title: The single, ancient origin of chromist plastids.
abstract: Algae include a diverse array of photosynthetic eukaryotes excluding land plants. Explaining the origin of algal plastids continues to be a major challenge in evolutionary biology. Current knowledge suggests that plastid primary endosymbiosis, in which a single-celled protist engulfs and "enslaves" a cyanobacterium, likely occurred once and resulted in the primordial alga. This eukaryote then gave rise through vertical evolution to the red, green, and glaucophyte algae. However, some modern algal lineages have a more complicated evolutionary history involving a secondary endosymbiotic event, in which a protist engulfed an existing eukaryotic alga (rather than a cyanobacterium), which was then reduced to a secondary plastid. Secondary endosymbiosis explains the majority of algal biodiversity, yet the number and timing of these events is unresolved. Here we analyzed a five-gene plastid data set to show that a taxonomically diverse group of chlorophyll c(2)-containing protists comprising cryptophyte, haptophyte, and stramenopiles algae (Chromista) share a common plastid that most likely arose from a single, ancient ( approximately 1,260 million years ago) secondary endosymbiosis involving a red alga. This finding is consistent with Chromista monophyly and implicates secondary endosymbiosis as an important force in generating eukaryotic biodiversity.
authors.affiliation: Department of Biological Sciences and Center for Comparative Genomics, University of Iowa, 210 Biology Building, Iowa City 52242, USA.
authors.forename: Hwan Su
authors.initials: HS
authors.lastname: Yoon
authors.forename: Jeremiah D
authors.initials: JD
authors.lastname: Hackett
authors.forename: Gabriele
authors.initials: G
authors.lastname: Pinto
authors.forename: Debashish
authors.initials: D
authors.lastname: Bhattacharya
mesh_terms: D020418:Algal Proteins / Q000235:genetics; D002734:Chlorophyll / Q000032:analysis; D018076:DNA, Complementary / Q000235:genetics; D017753:Ecosystem; D017343:Genes, Plant; D008957:Models, Genetic; D008969:Molecular Sequence Data; D010788:Photosynthesis; D010802:Phylogeny; D018087:Plastids / Q000235:genetics; D020133:Reverse Transcriptase Polymerase Chain Reaction; D000461:Rhodophyta / Q000235:genetics / Q000648:ultrastructure; D016415:Sequence Alignment; D012689:Sequence Homology, Nucleic Acid; D013559:Symbiosis
chemical_list: D020418:Algal Proteins; D018076:DNA, Complementary; D002734:Chlorophyll