How to solve polyploidy problems

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Polyploidy definition

Polyploidy is the mutation that leads to the multiple increase in the number of sets of chromosomes. We know, that genes are localized in the chromosomes. If the chromosomes have the same set of genes and have the same shape and size, they are called homologous chromosomes. Normally, most organisms have a double set of chromosomes, in other words, this set consists of pairs of homologous chromosomes. For example, in the record of the genotype "Aa" each character represents allele of the gene in the pair of homologous chromosomes. The number of sets of chromosomes in the cells of every organism is called ploidy, and the organism with a double set of chromosomes is diploid. Normally, in the process of meiosis only one chromosome from each pair of homologous chromosomes can get into the gamete. For individual with genotype "Aa" we get the gametes "A" and "a", i.e. gametes have only one haploid set of chromosomes. Later, in the process of fusion of gametes with haploid set of chromosomes, the diploid organism formed again. He has two sets of chromosomes, one of which came from the maternal organism, and another from paternal. But the process of cell division can be broken, which could lead to a change in the number of sets of chromosomes in the cell. And as result of this we can get polyploids - the organisms with multiple set of chromosomes.

Polyploidy in animals

In the animal world polyploids occur in species that have lost their normal mechanism of gender reproduction and the species in which eggs develop without fertilization (Parthenogenesis), for example, roundworms(Nematodes and Ascarides), leech (Hirudinea), in some species of insects, amphibians and fishes. Polyploid cells are found in specific organs (liver, etc.) of many mammalians, but known only one the complete polyploidy example in mammalians - this is tetraploid - the South American red viscacha rat (Tympanoctomys barrerae) (the species related to guinea pigs and chinchillas). These animals belong to the Octodontidae family, the Rodents order. Living in Argentina on salt marshes and are extremely rare. The main reason that polyploidy in animals is rare, is that this type of mutations disrupt the chromosomal mechanism of gender determination: if the number of X - chromosomes greater than two, in individuals observed abnormalities in development and they either die or are unable to reproduce. In the genome of red viscacha rat quadruple only autosomes, and the number of X - chromosomes is normal as for diploid organisms. Researchers believe that because of that red viskashi maintaining the vitality and fertility.

Polyploidy in plants

For most plants this limitation does not exist because they do not have X - chromosomes at all and they have the possibility to self-pollination. Therefore, polyploidy is much more common among plants than among animals. There are natural and artificially derived polyploids. For example, bread wheat - a natural polyploid, consisting of six haploid chromosome sets of related species of cereals. Polyploids obtained as a result of the impact on plant by temperature, ionizing radiation, chemicals (colchicine), which destroy the cell division spindle. Many cultivated plants are polyploid, in other words, contain more than two sets of chromosomes. Among the polyploids are many major food crops: wheat, potatoes and oats. In the crop known more than 500 plant's polyploids (rye, sugar beets, grapes, buckwheat, mint, radishes, onions, etc.). Some polyploids possess high resistance to unfavorable factors and a good yield. The great variety of polyploids observed in floriculture: if the original one form in the haploid set had 9 chromosomes, the cultivated plants of this species may have 18, 36, 54 or more chromosomes. Distinguish autopolyploids and allopolyploids.

Autopolyploidy and Allopolyploidy

Autopolyploidy - hereditary variation with fold increase in the number of sets of chromosomes in cells of the same species. On the basis of artificial autopolyploidy synthesized new forms and varieties of rye, buckwheat, sugar beets and other plants.

Allopolyploidy - is the multiple increase in the number of chromosomes in hybrid organisms. Occurs when crossing plants that belong to different species or genus. First generation hybrids are usually infertile. Cause of infertility is in violation of conjugation of chromosomes in meiosis. The increase in the number of sets of chromosomes in such hybrids leads to the restoration of fertility.

Triploids

Triploids have three sets of cromosomes. Known such triploids as banana, watermelon, ginger, citrus. Most from these plants is sterile and very minor part of them is fertile. Using a genetic calculator you can see what kind of gametes will give us a triploid individual. For this we need conclude in the ampersands characters the each group of chromosomes (genes) in the parents genotype notation, like this: &AAA&. This write takes into account all the possible combinations, including those that are the result of nondisjunction of chromosomes. And as chromosomal nondisjunction occurs infrequently, the ratio of gametes in this case does will be not to match the reality, as well as the ratio of genotypes and phenotypes in this cross. By selecting "Gametes for genotype 1" for the results of crossing you will see a combination of gametes that can be formed. As you can see the gametes can be haploid, diploid, or have an unbalanced number of chromosomes. Gametes of the last type is formed quite a lot, and this determines the sterility of the most individuals with uneven set of chromosomes, in this case for triploids.

But we must say that there are plants for which triploidy is the normal, for example, it is aspen(57 chromosomes), red beet(27), apple(51). Such plants produce normal gametes. In order to select only normal gametes it's necessary to add percent characters in the end of the parents genotype notation, like that: &AAA%%&. How the program determines which gametes is normal? In the case of an even number of sets of chromosomes ploidy simply divided by 2. In the case of an uneven number, the ploidy is also divided by 2, but then to the obtained result added 0.5 and subtracted 0.5, and thereby we get two values. For example, for triploids normal gametes is haploid A) and diploid (AA) (3/2 = 1.5, 1.5 + 0.5 = 2 (AA); 1.5 - 0.5 = 1 (A)). If you want to select only gametes with certain ploidy, than you need to write necessary value between the percent characters. For example, &AAA%1%& - if only the haploid gametes are needed or &AAA%2%& - only the diploid gametes are needed. The ratio of normal gametes is completely reliable and if you use the selection of normal gametes for both parental genotypes in the cross, then the ratio of genotypes and phenotypes will be also reliable.

Polyploids may be homozygous and heterozygous. Homozygous triploids is "AAA" and "aaa". And heterozygous is "Aaa" and "AAa". There is one important rule for polyploidy. In each group of polyploids genes you can use only one type of dominant and recessive alleles, for example like this: &AAa%%&&Bbb%%&, but notation like that - &Aab%%& will be wrong. In contrast to polygenic inheritance - for polyploidy the order of writing dominant and recessive alleles is not important. So writing genotypes like this &aAa%%& or &aaA%%& you still get the correct results. By selecting "Gametes for genotype 1" for the results of crossing you will see a combination of gametes, which will give the triploids "&AAA%%&", "&aaa%%&" and "&Aaa%%&", "&AAa%%&". Using a genetic calculator, you can also see what ratio of phenotypes may give these triploids in crosses.

Tetraploids, hexaploids and other polyploids

In nature, you can also find a plants with five haploid sets of chromosomes - pentaploids. For example, Kenai Birch (Betula papyrifera) it is pentaploid. As triploids, pentaploids is polyploids with uneven set of chromosomes. For pentaploids the notmal gametes is triploid (AAA) and diploid (AA) (5/2 = 2.5; 2.5 + 0.5 = 3(AAA); 2.5 - 0.5 = 2(AA)). Using a genetic calculator, you can also see what the ratio of gametes and phenotypes may give a pentaploids.

But the more better studied polyploids with even set of chromosomes - tetraploids ( four sets of chromosomes ) and hexaploids ( six sets of chromosomes ). Known, such tetraploids as durum wheat, potato, cabbage, tobacco, leek, peanut, cotton, kinnow, apple, Pelargonium. The normal gametes for tetraploids is diploid gametes (AA) (4/2 = 2(AA)). And variants of parents genotypes, that you can use in genetic calculator is - &AAAA%%&, &AAAa%%&, &AAaa%%&, &Aaaa%%&, &aaaa%%&.

From hexaploids is known, such as bread wheat, triticale, oat, chrysanthemum and kiwifruit. The normal gametes for hexaploids is triploid gametes (AAA) (6/2 = 3(AAA)). And variants of parents genotypes, that you can use in genetic calculator is - &AAAAAA%%&, &AAAAAa%%&, &AAAAaa%%&, &AAAaaa%%&, &AAaaaa%%&, &Aaaaaa%%&, &aaaaaa%%&.

Also known octaploids ( eight sets of chromosomes ) such as pansies, sugar cane, dahlia and strawberry ( normal gametes - tetraploids (8/2 = 4(AAAA)). Decaploids ( ten sets of chromosomes ), for example certain strawberries ( normal gametes - pentaploids (10/2 = 5(AAAAA)). Dodecaploids ( twelve sets of chromosomes ), for example the plant Celosia argentea ( normal gametes - hexaploids (12/2 = 6(AAAAAA)).

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