Inheritance of a mutable phenotype that is activated in alfalfa tissue culture

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Abstract
Mutable alleles, typically regulated by transposable elements, have been identified in a number of plant species. Tissue culture induced genomic shocks may activate such elements. A mutable recessive condition arose in tissue culture of alfalfa and was highly unstable in subsequent cycles of culture. The mutable allele, designated c2-m4, is allelic to the C2 locus, a basic color factor locus involved in anthocyanin synthesis. Current research has focused on the inheritance and instability of c2-m4 in new genetic backgrounds as well as on dosage effects of the allele. We have confirmed a previous report that c2-m4 reverts to function at much higher frequencies in vitro (reversion frequency ca. 0.23) than in planta (reversion frequency < 0.001). Over sexual generations c2-m4 continues to be unstable. Transmission of the mutable phenotype to selfed, testcross, and F2 populations demonstrated monofactorial inheritance patterns (P > 0.25). In populations expected to have some plants carrying two or more c2-m4 alleles, individuals were found that reverted in vitro at significantly higher frequencies than their parent (0.67 ± 0.04 and 0.44 ± 0.08 versus 0.20 ± 0.09). In planta reversion also increased with increasing c2-m4 dosage. Preliminary evidence indicates that as c2-m4 dosage increases, each allele maintains its original capacity to revert.Key words: somaclonal variation, transposable element, tissue culture, mutable allele.

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