Journal of Molecular Evolution

, Volume 59, Issue 6, pp 849–858

Mutation Exposed: A Neutral Explanation for Extreme Base Composition of an Endosymbiont Genome


DOI: 10.1007/s00239-003-0192-z

Cite this article as:
Wernegreen, J.J. & Funk, D.J. J Mol Evol (2004) 59: 849. doi:10.1007/s00239-003-0192-z


The influence of neutral mutation pressure versus selection on base composition evolution is a subject of considerable controversy. Yet the present study represents the first explicit population genetic analysis of this issue in prokaryotes, the group in which base composition variation is most dramatic. Here, we explore the impact of mutation and selection on the dynamics of synonymous changes in Buchnera aphidicola, the AT-rich bacterial endosymbiont of aphids. Specifically, we evaluated three forms of evidence. (i) We compared the frequencies of directional base changes (AT→GC vs. GC→AT) at synonymous sites within and between Buchnera species, to test for selective preference versus effective neutrality of these mutational categories. Reconstructed mutational changes across a robust intraspecific phylogeny showed a nearly 1:1 AT→GC:GC→AT ratio. Likewise, stationarity of base composition among Buchnera species indicated equal rates of AT→GC and GC→AT substitutions. The similarity of these patterns within and between species supported the neutral model. (ii) We observed an equivalence of relative per-site AT mutation rate and current AT content at synonymous sites, indicating that base composition is at mutational equilibrium. (iii) We demonstrated statistically greater equality in the frequency of mutational categories in Buchnera than in parallel mammalian studies that documented selection on synonymous sites. Our results indicate that effectively neutral mutational pressure, rather than selection, represents the major force driving base composition evolution in Buchnera. Thus they further corroborate recent evidence for the critical role of reduced Ne in the molecular evolution of bacterial endosymbionts.


MutationSelectionBase compositionDriftDNA repairBuchneraNeutrality

Copyright information

© Springer Science + Business Media Inc. 2004

Authors and Affiliations

  1. 1.Josephine Bay Paul Center for Comparative Molecular Biology & EvolutionThe Marine Biological LaboratoryWoods HoleUSA
  2. 2.Department of Biological SciencesVanderbilt UniversityNashvilleUSA