Molecular Biology and Evolution, Vol 16, 83-97, Copyright © 1999 by Society for Molecular Biology and Evolution
JJ Wernegreen and NA Moran
Buchnera, the bacterial endosymbionts of aphids, undergo severe population
bottlenecks during maternal transmission through their hosts. Previous
studies suggest an increased effect of drift within these strictly asexual,
small populations, resulting in an increased fixation of slightly
deleterious mutations. This study further explores sequence evolution in
Buchnera using three approaches. First, patterns of codon usage were
compared across several homologous Escherichia coli and Buchnera loci, in
order to test the prediction that selection for the use of optimal codons
is less effective in small populations. A chi 2-based measure of codon bias
was developed to adjust for the overall A + T richness of silent positions
in the endosymbionts. In contrast to E. coli homologues, adaptive codon
bias across Buchnera loci is markedly low, and patterns of codon usage lack
a strong relationship with gene expression level. These data suggest that
codon usage in Buchnera has been shaped largely by mutational pressure and
drift rather than by selection for translational efficiency. One exception
to the overall lack of bias is groEL, which is known to be constitutively
overexpressed in Buchnera and other endosymbionts. Second, relative- rate
tests show elevated rates of sequence evolution of numerous protein-coding
loci across Buchnera, compared to E. coli. Finally, consistently higher
ratios of nonsynonymous to synonymous substitutions in Buchnera loci
relative to the enteric bacteria strongly suggest the accumulation of
nonsynonymous substitutions in endosymbiont lineages. Combined, these
results suggest a decreased effectiveness of purifying selection in purging
endosymbiont populations of slightly deleterious mutations, particularly
those affecting codon usage and amino acid identity.
ORIGINAL ARTICLE
Evidence for genetic drift in endosymbionts (Buchnera): analyses of protein-coding genes
Department of Ecology and Evolutionary Biology, University of Arizona, USA. werjen@u.arizona.edu
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