Selection on Transposable Element Families

David Witherspoon dwithers at genetics.utah.edu
Mon Sep 11 17:21:04 EST 1995

	I am studying a family of transposable elements (TBE1s) in a 
couple of species of ciliates (Oxytricha fallax and trifallax.)  These
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
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 


David Witherspoon,	dwithers at genetics.utah.edu	(801)581-
Oncological Sciences, Division of Molecular Biology and Genetics
5C334 SOM, University of Utah, Salt Lake City, Utah 84132

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