What is the cause of polyploidy



 

As Polyploidy In biology, this is the term used to describe the phenomenon observed in some species of having more than two sets of chromosomes in the cells.

A single (haploid) chromosome set contains each chromosome once, a double (diploid) chromosome set twice. If there are three or more sets of chromosomes, one speaks of polyploidy:

  • triploid: three sets of chromosomes, e.g. B. in some tardigrades (Tardigrada) and rarely in amphibians (compare: pond frog).
  • tetraploid: four sets of chromosomes, e.g. B. in some Salmonidae
  • hexaploid: six sets of chromosomes, e.g. B. in wheat
  • octoploid: eight sets of chromosomes, e.g. B. with some disturbances

Polyploidy occurs during meiosis when chromosomes are copied. 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 of such failures in separations can be metabolic disorders, environmental influences (cold) or toxins added by humans (colchicine or 8-hydroxyquinoline).

Occurrence

Polyploidy occurs particularly frequently in plants; Examples are wheat as well as many ferns and orchids. Nowadays, practically every cultivated fruit and vegetable variety shows polyploidy, since the largest and best - often from different species - are always being bred during breeding and polyploidy can occur especially with such crosses, but also as a random mutation.

In mammals, the risk of partial polyploidy - mostly trisomy - increases significantly with the age of the dam, although the possibility exists at any age.

Allopolyploidy

This is understood as the spontaneous or artificial multiplication of added, dissimilar sets of chromosomes. Alloploid plants or animals are obtained by crossing between different species. By doubling the chromosomes, sterility of the offspring is avoided. The germ cells each contain one (allodiploidy) or several (allopolyploidy) sets of chromosomes from each parent. The chromosomes of the resulting hybrid species are called homeologous.

Wheat has a special form of polyploidy, allo- or amphipolyploidy. Here, the sets of chromosomes from three different species are combined to form a new species (here wheat). Another example is the tetraploid potato, which contains two diploid sets of chromosomes from each of its (different) parents.

This type of polyploidy plays a significant role in agriculturally important species such as B. the rapeseed.

Endopolyploidy

Another form of polyploidy is endopolyploidy; certain tissues or cells of an organism are separately polyploid. Examples of this are the stinging hairs of the nettle or the megakaryocytes of humans. It is also genetically significant that an individual can thus have several different alleles of a gene and therefore - with mixed inheritance - an increased heterosis effect.

Advantages and disadvantages

Polyploidy can often manifest itself in increased vitality, since the transcription of protein synthesis can take place more closely in parallel and therefore the production of proteins, e.g. B. enzymes, is possible faster.

Parents with varying degrees of ploidy are usually unable to produce compatible sex cells. 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.

Artificial production

In plant breeding, the formation of microtubules (spindle fibers) is artificially prevented. The poisons of the autumn crocus colchicine or 8-hydroxyquinoline cause polyploidy in addition to their other toxic effects and are therefore used to artificially induce polyploidy in plants. Such methods are used, for example, in agriculture to grow stronger, more robust and more productive types of grain.

literature

  • P. Schopfer and A. Brennicke: Plant physiology. 6th edition Elsevier, 2005, ISBN 3-8274-1561-6

Category: Genetics