Homeotic Sexual Translocations and the Origin of Maize (Zea Mays, Poaceae): A New look at an old problem

In the Origin of Maize Controversy, the Orthodox Teosinte Hypothesis (OTH; Beadle 1939, 1972; Iltis 1971), five key mutations change 2-ranked (distichous) ears of teosinte (wild Zea) with a single row of grains per rank to 4- to many-ranked (polystichous) maize ears with a double row of grains per rank. BUT teosinte ears are lateral to the 1° branch axes, maize ears, like their male homologues, the teosinte I° branch tassel spikes, terminal, an enigma long unrecognized, hence ignored. In the Catastrophic Sexual Transmutation Theory (CSTT; Iltis 1983b, 1987), now abandoned, the I° branch tassel (male) of teosinte (spikelets soft-glumed, paired, i.e., double-rowed per rank, as in maize ears), when brought under female hormonal control by branch condensation, becomes feminized into a maize proto-ear. BUT lateral ears should then have remained teosintoid (2-ranked, each rank with a single row of grains), yet are in fact double-rowed. Combining OTH and CSTT, the new Sexual Translocation Theory (STLT) is based on: first, the branching pattern of teosinte ear clusters (Camara-H. & Gambino 1990), sequentially maturing, sympodially branching, typically Andropogonoid systems, called rhipidia (sing, rhipidium), where each higher order (younger) ear originates as a lateral branch of its lower order, earlier maturing predecessor; and second, on 3 or 4 key mutations [cupule reduction, softening of glumes, doubling of female spikelets], which, by projecting outward the grains, invited human domestication by making them accessible. Within each ear cluster, the earliest maturing, hence nutrient-monopolizing and largest ear would be selected, all younger ears, already nutrientinhibited, suppressed. As fewer, larger ears evolved, and branch internode condensation moved male tassels into female hormonal zones, homeotic conversions translocated female morphology to terminal male positions: first replacing each of the II° branch tassels, and ultimately the 1° branch tassel (male), with an ear (female). With this, now female structure in the apically dominant, hence most nutrient-demanding terminal position gradually suppressing all subsidiary ears on the 1° branch beneath it, mutations for polystichy (contingent on nutrient overload) were finally allowed to become expressed, and the multi-rowed maize ear (at first with an atavistic male tail) evolved. Favored by human selection, these increases in apical dominance by stepwise homeotic sexual conversions explain both archeological and morphological realities, but need to be harmonized with recent results of developmental genetics. Current evidence suggests that teosinte was first tended for its green ears and sugary pith by hunter-gatherers as an occasional rainy-season food in small “garden” populations away from its homeland, and not for its abundant grain-containing, hard fruitcases, which easily mass-collected but useless as food, are as yet unknown from the archeological record. A rare grain-liberating teosinte mutation (probably expressed in only one “founder” plant, a mazoid “Eve”), which exposed the encased grain for easy harvest, was soon recognized as useful, collected and planted (or self-planted). Thus maize was started on its way to a unique horticultural domestication that is not comparable to that of the temperate Old World mass-selected agricultural grains.