Selection on Transposable Element Families

Daivd Witherspoon dwithers at genetics.utah.edu
Fri Oct 27 12:12:41 EST 1995

Subject: Re: Selection on Transposable Element Families
From: Robert Dejournett, hermes at rtd.com
Date: 19 Oct 1995 05:44:35 GMT
In article <464ok4$am9 at baygull.rtd.com> Robert Dejournett, 
hermes at rtd.com writes:
>>In article <432cog$kcq at news.cc.utah.edu>,
>David Witherspoon  <dwithers at genetics.utah.edu> wrote:
>>	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
>>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
>>elements over a long period within the host population.

>Could it be that these two species have only recenly arisen from a common
>ancestor, and thus have not had time to change their genome?  About the
>lack of mutations, is there no exons in the genome?  Those would tend to
>mutate more I would think...

These elements have had time to change their genome.  Many synonymous 
mutations (those that do not change the encoded amino acids) have 
accumulated between elements, but only a few nonsynonymous ones.  Given 
the genetic code and assuming these elements were not under selection,
would expect about three times as many nonsynonymous as synonymous 
changes to arise between diverging elements.  The data show that
changes are significantly more common than nonsynonymous ones, however.  
This is the same result one would expect if one compared copies of the
gene in two species that had separated tens of millions of years ago.

	David Witherspoon.

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