Figure 2.

Asymmetrically linked reaction pairs (A→B) related to asymmetry in gene essentiality, growth defects, gene expression and genome evolution. The fraction (f_0/1 = n_0/1/(n_0/1 + n_1/0)) where only B is essential in rich medium (essentiality) or has an effect on the growth across conditions (growth), where only B is expressed across conditions (expression), where only B is present across species (occurrence), where only B is present after gain, loss or maintenance over evolutionary lineages, and where A is contingently gained over evolutionary lineages (contingent gain A) is averaged over all reaction pairs (see Materials and methods). For conserved pairs there is no relevant result on gain, because too few (n = 2) events were found.