Evolution of subspecies of Francisella tularensis

Abstract

Analysis of unidirectional genomic deletion events and single nucleotide variations suggested that the four subspecies of Francisella tularensis have evolved by vertical descent. The analysis indicated an evolutionary scenario where the highly virulent F. tularensis subsp. tularensis (type A) appeared before the less virulent F. tularensis subsp. holarctica (type B). Compared to their virulent progenitors, attenuated strains of F. tularensis exhibited specific unidirectional gene losses.

FIG. 1.

Unidirectional repeat-mediated deletion mechanism exemplified by RD4. The symbol Δ represents deleted sequence. Flanking direct repeats (DR1 and DR2) and the resulting composite repeat are depicted as hatched bars. Open bars represent open reading frames that were truncated by the deletion. Partial nucleotide sequences are shown before (DR1 and DR2) and after (composite repeat) the deletion event and allowed analysis of evolutionary direction.

FIG. 2.

Evolutionary scenario of F. tularensis based on deletion characters assuming a successive loss of genetic material. The scheme reads from left (F. tularensis ancestor) to right. F. tularensis subspecies are indicated. The horizontal distance may not correspond to actual phylogenetic distances calculated by other methods.

FIG. 3.

Phylogenetic analysis of F. tularensis using a maximum-likelihood algorithm and based on concatenated sequences of gene fragments. A. Four-gene tree constructed using amino acid sequences (631 amino acids). The tree was rooted using A. tumefaciens and Y. pestis as outgroups. B. Seven-gene analysis using nucleotide sequences (3,135 bp). The tree was rooted using F. tularensis subsp. novicida as outgroup. F. tularensis subspecies and the division of type A isolates into two clades (A.I, A.II) are indicated. Bootstrap values from 1,000 replications are indicated at interior branch nodes (values of <50% not shown).