Figure 1

Schematic representation of orthologous co-regulation. For two genes A and B in one species, co-expressed genes are more likely to functionally interact than is expected by chance. If the co-expression is conserved, i.e. also found in an orthologous gene pair, A' and B' in another species, where A is orthologous to A' (A and A' arouse from a speciation event, red circle in the figure) and B is orthologous to B', the probability of a functional interaction is increased. This likelihood might be even further increased if the conservation of co-expression occurs across three species at the same time. One might speculate that such conserved co-regulations have an ancient origin and constitute basic functionalities.