The transposable element in R-mb:cc - Composition and test for homology with some two-element systems
-- B.M. Prasanna and K.R. Sarkar

The newly discovered R-mb:cc, a mutable allele derived from the R-marbled (R-mb) genetic stock in our laboratory (MNL 67:87-88, 1993; MNL 68:63-64, 1994) is further studied to: (i) genetically analyze the nature of the element system operating at this allele; and (ii) test the homology, if any, of the element in R-mb:cc with the known element systems in maize.

(i) Composition of the element system in R-mb:cc: The R-mb:cc allele frequently gives rise to completely colorless somatic revertants as well as kernels with very light striping (1-2 concentric colored stripes/rings). Kernels of the former category gave rise to some germinal revertants with completely colorless expression; a majority of the 'colorless' category as well as the 'very light striped R-mb:cc', on progeny testing, yield a high percentage of colorless kernels with 1-2% very light striped kernels. This is in stark contrast to the segregation profiles of medium/heavily striped kernel categories, where the frequency of occurrence of colorless/very light striped kernels is substantially low (1-2%). As per the procedure to verify the composition of the element system (one/two-element system), we crossed various categories of R-mb:cc with ACR lines and the F1 progeny (colored/mottled) were selfed. If the transposable element system in R-mb:cc is a two-element system, the F2 progeny should segregate in a 12:3:1 (colored: mb:cc: colorless) ratio; segregation in a 3:1 (colored: mb:cc) ratio would indicate a one-element system. Results from this study are presented in Table 1.

Table 1. Segregation profiles in the F2 progeny obtained by selfing ACR/R-mb:cc plants.
 
     
F2 progeny segregation
No. of ears fitting
Culture
Origin*
No. Ears
Cl
mb:cc
cl
Total
3:1
12:3:1
6903
ACR x cc1
8
1595
630
266
2491
4
1
6904
ACR x cc2
4
599
334
156
1089
-
-
6905
ACR x cc2
2
269
112
99
480
-
-
6906
ACR x cc2
6
1489
437
84
2010
6
3
 
Subtotal
12
2357
883
339
3579
6
3
                 
6907
ACR x cc3
1
158
42
17
217
1
1
6908
ACR x cc3
5
907
381
117
1405
2
2
6909
ACR x cc3
12
2090
966
991
4047
-
-
6910
ACR x cc4
7
1231
445
226
1902
1
1
6911
ACR x cc4
2
383
134
71
588
1
-
 
Subtotal
27
4769
1968
1422
8159
5
4
                 
6912
ACR x cc6
3
478
222
27
727
1
1
6913
ACR x cc6
5
812
561
169
1542
-
-
 
Subtotal
8
1290
783
186
2269
1
1
                 
Total
55
10011
4264
2223
16498
16
9
 
                 
*cc1 = near colorless/very light striped R-mb:cc; cc2 = light striped; cc3 and cc4 = medium striped; cc6 = heavy striped

It can be observed that the F2 progeny defy either 3:1 or 12:3:1 segregations. For computing the 3:1 ratio, the mb:cc and colorless categories were pooled, on the basis that the colorless kernel category originated due to the transposable element activity in R-mb:cc. Also, wherever the segregation pattern fitted a 12:3:1 ratio, the 3:1 segregation for colored:mb:cc is naturally followed, but not vice versa, as is evident from Table 1. Significant also was the occurrence of ears where the segregation of colored:mb:cc fitted a ratio of 1:1. One plausible reason for the above observations may be the presence of the regulator component of the R-mb:cc element system in the ACR lines (6186, 6187 and 6188) used in crossing with R-mb:cc. Additional tests are being carried out to verify the composition of the element system in R-mb:cc.

(ii) Test for homology with other transposable element systems: R-mb:cc was also tested for its ability to induce mutability of heterologous responding alleles of some two-element transposon systems, the MERL series of which were generously provided by Ellen Dempsey, Peter Peterson, F. Salamini and G.F. Sprague. The crossing scheme adopted was as per the standard procedures. Plants homozygous for R-mb:cc with an active element were crossed by plants containing the appropriate responding allele (MERL) of the other element system but lacking the regulator element of that system. The resultant F1s were selfed or testcrossed and the F2/BC progeny were screened to detect mutability of the MERL allele. The results obtained are presented in the following table:
 
En Uq Mrh Cy Bg
R-mc:cc/a-m(r) (-)
R-mb:cc/a-ruq (+)
R-mb:cc/c-ruq (+)
R-mb:cc/a-mrh (-)
R-mb:cc/bz-rcy (-)
R-mb:cc/o2-m(r) (-)

Out of the five transposable element systems tested, only Uq (Ubiquitous) was found in the R-mb:cc genetic stock. Absence of En (Enhancer), Mrh (Mutator of Rhoades), Cy (Cycler) and Bg (Bergamo) clearly rules out the possibility of the regulatory elements of these systems in triggering the mutability pattern of R-mb:cc. On the other hand, presence of Uq does not necessarily provide evidence for its involvement. In the case of Uq, mutability characteristic of Uq indicated by + was found in all the 8 plants tested with the a-ruq allele, whereas mutability for c-ruq was found in 3/5 plants for which testing was carried out. The presence of Uq in only a few of the lines that carry the R-mb:cc phenotype suggests that Uq may not be an integral component of the R-mb:cc allele. Also, the presence of Uq in a 'near colorless' line from R-mb:cc suggests that Uq cannot substitute for the transposable element system in R-mb:cc. Additional tests are now being carried out to ascertain if there is any interaction of R-mb:cc with the components of r-cu/Fcu and Ac/Ds transposable element systems. 


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