Ear shape as a correlate of differences in rachilla and cupule development that trace to two independent domestications of two different teosintes

--Walton C. Galinat

A natural grouping of maize into two different ear shapes, pyramidal and cylindrical, was observed by Anderson and Cutler (Ann. Mo. Bot. Gard. 29:6988, 1942). These two shapes correlate with my findings on differences in rachilla and cupule development that trace to two different systems for kernel exposure by independent domestications of two different teosintes. The pyramidal corns have short sharply tapering ears and elongated kernels. They include Palomero Toluqueno, Conico, Pepitilla and Chalqueno as well as the U.S. Gourdseed corn. The ancestral system of kernel exposure during domestication of their teosinte progenitor was based on cupule reduction and to this day these races have short rachillae and reduced cupules.

When multiranking is added to cobs that have short rachillae and reduced cupules on the one hand and cobs with long rachillae and pronounced cupules on the other, the cob assumes two different shapes. The circumferential space necessary to accommodate multiranking in the presence of short rachillae and reduced cupules can only come from increases in pith diameter. Without the mechanical support of cupules, such cobs rapidly taper into a pyramidal shape except in the presence of fasciation which may be either terminal or subterminal.

In contrast, cobs with long rachillae and well-developed cupules have a cylindrical shape that has structural support from a rigid framework of these cupules. This cylindrical shape is most prevalent in modern corn as a result of selection by commercial corn breeders. It represents another natural grouping of the races of corn, probably tracing back to domestication from Guerrero teosinte.


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