title: Diversity and host specificity of the Verminephrobacter-earthworm symbiosis.
abstract: Symbiotic bacteria of the genus Verminephrobacter (Betaproteobacteria) were detected in the nephridia of 19 out of 23 investigated earthworm species (Oligochaeta: Lumbricidae) by 16S rRNA gene sequence analysis and fluorescence in situ hybridization (FISH). While all four Lumbricus species and three out of five Aporrectodea species were densely colonized by a mono-species culture of Verminephrobacter, other earthworm species contained mixed bacterial populations with varying proportions of Verminephrobacter; four species did not contain Verminephrobacter at all. The Verminephrobacter symbionts could be grouped into earthworm species-specific sequence clusters based on their 16S rRNA and RNA polymerase subunit B (rpoB) genes. Closely related host species harboured more closely related symbionts than did distantly related hosts. Co-diversification of the symbiotic partners could not be demonstrated unambiguously due to the poor resolution of the host phylogeny [based on histone H3 and cytochrome c oxidase subunit I (COI) gene sequence analyses]. However, there was a pattern of symbiont diversification within four groups of closely related hosts. The mean rate of symbiont 16S rRNA gene evolution was determined using a relaxed clock model, and the rate was calibrated with paleogeographical estimates of the time of origin of Lumbricid earthworms. The calibrated rates of symbiont 16S rRNA gene evolution are 0.012-0.026 substitutions per site per 50 million years and thus similar to rates reported from other symbiotic bacteria.
authors.affiliation: Department of Biological Sciences, Microbiology, Aarhus University, Aarhus, Denmark.
authors.forename: Marie B
authors.initials: MB
authors.lastname: Lund
authors.forename: Seana K
authors.initials: SK
authors.lastname: Davidson
authors.forename: Martin
authors.initials: M
authors.lastname: Holmstrup
authors.forename: Sam
authors.initials: S
authors.lastname: James
authors.forename: Kasper U
authors.initials: KU
authors.lastname: Kjeldsen
authors.forename: David A
authors.initials: DA
authors.lastname: Stahl
authors.forename: Andreas
authors.initials: A
authors.lastname: Schramm
mesh_terms: D000818:Animals; D042621:Comamonadaceae / Q000235:genetics* / Q000254:growth & development; D004269:DNA, Bacterial / Q000235:genetics; D019143:Evolution, Molecular; D005798:Genes, Bacterial; D020459:Genes, rRNA; D017404:In Situ Hybridization, Fluorescence; D009835:Oligochaeta / Q000382:microbiology*; D010802:Phylogeny; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D013559:Symbiosis
chemical_list: D004269:DNA, Bacterial; D012336:RNA, Ribosomal, 16S