The system you describe is an interesting one...an evolutionary
scheme explaining the origin of these transposons might possibly
be similar to schemes which have been used to explain the appearance
of the phenomenon of RNA-editing in trypanosome or other mito
genomes. The scheme, goes something like this:
1) appearance of an editing enzyme activity (side reaction or something)
2) nucleotide substitutions in the DNA occur and are edited on the RNA
3) once several substitutions have occurred the editing activity is
required (loss would lead to lethal consequences)
The case for IES evolution might go something like this.
1) appearance of transposon which cannot move much (large selective
2) appearance of sequence eliminating "activity" during macronuclear
3) the spread of transposon throughout genome- such spread having
been made selectively neutral by the "eliminating activity" during
What do you think?
Perhaps the editing activity is a property of the transposons....
is this known? They would then be ultimately selfish elements
taking advantage of the macro/micro-nuclear innovation in