4. Somaclonal variability and early selection in vitro

Question 1

Initialy define somaclonal variation and genetic variability?

Answer 1

Question 2

What are potential problems associated with Somaclonal variability?

Answer 2

Somaclonal variability has to be inherited by gamic reproduction (at least for some authors)

1. In fruit trees is difficult to early detect gamic inheritance →long juvenile phase
2. problem for commercial labs→max uniformity of propagated material

• But also great interest for breeding→ potential source of new variability (selection of improved genotypes)

Question 3

What are Epigenetic variations?

Answer 3

1. reversible variations
2. can not be transmitted to the progeny
3. temporary changes of gene expression induced by the action of external stimulus (as in vitro culture)
4. regression after a few ex vitro seasons or after agamic propagation

Question 4

What are examples of epigenetic variations?

Answer 4

In in vitro conditions

• different growth speed
• callus diversity (colour & consistency)
• loss of hormons and vitamines needs in callus culture

In field conditions

• thorny
• plant habitus (rapid flower induction and increased shoot production)
• increased ex vitro vigour (juvenility and/or virus removal)
• dwarfing (hormone “carryover”; normal plants after 1-2 seasons)

Question 5

What are possible causes of the epigenetic variations?

Answer 5

1. Demethylation events
2. Methylation events

Question 6

What are Genetic variations?

Answer 6

1. DNA Mutations at nuclear or extranuclear (plastids or mithocondria) level
2. Genomic mutation (ploidy level, aneuploids)
3. Chromosomic mutations (chromosome number variation, deletions, duplications, inversions and translocations of chromosome parts)
4. Genic mutations (changes, deletions, insertions)

Question 7

What is the origin of somaclonal variability?

Answer 7

1. Pre-existing variability in the original explant

• frequent in fruit tree species (mainly in very old cvs)
• accumulation of mutations after agamic propagation
• fruit tree plants can be complex chimera
• mutated traits cannot be easily visible in the phenotype

Question 8

What are the Limitations of Somaclonal Variations?

Answer 8

1. Most of the somaclonal variations may not be useful
2. The variations occur in an unpredictable and uncontrolled manner.
3. Sometimes the genetic traits obtained by somaclonal variations are not stable and heritable.
4. Somaclonal variations are cultivar-dependent which is frequently a time consuming process.
5. Somaclones can be produced only in those species which regenerate to complete plants.

Question 9

What are the advantages of in vitro selection?

Answer 9

1. Reduction of selection spaces, times and costs
2. High selection pressure (very wide populations)
3. in vitro environmental conditions→homogeneous (in respect to the field environment)

Question 10

What are possible types of explants?

Answer 10

1. explants in regeneration or propagation phase
2. callus (external cells are in direct contact with the selection agent → higher selection pressure)
3. cell culture (but Wide populations)
4. protoplasts (but More difficult regeneration)

Question 11

What are Pre-requisite for in vitro selection?

Answer 11

1. The selection trait must have a cellular component Examples: Selection for drought resistance is possible
2. Component that can be selected at a cellular level Examples: increased osmotic resistance
3. Narrow correlation between in vitro and in vivo traits Examples: in vitro growth and in vivo vigour
4. Trait to select → simple genetic control
5. Ability to regenerate from callus cells or protoplast →main limit for fruit tree species

Question 12

Please explain the model for early in vitro selection

Answer 12

Positive Selection

1. explant development on selective substrate
2. Elimination of highly susceptible material
3. Survival and selection of resistant lines

Main selective agents:

1. toxins or culture filtrates - heavy metals
2. salinity
3. pH

Question 13

What are Positive selection inidcations?

Answer 13

1. Resistance to disease (different examples in fruit tree species)
2. Resistance to extreme temperatures
3. Resistance to salinity (Citrus and Colt cherry)
4. Resistance to toxic ions
5. Resistance to anomalous pH (resistance to limestone in actinidia and quince BA29)
6. Resistance to herbicides (i.e. glyphosate)

Question 14

There is also a Model of early in vitro selection with Negative selection but it is very Difficult to realize