Genetic characterization of R-mb:cc, a mutable derivative from R-mb
--V. Niral, B. M. Prasanna and K. R. Sarkar

R-mb:cc, a new variant from R-mb, was reported by Prasanna and Sarkar (MNL 67:87-88, 1993). The phenotype characterized by R-mb:cc on the aleurone has some exceptional features. Colored sectors on colorless background appear in the form of concentric rings or stripes on either side of the scutellum. The sectors may extend onto the crown and the abgerminal side (Fig. 1). However, the flow region on the abgerminal side might show irregular spots as in R-mb. We tried to further characterize this derivative from R-mb through genetic analysis.

Wide variation could be observed in the degree of pigmentation of R-mb:cc kernels, evidenced by varying number of stripes on the kernels from the same ear. To test if this variation has any genetic basis, kernels were categorised into six different scores based on a 'striping scale' (cc1 with only one colored stripe to cc6 with almost full coloration on the aleurone except for one or two colorless sectors). Although each of these scores segregate for different scores on selfing, distinct segregation profiles could be observed. By analysing the mean visual scores of ears belonging to the different classes through Student's t-test, we could categorise the striping pattern of R-mb:cc into three classes: very light striping (cc1), light striping (cc2) and medium striping (cc3, cc4 and cc5). The very heavily pigmented class (cc6) could not be statistically analysed due to small sample size.

Unlike the 'sister' pattern alleles R-nj and R-st, the R-mb allele shows a drastic reduction in both penetrance and expressivity when transmitted through the pollen parent in a single dose (Weyers, Genetics 47:1061-1067, 1961; Prasanna and Sarkar, MNL 67:85-86, 1993). To determine if R-mb:cc displays a genetic behaviour similar to that of the parental allele R-mb, we carried out reciprocal crosses between homozygous R-mb:cc lines and the recessive tester lines carrying r-g, besides selfing R-mb:cc lines. Analysis of the mean visual scores in R-mb:cc R-mb:cc R-mb:cc, R-mb:cc R-mb:cc r-g and R-mb:cc r-g r-g by t-test showed statistically significant differences in striping potential of the three classes (Fig. 2). In a single dose the penetrance of R-mb:cc was drastically reduced, with only 5-6% of the kernels showing the mb:cc phenotype as compared to 61-62% in two doses and 88-89% in three doses. We could also observe that such a significant effect of dosage on penetrance and expressivity is restricted only to the R-mb and R-mb:cc among the pattern alleles at the R locus.

Reversions to the self-colored form in a high frequency signify the influence of a transposable element on a specific pattern. The R-mb:cc ears showed frequent reversions to the fully colored form (Fig. 1). Progeny testing showed that the self-colored kernels were either somatic or germinal revertants. The germinal reversion rate from R-mb:cc to R-scm was 17.97 x 10-4. However, in the case of test crosses of R-mb:cc/g r-g with recessive tester g r-g the germinal reversion rate was found to be higher (26.98 x 10-4). In addition, discordant endosperm-embryo phenotypes were also recovered from R-mb:cc. Kernels with mb:cc endosperm and colored scutellum from test crosses were observed at a frequency of 18.40 x 10-4 while the frequency of colorless endosperm with colored scutellum was 3.07 x 10-4. Both classes of discordant endosperm-embryo phenotypes were found to be germinal revertants on progeny testing by selfing or crossing onto the recessive tester.

To ascertain whether the element system at R-mb:cc is influenced by other pattern alleles at the R locus, we made reciprocal crosses of R-mb:cc with R-nj, R-st and the parental allele R-mb. It was found that there are no dominance-recessive relationships between R-mb:cc, R-nj and R-st, evident by occurrence of a codominant pattern. However, when R-mb:cc as a female was crossed with R-mb, mb:cc phenotype was predominant and the converse was true when R-mb was used as a female parent. The observations indicate that the R-mb:cc pattern is not influenced by the parental allele R-mb in a R-mb:cc/R-mb heterozygote, and the differences in phenotypic segregation in the reciprocal crosses can be attributed to the single dose effect.

A plausible mechanism for the origin of R-mb:cc from R-mb can be a 'change in state' of the element in the parental allele where the element at R-mb:cc might be responding to different developmental signals or host factors. Characterization at the molecular level can help us dissect out the differences between R-mb and R-mb:cc. This can also provide significant clues in understanding the genetic and epigenetic phenomena underlying the formation of this symmetric and germinally transmissible pattern.

Figure 1. Ear showing the characteristic striping pattern of R-mb:cc. The two phenotypic extremes, a self-colored revertant (R-scm) and a colorless kernel, can also be seen.

Figure 2. Segregation profiles of various doses of R-mb:cc.

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