R-marbledas a transposable element system
--B. M. Prasanna and K. R. Sarkar

R-marbled (R-mb), a 'pattern' allele at the R locus, gives a characteristic aleurone variegation with irregularly shaped, coarse colored sectors on a colorless background. This allele has not been studied extensively so far. We have made an attempt to understand the genetic mechanisms behind the pattern formation and variability in R-mb and how this allele differs from its sister alleles, R-nj and R-st.

Wide variation was observed in the extent of anthocyanin pigmentation of R-mb kernels in the same stock background. Elaborate progeny testing of different marbled scores (score 1, only one colored spot, to score 6, almost fully colored) revealed that variations in R-mb expression cannot be solely attributed to developmental effects. On the basis of distinct segregation profiles, R-mb could be categorized into four classes: very light spotting, light spotting, medium spotting and heavy spotting. Intensity of pigmentation also varied in some marbled kernels (dark, pale etc.) and these differences persisted upon selfing.

At least two doses of R-mb are necessary for the characteristic spotted phenotype. The marbled pattern expressed in only 1.5 percent of the R-mb r r kernels, with a reduced intensity of pigmentation. Analysis of mean visual scores of R-mb R-mb R-mb, R-mb R-mb r and R-mb r r endosperm genotypes by Student's t-test showed significant differences in aleurone pigmenting potential. Such a drastic dosage effect appears to be a characteristic feature of only R-mb among the pattern alleles, since R-nj (Kumar and Sarkar, Indian J. Exp. Biol. 24:270-273, 1986) and R-st (Ashman, Genetics 45:19-34, 1960) were found to express even in a single dose. It is to be confirmed whether this effect is dependent only on the transmission of R-mb by the pollen parent.

We also found that R-mb reverts at a high frequency to the fully colored form in both somatic and germinal tissue. Different classes of mutants with discordant endosperm-embryo phenotypes were obtained. Only germinal reversions (R-mb-->R-sc) were found to be stably transmissible. Reversion of R-mb to R-sc appears to occur at a slightly higher frequency than that of R-st to R-sc (18.54 x 10-4 and 17.0 x 10-4, respectively).

There were no dominance-recessive relationships observed among the pattern alleles. R-mb, in crosses with R-nj or R-st, gave rise to progeny with expression of both the patterns on the same kernel (nj+mb or mb+st). In the reciprocal crosses, the marbled phenotype expressed only in 1.5 to 1.7 percent of the cases, confirming the earlier observation on the single dose effect of R-mb on aleurone pigmentation. An interesting situation arose in the crosses of R-mb and R-nj; in F2, there is a preponderance of the Navajo phenotype. The possibility of contamination is excluded since kernels borne on heterozygous R-mb R-nj ears showing both Navajo and marbled kernels were planted and selfed. The influence of a maternally accumulated signal that can 'preset' the expression of R-nj might account for the preponderance of the Navajo phenotype. We observed that the interaction of R-mb with R-st was not identical to that of R-mb with R-nj. Occurrence of R-mb/R-st kernels with discordant endosperm-embryo phenotypes strongly indicated somatic and germinal instability at R-mb and R-st.

For an explanation of the phenomena concerning spotting in R-mb, it would be necessary to postulate the influence of a tranposable genetic element, on the basis of the following observations: 1) mutations within R-mb might result in the appearance of coarse, colored spots in aleurone as well as changes in color intensity and these changes can occur independently; 2) both somatic and/or germinal reversions to wildtype can occur at a very high rate, and 3) only germinal reversions from R-mb to R-sc were found to be transmissible. The phenomenon of excision of the controlling element at R-mb might explain the size, shape and location of the colored spots in aleurone. Chang and Neuffer (J. Hered. 78:163-170, 1987) reported earlier that R-mb behaves like R-st except that the timing is much earlier. Observations in our study showed that although R-mb shares some similarities with R-st, the distinct genetic behavior of this allele might be attributed to the influence of a specific controlling element. Our original R-mb genetic stock did not carry Ac, Spm or Mu transposable element families. Further work is being carried out to ascertain the nature of the controlling element influencing the marbled pattern. 

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