Studies of dotted, a regulatory element in maize : I. Inductions of dotted by chromatid breaks II. Phase variation of dotted.

Dotted (Dt) is the regulatory element of a two-unit controlling system in maize. Dt causes the inherited change from the recessive a 1 (colorless) to its dominant allele, A 1 (anthocyanin production), during the development of the stalk, leaves, and endosperm. The mutation events are observed as sectors of color in an anthocyaninless background.One of the most puzzling, but perhaps significant, aspects of controlling elements in maize is that they originate in conjunction with chromosome or chromatid breaks. This fact invokes a requirement that either an existing regulatory mechanism is disturbed by the breakage or that a foreign element is incorporated before fusion of the broken chromatids.Experimental crosses were made between Dt tester stocks and a pollen parent, a large proportion of whose chromosomes 9 were undergoing the chromatid type of bridge-breakage-fusion cycle. New Dt's were induced in endosperm sectors of 250 of 154,422 kernels tested (1/600); among these, two germinal Dt's (Dt (crown) 4 and Dt 5) were recovered, presumably due to chromatid breaks during meiosis or the first microspore division. Dt 5 produces a mutation pattern very similar to the original Dt 1 and is located 0.33 crossover units away from the yg 2 locus. This is close to the known location of Dt 1 (7 crossover units distal to the yg 2 locus) and is suggestive of a specific site for Dt inductive breaks. Dt (crown) 4, on the other hand, is inherited independently of the yg 2 locus and does not support this contention. Dt (crown) 4 represents a new "state" causing a high concentration of fine dots in the crown of the kernel, with little or no dotting at the base.The phase variation of Dt (crown) 4 is discussed together with the tissue-dependent expression of Dt (in-ac) 1 (Dotted, inactive-active). Dt (in-ac) 1 is a new "state" of Dt 1 and shows inactive (no a 1 to A 1 mutations) and active (a 1 to A 1 mutations) phases in the endosperm, whereas it is only in the active phase in the diploid scutellum. The observed phase variation was shown to be a property of the regulatory elements, Dt, responding to differences in the cellular environment.