A "retrotransposon" is an element that copies itself and then pastes copies back into genomes at multiple places. It does this by initially copying itself into RNA, but this RNA element is then copied into DNA by an enzyme called reverse transcriptase.
"In this study, we specifically wanted to assess the pattern of selection on these elementsa pattern that could derive from the effect of the elements on the host genome, or the effect of host silencing mechanisms on the elements," Baucom said. "Our expectation was that if the elements are adapting to the host genome, we should see evidence of positive selection in the genes involved in transposition."
The researchers examined selection pressure on retrotransposons using Oryza sativariceas a model plant genome. They analyzed more than 1,000 LTR retrotransposon sequences from 14 separate families that varied in both the dates they were inserted into the rice genome and the numbers of copies that were inserted.
"Overwhelmingly, we found that LTR retrotransposons are under significant evolutionary constraint, by finding strong purifying selection on genes involved in their replication and life-cycle, regardless of the family that any the LTR retrotransposon sequences might belong," says Baucom.
This evidence of so-called "purifying selection" across all gene regions is important in understanding how retrotransposons work. But the research also shows there are rare episodes of positive selection and even adaption to a host genome when these Clark Kents get busy.
It has been known for a long time that the insertion of transposable elements can harm the host, but few studies have been done to determine if there is evidence of selection pressure on LTR retrotransposons.
What the scientists found helps explain why these elements can, while
|Contact: Phil Williams|
University of Georgia