Lesson Explainer: Methods of Asexual Reproduction Biology

In this explainer, we will learn how to distinguish between asexual and sexual reproduction and describe different ways that organisms asexually reproduce.

All living organisms reproduce to perpetuate their species. There are many different ways for reproduction, such as giving birth (e.g., in humans), laying eggs (e.g., in reptiles), and producing seeds (e.g., in flowering plants). Some unicellular organisms reproduce by simply copying their genetic material and splitting it into two.

Reproduction may be described as either sexual or asexual. Many organisms can use more than one method of reproduction and have both sexual and asexual reproductive abilities. Figure 1 shows a diagram illustrating sexual and asexual reproduction.

Sexual reproduction typically occurs when two parents combine their genetic material to produce genetically unique offspring. These offspring are different from either parent and likely also different from each other. Sexual reproduction has the advantage of being able to increase the genetic variation in a population, which increases their resilience to environmental changes.

Asexual reproduction is when just one parent generates offspring. The offspring of asexual reproduction are usually genetically identical to their parent and to each other. Asexual reproduction has the advantage of being a generally more rapid process than sexual reproduction. It also does not require finding a mate, which can be time-consuming and dangerous. However, asexual reproduction does not give rise to the same level of genetic variation that occurs as a result of sexual reproduction. This means that a reproductive strategy that relies only on asexual reproduction can leave populations vulnerable to environmental changes.

Key Term: Asexual Reproduction

Asexual reproduction is a type of reproduction that only involves one parent and does not involve the fusion of gametes or a change in the number of chromosomes.

Example 1: Recalling a Feature of Asexual Reproduction

What is a distinguishing feature of asexual reproduction?

  1. A wide range of alleles increases the chance of resistance to disease.
  2. New offspring can be produced rapidly.
  3. There is variation in the next generation.
  4. It requires a lot of time and energy.
  5. More than one individual is necessary.

Answer

All living organisms reproduce to perpetuate their species. Reproduction may be described as either sexual or asexual.

Sexual reproduction typically describes the process of two parents combining their genetic material to produce genetically unique offspring. These offspring are different from either parent and likely also different from each other. Sexual reproduction has the advantage of being able to increase the genetic variation in a population, which increases their resilience in the case of environmental changes.

Asexual reproduction is when just one parent generates offspring. The offspring of asexual reproduction are usually genetically identical to their parent and to each other. Asexual reproduction has the advantage of being a generally more rapid process than sexual reproduction. It also does not require finding a mate, which can be time-consuming and dangerous. However, asexual reproduction does not give rise to the same level of genetic variation that occurs as a result of sexual reproduction. This means that a reproductive strategy that relies on only asexual reproduction can leave populations vulnerable to environmental changes.

Using this information, we can conclude that a distinguishing feature of asexual reproduction is that new offspring can be produced rapidly.

Asexual reproduction occurs in a number of ways. These include binary fission, budding, regeneration, vegetative propagation, reproduction by spores, parthenogenesis, and reproduction facilitated in a laboratory setting.

Binary fission is a type of asexual reproduction that occurs in unicellular organisms. During binary fission, one parent cell replicates its genetic material and then splits, or divides, into two identical daughter cells.

In prokaryotic organisms, such as bacteria, binary fission involves first replicating the genetic information stored in the circular chromosome, followed by the cell dividing into two. In eukaryotic organisms, such as amoeba or Paramecium, the nucleus first makes a copy of itself through mitosis, and then the cell divides into two smaller cells. During unfavorable conditions, amoeba can form a tough coat for protection and undergo multiple cycles of binary fission. Once the conditions become favorable again, these young amoeba can be released.

Figure 2 illustrates binary fission in a prokaryotic cell and in a eukaryotic amoeba.

Remember that while we often describe the asexual reproduction of eukaryotic cells as mitosis, mitosis actually specifically describes the process of a cell making an exact copy of its nucleus and the chromosomes it contains. Prokaryotes do not possess a nucleus and do not undergo mitosis. Cytokinesis is the name of the last step when the parent cell divides into two daughter cells in both mitosis and binary fission.

Mitosis and binary fission can be easily confused. We call cell division mitosis when it is used for the growth or repair of a multicellular organism, whereas we call it binary fission when it is used in the asexual reproduction of a unicellular organism.

Key Term: Binary Fission

Binary fission is a type of asexual reproduction in unicellular organisms in which the organism duplicates its genetic material and then divides into two parts (cytokinesis).

Budding is a type of asexual reproduction that occurs in some unicellular organisms and some simple multicellular organisms. During budding, an offspring begins its life attached to the parent. The offspring usually eventually breaks off to continue its life as a separate individual.

Yeast is the name of a type of unicellular fungus that you may be familiar with as an ingredient used in baking bread. Yeast cells reproduce by budding and typically have a round, slightly elongated shape. When they reproduce, a small projection starts to grow from one end or the other. The cell copies its nucleus via mitosis, and one copy stays in the parent cell while the other enters the growing bud, or daughter cell, as seen in Figure 3.

The daughter cell remains attached to the parent until it reaches maturity. Then, the daughter cell breaks off, leaving a bud scar at the former site of attachment on both the parent and the daughter cell. Sometimes yeast cells stay connected to each other after reaching maturity, forming colonies of branched strands of yeast cells.

Budding in unicellular organisms is similar to binary fission. However, in binary fission, one parent cell splits into two immature daughter cells that separate from each other and grow to maturity as separate individuals. During budding, an immature daughter cell sprouts from a mature parent cell and grows to maturity while it is still attached.

Key Term: Budding

Budding is a type of asexual reproduction in some unicellular and multicellular organisms, in which a new organism develops from a small projection or bud that develops to maturity while it is still attached to the parent organism.

Example 2: Contrasting Reproduction in Yeast and Bacteria

How does asexual reproduction in yeast differ from asexual reproduction in bacteria?

  1. In yeast, the process generates multicellular offspring.
  2. In yeast, daughter cells are formed by parthenogenesis.
  3. In yeast, there is no growth and separation of a cell wall.
  4. In yeast, the daughter cell forms from a small outgrowth in the original cell.
  5. In yeast, mitosis does not occur during the fission process.

Answer

Yeast is a unicellular fungus that reproduces by budding. Bacteria are prokaryotic organisms that reproduce by binary fission.

Budding is a type of asexual reproduction that occurs in some unicellular and simple multicellular organisms. During budding, an offspring begins its life attached to the parent. The offspring usually eventually breaks off to continue its life as a separate individual.

Yeast cells reproduce by budding. When they reproduce, a small projection starts to grow from one end of the yeast cell. The cell copies its nucleus via mitosis, and one copy stays in the parent cell while the other enters the growing bud, or daughter cell. The daughter cell remains attached to the parent until it reaches maturity. Sometimes yeast cells stay connected to each other after reaching maturity, forming colonies of branched strands of yeast cells.

During binary fission, one parent cell replicates its genetic material and then splits or divides into two identical daughter cells. Binary fission in bacteria involves first replicating the genetic information stored in the circular chromosome, followed by the cell dividing into two.

Budding and binary fission are both types of asexual reproduction that occur in unicellular organisms. The two processes have several similarities. However, the main difference is that, in binary fission, one parent cell splits into two immature daughter cells that separate from each other and grow to maturity as separate individuals, whereas in budding, an immature daughter cell sprouts from a mature parent cell and grows to maturity while it is still attached.

This means that asexual reproduction in yeast differs from asexual reproduction in bacteria, because in yeast the daughter cell forms from a small outgrowth in the original cell.

Hydra and sea sponges are examples of simple multicellular animals that can reproduce by budding. Both hydra and sea sponges also have the ability to reproduce sexually. These organisms are considered simple rather than complex because they possess organization at the cellular level but do not have true tissues or organs.

Hydras are small organisms that live in fresh water. They have stinging tentacles that they use to catch and eat their prey. When a hydra reproduces by budding, a small offspring starts to grow from the side of the parent organism. The offspring hydra remains attached to the parent, where it can absorb nutrients to grow and develop until it reaches maturity. When the offspring hydra is ready to live on its own, it separates from the parent and moves away. We can see a photograph of a hydra budding on the bottom right of the photograph below.

Hydra budding

Figure4

Regeneration occurs when a missing or damaged part of an organism repairs itself and grows back. Many organisms possess some level of regenerative ability. However, the capacity to regenerate decreases in more advanced animals. The human body can repair skin cells, blood vessels, and muscle cells when they are damaged. Some lizards can regrow their tails or even a toe. Crustaceans like crabs and lobsters can replace an entire missing limb.

In some organisms, an entirely new organism can be produced via regeneration, such as in hydras or sponges. In this case, regeneration represents a type of asexual reproduction.

Key Term: Regeneration

Regeneration is the ability of an organism to repair or regrow a damaged or missing part and is sometimes considered a type of asexual reproduction.

Starfish, also sometimes called sea stars, are an example of an organism that can reproduce via regeneration. Starfish are invertebrates that live in salt water. They possess several limbs surrounding a central body disc, and they use these limbs to pry open shellfish, their prey. If a starfish is broken apart, only one limb with some of the central disc attached can eventually grow into an entirely new organism, as shown in Figure 5.

Another common example of an organism that reproduces by regeneration is flatworms, also called Planaria. This family of invertebrates has remarkable regenerative capabilities. A flatworm can be cut into several pieces, and each piece can eventually regenerate into a new organism under the suitable conditions. You can see this in Figure 6.

Parthenogenesis is a term that refers to a type of asexual reproduction that occurs in complex, multicellular animals. Parthenogenesis is when an embryo forms and develops without fertilization by a male gamete. An example occurs in a large lizard called the Komodo dragon. When mates are scarce, female Komodo dragons can lay eggs that develop into offspring, which are exact genetic replicas of their mother. Another example is the freshwater crustacean Daphnia, which reproduces asexually by producing offspring from unfertilized haploid eggs.

Key Term: Parthenogenesis

Parthenogenesis is when an embryo forms and develops without fertilization by a male gamete.

Example 3: Recalling the Name of a Type of Asexual Reproduction

When conditions are favorable, the freshwater crustacean Daphnia (shown in the figure) reproduces asexually by producing offspring that develop from unfertilized haploid eggs. What is the name of this process?

Daphnia water fleas from the pond

Answer

All living organisms reproduce to perpetuate their species. There are many different ways for reproduction. Reproduction may be described as either sexual or asexual. Sexual reproduction usually involves two parents, whereas asexual reproduction involves only one parent.

Asexual reproduction occurs in a number of ways. These include binary fission, budding, regeneration, vegetative propagation, reproduction by spores, parthenogenesis, and reproduction facilitated in a laboratory setting.

Parthenogenesis is a term that refers to a type of asexual reproduction that occurs in complex, multicellular animals. Parthenogenesis is when an embryo forms and develops without fertilization by a male gamete. An example occurs in a large lizard called the Komodo dragon. When mates are scarce, female Komodo dragons can lay eggs that develop into offspring, which are exact genetic replicas of their mother.

Therefore, the name of the process in which Daphnia reproduce asexually by producing offspring that develop from unfertilized haploid eggs is parthenogenesis.

Plants can also reproduce by parthenogenesis. Sometimes a plant can make a fruit without first being pollinated. This fruit will contain seeds with embryos that are exact genetic copies of the parent plant.

Parthenogenesis plays a role in determining the sex of honeybees. The queen bee can lay eggs that have been fertilized, which means they are the result of sexual reproduction. The fertilized eggs develop into female worker bees or more queens. The fertilized eggs and offspring that develop from them are diploid and possess the typical number of chromosomes (2n).

The queen may also lay haploid, unfertilized eggs. These unfertilized eggs are produced by asexual reproduction via parthenogenesis. The unfertilized eggs develop into male bees called drones. These eggs, as well as the drones that hatch from them, are haploid and possess half as many chromosomes as in a diploid female bee’s cells, as shown in Figure 7. This is in contrast to aphids, which can produce female diploid eggs by parthenogenesis.

Parthenogenesis can also be stimulated artificially by scientists. Sometimes the unfertilized egg cells of certain organisms, such as frogs or sea stars, can be stimulated by electricity, irradiation, exposure to certain salts, agitation, or physical pricking. This stimulation causes the haploid egg cell to duplicate its chromosomes and become a diploid cell. The diploid egg cell can then grow into an embryo without fertilization.

Sporogenesis is the process of producing spores that can be used in reproduction. Many different types of organisms can reproduce by sporogenesis, including nonflowering plants (e.g., mosses and ferns), multicellular fungi (e.g., mushrooms and molds), and some types of algae. A spore is a single cell that is generated by a parent organism and is able to grow into an offspring organism under favorable conditions.

Key Term: Sporogenesis

Sporogenesis is the process of forming spores that can be used in reproduction.

For example, mushrooms is the name that we give to the fruiting bodies of several types of multicellular fungi. These mushrooms are the reproductive organs of underground fungi that grow above the ground when the environmental conditions are favorable. They develop quickly and release clouds of spores, which are carried by the wind and sometimes by animals. The mushroom itself quickly withers once the spores are released.

When a spore lands on a suitable, moist surface, it begins to grow, or germinates, and eventually becomes a new fungus. Spores can be produced sexually or asexually. If the spores are made by asexual reproduction, they grow into offspring with an identical genetic makeup to that of the parent organism. The life cycle of a mushroom-producing fungus can be seen in Figure 8. Bread mold is a different type of fungus that also reproduces by spores in a similar way.

Ferns are a type of plant that reproduce using spores. The large, leafy fern plant that you typically picture is referred to as a sporophyte. The sporophyte releases spores from structures called sori (singular: sorus) on the underside of its leaves. Just like the fungal spores released by mushrooms, these germinate and grow into an offspring organism if they land on a suitable surface.

The spores of a fern plant are haploid, which means that they possess half the number of chromosomes found in a typical fern cell, or 1n. These haploid spores grow into a small offspring plant that also has haploid cells. This smaller version of the fern is called a gametophyte. This portion of the life cycle of the fern, the production of spores and the growth of the gametophyte, represents a phase of asexual reproduction.

However, in ferns, the asexual phase is only part of the life cycle of the plant. The gametophyte reproduces sexually by making gametes, egg cells and sperm cells, that fuse to produce an offspring. The offspring that results from this sexual reproduction phase is a new sporophyte. The sporophyte grows into the familiar fern plant and produces spores to begin the life cycle again.

This scheme, where an organism has both haploid sexual and diploid asexual life stages, is called alternating generations and is illustrated in the fern life cycle in Figure 9.

Example 4: Identifying a Stage in the Life Cycle of a Fern

The figure shows the life cycle of a fern. Identify the stage that corresponds to reproduction by spores.

Answer

Ferns are a type of plant that has both haploid sexual and diploid asexual life stages. This reproductive scheme is called alternating generations.

During the asexual phase, ferns reproduce using spores. The large, leafy fern plant that you typically picture is referred to as a sporophyte. The sporophyte releases spores from structures called sori (singular: sorus) on the underside of its leaves. These then germinate and grow into an offspring organism called a gametophyte if they land on a suitable surface.

The gametophyte reproduces sexually by making gametes, egg cells and sperm cells, that fuse to produce an offspring. The offspring that results from this sexual reproduction phase is a new sporophyte. The sporophyte grows into the familiar fern plant and produces spores to begin the life cycle again.

This scheme, where an organism has both haploid sexual and diploid asexual life stages, is called alternating generations and is illustrated in more detail in the diagram below.

Using this information, we can infer that the stage in the fern life cycle that corresponds to reproduction by spores is stage B.

Table 1 lists some of the different organisms outlined so far and the method of asexual reproduction they may use.

Vegetative propagation is similar in some ways to regeneration, which we have discussed earlier. Vegetative propagation is the name given to the asexual reproduction methods that occur in plants when an offspring grows from a fragment or piece of the parent plant. Vegetative propagation can involve a new plant growing from a broken branch, stem, or root. It also includes reproduction by runners, commonly seen in strawberry plants, which you can see in Figure 10. These runners are modified stems that branch out from the mature, adult plant, and they develop roots and leaves and eventually grow into a new adult strawberry plant. Different plants have different methods of vegetative propagation, including those involving bulbs, tubers, and plantlets.

Sometimes scientists need to cultivate plants in a controlled environment. This may be done for research purposes with new plant strains with beneficial properties, such as disease resistance. Plants can be grown in a lab in a process called tissue culture. Tissue culture is when a sample of cells taken from living tissues are grown in a laboratory setting.

Key Term: Tissue Culture

Tissue culture is when a sample of cells taken from living tissues are grown in a laboratory setting.

In tissue culture involving plants, an entirely new plant specimen can be grown from a very small sample, even as small as a single cell. This new plant is often referred to as a clone of the parent cell, as it is genetically identical. The tissue sample is placed into a growth medium, which can be a liquid or a gel. Sometimes coconut water is used as a growth medium in these experiments because it possesses a mixture of nutrients and growth hormones. Over time, the cells from the tissue sample multiply and differentiate. Eventually, an entirely new plant specimen grows from the original tissue sample under the correct conditions.

Because a new plant is grown from a small sample taken from just one parent plant, tissue culture represents a type of asexual reproduction in plants. It demonstrates the principles of vegetative propagation and regeneration. Tissue culture also shows us that just a single plant cell possesses all the genetic material and structures necessary to develop into an entirely new individual. The photograph below shows plants grown via tissue culture at different stages of development in test tubes.

Cloned decorative micro plants in test tubes

Figure11

Most organisms that reproduce asexually are also able to reproduce sexually. Because asexual reproduction is more rapid and does not require successful mating or fusion of gametes, it is often considered a more reliable reproductive method that is always available. However, asexual reproduction has the disadvantage of creating a genetically identical population, which are all vulnerable to the same environmental threats. Sexual reproduction creates genetic variation and produces a more diverse population, which is more resilient in the case of environmental changes, even though it requires more energy and often involves a higher risk. Hence, we can see why it may be advantageous for some organisms to be able to do both.

Let’s summarize what we have learned in this explainer.

Key Points

  • Sexual reproduction usually involves two parents and produces genetically unique offspring.
  • Asexual reproduction involves one parent and usually produces genetically identical offspring.
  • There are many different methods of asexual reproduction, including binary fission, budding, regeneration, sporogenesis, parthenogenesis, and tissue culture.
  • Many organisms can reproduce in more than one way. Some can reproduce both sexually and asexually.

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