I am studying a family of transposable elements (TBE1s) in a
couple of species of ciliates (Oxytricha fallax and trifallax.) These
are
DNA "Cut and Paste" transposons. The most striking feature of this
family is the degree of conservation between elements: virtually all of
the several thousand elements in any given host genome are intact,
meaning that no stop codons, no deletions or insertions, and few
nonsynonymous changes (relative to synonymous changes) have
occurred during their divergence from each other. I take this to be
evidence that some force of purifying selection has been acting on these
elements over a long period within the host population.
I would like to contrast this pattern with that of other DNA cut
and paste transposons, such as mariner elements: when a 'population'
of mariners is found in a host population, virtually all of the element
are
defective, meaning that their ORFs are interrupted by stop codons and
deletions (Hugh Robertson's recent papers.) This pattern apparently
results from the fact that, in eukaryotes, the transposase encoded by
one copy of a transposon can be used to replicate other (possibly
defective) elements in the same genome; there is therefore no selection
acting to conserve the transposase ORF, and the elements 'crumble' as
mutations accumulate (Kaplan, Darden & Langley, 1985, in Genetics).
Does anyone know of other eukaryotic DNA cut and paste
transposons which seem to have evolved under conservative selection,
as TBE1s have? I'd like to say this observation is 'unprecedented', but
I'm not sure it is. While I'm asking, if anyone knows of papers which
demonstrate selection operating during the evolution of other sorts of
repeated sequence families, I'd appreciate their sending me the
references.
Thanks,
David Witherspoon, dwithers at genetics.utah.edu (801)581-
3747
Oncological Sciences, Division of Molecular Biology and Genetics
5C334 SOM, University of Utah, Salt Lake City, Utah 84132
