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.


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