Location of a putative regulator
of Mutator-induced mutability of a1 on chromosome 2
--Donald S. Robertson and Philip S.
Stinard
In a soon to be published paper in Developmental
Genetics, we present extensive evidence that in some stocks of a1-Mum2
and a1-Mum3, a regulator element of the somatic mutability of these
mutable alleles may be present. If a regulator is involved, it should be
possible to map it. In an endeavor to accomplish this, mutable kernels
were planted from ears of the cross a1-Mum/a1 sh2
x a1 sh2, in which half of the plump kernels were mutable
and half were stable. If a single regulator gene is present, as the 1:1
ratio would suggest, the mutable seeds will be heterozygous for the a1-Mum
allele and the regulator. These plants were crossed to an extensive series
of waxy (wx) marked chromosome nine translocations. The F1 plants
from these crosses were outcrossed as males to an a1 a1 wx
wx tester stock. If linkage is present between wx and a regulator
of mutability there should be a surplus of mutable kernels in the starchy
(Wx) class and a deficiency of mutable kernels in the wx
class. Table 1 shows the results of crosses of a1-Mum3 with wx
T2-9d. Cross number 1 was made in the '88-'89 winter nursery. The results
were suggestive of linkage but the classes are not balanced (the imbalance
is significant at the 1% level). The imbalance is due to an excess of stable
kernels in both the Wx and wx classes. Methylation of the
Mu element inserted at a locus is known to result in loss of mutability.
If methylation of the Mu element was occurring in some kernels of
this cross, the observed excess of stable kernels would be expected. (It
also is possible that methylation of the putative regulator could result
in a loss of its function and as a consequence somatic mutability would
cease.) These results were encouraging enough to make additional testcrosses
in the 1989 summer nursery (the second and third crosses, Table 1). The
second cross has balanced parental classes but a large excess of stables
in the Wx crossover class. This is not the only imbalance in this
cross since the combined Wx classes make up 61.94 percent of the
progeny (these imbalances are significant at the 1% level.). It has been
frequently observed that in heterozygous plants the wx allele shows
reduced transmission through the male. A combination of some Mu
element modification and reduced wx transmission could explain the
results of the second cross.
Table 1. Results of testcrosses of Wx
a1-Mum3 R*/wx T2-9d-* plants with a1 a1 wx
wx - - .
Cross No. |
Parental
|
Crossover
|
|
|
|
Wx mutable
|
wx stable
|
Wx stable
|
wx mutable
|
Totals
|
% c.o.
|
|
|
|
|
|
|
|
#1 |
|
|
|
|
|
|
88-89-9522-6 |
32
|
56
|
28
|
4
|
|
|
8522-7 |
|
|
|
|
|
26.67
|
Totals |
88
|
|
32
|
120
|
|
|
|
|
|
|
|
|
#2 |
|
|
|
|
|
|
89-6253-2 |
90
|
92
|
63
|
2
|
|
|
5253-3 |
|
|
|
|
|
26.32
|
Totals |
182
|
|
65
|
|
247 |
|
|
|
|
|
|
|
#3 |
|
|
|
|
|
|
89-6255-6 |
78
|
83
|
8
|
16
|
|
|
5254-4 |
|
|
|
|
|
14.91
|
Totals |
161
|
|
24
|
|
185 |
*R = Symbol for putative regulator,
- = absence of regulator.
Cross number three has statistically
balanced classes and gives a value of 14.91 percent recombination between
wx and the putative regulator. Because the breakpoint of T2-9d in
chromosome two is 2L.83, and because the regulator did not show linkage
with other wx translocations, these data would suggest that in this
a1-Mum2 stock there is one regulator (autonomous element?) in the
long arm of chromosome two.
Please Note: Notes submitted to the Maize Genetics Cooperation
Newsletter may be cited only with consent of the authors
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