ALGAE OR PROTOZOA - PHYLOGENETIC POSITION OF EUGLENOPHYTES AND DINOFLAGELLATES AS INFERRED FROM MITOCHONDRIAL SEQUENCES

The chloroplasts of euglenophytes and dinoflagellates have been sugges ted to be the vestiges of endosymbiotic algae acquired during the proc ess of evolution. However, the evolutionary positions of these organis ms are still inconclusive, and they have been tentatively classified a s both algae and protozoa. A representative gene of the mitochondrial genome, cytochrome oxidase subunit I (coxI), was chosen and sequenced to clarify the phylogenetic positions of four dinoflagellates, two eug lenophytes and one apicomplexan protist. This is the first report of m itochondrial DNA sequences for dinoflagellates and euglenophytes. Our COXI tree shows clearly that dinoflagellates are closely linked to api complexan parasites but not with algae. Euglenophytes and algae appear to be only remotely related, with euglenophytes sharing a possible ev olutionary link with kinetoplastids, The COXI tree is in general agree ment with the tree based on the nuclear encoded small subunit of ribos omal RNA (SSU rRNA) genes, but conflicts with that based on plastid ge nes. These results support the interpretation that chloroplasts presen t in euglenophytes and dinoflagellates were captured from algae throug h endosymbioses, while their mitochondria were inherited from the host cell. We suggest that dinoflagellates and euglenophytes were original ly heterotrophic protists and that their chloroplasts are remnants of endosymbiotic algae.