Average organism degree (OD) as function of the distance from the most central reaction node. The centre node is defined as the reaction node with the highest betweenness centrality conditioned that it is present in all organisms. The horizontal line is the average OD of all reaction nodes in the network. Both on the family level (Chlamydiae and Proteobacteria) and for the union of bacteria there is a decay in OD when moving outward from the core. This indicates that reactions with high OD are in general found in the centre while species specific reactions with low OD are found further out.
Bernhardsson et al. BMC Evolutionary Biology 2011 11:20 doi:10.1186/1471-2148-11-20