8. Marker Assisted Selection

Question 1

What´s the main limiting factor of breeding in fruit tress species?

Answer 1

1. The long juvenile phase
2. Selection based on phenotype analysis→ traits difficult or complex to score

Question 2

What made early selection methods possible and what are their advantages?

Answer 2

• molecular markers made it possible to made the early selection in fruit tree species:

1. Early recognition of the plants carrying determined traits: even if difficult to score
2. even if masked by other traits
3. even if not yet directly scorable on the phenotype
4. even to improve the selection efficiency for traits relatively easy to score (i.e. identification of the homozygous andheterozygous plants)

Question 3

What are the advantages of markers?

Answer 3

1. Illimitate marker availability
2. Represent directly the genotype
3. No influences of the environment or development stage of a plant
4. Selection can be directly made on the plant genotype for the marker linked to the disired trait

Question 4

How close must be a marker to the trait for which we would like to make the selection?

Answer 4

1. A good marker in order to be used for MAS must be tightly linked to the trait (preferably at a distance < 1 cM)
2. The higher the distance, the higher the number of plants that can be erroneously selected or eliminated because of recombination between gene and marker

Question 5

What are the main advantages of using molecular markers for MAS?

Answer 5

1. can be used in the absence of the trait expression (traits shown after the end of the juvenility period or traits induced by stresses)
2. ‘positive’ selection for the trait of interest
3. Identification of homozygous plants
4. ‘negative’ selection for the trait of interest
5. When trait expression is ‘masked’ by other gene effects

Question 6

What are disadvantages of MAS?

Answer 6

1. High initial costs for the identification of tightly linked markers
2. Higher costs if the traits to be selected are polygenic
3. Optimisation of marker identification techniques simple and easy to score →transform marker difficult to score as. RAPD & AFLP in SCAR or CAPS markers
4. Improve scoring technique in order to analyse a lot of samples in short times (‘high throughput’) and with low cost for single analysis