POTATOHANDBOOK The P of Planting stock The recently published Potato Handbook, written by Professor Anton Haverkort, covers in around six hundred pages all conceivable aspects that relate to the potato. Never before has a unique and complete reference book about the nutritious tuber been published, intended for everyone in the world who works with the potato in any way whatsoever and/or wants to know more about it. In order to establish a clear line in the format of the extensive manual, Haverkort has used the formula S ociety x P lant = P lanting material x E nvironment x C ultivation as a guideline. This brings together all the factors related to the potato. To give a first impression of all the information that the Potato Handbook has to offer, in a few consecutive editions of Potato World magazine you will find a very brief summary of each chapter in the column ‘Potato Handbook’. Last month, chapter 2 was discussed, this time you can read two instructive excerpts from Chapter 3 about ploidy level and crossing varieties with a different number of chromosomes. Ploidy level All potato varieties have a multiple of twelve chromosomes. The Irish potato (Solanum tuberosum L.) is tetraploid, which means that each of the twelve chromosomes occurs four times in the genome. Each allele (a place on the chromosome with a gene coding for a specific property, such as the colour of the skin of the tuber) can also occur in four doses (simplex, duplex, triplex and quadruplex). In diploid potato varieties such as Solanum phureja, the situation is less complex. An allele that codes for the same characteristic occurs twice instead of four times. If the characteristic is the same on both alleles, for example a pink skin, it is homozygous. If the characteristic differs on both alleles, for Potato handbook Crop of the future Anton J. Haverkort example a pink and a yellow skin, then this is heterozygous. Breeding tetraploid potato varieties, usually with a different characteristic on each of the four alleles on the four chromosomes, is therefore much more complicated than growing diploid crops. Breeding for desired characteristics in the tetraploid Solanum tuberosum requires more than 100,000 seedlings for selecting a new variety. For the diploid species, this is only a fraction of that number and the characteristics are then also transferred more accurately. It is possible to study linkage: alleles that are closer together on the chromosome inherit together more often. It is also possible to make maps of the location of the genes on the chromosomes. In order to do this, breeders must reduce the ploidy level from four to two. Gametes of the 4x (tetraploid) species are 2x (female ova and male pollen) from which 2x plants can be derived from cells of the anther (part of the flower where pollen is produced) or via microspore culture. Alternatively, crossing a tetraploid S. tuberosum with diploid S. phureja is also possible. The diploid species must first form diploid female gametes through parthenogenesis (female propagation without male intervention). All potato varieties contain a multiple of 12 chromosomes (x + 12). The number of chromosomes in a gamete (n) is the haploid number. A gamete (pollen or egg) has at least 12; after crossing, it is 2x12 = 24 chromosomes. So, when a potato species has two sets of chromosomes (n = 2, a socalled diploid), it has 24 chromosomes. A triploid (n = 3) has 36 and a tetraploid (n = 4) has 48 chromosomes. The number of diploid (n = 2) species is limited. These are grown as S. ajanhuiri in the south of Peru, S. phureja is widespread and is eaten in Venezuela, Colombia, Ecuador, central Peru and in Bolivia and S. stenotomum in central Bolivia and the interior of Peru. The Solanum tuberosum species has 4 sets of 12 chromosomes (n = 4), it is widely grown and has two subspecies (abbreviation = ssp.). These are S. tuberosum ssp. andigena grown in Latin America in the Andes from Argentina to Northern Venezuela and S. tuberosum ssp. tuberosum is originally grown in Chile. Neotuberosum is a collection of genetic sources (germoplasm) that was thought for a long time to have been derived from S. andigenum and was later identified as originating from S. tuberosum material from central Chile. Crossing species with different numbers of chromosomes In order to improve their resistance to pests and diseases, breeders look at the wild potato varieties that are abundant in North and South America. Most commonly used are genes from wild varieties, namely S. stoloniferum for virus resistance and S. demissum for resistance to potato blight. A limited number of other wild species were also used, but they only add a fraction to what is already there on these continents. Most 12 Potato World 2019 • number 3 Pagina 11
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