Molecular Biology and Evolution, Vol 16, 472-478, Copyright © 1999 by Society for Molecular Biology and Evolution
DJ Witherspoon
P elements, like mariners, inhabit eukaryotic genomes and transpose via a
DNA intermediate. Mutant and wild-type elements in the same genome should
be transposed with equal probability by trans-acting transposase, and so no
selection should counteract the accumulation of inactivating mutations in
transposase genes. Thus, copies of mariner elements diverge within a host
species under no selection (Robertson and Lampe 1995). It is unknown
whether or not this pattern holds for P elements, which are unrelated to
mariner elements but share the same life history. Publicly available
P-element sequences were analyzed for evidence of conservative selection
for the function of P-element- encoded proteins. Results were compared to
predictions derived from several hypotheses that could explain selection,
or the lack of it. P- element protein-coding sequences do evolve under
conservative selection but apparently because of more than one selective
force. Of the four exons in the P-element transposase, the first three
(exons 0, 1, and 2) can be translated alone into a repressor of
transposition, while the last (exon 3) is only expressed as part of the
full-length transposase and probably serves a transposition-specific role.
As full-length P- element copies diverge from each other within a host
population, selection maintains exons 0-2 but apparently not exon 3. The
selection acting on exons 0-2 may act at the host level for repression of
transposition (since host level selection does act on orthologous truncated
elements that contain only exons 0-2). Evidence of selection on exon 3 is
only found in comparisons of more diverged elements from different species,
suggesting that selection for transposition acts primarily at horizontal
transfer events. Thus, horizontal transfer events may be the sole source of
the selection that is crucial to the maintenance of autonomous P elements
in the face of mutation (as suggested by Robertson and Lampe 1995). The
predictions derived here suggest a strategy for collecting sequence data
that could definitively answer these questions.
ORIGINAL ARTICLE
Selective constraints on P-element evolution
Department of Biology, University of Utah, Salt Lake City, USA. dwithers@hci.utah.edu
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