Dosage of MuCOLD SPRING HABOR, NEW YORK
Cold Spring Harbor Laboratory
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|
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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