As in the case of genes encoding pox-D5 family of NTPases in all ascoviruses, iridoviruses, and GfIV, genes encoding virion proteins cannot result from a horizontal transfer from a Campoplegine or Banchine ichnovirus genome to all ascovirus, iridovirus, phycodnaviruses and asfarvirus genomes. As the ascovirus genes encoding the two virion proteins investigated here are the closest relatives of virion proteins in CsIV, they can be considered a landmark reflecting the symbiogenic origin of the two ichnovirus lineages from ascoviruses closely related to DpAV4. In fact, the difficulty encountered in elucidating their sequence relationships can be explained by a combination of the marked transition from ascovirus to ichnovirus, and the significant selection constraints that resulted as the latter virus type evolved from the former.