Multiple sequences were obtained for each of the P. marinus histones; in most of the phylogenetic trees, these sequences clustered together and allied with apicomplexans except for the H3 tree, in which one P. marinus H3 grouped with the apicomplexan Toxoplasma gondii, the other with dinoflagellate/ ciliate clade (Figures 8, 9 ). Histone 2A in many organisms has acquired an isoform referred to as H2A.X. In both dinoflagellates and P. marinus, H2A.X seems to be the dominant, if not the only, form. The homolog retrieved from the P. marinus genome was clustered with H2A.X in the clade of apicomplexans (Figure 8 ).