The string cob trait that became the basis of the slender, eight rowed sweet corn inbred MA-W401 and its hybrid, Candystick, was initially extracted from Confite Morocho of Peru (WC. Galinat, Mass. Agric. Exp. Sta. Bul. 577, 1969.). It is characterized by a very slender cob resulting from three factors: reduced pith, reduced cupules and short rachillae. In most backgrounds the thick so-called normal cob segregates out in only 1/16 of the F2 as if two incompletely dominant genes controlled string cob. In teosinte the short rachillae trait is combined with well-developed cupules as if these were independent traits.
Allelism tests made by hybridizing W401 with White Cloud Popcorn (a white rice variety related to Palomero Toluqueno) have continued to breed true for the short rachillae. Selection for kernel row numbers above 20 increased the pith area but the cobs remained slender (semi-string cob) due to their short rachillae. The kernels of these high-row selections became long and thin, of the Shoe Peg type, and the cupule became narrow and greatly reduced, sometimes to the noncupulate condition.
It is suggested that the domestication of teosinte occurred at least twice and by different pathways. It seems that Chalco teosinte may have been domesticated by a combination of cupule reduction and kernel elongation that led to such diverse modern derivatives as Palomero Toluqueno, Confite Morocho and Gourd Seed Dent. Most maize may be predominantly from another independent domestication, apparently involving the tunicate locus and Guerrero teosinte. In this case the glumes become soft and the rachilla elongate in a way that elevates the grain almost beyond the chaff.
Selection by man for recessive alleles for a thick cob at the string cob loci increased the vascular supply that was necessary for the development of a more productive ear. The long rachillae plus the thickened pith area combine to make the enormous cob of modern maize.
Walton C. Galinat
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