Download Somaclonal Variation

Somaclonal Variation
• Tissue-culture cycle involves:
– dedifferentiation in culture
– proliferation of cells (implies sev. cell generations
removed from original differentiated cell)
– subsequent regeneration to plants
– no selection applied (although screening may be
done, e.g. for pathogen resistance
Somaclonal Variation
• Negative consequences (of somaclonal var.)
– introduction of deleterious genes
• dominant alleles – albinos, pollen sterility
• recessive alleles – off-types that show up in
progeny of seed-propagated cultivars
– carry-over effect from tissue culture
• common with asexually propagated plants
• symptom – excessive and multiple branching of
regenerated plants (lack of establishment of apical
dominance)
Somaclonal Variation
• Applications to crop improvement
– improvement of existing clonal (asexually propagated
cultivars)
• sugarcane – selections for higher yield & disease
resistance
• potatoes – yield & disease resistance
• improved geraniums (esp. scented varieties)
• woody ornamentals (e.g., Paulownia – selection for
leaf variegation
Somaclonal Variation
• Applications to crop improvement
– overcoming barriers to genetic exchange
• embryo culture of interspecific hybrids (Lolium,
Hordeum)
• selection for stable mitosis, fertility in a hybrid; e.g.,
intergeneric hybrids of barley x canada wild rye
– seed cultivar improvement (e.g., Fusarium resistance
in celery, white-flowered Torenia, tomato cultivars
with higher soluble solids, better "flesh" color)
Somaclonal Variation
• Types of somaclonal variants
– genetic changes
• point mutations (e.g., Adh mutants in wheat)
• cytoplasmic (maternal inheritance)
• gene amplification (e.g., incr. gene copy no.)
• activation of transposable element
Somaclonal Variation
• Types of somaclonal variants
– genetic changes
• cytogenetic (changes to genome structure)
» aneuploidy – gain or loss of 1 or more
chromosomes
» polyploidy – gain or loss of an entire
genome
» translocation – arms of chromosomes
switched
» inversion – piece of chromosome inverted
Somaclonal Variation
• Types of somaclonal variants
– epigenetic
• change in phenotype that isn't stable during sexual
propagation
• may or may not be stable during asexual
propagation
• usu. undesirable in a breeding program, not
always undesirable in propagation
• habituation (most studied epigenetic change)
Somaclonal Variation
• Types of somaclonal variants
– epigenetic
• habituation (most studied epigenetic change)
– defn – loss of exogenous requirement for a
growth factor ( usu. a PGR); e.g., auxin,
cytokinin habituation
– detection – callus may lose requirement for a
PGR in the process of several transfers to fresh
medium
Somaclonal Variation
• Types of somaclonal variants
– epigenetic
• characteristics
– often occur gradually
– are regularly reversible (esp. in regenerated
plants)
– are not seed-transmitted
Somaclonal Variation
• Possible causes
– the dedifferentiation – redifferentiation process
• axillary shoot proliferation vs. organogenesis &
embryogenesis
• hypothesis of D'Amato
– somaclonal variants are rare in
micropropagated plants (when multiplication is
by axillary branching of shoot tips/buds)
– more common during shoot organogenesis &
somatic embryogenesis (esp. w/a callus phase)
Somaclonal Variation
• Possible causes
– the "culture environment"
• the hypothesis – that tissue culture is inherently
stressful to cultured plant cells
• environmental stress is known to cause:
– DNA methylation – the methylation of cytosine
is known to cause gene inactivation; this may
occur during the redifferentiation process
– gene amplification – can result in incr. gene
expression
Somaclonal Variation
• Possible causes
– the "culture environment"
• environmental stress is known to cause:
– transpositional changes
– inadequate control of the cell cycle (errors in
microtubule synthesis, spindle formation)
– importance of PGRs
» scant evidence of direct mutagenic action
» more evidence for transient modifications of
phenotype (e.g., dwarfing)