There is morphological evidence that the switching off of one phase and turning on of the next phase are independent genetic events. In the case of the corngrass (Cg) and teopod (Tp) mutants, the floral phase may be turned on without the vegetative phase being switched off. The overlapping and sometimes simultaneous expression of both phases accounts for the phenotypes of these mutant genes. When the vegetative phase is finally shut down, the demarcation is not always at right angles to the axis but the switching may follow along wedge-shaped sectors flowing up into the pure floral region. This is suggestive of an Activator-Dissociator type of action such as the somatic cell heredity known for aleurone color.
If two independently acting genes are responsible for this type of phase change, then a state should also be found in which the vegetative phase is turned off but the floral phase is not yet turned on. Since both the vegetative and floral phases have nodes, this hypothetical state with a barren axis might also have nodes or at least rudimentary nodes. Since the pre-phase non-specialized condition is one of short internodes, the barren nodes would be close together. Since an Activator-Dissociator type of switching is envisioned, the interface between the pre-phase barren axis and any eventual switch to the floral phase should sometimes follow wedge-shaped sectors. The delayed switch to the floral phase should affect both tassel and ear. It is expected that by a year from now, I shall be able to report on the inheritance of such an interrupted phase change condition.
One problem is how a given complex of genes becomes identified with one switching element, like an antibody with an antigen, and another complex of genes with a second switching element. Another problem is how the two switching elements normally synchronize their on-off activities during phase change. Perhaps the mutant genes that cause defective switching may be utilized to reveal the genetic process of switching and the control of morphogenesis.
Walton C. Galinat
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