Activation of a Mutable Allele in Alfalfa Tissue Culture
A white-flowered mutant (WFM) was regenerated from tissue culture of a purple-flowered alfalfa donor carrying one dominant allele (C2) for anthocyanin synthesis. Chromosome counts confirmed that WFM was not due to loss of the chromosome carrying C2. The dominant allele had mutated. This mutation has occurred only once among about 2,500 plants regenerated from the original donor. When WFM was recultured, many regenerated plants (about 20%) were purple-flowered. Genetic transmission of the unstable recessive through the zygote (single cell) to progeny that reverted proved that revertants of WFM were not due to culturing chimeral tissue. Hence, the dominant allele (C2) in the donor mutated to an unstable recessive (mutable), c2-m4, which is carried by WFM, and transmitted to its progeny. The c2-m4 allele has now been transmitted through three sexual generations, and in single dosage behaves as it did in WFM.
Allele c2-m4 is unstable in vitro and frequently reverts to the functional state. Reversion occurs early in culture and may be the result of a genome shock associated with callus formation. Callus growth studies indicated that the high frequency of revertant regenerated plants was not due to more rapid growth of revertant callus. Nonrevertant regenerated plants are alike phenotypically, but differ in reversion frequency upon reculture. Allele c2-m4 is relatively stable in planta, and revertant sectors in flowers and revertant gametes are fewer than 1/1,000. The data are discussed in terms of a transposable element which is especially active in vitro.
KeywordsTransposable Element Mutable Allele Somaclonal Variation Genetic Transmission Test Cross
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