Further studies on the regulation of Ac-dependent events
--Manfred Heinlein and Peter Starlinger
Two Ac-dependent events usually do not occur at the same time and in the same cells (MNL 65). Therefore, the rate-limiting step of the decision, whether transposable elements Ac/Ds undergo excision/transposition events in the cell, is not the synthesis or activation of a trans-acting factor. However, the probability of Ds-excision is different at a certain time during development if different Ac-elements are used for trans-activation. This indicates that Ac elements differ in trans-activity. This is further supported by the observation that different Ds elements respond similarly in the presence and dose of the particular Ac element. In last year's report the responses of bz-m2(DI) in kernels carrying one or two doses of the wx-m9 Ac or wx-m7 Ac alleles, respectively, have been described (also in: Maydica 36:309). The two Ac elements are identical in sequence and are inserted in the same gene and in the same orientation. Hybrid F1 wx-m7 Ac/wx-m9 Ac plants have now been crossed to bz-m2(DI) and other tester plants to test whether the observed differences are due to varying genetic backgrounds of the plants used in the analysis. The postulated factors of the genetic background which should have been mixed in the hybrid should segregate independently and because of this should cause the appearance of new patterns on the tester ears. However, the known Ac-specific patterns appeared in the progeny. The kernels showing the particular reversion pattern in the aleurone also showed the particular Wx reversion pattern in the inner endosperm. The kernels also contained the appropriate Ac element as has been proven by PCR. The observed regulation of excision events is therefore rather due to Ac-specific properties than due to background effects. However, the possibility that the differences in phenotypes are due to genes closely linked to the Ac element cannot be excluded.
Ac elements which differ in their action in trans most likely should be differentially expressed. Comparisons between wx-m7 and wx-m9 Ac did not reveal convincing evidence for differences in Ac-mRNA levels in seedlings. This might be different in endosperm, where the Wx-locus is expressed. First experiments were performed to test the influence of host-gene expression on excision. Preliminary results suggest that the Ac element of bz-m2 is excised 20-30 times less often in C-I C-I C and C-I C-I C-I kernels than in C C C kernels as revealed by comparison of excision bands in Southern blot experiments. Since the effect is seen by excision of an Ac element it cannot be distinguished whether the excision of the element is influenced directly or rather indirectly through altered Ac expression.
Ac dosage effects are often measured by comparing patterns on progeny kernels of reciprocal crosses between Ac-bearing maize lines and Ds-bearing maize lines. The progeny of the two crosses not only differ in Ac dose but also in Ds dose. To measure the influence of Ds dose (not the overall Ds copy number, but the dose of a particular Ds element with visible phenotype), the reversion pattern of wx-m9 Ds has been compared between kernels carrying a constant dose of bz-m2 Ac but one, two and three doses of wx-m9 Ds. It appeared that the difference in reversion frequency was much stronger between two and three Ds than between one and two Ds. The two individual copies of Ds derived from the female are excised much less frequently than the one copy derived from the male, which suggests that probably paternal imprinting is involved. The frequency of the individual Ds elements in three-Ds kernels in which the elements of maternal and paternal origin have been combined clearly exceeds the sum of the observed values in one- and two- Ds kernels. This indicates that the combination of paternally and maternally inherited elements might promote excision. Unless the Ds9 element does encode a participating trans-acting product, one might speculate that the element derived from the male is able to complement some inhibitory properties of the element derived from the female, probably by interphase pairing. Further studies employing the Ds element present in the bz-m2(DI) allele are in progress.
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