Gene duplicability dependent on degrees. Correlation between the degree and the duplicability of proteins in the ( A ) yeast, ( B ) worm, ( C ) fly, ( D ) human, and ( E ) malaria parasite PINs. L, M, and H represent low- (k = 1), middle- (k = 2-6), and high-degree (k > 7) proteins, respectively. A vertical axis indicates the mean duplicability in each category. A species name above each diagram denotes the species with which the orthologous relationships were examined. For example, in the top left diagram in ( A ), gene duplicabilities were investigated using a phylogenetic tree containing S. cerevisiae and S. paradoxus genes. In ( A ) and ( C ), the results for MIPS and Rual et al. datasets, respectively, are shown, and those for other yeast and human datasets are provided in Additional file 5: Figure S5. In each diagram, the duplicability of proteins in each category is compared to one another by using the Wilcoxon rank-sum test with the Bonferroni correction. *, P < 0.05; **, P < 0.01; ***, P < 0.001.