The Ac-dosage effect

--Manfred Heinlein and Peter Starlinger

Reciprocal crosses between Ac bearing and Ds bearing lines were done to learn more about the Ac-dosage effect (Heinlein and Starlinger, Maydica, submitted). The Ds carrying maize line was C Sh bz-m2(DI) wx, which was used as the tester line to measure the dosage effects of different Ac-elements in trans. The Ac-containing maize lines were homozygous C sh bz wx-m9Ac, C sh bz wx-m7, and C sh bz wx Ac. The latter Ac-element we call Ac u.p. for “unknown position” (on chr. 9). The dosage effects measured were compared with the dosage effect of bz-m2Ac, in which case the Ac is acting on itself. The reversion patterns seen on the kernels all were different. If bz-m2(DI) is transactivated by wx-m9, reversions occur later during development, with higher dose giving rise to smaller spots. However, if wx-m7 is used, larger spots are seen with 2 Ac than with 1 Ac. We counted the number of spots of each revertant clone size separately and plotted the number of sectors against clone size. In every case the curves for 1 Ac and 2 Ac cross each other, which means that the sign of the dosage effects is altered during development. Whereas wx-m9 shows a negative dosage effect during early developmental stages but a positive dosage effect during later stages, wx-m7 shows a positive effect during early stages, but a negative effect at later stages. wx-m9 is very similar to bz-m2Ac in respect to time and frequency of events as well as the dosage effect. Therefore we think that beside the known negative dosage effect positive dosage effects can also be measured in maize, and the positive dosage effect is therefore not restricted to heterologous plants transgenic for Ac (Jones, JDG et al., Science 244:204, 1989). The Ac-elements in wx-m7 and wx-m9 are identical in sequence but show opposite dosage effects in our test. This might either be due to position effects or to differences in the genetic background.

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