The sh2 mutant discovered by E.B. Mains (J. Hered. 40:21, 1949) and characterized biochemically by J.R. Laughnan (1953) is the basis for the significant supersweet corn industry. However, the useful sh2 allele has two major disadvantages: it conditions poor germination and, being a recessive like su1 (the major sweet corn gene), produces normal starch when crossed by any other corn (all of which carry the starchy Sh2 allele). The first disadvantage was overcome by mass selection in heterogeneous populations (R.D. Bell, et al., Crop Sci. 23:461, 1983). The second can be overcome by incorporating the Ga factor commonly found in popcorn. For this purpose, 4 fairly separate populations of sh2 Ga were created using 4 disparate ears of M.S. Zuber's high-germination sh2 composite, crossing these by a commercial yellow popcorn, crossing the 4 F1's as male to 4 different commercial su1 sweet corn varieties, then selecting the sh2 heterozygote and crossing these as males to 4 Missouri white Ga inbreds. Finally, the progenies from these crosses, which were Ga Ga and half heterozygous for sh2, were selfed and the sh2 seed bulked, to produce 4 sh2 Ga composites designated sh2 Ga 501, 503, 504, and 505, based on the Missouri white Ga line used in each case.
Desirable characteristics of these separate composites were increased by planting in isolation from each other ten 50-plant rows, allowing to open pollinate, harvesting only the best ears from the middle 8 rows, and bulking the resultant seed. This was done for 7 generations. Germination was improved by overplanting under adverse conditions. Ga stability was improved by planting every fifth hill to a colored aleurone Sh2 ga stock and discarding all ears with any colored Sh2 kernels. Disease resistance and agronomic characteristics were improved by discarding all diseased or poor plants. No selection was attempted for flavor or tenderness.
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