Asexual reproduction

Under asexual or asexual reproduction means a reproduction of living things increased the number of individuals in which the offspring only the genes alone contain an ancestor and this - apart from mutations - in an identical copy. Synonyms are monogony (from ancient Greek μόνος monos ' alone, unique 'and γονή gonē ' generation, birth ') or agamogony ( γάμος gamos , marriage, marriage').

The terms asexual and asexual reproduction leave open whether the number of individuals increases during reproduction .

Mechanisms

The decisive feature of asexual reproduction is that the offspring do not arise from sex cells and without recombination . In sexual reproduction, on the other hand, sex cells with reduced and recombined chromosome sets are formed via meiosis and the offspring resulting from fertilization with subsequent karyogamy have received genes from both parents.

Asexual reproduction does not include self-fertilization in some hermaphrodites , especially plants . Here the offspring receive their genes from only one parent , but still recombined. The parthenogenesis (unisexual reproduction) is depending on the subtype for either asexual reproduction (see also apomixis ) or for sexual reproduction (Automixis).

The main advantage of asexual reproduction over sexual reproduction is that the offspring are largely genetically identical (no recombination). Advantageous gene combinations or those in which no regulated meiosis is possible (see polyploidy ) can be obtained in this way. In addition, with asexual reproduction, just as with self-fertilization, the sometimes time-consuming and resource-consuming search for sexual partners is no longer necessary.

The disadvantage of asexual reproduction compared to sexual reproduction is that there is no recombination, i.e. no exchange of genetic material . This prevents a new generation of possibly advantageous gene combinations. Only a few eukaryotes therefore completely renounce sexual reproduction. The prokaryotes , who can only reproduce asexually, have solved the problem of recombination in other ways (see horizontal gene transfer ).

Examples

Asexual reproduction can be found in living things from different systematic units.

Transverse or longitudinal division of the cells in single cells , e.g. B. in archaea , bacteria and unicellular algae

Cytogony , the pinching off of daughter cells from a mother cell in protozoa

Schizogony , the disintegration of one cell into many cells in some protozoa

Sprouting and constriction in yeasts

Splitting of fungal threads (hyphae) - this is how infection with athlete's foot occurs in swimming pools

Molds form spores through mitosis , which are recognizable as the ends of fungal hyphae ( conidia )

Runners or brood buds on plants

In the case of higher plants , so-called cuttings can be produced by keeping leaves or twigs moist for several days

Potatoes form underground tubers , dahlias and celandine form root tubers - a new plant then grows from the tubers in the following growth period

Few species of annelids shed body segments from which new worms grow

The polyembryony can to the development of monozygotic twins result of sexual reproduction

The generation of induced pluripotent stem cells and somatic cell nucleus transfer are artificial methods for the asexual generation of embryos . On the other hand, artificial insemination still requires meiosis in the gametes of both donors and thus still has essential characteristics of sexual reproduction.

evolution

The first creatures to emerge reproduced asexually. Sexual reproduction did not occur until the late Proterozoic (around 700–800 million years ago) . With this development came many new forms of life. It is therefore assumed that the diversity of living beings on earth was only possible through the emergence of sexual reproduction.

literature

Lexicon of biology : asexual reproduction . Spectrum, Heidelberg 1999.

Individual evidence

^ ^A ^b Friedrich W. Stöcker, Gerhard Dietrich (Ed.): Brockhaus abc Biology . 7th edition. Brockhaus, Leipzig, 1986, ISBN 3-325-00071-1 .

[Stöcker1986-1] A ^b Friedrich W. Stöcker, Gerhard Dietrich (Ed.): Brockhaus abc Biology . 7th edition. Brockhaus, Leipzig, 1986, ISBN 3-325-00071-1 .