region to its side. This region becomes a small ‘bud’ (or miniaturized clone) off of the mother cell. The bud will grow and develop as it receives nourishment from the mother cell.
budding.JPG
Yeast cells can reproduce asexually through a method called budding.


In some organisms, such as coral, the offspring remains attached to the parent, forming a structure composed of many identical individuals. In the picture below, each of the different types of coral are made up of many individuals of the same species, although they appear to be a single animal.


coral

In other organisms, the daughter cell will be big enough to break off and survive on its own. The final result will be two cells that are genetically identical, but the daughter cell will be a bit smaller than the mother cell when it first breaks off.
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Hydra
Hydras are simple, hollow organisms which live in fresh water systems such as lakes, streams, and ponds or rivers. They are usually only a few millimeters long and are able to be easily seen with a microscope. Hydras have a tubular body that is topped with tentacles. They have a primitive nervous system, and move in a tumbling or cartwheel motion by continually bending their body over and gripping the surface that they are living on. They have a mouth area in the center of their tentacles, and a digestive region in the center of their tubular body. Hydras eat small aquatic insects by catching them with their tentacles and bringing them into their mouth.
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Hydras are small water-dwelling organism. They reproduce both sexually and asexually.
Hydras are small water-dwelling organism. They reproduce both sexually and asexually.

Hydras can reproduce both sexually and asexually. Their method of asexual reproduction is the same method that yeast cells use to reproduce: budding. An adult hydra will begin the process of budding by producing a bulging region along their tubular body. This bulging region will grow and develop and form small tentacles at its tip. The small region will continue to grow and develop until it is large enough to support itself. Eventually the daughter hydra will break off from the mother hydra and will be free-living. The daughter hydra, being produced asexually, will be genetically identical to the mother hydra, but will begin its independent life smaller than its mother.

hydra.jpg
An adult hydra will produce a tiny bud on its tubular body. In time, this bud will grow to become a daughter hydra.
An adult hydra will produce a tiny bud on its tubular body. In time, this bud will grow to become a daughter hydra.

Sporulation:

You’ve likely seen a piece of bread, cheese, or even fruit with white-ish green mold on it. What you probably didn’t know is that the mold that you see is actually an entire colony of millions of small individual fungus bodies. In reality, the moldy bread likely has fungus growing all over it by the time you see an entire colony.

Bread mold spreads by air-born spores. Spores are haploid cells that contain a full copy of the parent organisms DNA. Spores are specialized cells that are able to remain dormant for a period before germinating and growing. Spores are similar to a plant's seeds in that it contains and protects the DNA. The fungus' spores are extremely tiny and can travel long distances by being blown through the air. Spores in are in a state of suspended animation--they are alive but do not grow--these spores will germinate when they land on in a suitable environment (right temperature, moisture, sunlight livels, etc.) and will grow tiny hyphae into their food to gather nutrients. Hyphae are small specialized roots that help the fungus eat its food source. As the fungus grows and develops, it will grow what are called sporangia which are tall, skinny, spore-bearing structures. As these sporangia develop, they will release spores, which will spread and continue the life cycle of the fungi. Thus, one individual will produce many spores, and each spore can develop into a new individual.

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Tiny fungi organisms produce sporangia which release spores, which will spread about.
Tiny fungi organisms produce sporangia which release spores, which will spread about.

Organisms that undergo sporulation include fungi, green algae, some moulds, and non-flowering plants such as ferns.

Fragmentation:

In fragmentation, new individuals are formed from a piece of the existing parent organism. Some animals, such as some species of worms, can grow two identical individuals after being cut in half. Starfish have this same ability, as they can grow a whole new organism from one part of an arm. Individuals produced through fragmentation are clones of the parent.

Planaria, a species of flat worm, provides an example of just how remarkable fragmentation is.
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Although you may have heard otherwise, earthworms are not like the Planarian worms you just saw. If an Earthworm is cut in half, only the top half can grow the rest of the body back. The tail end of the worm CANNOT grow a new head and will die.

Some plant species can reproduce via fragmentation. This topic will be discussed in the next section in artificial vegetative propagation.


Vegetative Propagation:


Vegetative propagation involves asexual plant reproduction. It is important to remember before we begin to discuss the methods of asexual reproduction in plants, that plants are an entire kingdom, and a very large one at that. Not all of the methods that are discussed here will apply to all types of plants. Many plants naturally reproduce asexually by various methods. Many other plants don't naturally reproduce asexually, but can be made to reproduce by man-made methods. In this section, we will first examine a few of the ways that plants naturally reproduce asexually. Sometimes, however, gardeners and farmers want to reproduce plants that don't naturally reproduce asexually. The second part of this section will explore the methods that are often used to for these goals.

Natural methods of vegetative propagation:
1. Strawberry plants and spider plants, for instance, reproduce by growing a plantlet at the end of a runner. A runner is a specialized horizontally growing, above ground stem that produces a plantlet at its end or at its nodes. Runners are fast growing and some plants have runners that can scale up buildings or across landscapes.
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here, the runners of a strawberry plant are producing plantlets.

Here, the runners of a strawberry plant are producing plantlets.

2. Another method that plants use to reproduce asexually is the growth of bulbs. Bulbs are thick, fleshy leaves that are wrapped around each other to create a round mass under the surface of the soil. These thick leaves grow on a modified short stem that lies beneath the leaves. The roots of the plants grow out of this stem region. Onions and tulips are examples of plants that grow a bulb. Bulbs act as an asexual reproductive structure by allowing the plant to die back nearly entirely during the winter and then grow back in the spring.
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The bulb of an onion is technically composed of leaves. A bulb is an asexual reproductive structure for the plant.

The bulb of an onion is technically composed of leaves.
A bulb is an asexual reproductive structure for the plant.

3. Rhizomes are stems that grow underground like a root. Rhizomes travel underground away from a mother plant to produce daughter plants nearby. Rhizomes are similar to runners, except that they have nodes from which a new individual can grow. Various plants, from bamboo to water lilies reproduce asexually through the use of rhizomes. Most types of grasses produce rhizomes as a method of asexual reproduction. Rhizomes produce genetically identical plants in an area surrounding the mother plant.
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The rhizomes of this grass plant will allow it to spread throughout the area.

The rhizomes of this grass plant will allow it to spread throughout the area.

Artificial methods of vegetative reproduction.
Many times throughout the course of human history we have discovered plants that have beneficial aspects. When we found a plant that we wanted more of, we often gathered seeds from it and then planted these seeds elsewhere. However, seeds are the product of a plant’s sexual reproduction, and many times these seeds don’t have the exact same qualities that the mother plant did.

In times when we have wanted an exact copy of a plant, we found ways to make plants reproduce asexually. The first of these methods that we will discuss here is called taking cuttings. When we take a cutting of plant, we remove a portion of the mother plant that can be safely lost without damaging its health. This portion may be a root, stem, or leaf portion. In order to make the removed portion of the plant become a new plant, gardeners will put the plant in damp soil or water for a period. Since most plants have undifferentiated cells (cells that aren't designated as leaf, stem or root cells-but are "undesignated") at key locations throughout the plant, the cut plant can use its stored sugars and starches to produce the necessary organs that it needs. Once established, the gardener can plant the newly grown plant, and it will grow into a healthy, mature plant. Thus, fragmentation has occurred.
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Small portions of one plant were removed. In time, these peices have grown roots. They are genetically identical to their mother plant

Small portions of one plant were removed. In time, these pieces have grown roots. They are genetically identical to their mother plant

The second artificial method that we will explore about is called grafting. In grafting, two or more plants are creatively attached together. In time, the region between these two similar plants heals together, and the plants mend to create a continuous system of cells. Once this has occurred, the foreign stem will receive nourishment and grow as if it was still growing on its own mother plant.

Grafting is used very commonly for fruiting trees. Sometimes, a fruit tree may have desirable fruit, but its root system will be poor. To resolve this issue, gardeners will choose a similar tree species that has a strong root system. They will remove most of the stems of the plant with the strong root system, and will graft on healthy stems from the fruit tree with desirable fruit.

Plants that are grafted together will not modify their own genetic information in any way, and they also won’t exchange their DNA with each other. Instead, the original portions of the plant will continue to grow and develop as their DNA instructs, and the portions of the plant that have been grafted on will grow and develop as their own DNA instructs.
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Plant stems of one plant were removed and strategically attached to a closely related plant. In this instance, the root-stock plant species will help the stems grow into strong and healthy plants.

Plant stems of one plant were removed and strategically attached to
a closely related plant. In this instance, the root-stock plant species
will help the stems grow into strong and healthy plants.

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Stems from an apple tree with desired fruit are grafted onto the stem of an established tree as a method of producing more of the desired fruit.

Stems from an apple tree with desired fruit are grafted
onto the stem of an established tree as a method of
producing more of the desired fruit.



Parthenogenesis: When animals reproduce without a mate.

*information adapted from
http://www.findingdulcinea.com/features/science/environment/Parthenogenesis--When-Animals-Reproduce-Without-a-Mate.html#1

In the wild, some female animals are able to create offspring without the help of a mate, a process known as parthenogenesis. Parthenogenesis happens when an unfertilized egg produces an offspring.



Depending on the species, parthenogenesis happens for different reasons. Some species that normally reproduce sexually will sometimes reproduce asexually, either for lack of males, for population sex control, or in some cases because of an abundance of resources.



How the cell division happens during reproduction in each species determines if the resulting offspring from parthenogenesis will be an exact clone of the mother or not. Parthenogenesis occurs in lower animals such as insects, and in only about .1 percent of vertebrate species, according to Scientific American.


While there are a number of species that are capable of parthenogenesis, below are a few of the more interesting cases.

The New Mexico whiptail (see below) is one of few species that has only female members. This animal, which is also the state reptile of New Mexico, reproduces solely through parthenogenesis, and males have become obsolete.
Image result for new mexico whip tail
Image result for new mexico whip tail


In 2006 two different cases of virgin births by Komodo dragons were discovered in England. In the first, a group of eggs hatched in April from a Komodo dragon named Sungai at the London Zoo. The second set of eggs hatched from a Komodo dragon named Flora at the end of the same year, according to National Geographic. Scientific American reported that some female reptiles are able to store sperm for years after copulation, so initially some scientists believed the hatchlings might actually have a father, but DNA testing proved that the baby lizards were in both cases the product of parthenogenesis.

Komodo dragons are one species with the WZ chromosome makeup, so all of the parthenogenesis offspring are male. What this means, explains LiveScience, is that a female could become isolated from her population—as might happen in the island areas where Komodo dragons originate—and she could create an entire population of her own by mating with her male offspring.
Image result for komodo dragon
Image result for komodo dragon
Sharks are another animal not known, until recently, to be able to reproduce by parthenogenesis. In December of 2001, according to New Scientist, in a tank of three female hammerhead sharks, a shark pup suddenly appeared. Scientists argued for years about how the baby had appeared, and in 2007, after analyzing the pup’s DNA, they found that it was the product of only one animal, its mother.
All shark pups resulting from parthenogenesis will be female.


Image result for hammerhead shark
Image result for hammerhead shark