The cupule is a secondary sex trait of the female spike in both teosinte and the oldest known archaeological maize as well as in most stocks of modern maize. While the non-cupulate cob or reduced-cupulate cob now occurs in certain maize strains, it has evolved only in coadaptive association with certain derived kernel types. Paired female spikelets, when transferred from maize to teosinte, flatten and reduce cupule development similar to the cupules of the oldest known maize.
When sex reversal suddenly occurs during the ontogeny of growing plants of teosinte, maize and many other species, the expression of secondary sex traits changes accordingly. The synchronization is thus developmental with a somatic basis. The primary and secondary sex traits may have different genetic bases. The genes for secondary male traits also are distinct from those for secondary female traits. A change in the primary sex of the teosinte tassel could not fail to activate genes for the secondary female trait of a cupulate fruit case. Genes separate from those controlling the primary sex are known to produce secondary female traits of paired spikelets, long soft glumes, a narrow flexible rachis and reduced cupules. A change from the cupulate fruit case to the above as female secondary sex traits in teosinte would evolve by domestication from changes at their controlling loci, not that for the primary sex. Primary sex genes such as "anther ear" and "tassel seed" activate changes in the expression of the secondary sex genes accordingly in a form of controller to structural gene relationship. The common presence of the cupule in the female rachis of both teosinte and maize is evidence that the maize ear is derived from the teosinte ear, not the tassel.
Walton C. Galinat
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