Fig. 1.

Schematic representation of relationships between apicomplexan parasites and their closest relatives and the evolution of their plastids. The closest known branch to the ‘true’ apicomplexans (at top, including Coccidia, Piroplasms, Haemosporidians and the paraphyletic Gregarines) is a biologically undescribed lineage known only from plastid environmental surveys, the so-called ARL-V lineage. The nearest relatives that have been biologically characterized include a diverse array of predatory flagellates (Colpodella, Voromonas and Alphamonas), photosynthetic coral symbionts (Chromera and Vitrella) and a large number of unknown environmental lineages (many from coral, but also many from other environments). These are all in turn related to a large group including dinoflagellates and their closest relatives, the Perkinsids and Psammosa, both of which possess structures homologous to the apical complex, and the enigmatic predator Acavamons. The column to the right summarizes what we know about plastids in each lineage: red plastids indicate photosynthesis, colourless plastids indicates plastids that are known but non-photosynthetic. ARL-V is hypothesized to be photosynthetic but this has not been tested, and dinoflagellates contain about 50% photosynthetic and non-photosynthetic species. Lineages for which no plastid has been detected are indicated by a question mark.