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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)