Figure 1.

Network growth models. The duplication of proteins involved in metabolic reactions results in growth of the metabolic network. Paralogous pairs that process different metabolites increase the number of nodes and lead to a horizontal expansion of the network, as depicted in part A. Paralogs catalysing the same reactions, for example in different conditions, would result in a more specialized expression of the proteins and lead to a vertical growth of the metabolic network with the number of nodes remaining constant (part B). The colour scale goes from red (less recent duplications) to blue (more recent duplications).