--Michael G. Muszynski and Peter A. Peterson
The c2-m884259Y allele was thought to carry a receptor element
that responded to an independent regulator (MNL 64:9). Results from recent
crosses have led to a modification of this hypothesis. Spotted kernels
of genotype c2-m/c2, Wx/wx-m8, were crossed by an a-o wx line
and a C sh bz wx line to generate F1's for system tests. The expected
genotypes from those crosses are: 1/4 C2/c2 Wx/wx, 1/4 C2/c2
wx-m8/wx, 1/4 C2/c2-m Wx/wx, and 1/4 C2/c2-m wx-m8/wx.
Kernels on the progeny ears were colored due to the C2 from the
tester lines. Because of the c2 dosage effect, two classes of colored
kernels could be distinguished: kernels which were only colored (C2/c2)
and kernels which were colored plus spots (C2/c2-m). Because the
tester lines were both wx and the reporter allele wx-m8 was
segregating, half of the kernels could be scored for the presence of
En by assaying for wx mutability. If En is unrelated
to the c2-m mutability, there will be no correlation between spots
and wx mutability. If one En is segregating and controls
the c2-m mutability, the following ratios would be expected:
| C2/c2 | C2/c2 | C2/c2-m | C2/c2-m | |
| Wx/wx | wx-m8/wx | Wx/wx | wx-m8/wx | |
| +En | 1/8 Cl Wx | 1/8 Cl wx-m | 1/8 Cl+sp Wx | 1/8 Cl+sp wx-m |
| -En | 1/8 Cl Wx | 1/8 Cl wx | 1/8 Cl Wx | 1/8 Cl wx |
The following ratios were observed:
| Cl Wx | Cl wx | Cl wx-m | Cl+sp Wx | Cl+sp wx | Cl+sp wx-m |
| 3/8 | 3/8 | 0 | 1/8 | 0 | 1/8 |
All Cl+sp kernels are also wx mutable; there are no Cl+sp wx kernels, thus En is implicated in controlling mutability.
The En may be independent of the c2-m allele or linked. If an independently segregating En is controlling mutability, then a Cl wx-m class should also be segregating (C2/c2 wx-m8/wx En/-). Clearly this class is missing and the amount expected for this class has been incorporated into the Cl wx class. If En was closely linked to the c2-m, then the expected ratio for the Cl+sp wx-m class would be closer to 1/4. Any crossovers would be classified as Cl wx-m and they are not present. Neither of these hypotheses fits the observed data.
The data do indicate that an independent factor controls spotting at
c2-m and triggers wx-m8, implicating En. But this
factor does not elicit a response from wx-m8 when it is separate
from the c2-m allele. Therefore, this factor only has En
function when it is together with the c2-m allele. Since neither
the factor nor the c2-m provide an En function by themselves,
they must be classified as non-autonomous elements. Because En is
implicated, the element at c2-m884259Y and the independent factor
are defined as I(dSpm) elements. To date, all I's have been
found to be deletion derivatives of En that have lost sequences
which encode En specific functions. These two I elements
may possess complementary deletions that allow them to function together
as an intact En. Frey et al. (EMBO J. 9:4037, 1990) have demonstrated
that two components (TNPA and TNPD) are needed for complete En function
in transgenic tobacco. This mutant may be an "in maize" demonstration of
this aspect of En function. Also either the c2-m allele or
the factor needs to be present in at least two doses, since neither spots
nor wx mutability is male transmitted. Tests of the dosage effect
and the modified hypothesis are in progress.
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