ASEXUAL REPRODUCTION

Unit Overview

Asexual reproduction is the topic of discussion for this unit. We will study how living things reproduce from only one parent, how characteristics are passed down from one generation to the next, and how asexual reproduction relates to the survival of the species.

GLOSSARY OF KEY TERMS
asexual reproduction	mitosis	gene
interphase	prophase	metaphase
telephase	metaphase	anaphase
telophase	plantlets	cuttings
vegetative propagation	rhizome	bulbs
bulblets	fragmentation	fission
vegetative production	regeneration	budding plantlets

Asexual Reproduction

Reproduction is the creation of how living things reproduce. There are two different types of reproduction: sexual reproduction and asexual reproduction. Sexual reproduction usually requires two parents. In asexual reproduction, some living things reproduce from only one parent. When an organism divides into two parts, it reproduces asexually. Asexual reproduction is sometimes referred to as nonsexual reproduction. This unit will introduce asexual reproduction and the next unit will introduce sexual reproduction.

Sexual and asexual reproduction has an important function in common. In both types of reproduction, a special set of chemical “blueprints” are given to the offspring. The blueprints determine what characteristics that living thing will have. These characteristics are then passed from one generation to the next. In both cases reproduction is essential for the survival of the species.

Asexual reproduction is the simplest form of reproduction which occurs in many plants and simple animals. The offspring are always genetically identical to the parent, or sometimes commonly referred to as a clone. When we think in evolutionary terms, there are disadvantages with this type of reproduction. Only identical individuals are produced. This means that there is no variation. This can be useful in terms of agriculture and horticulture, however, in the wild, an asexual population that cannot adapt to a changing environment or evolve defenses against new diseases is at risk of extinction.

Asexual Reproduction in Plants

Asexual reproduction does not involve sex cells as would be common with sexual reproduction. This process is reproduction by mitosis. Chromosomes copy themselves when a cell reproduces by division into two new cells called daughter cells. Each daughter cell has the same genes as the original cell. This process is called mitosis. A gene determines the inheritance of a particular trait. Asexual plant reproduction requires only one organism. New plants have the same genetic structure as the parents.

There are 5 stages of plant mitosis: interphase, prophase, metaphase, anaphase, and telophase. In interphase, the cell is preparing for division by replicating DNA and organelles and increasing cell size. In the second stage, prophase, the cell prepares for nuclear division by condensing DNA into moveable packages called chromosomes. In metaphase the cell prepares the chromosomes for division by aligning the chromosomes at the cell equator (center), and attaching spindle fibers from each new daughter cell pole to each chromosome at the centromere. In anaphase, the spindle fibers pull the chromosomes apart. One half of each chromosome moves to a new daughter cell. In telophase the cytoplasm divides and the DNA condenses forming two nuclei. The new cell wall appears between the two new nuclei to form two new daughter cells.

The following diagram show plant cell mitosis.

Asexual reproduction includes plants that grow from bulbs, feelers, and rhizomes. Branches grafted to trees can also be classified as asexual reproduction, as can single-celled plants such as algae.

Asexual reproduction can take a number of forms in plants. One of these is vegetative propagation. Many plants develop underground food storage organs which over winter will develop into the following year’s plant. Some examples might include bulbs, tubers (potatoes), and rhizomes. Plantlets can take the form of runners that grow above ground and will start a new plant (strawberries). As the heavy stems bend over or break off coming in contact with the soil, the preformed roots will penetrate the soil and form a new attachment for the plant. Under the right conditions, cuttings, or grafts, will develop roots and grow into a new plant. Tissue culture involves taking a few cells from a plant and growing them into a complete specimen. Tissue culture is a type of cloning and will be discussed in a future unit on genetics. A rhizome is an underground runner that gives rise to new plants. Quackgrass is a good example of this. It is a notorious invader of home gardens due to its habit of sending long underground roots. Bulbs also have underground storage units that divide by mitosis allowing many new plants to form new bulbs. An amaryllis, a house plant, is an example of this. And then there are bulblets which is a name coined for a form of asexual reproduction which is quite unique. Some types of garlic will form small bulblets in the place of a flower head. Bulblets will form on the flower stem after the flowers have dried up. They send out a shoot while still on the stem and then will break away from the stem and fall on moist soil where they will start to grow again. Fragmentation is where parts of a plant will break off and grow into a new plant. This may also include vegetative reproduction in which a part of the plant, not specialized for reproduction, breaks off and grows into a new plant. An example of this is a cactus which has brittle stems and leaves. As animals walk by, they break off. Any piece of leaf tissue touching the ground will form roots and grow into a new plant when the conditions are right. Budding plantlets offer several plant buds of tissue that develop into a miniature version of the parent plant. These may even develop roots while still attached to the parent plant. Some examples of this include a spider plant and a Mexican hat plant. Budding involves one parent dividing its nucleus (genetic material) equally, but the cytoplasm unequally. Yeast is an example of this.

Asexual Reproduction in Animals

Asexual reproduction can be advantageous for certain animals. Animals that remain in one particular place and are unable to look for mates, would need to produce this way. Another advantage is that the numerous offspring can be produced without “costing” the parent a great deal of energy or time. Environments that are stable and have very little change are the best places for organisms that reproduce asexually. The cloned offspring are more likely to succeed in the same stable areas as their parents.

There are three main ways of asexual reproduction in animals. They are fission, fragmentation, and regeneration. In fission, or budding as it is called, one or more individuals are formed from the original. These clones may remain attached physically to the parent or break away from it. If the buds remain attached, colonies of coral form. Stony corals are an example of this type of reproduction.

Gemmules, a type of internal budding, allows an organism to survive under extreme conditions. An example of this type of reproduction is a sponge.

Fragmentation is another way to reproduce asexually. The parent breaks into different fragments, which will eventually form new individuals. As many worms grow to full size, they spontaneously break up into 8 or 9 pieces. Each of these fragments develops into a mature worm, and the process is repeated. An example of this is certain flatworms known as planarians.

In regeneration, when an animal capable of regeneration loses a body part, it can grow a replacement part. If the lost body part contains enough genetic information from the parent, it can regenerate into an entirely new organism. Planaria, a flatworm, and echinoderms are examples of animals that can regenerate the missing parts. They also have the ability to regenerate injured body parts as well.

There are 5 stages of animal mitosis: interphase, prophase, metaphase, anaphase, and telophase. In interphase the cell is preparing for division by replicating DNA and organelles and increasing cell size. If you were to look at the cell during this phase, you might think it appears to be resting most of the time. However, it is carrying on the ongoing functions of a cell. This is important to keep the cell in balance. In prophase the cell is preparing for nuclear division by condensing DNA into moveable packages called chromosomes. In this phase the cell is getting ready to divide. In metaphase the cell prepares the chromosomes for division by lining up the chromosomes across the middle of the cell. They divide so that there are now two copies of each one. The spindles are about to pull copies of the chromosomes to each end of the cell. In anaphase you can see the chromosomes being pulled toward the poles of the spindles. This process duplicates and separates the chromosomes so that each new cell will be an exact copy of the old one. In the last phase, telophase, the cell is ready to reconstruct a nucleus at each end of the cell. The chromosomes are ready to be enclosed in a nucleus again. The spindles are disappearing and their job is finished.

Mitosis and Humans

Why is the process of mitosis important in humans? How is it related to this topic? Actually mitosis is going on in our bodies all the time. As our hair grows, the cells are dividing asexually. When our fingernails grow, there is also asexual division again. But that’s not all. When we cut our finger and the skin heals, the cells go through this same process. And when our bodies grow, this is how our bones get longer and tissues grow.

Why choose asexual reproduction?

Maybe we should ask, why not? We already know that asexual reproduction is an efficient way of reproducing and avoids all sorts of problems. And it is also advantageous in that it can create individuals rapidly and in large quantities. Asexual reproduction tends to be genetically static. This may doom the species to extinction if they are unable to adapt to a changing environment. Plants and animals that reproduce asexually may have evolved this way because they are fixed in place or can’t move very far.

Maybe sexual reproduction, as we will discuss in the next unit, will remain the favorite because it provides a way to weed out harmful mutations that may arise. Perhaps the need to adapt to changing environments has caused sexual reproduction to remain the method of choice for most living things.

Unit Extensions:

Suggested field trips:

· Visit a greenhouse and try to identify the plants that reproduce asexually.

Careers to explore:

· Horticulturist

· Biologist

Suggested topics for further research:

· horticulture

· genetics

Unit Conclusion

Asexual reproduction is defined as reproduction from one parent. This type of reproduction plays an important role with plants and animals. This can be productive in the field of agriculture and horticulture, however, may not be the best means of reproduction in evolutionary terms. The stages of mitosis in both plants and animals were discussed as well as the various types of asexual reproduction.