Purifying Selection and Molecular Adaptation in the Genome of Verminephrobacter, the Heritable Symbiotic Bacteria of Earthworms

• DOI: 10.1093/gbe/evs014
• Publication Date: 2012-02-15T03:32:52Z
• Authors: Kasper U. Kjeldsen; Thomas Bataillon; Nicolás Pinel; Stéphane De Mita; Marie B. Lund; Frank Panitz; Christian Bendixen; David A. Stahl; Andreas Schramm
• Keywords: Negative selection; Comparative Genomics; Bacterial genome size; Genome size

While genomic erosion is common among intracellular symbionts, patterns of genome evolution in heritable extracellular endosymbionts remain elusive. We study vertically transmitted (Verminephrobacter, Betaproteobacteria) that form a beneficial, species-specific, and evolutionarily old (60–130 Myr) association with earthworms. assembled draft Verminephrobacter aporrectodeae compared it the genomes eiseniae two nonsymbiotic close relatives (Acidovorax). Similar to V. eiseniae, was not markedly reduced size showed no A–T bias. characterized strength purifying selection (ω = dN/dS) codon usage bias 876 orthologous genes. Symbiont exhibited strong 0.09 ± 0.07), although transition symbiosis entailed relaxation as evidenced by 50% higher ω values less symbiont reference genomes. Relaxation evenly distributed functional gene categories but overrepresented genes involved signal transduction cell envelope biogenesis. The same also harbored instances positive clade. In total, detected 89 genes, including DNA metabolism, tRNA modification, TonB-dependent iron uptake, potentially highlighting functions important symbiosis. Our results suggest accompanied molecular adaptation, while only moderately relaxed, despite evolutionary age stability host association. hypothesize biparental transmission symbionts rare genetic mixing during can prevent symbionts.