Permalink : http://hdl.handle.net/2115/38506

KEYWORDS : Chloroplast;Diatom;Dinoflagellate;Endosymbiosis;Eyespot

The origin and evolutionary scenario of a small group of dinoflagellates with unusual chloroplasts are discussed. These dinoflagellates are known to possess an endosymbiotic alga of diatom origin. These are Durinskia baltica, Kryptoperidinium foliaceum, Peridinium quinquecorne, Durinskia sp., Gymnodinium quadrilobatum, Peridiniopsis rhomboids, Dinothrix paradoxa and a new coccoid dinoflagellate from Palau (P-18 strain). Although these eight species share a similar type of endosymbiont, morphologically they are so diverse that they may be classified as different entities, even to the ordinal level, using the current taxonomic criteria. To investigate the origin(s) and phylogenetic affinities of these dinoflagellates, the SSU rRNA and rbcL genes of D. baltica, K foliaceum, Durinskia sp., Peridiniopsis rhomboids, Dinothrix paradoxa and P-18 strain were sequenced and analysed. Phylogenetic trees based on nuclear encoded SSU rRNA gene strongly suggested that all these endosymbiotic dinoflagellates are monophyletic. The phylogenetic analyses based on the plastid encoded rbcL gene also revealed that all the endosymbiotic algae formed a unique clade within the diatom clade. Therefore, the acquisition of the endosymbiont took place only once and species diversified later. Of the diatoms included in the alignment, a pennate diatom, Nitzschia longissima var. reversa (Bacillariaceae) was shown to be the closest relative of the dinoflagellate endosymbionts. According to the fossil record, the members of the genus Nitzschia first appeared in Late Oligocene to Early Miocene and therefore, establishment of this endosymbiotic event must have been post this era. The evolutionary scenario of these dinoflagellates after establishment of the endosymbiont has been presented.