Maize Genetics Cooperation Newsletter vol 84 2010
Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of authors.
Isolation and characterization of a dominant inhibitor of Bn1.
For several years, the Maize Genetics Stock Center grew a hybrid between Mo20W and 4Co63 as a p1‑ww standard for purposes of re‑extracting pericarp and cob color traits. Both inbreds have white endosperm due to the presence of y1 and Wc1. However, we were surprised to observe segregation for a low frequency of dark pale yellow kernels on selfed ears of this standard. The pale yellow kernel trait was tested against Bn1 (dominant brown aleurone color) and found to be allelic. Tests of the the two inbred lines revealed that 4Co63 is bn1 and Mo20W is Bn1, but Mo20W carries a dominant inhibitor of Bn1, which explains why Mo20W has white endosperm despite the presence of Bn1 (in retrospect, Mo20W has kernels that are slightly off‑white in comparison with 4Co63). This also explains the observed segregation for pale yellow kernels in the F2 between the two lines: the F1 is heterozygous for both Bn1 and the inhibitor; Bn1 expression is observed in the F2 kernels that carry Bn1, but do not carry the inhibitor due to independent segregation.
A second white endosperm y1 Wc1 inbred line, K55, was also found to carry a dominant inhibitor of Bn1 (although it carries a recessive allele at the bn1 locus). The inhibitors from both Mo20W and K55 were converted to a B73 inbred background by crossing to a B73 Ht1 conversion of y1 Bn1 for seven generations, selecting white endosperm kernels carrying the inhibitor each generation. Both conversions were self pollinated to homozygosity for the inhibitor and curiously, both homozygous conversions were also found to be homozygous for Wc1. This would indicate either that the inhibitor of Bn1 is tightly linked to Wc1 in both Mo20W and K55, or that the inhibitor is an allele of Wc1. Dominant alleles of Wc1 are known to reduce the colored carotenoid content of Y1 endosperms due overexpression of a carotenoid cleavage dioxygenase (Vogel et al. 2008, Journal of Biological Chemisty 283:11364‑11373; Tan et al. 2004, Maize Genetics Conference Abstracts 46:T14). The nature of the brown pigment that accumulates in the aleurones of Bn1 kernels is unknown, other than that it is water soluble (Kulkarni 1927, Mich Acad Sci Arts and Letters Papers 6:253‑273). It is conceivable that this pigment may provide a substrate that is degraded by carotenoid cleavage dioxygenase; however, if that is the case, then not all Wc1 alleles inhibit Bn1 expression since 4Co63 does not carry an inhibitor of Bn1 although it carries a dominant Wc1 allele. In the absence of data supporting the allelism of the inhibitor of Bn1 with Wc1, we have named the locus corresponding to the inhibitor isolated from Mo20W ibn1 (inhibitor of Bn1) and the inhibitor isolated from Mo20W has been named Ibn1‑Mo20W. Tests of allelism will be made between Ibn1‑Mo20W and the inhibitor isolated from K55. We are also testing two other isolates of Wc1 (Wc1‑Wh and a Wc1 allele isolated from a PI accession of South American Caragua maize, possibly the source of the dominant white endosperm trait named Wh by O. White; 1917, American Journal of Botany 4:396‑406) for the presence of linked inhibitors of Bn1.