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Kopie van doubling of chromosomes(new).pdfDoubling of chromosomes in tissue culture Breeding takes place throughcrossing (hybridisation) andselection. The breeder aims tojoin the good characteristicsfrom 2 different plants in theiroffspring through hybridisa-tion. He aims to select the off-spring of plants which showsthe best combination of quali-ties from both parents plants.
This process depends entirelyon the production of offspring:in other words the formation number of chromosomes fromone of the parent plants.
The inability to realize offspring from hybridisation may have many reasons. Some reasons can be eradicated bydoubling of chromosomes. If one of the parent plants is sterile and the sterility is due to an uneven number of chro-mosomes then doubling of the chromosomes will often result in a fertile parent plant. If the ploidy level of the par-ent plants differs too much there may be no viability of the seed or all the offspring will be sterile and thus furtherhybridisation is impossible. This happens when for example diploid and tetraploid plants are crossed. A tetraploidparent which can be crossed with a diploid parent plant can be obtained by doubling the number of chromosomesin the diploid parent plant. The offspring will then always be tetraploid. It is also of interest to develop tetraploidplants because they are often more robust than diploid plants.
The cell division involves various phases. In the first phase the number of chromosomes per cell is doubled fol-lowing which the chromosomes are divided between the two cell halves. These are then essentially separated by anew cell wall. By means of treating the dividing cells with specific chemicals the chromosomes will be preventedfrom dividing over the two halves and there will be no formation of a new wall. As a result of this a cell with thedouble number of chromosomes is created. Cell division and regeneration can be achieved by removing the chem-icals thus creating a new plant in which every cell contains twice the number of chromosomes. Due to the fact thatthe chemicals used can have an effect on dividing cells of other organisms as well, the safe use of the chemicals isvital. Because of this it is essential that this procedure takes place in a well-equipped laboratory where the chemi-cals can be used safely.
Doubling of chromosomes in tissue culture It appears from the above that the technique for doubling ofchromosomes only works on dividing cells which are provi-sionally treated with chemicals. By tissue culture a such likeprocess is easy to time and measure. This can take place all year round. Furthermore, tissue culture offers the possibility toobtain sufficient dividing cells in artificially made wound sur- faces or shoots. Following treatment the plant consists of cells,some of which have a single number of chromosomes and some of which have twice the amount of chromosomes. Cellswith an abnormal number of chromosomes or with twice thedoubled amount of chromosomes can also be found in thetreated plants. From these cells plants can be obtained bymeans of regeneration or normal vegetative propagation. Thisis easy to realize in tissue cultures. These plants are selected for a doubled number of chromosomes. This is generally per- formed with the aid of a flowcytometer which determines therelative DNA-level in each cell nucleus. In this manner plantswith a double number of chromosomes can be separated fromthose with a single number of chromosomes, or anymixture of both. Only plants with a complete doubled numberof chromosomes will be propagated and distributed.
With cross-breeders and self-pollinators it often takes 5 generations or more beforethere is complete homozygote present in the genes. For the breeder working with thisit may be important to produce di-haploid plants. These plants are completelyhomozygote and can be used to produce hybrid cross-breeds for example.
possible to get mono-haploid plants from anthers. After doubling of the chromosomesof these mono-haploids, homozygotes with recessive properties will be reached quitequickly and this is important for hybridisation.
SBW International has extensive experience in doubling of chromosome numbers inplants based on time controlled treatment of plants in tissue culture. Conditions for chromosome doubling in tissue culture is that there is a good procedure for propagation via tissue culture for the crop concerned. The plants are introduced into tissue culture and propagated to 250, these are treated with 4 different concentrations The NetherlandsPhone. :+31 71 - 331 49 00 of colchine or oryzalin. After the treatment the plants that result from the three propagation cycles are inspected for size, a selection of 200 plants are screened with a flow cytometer and the plants that have a doubled number of chromosomes are then propagation to 10 plants before they are distributed. From these 10 plants SBWInternational will conserve some in vtiro for possible follow up experiments or anotherproduction order.
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