title: Phylogenetic analysis of symbionts in feather-feeding lice of the genus Columbicola: evidence for repeated symbiont replacements.
abstract: BACKGROUND
  Many groups of insects have obligate bacterial symbionts that are vertically transmitted. Such associations are typically characterized by the presence of a monophyletic group of bacteria living in a well-defined host clade. In addition the phylogeny of the symbiotic bacteria is typically congruent with that of the host, signifying co-speciation. Here we show that bacteria living in a single genus of feather lice, Columbicola (Insecta: Phthiraptera), present an exception to this typical pattern.
  
  
  RESULTS
  The phylogeny of Columbicola spp. symbionts revealed the presence of three candidate clades, with the most species-rich clade having a comb-like topology with very short internodes and long terminal branches. Evolutionary simulations indicate that this topology is characteristic of a process of repeated symbiont replacement over a brief time period. The two remaining candidate clades in our study exhibit high levels of nucleotide substitution, suggesting accelerated molecular evolution due to relaxed purifying selection or smaller effective population size, which is typical of many vertically transmitted insect symbionts. Representatives of the fast-evolving and slow-evolving symbiont lineages exhibit the same localization, migration, and transmission patterns in their hosts, implying direct replacement.
  
  
  CONCLUSIONS
  Our findings suggest that repeated, independent symbiont replacements have taken place over the course of the relatively recent radiation of Columbicola spp. These results are compatible with the notion that lice and other insects have the capability to acquire novel symbionts through the domestication of progenitor strains residing in their local environment.
authors.affiliation: Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, UT 84112, USA.
authors.forename: Wendy A
authors.initials: WA
authors.lastname: Smith
authors.forename: Kelly F
authors.initials: KF
authors.lastname: Oakeson
authors.forename: Kevin P
authors.initials: KP
authors.lastname: Johnson
authors.forename: David L
authors.initials: DL
authors.lastname: Reed
authors.forename: Tamar
authors.initials: T
authors.lastname: Carter
authors.forename: Kari L
authors.initials: KL
authors.lastname: Smith
authors.forename: Ryuichi
authors.initials: R
authors.lastname: Koga
authors.forename: Takema
authors.initials: T
authors.lastname: Fukatsu
authors.forename: Dale H
authors.initials: DH
authors.lastname: Clayton
authors.forename: Colin
authors.initials: C
authors.lastname: Dale
mesh_terms: D000818:Animals; D001419:Bacteria / Q000145:classification* / Q000235:genetics / Q000302:isolation & purification; D018407:Bacterial Physiological Phenomena; D001483:Base Sequence; D019143:Evolution, Molecular; D058326:Ischnocera / Q000382:microbiology* / Q000502:physiology*; D008969:Molecular Sequence Data; D010802:Phylogeny; D013559:Symbiosis