If there are three or more sets of chromosomes, one speaks of polyploidy:
- triploid : three sets of chromosomes, for example in some tardigrades (tardigrada), common dandelion and rarely in amphibians (compare: pond frog )
- tetraploid : four sets of chromosomes, for example in trout fish , common dandelion
- hexaploid: six sets of chromosomes, for example in seed wheat
- octoploid: eight sets of chromosomes, for example in some sturgeons
- decaploid: ten sets of chromosomes, for example in strawberries
- dodecaploid: twelve sets of chromosomes, for example in the silver burnt
An even-numbered set of chromosomes is called orthoploidy , and an odd-numbered set is called anorthoploidy .
The origin of polyploidy in an organism is caused by meiosis during chromosome replication. If no spindle fibers are formed or if the homologous chromosome pairs are not separated during the reduction division for other reasons, diploid gametes are formed . The causes for such a failure to break up can be metabolic disorders, environmental influences (cold) or poisons added by humans ( colchicine or 8-hydroxyquinoline ).
Polyploidy of organisms is not a permanent feature over evolutionary periods. It is created by whole genome duplication (WGD). By mutations , deletions , insertions and chromosome aberrations the gene copies to drift in their properties away from each other and, as well as entire chromosomes, deleted, so that after a long development, the origin of the surviving genes is blocked. However, through computer analysis, traces of previous WGDs can be found in all living things.
Polyploidy occurs frequently in higher plants in particular ; Examples are wheat as well as many ferns and orchids . Many cultivated types of fruit and vegetables show polyploidy, since the largest and best - often from different species - are always bred during breeding and polyploidy can occur especially in such crossings, but also as a random mutation .
In the animal kingdom polyploidy percentage rare, but occurs for example in single species or forms of amphibians (z. B. triploidy in Teichfrosch ), reptiles and rodents (eg. B. tetraploidy in the Red and Golden viscacha rat ), in particular Wenigborstern , wherein species of fruit flies (Drosophilidae), as well as various genera of planorbidae on (tetra- through Oktoploidie). The entire family of trout fish (Salmonidae) also originated from polyploidization.
In humans, polyploidy occurs physiologically in some cell types, for example in the cells of the heart muscles , the seminal vesicle , the anterior pituitary gland , in epithelial cells of the liver and extravillous trophoblasts of the placenta. Polyploid embryos usually die during early pregnancy.
Polyploidy has also been observed in some bacteria . An extreme example can be found in the giant bacterium Epulopiscium fishelsoni , which is up to 0.6 millimeters in size and contains up to 200,000 copies of its genome.
Allopolyploidy is a form of polyploidy in which sets of chromosomes from different species are present. When two species are crossed, sterile offspring are usually produced, since the pairing of chromosomes is usually disturbed in inter-species hybrids and meiosis cannot therefore proceed correctly. Some chromosomes still pair correctly and are called homologous. Some chromosomes are no longer completely homologous and no longer pair in meiosis. They are called homeological .
In such hybrids , especially in plants, polyploidization of the chromosome set can occur, which is then allopolyploidy. If this polyploidization occurs after the crossing of two normal diploid parents, one speaks of a polyhaploid kind hybrid. It contains two homozygous double sets of chromosomes. If the chromosomes of the parent species are sufficiently different, the duplicate chromosomes of the father and the mother can mate, the offspring is fertile again and a constant hybrid species is created. They behave cytologically and genetically like diploids. If the chromosomes of the parents are very similar, problems with chromosome pairing during meiosis can arise and the offspring are sterile or have limited fertility.
When two tetraploid species are crossed, another tetraploid so-called “addition bastard” is created, which, in contrast to the polyhaploid species bastard, is heterozygous . Such species are called amphidiploid.
Allopolyploidy is quite common in some plant genera. Examples are Nicotiana , cotton ( Gossypium ), nightshade ( Solanum ), some cruciferous vegetables , e.g. B. the rapeseed , and many sweet grasses . A well-known example is wheat , where there are diploid species ( einkorn ), allotetraploid species such as spelled , emmer and durum wheat and even allohexaploid species ( seed wheat ). There are three types involved in the latter. Over 40 different allopolyploid forms are known in wheat, and around 60 in tobacco. Their chromosome numbers range from 36 to 144.
Polyploidy, which is based on the duplication of chromosome sets within a species, is called autopolyploidy, as opposed to allopolyploidy. It can occur in individual somatic cells (endopolyploidy), but it can also arise in the germline and thus be passed on to the offspring (germline polyploidy).
If there is no reduction in meiosis , diploid gametes develop instead of haploid gametes . Fusing with a haploid gamete creates a triploid zygote , fusing with another diploid gamete creates a tetraploid zygote.
Triploidy is more common in plants. Triploid plants are often superior to diploid in terms of their vitality and physiological productivity. They are therefore often used in plant breeding, but must either be propagated vegetatively (some poplar varieties) or, like sugar beets , be newly produced from diploid and tetraploid parents.
Autopolyploid plants usually have larger cells due to the increase in nuclear volume. In many cases the flowers are larger, which is used in plant breeding, where natural and experimentally produced polyploids are in use.
In endopolyploidy, only some tissues or cells in an organism are polyploid. Examples of this are the stinging hairs of the nettle or the megakaryocytes of humans. These polyploid cells result from endomitosis or endoreduplication . In both cases the chromosomes are doubled without the nucleus subsequently dividing. It is also known as somatic polyploidy because it is limited to somatic cells and does not affect the germline . It affects cells with high metabolic capacities. Special forms are the polytene chromosomes .
Advantages and disadvantages
Polyploidy in plants can often manifest itself in increased vitality, since the transcription of protein biosynthesis can take place more closely in parallel and therefore the production of proteins, e.g. B. enzymes, is possible faster. In animals, it is usually a case of lethal changes in the genome. As already described (see above), polyploidy in humans is excluded, as it leads to the destruction of the fruit at an early stage of pregnancy.
Parents with different degrees of ploidy are usually unable to produce compatible sex cells (for exceptions, see the section on non-diploid chromosome sets in the Chromosome article). Therefore, polyploidization often acts as a genetic barrier to speciation . It also enables the emergence of new species without geographical isolation, i.e. a sympatric speciation .
In plant breeding, the formation of microtubules (spindle fibers) is artificially prevented. The poison of the autumn crocus ( Colchicum autumnale ) Colchicine or 8-hydroxychinoline cause polyploidy in addition to their other toxic effects and are therefore used to artificially induce polyploidy in plants. Such methods are used, for. B. in agriculture to grow stronger, more robust and higher-yielding grains.
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- This section is based on: Wilhelm Seyffert (Ed.): Textbook of Genetics. 2nd Edition. Spektrum Akademischer Verlag, Heidelberg / Berlin 2003, ISBN 3-8274-1022-3 , p. 504.
- The next section is based on: Wilhelm Seyffert (Ed.): Textbook of Genetics. 2nd Edition. Spectrum Academic Publishing House, Heidelberg / Berlin 2003, ISBN 3-8274-1022-3 , p. 502 f.
- P. Schopfer, A. Brennicke: Plant physiology. 6th edition. Elsevier, 2005, ISBN 3-8274-1561-6 .
- http://www.wissenschaft-online.de/abo/lexikon/bio/6438 (on autopolyploidy)