Identity of the Peridinium foliaceum (Pyrrhophyta) endosymbiont: Analysis of the rbcLS operon

Joby Marie Chesnick, Clifford W. Morden, Andrea M. Schmieg

J. Phycol. 32(5): 850-857 (1996)

Abstract. Extant chromophytic algae have been suggested to have obtained their chloroplasts by the engulfment of a photosynthetic alga by a colorless protist. The unusual dinoflagellate, Peridinium foliaceum, contains a reduced chlorophyll c- containing endosymbiont, and thus, represents an evolutionary intermediate stage in chloroplast acquisition events. Although the exact phylogenetic relationship of the symbiont to extant algal species is unknown, it had been suggested that the P. foliaceum symbiont had its evolutionary origins in 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 of ribulose-1,5-bisphosphate carboxylase/oxygenase, or Rubisco) by the P. foliaceum symbiont was pursued to provide insight into its identity. Cloned restriction fragments from a cpDNA library were screened, and those containing the rbcLS operon were then sequenced and parsimony phylogenetic analysis performed for each gene. Our data strongly suggest that the symbiont originated from a photosynthetic diatom. A discussion of phagotrophic dinoflagellate food preferences, feeding strategies, and the establishment of diatom symbioses in other organisms is presented. In addition, the utility of rbcS and the Rubisco intergenic spacer region for use in chromophyte algal phylogenetic assessments is examined.