Asexual reproduction - is the reproduction by dividing the body into two or more parts or by specially adapted for this part of the body, as well as

Asexual reproduction (fission, budding, vegetative, fragmentation, cloning, spores)

Asexual reproduction — is the reproduction by dividing the body into two or more parts or by specially adapted for this part of the body, as well as spores. Asexual reproduction process carries out without any contact with individuals of its kind, however there is no exchange of genetic information with this method of producing progeny.

There are several forms of asexual reproduction. Each of them is peculiar to specific groups of organisms.

Binary fission

Propagation by binary fission of the body is characterized primarily for single-celled organisms. In eukaryotic creatures (unicellular, algae, fungi) the basis of cell division is mitosis. In bacteria occurs direct fission peculiar to prokaryotes.

At first glance, the binary fission is the most unprofitable way of reproduction. Not by chance it is inherent in primitive single-celled organisms. After all, the number of descendants of one generation in such a case may increase at most twice, whereas reproduction by sex cells progeny of a single pair may reach millions of individuals. But it turned out that due to the fact that cell division is inherent in the simplest creatures, they possess an extremely high speed of reproduction. Scientists estimated that a single bacterium, which can divide every 20 minutes, is capable to give rise of 250 thousand Bacteria in six hours in suitable living conditions. In a day, the weight of descendants will be 4.730 tons. And they can fill a World Ocean in three or four days. No one can reproduce so quickly and efficiently.

Budding

Reproduction of unicellular can be carried out by binary division, when the parent cell divides strictly in half, and can be, as in yeast and certain bacteria — by budding. In this case, in cell first occurs doubling of genetic system, one part of which migrates to the cell outer membrane, where it forms a protrusion, which is separated lately (outbuds) from maternal cells.

Budding may occur not only in unicellular fungi and bacteria, but also in the most primitive animals — sponges and coelenterates (diagram 2), which body is constructed of only two layers of cells.

Multiple fission

In some unicellular, such as foraminifera, Sporozoa the divisions occur the other way. Initially, in the cell occurs a number of nuclear fission, resulting in a plurality of cores, after which it divides on the corresponding number of mononuclear cells. This method of reproduction is called multiple fission.

Fragmentation

Primitive multicellular animals, in particular flatworms and segmented worms, can reproduce by dividing the body into fragments — a fragmentation. This method of reproduction is common in many algae, for example in the green alga Spirogyra.

Diagram 2. Stages of the asexual reproduction of Hydra: 1 — the beginning of the formation of vegetative buds; 2 — its growth; 3 — completion of the formation; 4 — separation

Vegetative propagation

Another way of asexual reproduction is a vegetative propagation that is inherent mostly in higher plants. It is carried out by special organs, such as various types of shoots, bulbs and tubers. Usually due to vegetative propagation form colonial organisms consisting of individuals linked to each other. They have the same metabolism and equally responsive to external stimuli. Colonial multicellular organisms include corals, wheat grass, lily of the valley and other vegetatively propagated plants.

The ability of plants for vegetative reproduction is often used in plant breeding. Due to the fact that many varieties of garden plants (tulips, roses, dahlias) sterile and unable to form seeds, vegetative propagation becomes the only possibility of their reproduction. At artificial vegetative reproduction usually use cuttings and slips — parts of the plant that can give rise to new plants.

Cloning

During the reproduction by fission, budding or special vegetative organs always forms genetically uniform offspring, an exact copy of the mother's body. And it is quite natural, because the basis of asexual reproduction processes is mitosis in which the daughter cells receive equal genetic material. This reproduction, carried out in artificial conditions in order to obtain genetically uniform progeny, is called cloning, and formed descendants are clones (from Greek “clone” — “branch”, “shoot”, “scion”).

Spores

A special type of asexual reproduction of living organisms is a reproduction by spores — microscopic primordia, often consisting of one, rarely — two, and even more rarely — a group of cells that develop multicellular organisms. Their purpose — is a reproduction or surviving of the body in adverse conditions. Taken from http://worldofschool.org

Diagram 3. Organisms that reproduce by spores: a — fungi; b — algae; c — mosses; d — ground pines; e — horsetails; f — ferns

Fungi, algae and lower spore plants (diagram 3) reproduce by spores. In order for a new body began to develop from spores, the spores do not require contact with each other, as it happens in sexual reproduction by gametes; for germination of spores it is quite enough of favorable conditions (humidity, temperature, etc.).

The spores of fungi and algae often form as a result of mitosis, and higher plants — only due to meiosis. In the first case the organisms that develop from the spores of the parent organism, are the clones, in the second case they are genetically different from each other.

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Questions:
  • What types of reproduction do unicellular organisms have?

  • What feature of vegetative propagation of plants?

  • What types of spores reproduce flowering plants, and what — algae and fungi?

  • Why spore formation in bacteria is not a process of reproduction?

  • Why cases of asexual reproduction of mammals are not known?