Figure 1

Schemes outlining two competing hypotheses for plastid origin(s) in algae containing secondary red algal symbionts. (a) New data from nucleus-encoded genes whose products are targeted to the plastid, and new data on dinoflagellate and Perkinsus plastids suggest that one secondary endosymbiotic origin spawned multiple groups, whereas (b) the canonical interpretation is that all these groups arose through independent secondary endosymbiotic events. Secondary endosymbioses are represented as introgressions of the red alga (or perhaps a green alga in the case of Apicomplexa) into circles within the lines of vertical descent. If scenario (a) is correct, ciliates have lost plastids secondarily, as have subgroups within the other lineages.