Phylogenetic affinities of symbiotic and non-symbiotic nitrogen fixing bacteria. Different divisions of nitrogen fixing bacteria (classes/orders for proteobacteria) are indicated. Groups containing symbiotic species are marked with grey, non-symbiotic groups with white ellipses. Further information on nitrogen-fixing bacteria, different interactions, hosts and localisation of symbionts is provided in the text and summarised in table 1. Branches receiving less than 70% non-parametric bootstrap support in analyses of ingroup taxa (428 base positions) are dotted.