Genetic characterisation of Act1, the activator of a non-autonomous transposable element from Petunia hybrida

The line W138 of Petunia hybrida has variegated flowers because it is homozygous for the mutable an1-W138 allele. Excision of the element, causing instability, depends on the presence of the activatorAct1. The previously characterised non-autonomous element dTph1 excises from the dfrC gene in response to Act1. This implies that both non-autonomous elements belong to the same transposable element family. In a range of distantly related cultivars we could detect a single functional Act1 element. Linkage analysis for 11 of these lines showed that Act1 was located on chromosome I in all cases, indicating that the element might be fixed in the genome. A group of cultivars that did not exhibit Act1 activity could be traced back to a recent common origin (Rose of Heaven). Cultivars within this group presumably harbour the same inactivated Act1 element. Among the lines tested were 7 lines representing the two species (P. axillaris and P. integrifolia) from which P. hybrida originated. None of these exhibited Act1 activity. We assume that Act1 is present in an inactive state in these lines and that it was activated upon interspecific crossing. In general, lines representing the two parental species and P. hybrida cultivars contain between 5 and 25 dTph1 elements. The lines R27 and W138, however, contain significantly more dTph1 elements (> 50) than all other lines.