Cold Spring Harbor Laboratory
Dosage of Mu

--V. Sundaresan

It has been observed that the mutability of Mu-induced mutant alleles can exhibit considerable variability from one generation to the next, and even between kernels on the same ear (for example, Robertson, MNL 60:9-10, 1986). As a result relationships of dosage to mutability are not easily measured in the case of Mu. Bennetzen (in Plant Genetics, UCLA Symposia 35:343-354, 1985) has demonstrated that mutability of a Mu insertion in bz1 does not correlate with the copy number of Mu1 elements in the genome. We have asked a different question: Is the mutability of a Mu-induced allele correlated with the dosage of that allele? This question was provoked by the following observation: In a cross of a bz-Mum9/bz female to a bz-Mum9/bz-Mum9 male, the resulting ear was segregating approximately 1:1 medium mutable (300-400 spots/kernel) and highly mutable (>1000 spots/kernel) kernels. The sizes of the spots were the same in all kernels, i.e., small, covering 1-20 cells. In our experience the bz-Mum9 allele (a Mu insertion at bz1 isolated by D. Robertson) is relatively uniform in its mutability within a single ear and does not exhibit the extreme variability from kernel to kernel that is seen with some other Mu alleles (e.g., a1-Mum2). Therefore, the above segregation of two classes of mutable kernels could reflect the difference between 1 dose and 3 doses of bz-Mum9 in the aleurone, depending on whether the kernel inherited bz or bz-Mum9 from the female. If so, the medium mutable kernels should be genotypically bz-Mum9/bz and the highly mutable kernels should be bz-Mum9/bz-Mum9. A few kernels from each class were planted and crossed to bz testers to determine their genotype. As shown below, the results were consistent
with the above interpretation.
Kernel Phenotype Number Tested bz-Mum9/bz-Mum9 bz/bz-Mum9
High mutable 5 5 0
Medium mutable 4 1 3

Although the number of plants examined was small, and there was one exception (i.e. a medium-mutable kernel that was bz-Mum9/bz-Mum9), it does appear that bz-Mum9 exhibits an additive dosage effect. This observation might be exploited in identifying the genotypes of progeny from a cross using their kernel phenotype. However, this rule breaks down when kernels from different crosses are compared (for example, we have observed many cases where the kernels from a bz-Mum9 self exhibited lower mutability than the outcross progeny of the same plant).

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