@article{fdi:010055753,
  title = {{P}urifying selection and molecular adaptation in the genome of verminephrobacter, the heritable symbiotic bacteria of earthworms},
  author = {{K}jeldsen, {K}. {U}. and {B}ataillon, {T}. and {P}inel, {N}. and {D}e {M}ita, {S}t{\'e}phane and {L}und, {M}. {B}. and {P}anitz, {F}. and {B}endixen, {C}. and {S}tahl, {D}. {A}. and {S}chramm, {A}.},
  editor = {},
  language = {{ENG}},
  abstract = {{W}hile genomic erosion is common among intracellular symbionts, patterns of genome evolution in heritable extracellular endosymbionts remain elusive. {W}e study vertically transmitted extracellular endosymbionts ({V}erminephrobacter, {B}etaproteobacteria) that form a beneficial, species-specific, and evolutionarily old (60-130 {M}yr) association with earthworms. {W}e assembled a draft genome of {V}erminephrobacter aporrectodeae and compared it with the genomes of {V}erminephrobacter eiseniae and two nonsymbiotic close relatives ({A}cidovorax). {S}imilar to {V}. eiseniae, the {V}. aporrectodeae genome was not markedly reduced in size and showed no {A}-{T} bias. {W}e characterized the strength of purifying selection (omega = d{N}/d{S}) and codon usage bias in 876 orthologous genes. {S}ymbiont genomes exhibited strong purifying selection (omega = 0.09 +/- 0.07), although transition to symbiosis entailed relaxation of purifying selection as evidenced by 50% higher omega values and less codon usage bias in symbiont compared with reference genomes. {R}elaxation was not evenly distributed among functional gene categories but was overrepresented in genes involved in signal transduction and cell envelope biogenesis. {T}he same gene categories also harbored instances of positive selection in the {V}erminephrobacter clade. {I}n total, positive selection was detected in 89 genes, including also genes involved in {DNA} metabolism, t{RNA} modification, and {T}on{B}-dependent iron uptake, potentially highlighting functions important in symbiosis. {O}ur results suggest that the transition to symbiosis was accompanied by molecular adaptation, while purifying selection was only moderately relaxed, despite the evolutionary age and stability of the host association. {W}e hypothesize that biparental transmission of symbionts and rare genetic mixing during transmission can prevent genome erosion in heritable symbionts.},
  keywords = {symbiosis ; evolution ; nonsynonymous substitutions ; purifying selection ; positive selection ; genome reduction ; extracellular symbiont},
  booktitle = {},
  journal = {{G}enome {B}iology and {E}volution},
  volume = {4},
  numero = {3},
  pages = {307--315},
  ISSN = {1759-6653},
  year = {2012},
  DOI = {10.1093/gbe/evs014},
  URL = {https://www.documentation.ird.fr/hor/fdi:010055753},
}