The alpha-proteobacteria: the Darwin finches of the bacterial world

Abstract

The alpha-proteobacteria represent one of the most diverse bacterial subdivisions, displaying extreme variations in lifestyle, geographical distribution and genome size. Species for which genome data are available have been classified into a species tree based on a conserved set of vertically inherited core genes. By mapping the variation in gene content onto the species tree, genomic changes can be associated with adaptations to specific growth niches. Genes for adaptive traits are mostly located in 'plasticity zones' in the bacterial genome, which also contain mobile elements and are highly variable across strains. By physically separating genes for information processing from genes involved in interactions with the surrounding environment, the rate of evolutionary change can be substantially enhanced for genes underlying adaptation to new growth habitats, possibly explaining the ecological success of the alpha-proteo-bacterial subdivision.

Figure 1

The α-proteobacterial species phylogeny. Circular tree of the α-proteobacterial phylogeny (Williams et al. 2007). The growth habitat, GC content and genome size are depicted for each species (numbers taken from the Genomes Online database, http://www.genomesonline.org).

Figure 2

Reduction and expansion of α-proteobacterial genomes. Example of mechanisms that contribute to genome (a) reductions ((i) genome streamlining, (ii) gene loss and (iii) gene transfer to host) and (b) expansions ((i) auxiliary replicon transfer, (ii) auxiliary replicon integration, (iii) island transfer and (iv) gene transfer from host).