The non-functionality of white pollen permits design of a male-sterile system to replace hand or mechanical detasseling in hybrid seed production. It is in part reliant on optical-mechanical sorting of seed for the presence or absence of distinctive anthocyanin pigmentation in the aleurone and in the embryo.
We perceive the following components:
1. Source Stock, resulting from the cross of homozygous c2 whp C1 R by C2/c2 whp C1 R, is sorted each generation by aleurone color into two classes: colored (purple) and colorless (non-purple). The colorless kernels are homozygous c2 whp C1 R, and serve as the major Seed Parent in the next cycle of the maintenance of the Source Stock. These plants have white pollen and are male sterile. The colored kernels are C2/c2 whp C1 R and serve both as the Pollen Parent for the Source Stock and as a minor Seed Parent. The ears of these plants, if harvested, must be kept separate and only the colorless kernels used. The colored kernels are discarded, since they are a mixture of C2/C2 and C2/c2 genotypes. R in the Source Stock can be any R or B allele suited to sorting scheme or seed quality requirements.
2. In hybrid seed production fields the colorless kernels of the Source Stock are planted as the male-sterile seed parent for the single cross hybrid. They are homozygous c2 whp C1 R in uniform inbred background.
3. In hybrid seed production fields the pollen parent of the single cross hybrid will be homozygous c2 Whp to achieve colorless seed. It can be a standard inbred line, C2/C2, if colored seed is desired or acceptable.
4. The hybrid seed produced will be c2/c2 Whp/whp and colorless, by the main scheme here.
All current hybrids in the U.S. are r/r and usually c1/c1 C2/C2. Therefore, the Source Stock requires the greatest gene replacement effort. The seed production male parent requires one gene replacement in order to achieve a non-purple hybrid. This can be done by backcrossing and verification of the recovered genotype by crossing to an appropriate tester line for visual scoring.
If one assumes that only colorless seed (excluding pericarp color) is desired in the F1 and F2 (producers' and farmers' fields respectively), this scheme can present a potential problem from cross pollination by or onto "standard" maize, which results in colored kernel contaminants. In the seed producer's field, these colored kernels will signal that isolation measures have failed, but will permit mechanical separation of offending kernels.
For the farmer's field, what can be done? A partial solution may lie in the use of the allele C2-Idf instead of c2 when deriving the male parent line. C2-Idf is a dominant color inhibitor which suppresses C2. The single cross hybrid would be C2-Idf/c2 Whp whp C1/c1 R/r. C2/c2 kernels due to contaminant pollination will still show up as colored, providing a warning and permitting mechanical separation. In the farmer's field, all plants will be C2-Idf/c2 and half the kernels set by contaminating C2 pollen will be suppressed to nearly or completely colorless. Further reduction of colored exceptions can result from the use of a Pollen Parent of C2/c2 whp c1 r constitution in the final production of Source Stock for hybrid seed production.
Heterofertilization occurs at the rate of about 1%. This will cause non-correspondence between endosperm and embryo. Thus 1% of the seed parent kernels, identified by aleurone color, will in fact shed yellow pollen inappropriately. Embryo color can be incorporated into the Source Stock (for example, using R-nj, R-sc, or B-Peru). The optical seed sorting, if sensitive to wave length and low intensity, would remove most non-corresponding embryos. Alternatively, visual screening of aleurone-sorted (colorless) seed will substantially reduce this source of error. As a follow-up, these alleles in concert with C2 or Whp confer anthocyanin color to the growing plants, permitting accurate roguing.
Stephen A. Modena and E. H. Coe, Jr.
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