Abstract 55 3.1 Introduction 56 3.2 Primary Plastids 56 3.3 One Origin 58 3.4 Three Lineages 59 3.5 Secondary Plastids 60 3.6 Red and Green Endosymbionts 61 3.7 How Many Secondary Endosymbioses? 63 3.8 Loss of Photosynthesis: How Common Is It? 65 3.9 Tertiary Endosymbiosis, Serial Secondary Endosymbiosis 67 3.10 A Second Primary Endosymbiosis? 68 Acknowledgments 69 References 69

Plastids, the light-harvesting organelles of photosynthetic eukaryotes, are derived from an ancient symbiosis between a eukaryote and a cyanobacterium. This process is called primary endosymbiosis, and accounts for plastids in glaucocystophytes, red algae, green algae and land plants. All other plastid-containing eukaryotes acquired their plastids from either a red or a green alga by secondary endosymbiosis, in which a eukaryotic cell swallows a second phototrophic eukaryote and retains its photosynthetic machinery. Secondary endosymbiosis accounts for the plastids found in most of the diversity of eukaryotic algae, including the chlorarachniophytes, euglenids, cryptomonads, haptophytes, heterokonts, dinoflagellates and apicomplexan parasites. This chapter discusses the changing views with respect to the origin and evolution of plastid-containing organisms, with an emphasis on the molecular phylogenetic evidence bearing on the number of primary and secondary endosymbioses that have occurred during eukaryotic evolution.