IDENTITY OF THE ENDOSYMBIONT OF PERIDINIUM-FOLIACEUM (PYRROPHYTA) - ANALYSIS OF THE RBCLS OPERON

Extent chromophytic algae have been suggested to have originated via t he engulfment of a photosynthetic alga by a colorless protist. The din oflagellate Peridinium foliaceum (Stein) Biecheler contains a reduced chlorophyll c-containing endosymbiont and, thus, represents an evoluti onary intermediate stage in the establishment of chloroplasts. Althoug h the exact phylogenetic relationship of the symbiont to extant algal species is unknown, it had been suggested that the P. foliaceum symbio nt was either a diatom or a chrysophyte. Identification of the closest living relative of the P. foliaceum symbiont would provide a free-liv ing model system with which the photosynthetic symbiont could be compa red. Nucleotide sequence analysis of rbcL and rbcS (encoding the large and small subunits of ribulose-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 libr ary were screened, and clones encoding the rbcLS operon were sequenced . Parsimony phylogenetic analysis was performed for each gene. Our dat a strongly suggest that the symbiont originated from a photosynthetic diatom.