Domain architecture of the PWWP-containing proteins in Chlamydomonas and Volvox. Domains in the five Chlamydomonas and four Volvox proteins are drawn to scale. a) - b) single PWWP-containing proteins that are most similar to each other’s respective homologs. These proteins do not have counterparts in land plants but are highly conserved in extant algal and marine metagenomes; c) two algal PWWP containing proteins from the nucleosome modifying SNF2 family. Only the Chlamydomonas/Volvox proteins have ‘gained’ PWWP domains as a volvocine-specific feature; d) two PWWP containing proteins from the ASH1 histone methyltransferase family. Only the Chlamydomonas/Volvox proteins have ‘gained’ PWWP domains as a volvocine-specific feature; e) the earliest occurrence of the Trithorax (ATX1/2) type architecture with a fully assembled PWWP- DAST-ePHD-SET domain combination in Chlamydomonas. f) The PWWP domain has been lost from the Volvox protein.