Do transposed Ac2 elements arise by extra replication of existing elements or by excision from a parent allele?

Transpositions of Ac elements from AcAc alleles so far detected have included the low-dot cases mentioned above, where it is believed that a single component was transposed from the parent AcAc to a heterologous chromosome. In addition, transpositions have been identified in plants still possessing the parental AcAc (i.e., AcAc + tr-Ac). Other possible transpositions are very strong alleles (AcAcAc) where the additional component may have come by transposition from the sister chromatid bearing AcAc. We do not now believe these Ac elements arise by extra replication although this is very difficult to rule out. The evidence favors excision of one Ac component from a chromosome 8 chromatid having the AcAc allele and its insertion into either a homologous or heterologous chromatid. The remnant single Ac component on chromosome 8 has been found among our low-dot kernels from AcAc/ac parents and similar remnant Ac alleles (weak alleles on chromosome 8) were present in testcross progeny of AcAc/AcAcAc plants. Thus, changes in Ac constitution at the original location do occur and these weaker alleles cannot arise by extra replication. However, both the weak alleles (remnant Ac's) and the transposed Ac's can be explained by excision of an Ac component followed by its loss in the first case or its transposition and recovery in the second case. There is no need to invoke extra replication; all of the data can be accounted for by the excision hypothesis and by assuming the stronger Ac alleles are either duplications or triplications.

M.M. Rhoades and Ellen Dempsey


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