There are a series of multiple alleles for degree and induration of cupule development on the short arm of chromosome 4. As reported in MNL 56:163-164, the type of cupule has become coadaptive with kernel type in terms of spatial accommodation. Hard kernels, which do not shrink significantly on drying, will fly off from a soft cob that shrinks about 10% as it dries. Thus, large, hard kernels are coadaptive with hard cobs. Because of the transfer of cupule function involved, the cupule of teosinte has been retained in maize these last 8,000 years since domestication.
But the cupule is not now needed in sweet corn and other defective endosperm types with kernels that do shrink at least 10% on drying. In fact, a soft cob resulting from cupule reduction becomes coadaptive with defective kernels so that the cob will shrink along with the kernels, which thereby do not shatter by becoming too loose. Furthermore, the physical space required by the cupule at higher kernel row numbers promotes objectionable fasciation. By removing the cupule with the cupuleless allele on chromosome 4, we may expect to increase kernel row numbers in sweet corn to 40 or more without fasciation.
Walton C. Galinat and Ann E. Kennedy
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