Studies of dotted, a regulatory element in maize : III. Transpositions of dotted and its stability at various locations.

Dotted (Dt) is the regulatory element of a two-unit controlling system in maize. Dt causes the inherited change from the recessive a (colorless) to its dominant allele, A (anthocyanin production), during the development of the stalk, leaves, and endosperm. The mutation events are observed as sectors of color in an anthocyaninless background.Since its discovery over 40 years ago, Dt has always been found in the terminal knob of the short arm of chromosome 9. This is puzzling because controlling and regulatory elements in general are not located permanently, but change positions (transpose) within the chromosomal complement. To resolve this seeming discrepancy, transpositions were looked for in a homozygous a Dt stock. Because the frequency of aleurone mutations is exponentially related to Dt dosage, a Dt transposition would result in a greatly increased number of dots if the egg or sperm nucleus contained both the transposed Dt and the Dt remaining on chromosome 9. A total of 6 transposed Dt's (Dt-T) were recovered in this manner. Dt-TA was found linked to the gene Y ("yellow endosperm") of chromosome 6. Dt-TB no longer showed linkage with yg2 of chromosome 9, but remains unlocated (the original Dt in this stock is separated from yg2 by 6 or 7 cross-over units.). The remaining transpositions (C-F) assorted independently of Dt on chromosome 9.The transposed Dt's had the same effect as Dt on the frequency and timing of aleurone mutations. An increase in transposition frequency and losses of Dt-T's was characteristic of several of the transposed Dt's. Dt-T's B-F transposed so frequently that testcross ratios of 7∶1 (three Dt' s) and 15∶ 1 (four Dt' s) were observed. No secondary transpositions or losses of Dt-TA were detected. Thus, Dt-TA resembles the original Dt with regard to its transposition frequency and stability.