title: Evolutionary relationships of Fusobacterium nucleatum based on phylogenetic analysis and comparative genomics.
abstract: BACKGROUND
  The phylogenetic position and evolutionary relationships of Fusobacteria remain uncertain. Especially intriguing is their relatedness to low G+C Gram positive bacteria (Firmicutes) by ribosomal molecular phylogenies, but their possession of a typical gram negative outer membrane. Taking advantage of the recent completion of the Fusobacterium nucleatum genome sequence we have examined the evolutionary relationships of Fusobacterium genes by phylogenetic analysis and comparative genomics tools.
  
  
  RESULTS
  The data indicate that Fusobacterium has a core genome of a very different nature to other bacterial lineages, and branches out at the base of Firmicutes. However, depending on the method used, 35-56% of Fusobacterium genes appear to have a xenologous origin from bacteroidetes, proteobacteria, spirochaetes and the Firmicutes themselves. A high number of hypothetical ORFs with unusual codon usage and short lengths were found and hypothesized to be remnants of transferred genes that were discarded. Some proteins and operons are also hypothesized to be of mixed ancestry. A large portion of the Gram-negative cell wall-related genes seems to have been transferred from proteobacteria.
  
  
  CONCLUSIONS
  Many instances of similarity to other inhabitants of the dental plaque that have been sequenced were found. This suggests that the close physical contact found in this environment might facilitate horizontal gene transfer, supporting the idea of niche-specific gene pools. We hypothesize that at a point in time, probably associated to the rise of mammals, a strong selective pressure might have existed for a cell with a Clostridia-like metabolic apparatus but with the adhesive and immune camouflage features of Proteobacteria.
authors.affiliation: Evolutionary Genomics Group, División de Microbiología, Universidad Miguel Hernández, Apartado 18, San Juan 03550, Alicante, Spain. alex.mira@umh.es
authors.forename: Alex
authors.initials: A
authors.lastname: Mira
authors.forename: Ravindra
authors.initials: R
authors.lastname: Pushker
authors.forename: Boris A
authors.initials: BA
authors.lastname: Legault
authors.forename: David
authors.initials: D
authors.lastname: Moreira
authors.forename: Francisco
authors.initials: F
authors.lastname: Rodríguez-Valera
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics; D001482:Base Composition / Q000235:genetics; D002874:Chromosome Mapping / Q000379:methods; D002876:Chromosomes, Bacterial / Q000235:genetics; D004798:Enzymes / Q000235:genetics; D019143:Evolution, Molecular; D016967:Fusobacterium nucleatum / Q000201:enzymology / Q000235:genetics*; D023061:Gene Order / Q000235:genetics; D022761:Gene Transfer, Horizontal / Q000235:genetics; D005798:Genes, Bacterial / Q000235:genetics; D016680:Genome, Bacterial; D023281:Genomics / Q000379:methods*; D009876:Operon / Q000235:genetics; D010802:Phylogeny; D012329:RNA, Bacterial / Q000235:genetics; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D012337:RNA, Ribosomal, 18S / Q000235:genetics; D012338:RNA, Ribosomal, 23S / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D004798:Enzymes; D012329:RNA, Bacterial; D012336:RNA, Ribosomal, 16S; D012337:RNA, Ribosomal, 18S; D012338:RNA, Ribosomal, 23S; D011993:Recombinant Fusion Proteins