The single, ancient origin of chromist plastids.
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.
Yoon
Hwan Su
HS
Department of Biological Sciences and Center for Comparative Genomics, University of Iowa, 210 Biology Building, Iowa City 52242, USA.
Hackett
Jeremiah D
JD
Pinto
Gabriele
G
Bhattacharya
Debashish
D
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
D020418:Algal Proteins; D018076:DNA, Complementary; D002734:Chlorophyll