The uniquely monstrous, many-ranked ears of Zea mays have for too long remained an enigma. What species, what structure could possibly be ancestral? Most evidence has pointed to Zea mays subsp. mexicana, the annual teosinte. Yet its many, minute, 6- to 12-grained ears are fundamentally lateral on the primary branches, while those of maize are always solitary and terminal. Other differences abound. Maize ears could not have evolved from teosinte ears, structures which are not only rigidly canalized, but also edible only when immature and green.
Teosinte is, however, unquestionably ancestral; yet, since its primary lateral branches are always terminated by male inflorescences, the tassels, and since the central spike of the tassel is the homologue of the maize ear, as has long been recognized, the CSTT proposes that the maize ear is the transformed, feminized central spike of the tassels terminating the primary teosinte branches. Developmentally dominant because of its femininity in an apical position, and once past the sexual threshold, the proto-maize ear progressively suppressed all teosinte ears on the secondary and tertiary branches beneath it, drawing all nutrients of the whole branch system to itself. Feminization, and the resulting rapid catastrophic changes in resource allocation, thus transformed the slender terminal tassel spike into an increasingly effective nutrient sink. Because of their terminal position and their many non-cupulate axis segments, each with a pair of already fully fertile, soft-glumed, and free spikelets, primary branch tassel spikes were eminently preadapted to turn into free-grained, many-grained maize ears. These primitive tassel attributes, basic as they are throughout the Andropogoneae, are well-buffered genetically by what must be a large collection of polygenes accumulated long before the origin of maize. Once nutrient-overloaded, the distichous apical meristem would become polystichous by condensation-twisting as outlined by Collins, through slippage of rachid initials early in ontogeny. Femininity expressed on a male background per se, and not any of the postulated but elusive major genetic mutations, led to this epigenesis of the proto-ear, a probably easily induced abnormality with macro-evolutionary consequences. Canalization and amplification of these and additional cultivar attributes by human selection soon followed, saving this hopeful monster for posterity.
What exactly caused the sexual transmutation is unknown at present, but a shortening (condensation) of the primary branch internodes, which placed the tassel into the zone of female expression, was evidently involved (as either cause or effect?), triggered perhaps by abnormal environment (short days, cold nights), growth-substance-releasing pathogens (viruses, smuts), or mutations for branch condensation per se. Supported by much previous physiological work, and resolving nearly all archaeological, morphological and genetic paradoxes, the CSTT is amenable to experimental verification.
Figures. Page 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92.
Hugh H. Iltis
[Ed. note: Accompanying material follows]
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