Unstable expression of a soybean gene during seed coat development.

The R gene of soybean is involved in anthocyanin synthesis in the seed coat, and its r-m allele conditions a variegated distribution of black spots and/or concentric rings of pigment superimposed on an otherwise brown seed coat. We describe an unusual feature of r-m that causes expression at the R locus to switch between active and inactive phases both somatically and germinally. Non-heritable somatic changes of the allele produce single plants containing mixtures of seed with different coat colors (black+striped or brown+striped). Heritable changes of the r-m allele are manifested in progeny plants which produce all black seed or all brown seed. Surprisingly, subsequent generations from revertant sublines show continued instability of the allele such that brown revertants (r*/r*) or homozygous black seed revertants (R*/R*) can give rise to striped or striped+black-seeded plants. Thus, the revertants produced by the r-m allele are not stable but interconvert between all three forms (R*, r*, and r-m) at detectable frequencies. Mutability of the r-m allele in a different genetic background has also been found after inter-crossing various soybean genotypes.