Bifurcation of the spike (ear), the rachis (mr) and the spikelet (pd) are shown as different target levels of expression in Figure 2. The rachis and spikelet levels of expression are for genes (mr, pd) involved in the origin of corn from teosinte. The pd gene has been described here in the item on the role of transposons.
The mr gene (multiranking on short arm chromosome 2) controls another key domestic trait with symptoms of transposon involvement. Normally in teosinte, two-ranking occurs in both the vegetative and floral phases, while maize retains this primitive condition in only the vegetative phase. In the floral phase of maize, the central spike of both tassel and ear are usually multiranked, while the branches, or secondary ears, remain at the primitive two-ranked level. Energy-deprived inflorescences that develop under a vascular system that is genetically reduced or diverted on one hand, or environmentally stunted on the other, may revert to the primitive condition of two-ranking. In contrast, the higher the level of energy and vascular supply endowment to the terminal spike, the higher its floral ranking and kernel row number. Mutations are well known to corn breeders that result in vegetative multiranking that is usually unstable and confined to the upper nodes between the ear and tassel. At very high kernel row numbers, the whole plant may have many-ranked leaves, and every plant in a homozygous family (mr-v, mr-v) will express the phenotype. All of these changes in ranking, including the environmentally induced phenocopies, are based on a certain genetic condition at the mr locus on chromosome 2S. The possibility cannot be ignored that the movement of transposons involves the consumption of energy. A role for transposons in both spikelet bifurcation and non-divergent rachis bifurcation during the origin of maize seems possible. The various levels, or movements, of bifurcation in floral morphogenesis are illustrated in Figure 2.
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