Regulation of secondary sex traits in Zea

Teosinte and maize differ primarily in the secondary sex traits associated with female development. The programs for secondary female traits in these species have been genetically assembled by different selective pressures, a natural one in teosinte and a domestic one in maize. The various secondary traits include developmental modifications to various structures: the cupule (large to rudimentary); spikelets (single, paired); outer glumes (hard, soft); pith (small, large); condensation (relaxed, tight); apical dominance (weak, strong) and others.

Cupules in the female spikes of both teosinte and maize are adaptive but for different reasons. As the maize cob emerged, the cupule was preadapted to a transfer of function from protecting the grain to a mechanical support of the cob that prevents dry-cob shrinkage and, thereby, kernel shattering. When the non-cupulate condition of the male rachis in both species is genetically transferred to the female rachis, the new combination is incongruous in maize except when there is an absence of kernel crowding in certain low condensation strains. The non-cupulate condition is non-adaptive in the female spike of teosinte because it removes bird protection.

When the paired condition of male spikelets normal to both species also occurs with the female spikelets, it has opposite selective values. A change from solitary to paired female spikelets in teosinte opens its cupulate fruit case and, thereby, reduces its value as a protective device. Because such protection in maize comes from enclosing husk leaves borne below the ear, pairing of the kernels, like multi-ranking, results in the domestic advantage of increasing productivity per ear.

Walton C. Galinat

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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