Link attachment occurs preferentially towards proteins of high connectivity. (a) The color-coded plot shows the fraction of duplicate pairs with connectivities (k, k') that have gained a mutual interaction (cross-interaction) since duplication, as a function of k and k'. Points where all duplicate pairs have cross-interactions are shown in white, points where none carry a cross-interactions are shown black. Points (particularly at high connectivities) where no data is available are also shown in black. The number of duplicate pairs with given connectivities ranges from 2 to 39. Points in the k, k'-plane where only a single pair of duplicates exists are excluded. (b) For this histogram the data from a) are binned for low, medium, and high k + k' and the average for each bin is shown against k + k'. The number of k, k' values contributing to each bin are 10, 14, and 11, from left to right. Error bars give the standard error. (c) Assuming the functional form f
+ fk'for the probability of a cross-interaction between nodes with connectivities k and k' (asymmetric attachment), the most likely values of f
may be deduced from the data (see text). The maximum-likelihood result shows an approximately linear increase of f
with k. The alternative scenario, symmetric attachment, yields a smaller maximum likelihood. Only duplicate pairs with K
≤ 0.4 were used in this analysis in order to avoid overcounting of cross-interactions of duplicates of even older duplicates.