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26 I EUROPEAN SEED I EUROPEAN-SEED.COM FIGURE 4. Schematic overview of the final outcomes of different breeding approaches. During conventional breeding recombination of chromosome pairs results in the reshuffling of genetic material and the unique combination of genetic variation will be lost. In reverse breeding a selected heterozygous offspring plant is crossed with itself while chromosomal recombination is suppressed by a transgene resulting in lines with homozygous chromosome pairs. The haploidisation step producing plants in which only one chromosome of each chromosome pair is present and the subsequent doubling of the chromosomes producing doubled- haploid plants with homozygous chromosome pairs are not shown here. For hybrid variety production parental lines in which the genetic variation of the chromosome pairs complement each other are selected from the reverse breeding program. Crossing such lines will result in uniform offspring hybrid plants seeds which are genetically similar to the plants with which the reverse breeding was started. FIGURE 5. Simplified illustration of grafting. Here a fruit-bearing non- GM scion has been grafted onto a GM rootstock. exact copy. However seed companies are geared to reproduce and commercialise their elite plant varieties by means of seeds as vegetative reproduction is often too expensive technically cumbersome and commercialization is often logistically i mpossible. Reverse breed i ng uses a genetic modification step to suppress the recombination of chromosomes followed by specific tissue culture to create homozygous parent lines. These lines are then used to stably produce the heterozygous elite plants through seed Figure 4. GM ROOTSTOCK GRAFTING With this technique the top part of a plant called the scion is grafted onto a GM rootstock Figure 5. The resulting combined plant is usually regarded as a GM plant but the products such as the flowers or the fruits that are harvested on the non-GM scion do not carry the genetic modification and are considered GM-free. This is particularly useful in cases where the rootstock conveys beneficial characteristics to the combination such as more efficient nutrient uptake from the soil better rooting ability in heavy soils or resistance to soil-borne diseases e.g. nematodes. INDUCED EARLY FLOWERING With this approach recombinant genes are introduced into a plant that promote flower- ing in the first year. This is particularly help- ful in trees that have a long juvenile phase in which they dont flower. The early flowering enables much faster breeding and selection in these species. In the final breeding step the recombinant early flowering genes are crossed out resulting in varieties that are free of any transgenes. The plants produced in this way are indistinguishable from varie- ties obtained through conventional breeding but are now achieved decades earlier. CISGENESIS Using this method specific traits of interest are introduced into a plant using genetic modification but only using genes from the same species or from a species that can be crossed with it Figure 6. The introduced cisgene is an identical copy of a gene from the sexually compatible pool including promoter introns and terminator. The donor plant must be crossable with the recipient plant. This means that the same result could be achieved through conventional plant breeding but this would take a much longer period often up to four or five times longer. This technique is especially promising for the development of plant varieties of vegetatively propagated crops such as potato apple banana cassava and grape. INTRAGENESIS Plants created with this technique contain new genes that originate from the species itself or from a crossable species. Intragenesis allows in vitro recombination of genetic ele- ments isolated from different genes within the sexually compatible gene pool. With cisgene- sis genes are a new combination of genetic ele- ments that cannot be obtained with traditional breeding. For example one can replace the natural promotor by a promotor from another gene that comes from the same species. As with every technique there are limi- tations. One of the limitations shared by both cisgenesis and intragenesis is traits outside the sexually compatible gene pool cannot be introduced. Additionally the creation of intra- genic crops requires new expertise and more time compared with transgenic crops. The desired genes or fragments of genes may not be readily available but have to be isolated from the sexually compatible gene pool. Also the production of selection mark- er-free plants often requires the implemen- tation or development of new methods since such methods may not be readily available for the crop. This means considerable efforts have to be spent especially on crops with low IllustrationcourtesyofWageningenUR IllustrationcourtesyofNBTplatform