Phylogenetic relationships among histone H2A proteins. The reconstruction was carried out by calculating the evolutionary amino acid p-distances from the H2A sequences of all the organisms analyzed (see Additional file 1). Histone H2A types are indicated on the right near the species names, including canonical H2A proteins (orange), and the variants H2A.X (blue), macro H2A (green), the H2A.Z fraction (red) and H2A.Bbd (purple). The root of the tree is labeled in black. Green, purple and red circles denote the monophyletic origin for the corresponding group of variants. Numbers for interior nodes indicate BP/CP confidence values. Numbers in colored boxes and in boldface account for the bootstrap values obtained in the reconstruction of the maximum parsimony trees using all the informative positions in the alignment. Confidence values were based on 1000 replications and are only shown if at least one of the values is >50%.