Wolbachia: evolutionary novelty in a rickettsial bacteria

Abstract

Background: Although closely related, the alpha-proteobacteria Wolbachia and the Rickettsiaceae (Rickettsia and Ehrlichia), employ different evolutionary life history strategies. Wolbachia are obligate endocellular symbionts that infect an extraordinary host range and, in contrast to the infectious and pathogenic Rickettsia and Ehrlichia, profoundly influence host reproductive biology.

Results: Phylogenies of the Rickettsia, Ehrlichia, and Wolbachia were independently inferred from 16S rDNA sequences and GroEL amino acid sequences. Topologies inferred from both sets of sequence data were consistent with one another, and both indicate the genus Wolbachia shared a common ancestor most recently with Ehrlichia. These two genera are a sister group to the genus Rickettsia. Mapping biological properties onto this phylogeny reveals that manipulation of host reproduction, characteristic of Wolbachia strains, is a derived characteristic. This evolutionary novelty is accompanied by the loss of the ability to infect vertebrate hosts.

Conclusions: Because of the contrasting transmission strategies employed by each, Wolbachia is expected to maximize efficiency of vertical transmission, while Ehrlichia and Rickettsia will optimize horizontal transfer of infection. Wolbachia manipulation of host reproduction could thus be viewed as strategy employed by this bacterium to foster its own propagation via vertical transmission.

Figure 1

Phylogeny derived from 1.3 kb of 16S rDNA sequence. Parsimony tree (PAUP vers. 4.0b4a). All branches had 100% bootstrap support, except where indicated by numbers on branches. Bold line indicates the lineage which has specialized in manipulating host reproduction, and which has lost the ability to infect vertebrate hosts. Wolbachia host genera are shown in parentheses. Wolbachia sequences used in this analysis were from infections in coleopteran (Diabrotica), dipteran (Drosophila), hymenopteran (Muscidifurax) and filarial worm (Brugia) hosts. Crossed line indicates ancestral loss of the ability to infect vertebrates, and acquisition of reproductive specialization.

Figure 2

Shortest maximum parsimony trees derived from GroEL amino acid sequence (left) and 16S rDNA sequences (right), showing congruence of 16S rDNA and GroEL trees. Rickettsia and Ehrlichia sequences used to construct both trees are from the same species, Wolbachia sequences used in the GroEL analysis are from different taxa than those used in the 16S analysis. Host genera of Wolbachia strains are shown in brackets.

Figure 3

Maximum likelihood tree derived from 16S rDNA sequences. Taxa included are the same as those in Figure 1, with the addition of E. sennetsu.