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Marker-Assisted Selection

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Potential advantages of MAS

MAS can theoretically enhance selection efficiency because:
  • It can be performed on seedling material, thus reducing the time required before a plant’s genotype is known. In contrast, many important plant traits are observable only when the plant has reached flowering or harvest maturity. Knowing a plant’s genotype before flowering can be particularly useful in order to plan the appropriate crosses between selected individuals.
    Fig. 6. MAS may be conducted based on DNA collected from young plants, such as the wheat seedlings shown here.
  • MAS is not affected by environmental conditions. Some crop production constraints (such as disease, insect pests, temperature and moisture stress) occur sporadically or non-uniformly. Therefore, evaluating resistance to those constraints may not be possible in a given year or location. MAS offers the chance to determine a plant’s resistance level independent of environment. See, for example, Peng et al. (2000).
  • When recessive alleles determine the trait of interest, they cannot be detected through phenotypic evaluation of heterozygous backcross plants, because their presence is masked by the dominant allele. In a traditional backcross program, plants with recessive alleles are identified by progeny evaluation after self-pollination or testcrossing to a recessive tester. This time-consuming step can be eliminated in a MAS program, because recessive alleles are identified by linked markers.
  • Similarly, when multiple resistance genes are 'pyramided' (combined) together in the same variety or breeding line, the presence of each individual gene is difficult to verify phenotypically. The presence of one resistance gene may conceal the effect of additional genes. This problem can be overcome if markers are available for each of the resistance genes.
  • Environmental variation in the field reduces a trait’s heritability , the proportion of phenotypic variation that is due to genetics. In a low heritability situation, progress from phenotypic selection will be slow, because so much of the variation for the trait is due to environmental variation, experimental error, or genotype x environment interaction, and will not be passed on to the next generation. If a reliable marker for a trait is available, MAS can result in greater progress than phenotypic selection in such a situation.
  • MAS may be cheaper and faster than conventional phenotypic assays, depending on the trait. For example, evaluating nematode resistance is usually an expensive operation because it requires artificial inoculation of plants with nematode eggs, followed by a labor-intensive technique to count the number of nematodes present (Concibido et al., 1996). Selecting on the basis of a reliable marker would probably be cost-effective in this case. On the other hand, plant height is cheap and easy to measure, so there may not be an economic advantage in using markers for that trait. Economic aspects of MAS in a maize breeding program are discussed in detail by Dreher et al. (2003) and Morris et al. (2003).
  • A consideration that may affect cost effectiveness of MAS is that multiple markers can be evaluated using the same DNA sample. Extraction of DNA from plant tissue is one of the bottlenecks of MAS. Once DNA is extracted and purified, it may be used for multiple markers, for the same or different traits, thus reducing the time and cost per marker.

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