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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