The brown kernel mutant of maize consists of a duplicate factor pair brn1 brn2 --Stinard, PS The mutant brown kernel1 (brn1) was previously described by this author as being a simple Mendelian recessive mutation (Stinard. 1994. Maydica 39:273-278). Several mutant alleles have been isolated, and in order to study and compare them in a uniform genetic background, backcrosses of the different alleles were made to the inbred B73. Self-pollination of the F1 ears yielded considerably fewer than the expected 1/4 brown mutant kernels, approximating instead a 15:1 ratio of nonmutant to mutant kernels suggestive of a duplicate factor relationship. Further generations of backcrossing to B73 were done, and self-pollinations made after each generation of backcrossing also gave 15:1 ratios. At the fifth backcross generation, nonmutant kernels from 15:1 segregating ears of brn1-R were planted, and the resulting plants were self-pollinated and scored for brn1 mutant kernels. Four ears segregating 3:1 for nonmutant to brown kernels were identified; these ears would be presumably homozygous mutant for one of the two duplicate factors and segregating for the other. Nonmutant kernels from each of these 3:1 ears were planted and the resulting plants self-pollinated. Ears segregating 3:1 as well as ears with only nonmutant kernels were obtained. The latter ears would be expected to be homozygous mutant for one duplicate factor and homozygous nonmutant for the other duplicate factor; such ears will be referred to as single factor lines.

Crosses were made between all possible combinations of the single factor lines, and the F1 ears were self-pollinated to produce F2 ears. Intercrosses among three of the single factor lines yielded only nonmutant ears in the F2, but crosses of these three single factor lines with the fourth single factor line gave all F2 ears segregating 15:1. Thus, we succeeded in isolating the two duplicate factors as single factor lines. I name these two factors brn1 and brn2. brn1 is the factor that was located to the short arm of chromosome 3; brn2 remains unmapped.

The question remains as to why when brn1 was originally isolated, it was thought to be only a single factor. brn1 originally arose in a Robertsonís Mutator population that had been propagated by crossing an active Mutator line alternate generations to the two non-Mutator hybrids Standard B70 (B77 X B79) and Standard Q60 (Q66 X Q67). Apparently, these two hybrid lines are already homozygous mutant for the other factor, brn2, so when the brn1 mutation occurred, it segregated 3:1 on a self-pollinated ear. Outcrosses of brn1 to Standard B70, Standard Q60, and M14/W22 all give F2ís segregating 3:1, proving that they are homozygous for brn2. Similarly, the TB-3Sb line used in the arm-locating cross as well as the linkage stocks used for mapping brn1 must also have been homozgyous for brn2. Crosses to inbred B73 gave F2ís segregating 15:1; therefore B73 is homozygous nonmutant for both factors. Crosses are being planned to confirm the brn2 status of the Robertson hybrid lines.

brn1 brn2 is not the first duplicate factor pair discovered upon crossing a mutant into B73. Two other mutants previously considered to be single factor traits, su3 (now su3 su4; Stinard. 1997. MNL 71:83) and ns1 (now ns1 ns2; Scanlon and Freeling. 1995. MNL 69:23), were also shown to have duplicate factor inheritance upon crossing into B73.
 
 


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