Differences in phenotypic expression between brn1-R brn2-R and brn1-Nelson brn2-Nelson mutant kernels are attributable to the brn1 locus --Philip Stinard Phenotypic variation in mutant kernel expression exists among the various alleles of the brown aleurone duplicate factor pair brn1 brn2 (Stinard, PS, Maydica 39:273-278, 1994). In the B73 inbred background, homozygous mutant kernels of the reference alleles, brn1-R brn2-R are normal sized with normal embryos and dark brown pigmentation of the aleurone. Homozygous mutant kernels of Oliver Nelson�s alleles, brn1-Nelson brn2-Nelson, are small, with defective embryos and lighter brown aleurone pigmentation (Figure 1). In order to better understand the nature of the phenotypic differences, we decided to isolate the four alleles (brn1-R, brn1-Nelson, brn2-R, and brn2-Nelson) and combine them in all possible pairs in order to determine the effect of allelic differences on mutant expression.

All four mutant alleles were isolated in the B73 background (six backcross generations) according to the procedure previously outlined by the author (Stinard, P, MNL 74:70, 2000). The nonmutant alleles in these lines are provided by the inbred B73, which is Brn1 Brn2. Intercrosses were made between the complementary reference and Nelson alleles to produce the combinations brn1-R Brn1 brn2-Nelson Brn2 and brn1-Nelson Brn1 brn2-R Brn2, and the resulting F1�s were self-pollinated to produce segregating F2 ears.

Mutant and nonmutant kernels from these ears are shown in comparison with kernels from segregating ears of brn1-R Brn1 brn2-R Brn2 and brn1-Nelson Brn1 brn2-Nelson Brn2 in Figure 1. Mutant kernels of the combination brn1-R brn2-Nelson share the normal kernel and embryo size, and dark brown aleurone pigmentation with the reference allele combination brn1-R brn2-R. On the other hand, mutant kernels of the combination brn1-Nelson brn2-R share the small kernel size and defective embryo, and light brown aleurone pigmentation with the Nelson allele combination brn1-Nelson brn2-Nelson. Thus, it is allelic differences at the brn1 locus that determine the differences in mutant kernel expression between the reference and Nelson alleles.

It should be noted that at least a few inbred and hybrid lines are homozygous mutant at the brn2 locus. Inbred B73 is the only exception found to date (Stinard, P, MNL 74:70, 2000). Although the brn2-R and brn2-Nelson alleles have been maintained separately from each other since they were received by the author, it is conceivable that they could share a common ancestry, as could all lines that "naturally" carry a mutant allele at the brn2 locus. Only a molecular analysis will resolve this issue.

Figure 1. Segregating mutant (upper row) and nonmutant (lower row) kernels from F2 ears of brn1-R brn2-R (upper left), brn1-Nelson brn2-Nelson (upper right), brn1-R brn2-Nelson (lower left), and brn1-Nelson brn2-R (lower right). All mutant alleles are in inbred B73 background (six backcross generations).
 
 

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