POTATOHANDBOOK Gameten (haploïde cellen: pollen en eicellen) Zygoten (producten van bevruchting) van diploïden and tetraploïden IIIIIIIIIIII Haploïd pollen, x=12, n=1 IIIIIIIIIIII IIIIIIIIIIII Diploïde 2x=24, 2n IIIIIIIIIIII Haploïde eicel x=12, n=1 IIIIIIIIIIII IIIIIIIIIIII Haploïd pollen, 2x=24, n=1 IIIIIIIIIIII IIIIIIIIIIII IIIIIIIIIIII IIIIIIIIIIII Haploïde eicel, 2x=24, n=1 IIIIIIIIIIII IIIIIIIIIIII Diploïde 4x = 48, 2n Conventioneel veredelen met diploïde (boven) en tetraploïde (onder) aardappelrassen of these wild species are diploid, while most cultivated varieties are tetraploid. Hence, the number of chromosomes of wild species must be doubled or halved where tetraploids are concerned. Researchers do this by using targeted techniques. Other cultures halve the number of chromosomes, so that tetraploids become diploid. Polyploidisation using the chemical substance colchicin doubles the number of diploids to become tetraploids. The same ploidy number (number of sets of chromosomes) is necessary to be able to cross two species with the conventional pollen and egg cell technique and transfer resistance. A fusion technique is available for combining genotypes with different numbers of chromosomes, for example 2x = 24 or 48. Somatic fusion or protoplast fusion refers to the fusion of a species with different numbers of chromosomes (ploidy). To this end, in one cell of each of the species that the breeders wants to fuse (merge), the cell wall is removed with the cellulose enzyme. This produces a so-called protoplast or somatic cell. The two cells merge using a chemical (PEG: polyethylene glycol C2nH4n2On+1) or an electric shock. The resulting new cell is provided with a cell wall with the aid of hormones. On a culture medium, the cells multiply into callus which, under the influence of gravity, produces roots and stems in order to form a new plant. As a result of such somatic hybridisation, varieties have been obtained with resistance to leaf roll virus (PLRV) in the United Kingdom, PVY in the Netherlands and black leg in Poland. Somatic hybrids created in this way are used by researchers not only between plants of species that cannot be crossed, but also within species that are sterile. The backcrossing process that takes about six years is thus bypassed. With diploid potato varieties, a characteristic of an allele, such as the colour of the flower, can be dominant A for red and a for white. Genotypes with AA as well as Aa have red flowers, while only aa has white flowers. The gametes (germ cells) of the genotype with twice the dominant species both contain A. The genotypes with Aa have 50% A and 50% a gametes. The genotypes with an a gene on both chromosomes, so aa, only have a gametes. Crossing AA parents only produces offspring with red flowers, because the offspring only has AA alleles. The crossing of two red flowers with Aa parents gives 25% AA, 50 Aa and 25% aa offspring, so 75% of the offspring has red flowers. There are genotype x environmental actions for many multiple gene characteristics such as resistance to drought, high and low temperature and salt and lateness (or earliness). The latter is the most sensitive to day length and responds to it. This means that some varieties are strongly influenced by photoperiod (or day length, the number of hours per day that the sun is above the horizon), whereas others are not. Similarly, tolerance to drought is genotype-dependent, albeit through lateness. Therefore, early varieties escape a late drought period and late varieties survive a temporary dry period. ● Potato World 2019 • number 3 13 Pagina 12

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