Molecular Biology and Evolution, Vol 14, 696-706, Copyright © 1997 by Society for Molecular Biology and Evolution
DJ Witherspoon, TG Doak, KR Williams, A Seegmiller, J Seger and G Herrick
TBE1s are "cut-and-paste" transposable elements found in high copy number
in the germline genomes of the ciliates Oxytricha fallax and O. trifallax.
TBE1 "family" sequence (sequence of mixed polymerase chain reaction
products generated using primers that match roughly half the TBE1s in host
whole-cell DNA) was obtained from both host species. Although family
sequence autoradiograms represent thousands of different elements, they are
as legible as those representing corresponding sequences of a single TBE1,
implying that ideal polymorphisms are rare within the genes examined.
Nucleotide polymorphisms among TBE1s (indicated by ambiguities in family
sequence) are far more common at third than at first or second positions of
codons of genes, implying that selection has conserved the amino acid
sequences of these genes in the majority of TBE1s. Portions of the
transposase gene and another TBE1 gene have been sequenced from 10
individual TBE1s. None of these portions is interrupted by stop codons or
frameshifts, and, for both genes, pairwise comparisons of these sequences
show that nonsynonymous differences are significantly less common than
synonymous differences, again implicating conservative selection
Phylogenetic analysis shows that multiple divergent lineages of TBE1s have
evolved under this selection within O. fallax. All these results are
unexpected for cut-and-paste transposons in eukaryotic hosts: since
transposase encoded by intact elements presumably acts in trans, it can
duplicate mutant copies (those that do not encode functional transposase)
found in the same genome, and thus no selection is expected to maintain the
transposase gene. The selection demonstrated here could act at
transposition (if functional TBE1s are preferentially transposed) or at the
level of the host (if the host's fitness depends on functional TBE1 genes).
TBE1-encoded proteins might be responsible for the precise excision of
TBE1s that occurs during development of the host somatic nucleus; selection
on hosts for uninterrupted somatic genes would then translate into
selection for TBE1 protein-coding competence. We suggest a method for
distinguishing between these two classes of explanations by finding and
analyzing divergent alleles of ancestral transposable element insertions.
ORIGINAL ARTICLE
Selection on the protein-coding genes of the TBE1 family of transposable elements in the ciliates Oxytricha fallax and O. trifallax
Department of Oncological Sciences, University of Utah, Salt Lake City, USA.
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