The establishment of chloroplasts as cellular organelles in the dinoflagellate, heterokont (stramenopile), haptophyte, and cryptophyte algae is widely accepted to have been the result of secondary endosymbiotic events, that is, the uptake of a photosynthetic eukaryote by a phagotrophic eukaryote. However, the circumstances that promote such associations between two phylogenetically distinct organisms and result in the integration of their genomes to form a single functional photosynthetic cell is unclear. The dinoflagellates Peridinium foliaceum and Peridinium balticum are unusual in that each contains a membrane-bound eukaryotic heterokont endosymbiont. These symbioses have been interpreted, through data derived from ultrastructural and biochemical investigations, to represent an intermediate stage of secondary endosymbiotic chloroplast acquisition. In this study we have examined the phylogenetic origin of the P. foliaceum and P. balticum heterokont endosymbionts through analysis of their nuclear small subunit ribosomal RNA genes. Our analyses clearly demonstrate both endosymbionts are pennate diatoms belonging to the family Bacillariaceae. Since members of the Bacillariaceae are usually benthic, living on shallow marine sediments, the manner in which establishment of a symbiosis between a planktonic flagellated dinoflagellate and a bottom-dwelling diatom is discussed. In particular, specific environmentally-associated life strategy stages of the host and symbiont, coupled with diatom food preferences by the dinoflagellate, may have been vital to the formation of this association.
Extant chromophytic algae have been suggested to have originated via the engulfment of a photo synthetic alga by a colorless protist. The dinoflagellate Peridinium foliaceum (Stein) Biecheler contains a reduced chlorophyll c–containing endosymbiont and, thus, represents an evolutionary intermediate stage in the establishment of chloroplasts. Although the exact phylogenetic relationship of the symbiont to extant algal species is unknown, it had been suggested that the P. foliaceum symbiont was either a diatom or a chrysophyte. Identification of the closest living relative of the P. foliaceum symbiont would provide a free‐living model system with which the photosynthetic symbiont could be compared. Nucleotide sequence analysis of rbcL and rbcS (encoding the large and small subunits ofribulose‐1,5‐bisphosphate carboxylase/oxygenase) by the P. foliaceum symbiont was performed to provide insights into its identity. Cloned restriction fragments from a chloroplast DNA library were screened, and clones encoding the rbcLS operon were sequenced. Parsimony phylogenetic analysis was performed for each gene. Our data strongly suggest that the symbiont originated from a photosynthetic diatom.
A gene encoding a thioredoxin protein was identified in the chloroplast genome of the rhodophyte Porphyra yezoensis. The P. yezoensis trxA gene contains 324 bp and is transcribed into a 0.7 kb messenger RNA. Analysis of the transcription start site demonstrates that canonical chloroplast -10 and -35 sequences are not present. The deduced amino acid sequence of the thioredoxin gene from the red algae has the greatest similarity to type m thioredoxins, providing strong support for the hypothesis that type m thioredoxins in photosynthetic eukaryotes originated from an engulfed bacterial endosymbiont. Hybridization analysis of nuclear and chloroplast DNAs from several members of the phyla Chromophyta and Rhodophyta using P. yezoensis DNA as a probe demonstrated strong hybridization to the chloroplast and nuclear genomes of Griffithsia pacifica and a weak cross-hybridization to the chromophyte P. foliaceum. The G. pacifica chloroplast gene has a 66% identity with the P. yezoensis DNA, contains conserved active site amino acid residues, but lacks a methionine start codon.
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