Helicosporidia are single-celled obligate endoparasites of invertebrates. They have a unique morphology and infection strategy, which make them unlike any other eukaryote. Molecular data were produced to clarify their phylogenetic relationship and to examine the evolution of their cryptic plastid. Phylogenetic analyses of 69 ribosomal proteins identified from an expressed sequence tag (EST) library showed that Helicosporidia are derived green algae and more specifically, are related to the trebouxiophyte algae. An obligate parasitic lifestyle is rare among plant and algal groups, and because Helicosporidium possesses no pigments and no chloroplast-like structure has been identified, photosynthetic ability has presumably been lost in this organism. I sought to examine the role that a relict plastid might play in Helicosporidium. I identified ESTs of 20 putatively plastid-targeted enzymes that are involved in a wide variety of metabolic pathways. As expected, no components of photosynthesis were found, but components of other metabolic pathways including sulfur metabolism and fatty acid, isoprenoid and heme biosynthesis suggest that Helicosporidium retains its plastid for these functions. The complete plastid genome of this species of Helicosporidium was sequenced and revealed only four protein-coding genes not involved in transcription or translation, with two of these confirming the metabolic functions suggested by the nuclear-encoded, plastid-targeted genes identified from the ESTs. In addition, the Helicosporidium plastid genome is one of the smallest known (37.5 kb). Its reduced size results from loss of many genes commonly found in plastids of other plants and algae (including all proteins that function in photosynthesis), elimination of duplicated genes and redundant tRNA isoacceptors, and minimization of intergenic spaces. The Helicosporidium plastid genome is also highly structured, with each half of the circular genome containing nearly all genes on one strand. Both the structure and content of the plastid genome and the deduced function of the organelle show parallels with the relict plastid found in the malaria parasite, Plasmodium falciparum. These unrelated organisms each evolved from photosynthetic ancestors, and the convergence in form and function of their relict plastids suggest that common forces shape plastid evolution, following the switch from autotrophy to parasitism.

Helicosporidia are single-celled obligate endoparasites of invertebrates. They have a unique morphology and infection strategy, which make them unlike any other eukaryote. Molecular data were produced to clarify their phylogenetic relationship and to examine the evolution of their cryptic plastid. Phylogenetic analyses of 69 ribosomal proteins identified from an expressed sequence tag (EST) library showed that Helicosporidia are derived green algae and more specifically, are related to the trebouxiophyte algae. An obligate parasitic lifestyle is rare among plant and algal groups, and because Helicosporidium possesses no pigments and no chloroplast-like structure has been identified, photosynthetic ability has presumably been lost in this organism. I sought to examine the role that a relict plastid might play in Helicosporidium. I identified ESTs of 20 putatively plastid-targeted enzymes that are involved in a wide variety of metabolic pathways. As expected, no components of photosynthesis were found, but components of other metabolic pathways including sulfur metabolism and fatty acid, isoprenoid and heme biosynthesis suggest that Helicosporidium retains its plastid for these functions. The complete plastid genome of this species of Helicosporidium was sequenced and revealed only four protein-coding genes not involved in transcription or translation, with two of these confirming the metabolic functions suggested by the nuclear-encoded, plastid-targeted genes identified from the ESTs. In addition, the Helicosporidium plastid genome is one of the smallest known (37.5 kb). Its reduced size results from loss of many genes commonly found in plastids of other plants and algae (including all proteins that function in photosynthesis), elimination of duplicated genes and redundant tRNA isoacceptors, and minimization of intergenic spaces. The Helicosporidium plastid genome is also highly structured, with each half of the circular genome containing nearly all genes on one strand. Both the structure and content of the plastid genome and the deduced function of the organelle show parallels with the relict plastid found in the malaria parasite, Plasmodium falciparum. These unrelated organisms each evolved from photosynthetic ancestors, and the convergence in form and function of their relict plastids suggest that common forces shape plastid evolution, following the switch from autotrophy to parasitism.

This thesis deals with the question of whether planktonic protits of the genus Dinophysis have permanent plastids (=chloroplasts) or practice kleptoplasty, i.e. acquire plastids via predation on other microorganisms. Sequencing the plastid 16S rDNA of Dinophysis spp. collected from 4 different geographical regions unveiled two different plastid genotypes within this genera: one that was found at all locations investigated, identical to that of the free-living cryptophyte Teleaulax amphioxeia, and another found only in the Greenland Sea, closely related to that of the cryptophyte Geminigera cryophila. Both types were found within the species D. acuminata. These findings imply that the plastids in Dinophysis spp. were not inherited from a common ancestor, but acquired from feeding. By using flow cytometry in combination with an acidotrophic probe, it was shown that 71 % of the cells in a D. norvegica population in the aphotic zone of the Baltic Sea had food-vacuoles. Dinophysis used to be regarded as a primarily phototrophic organism, and this was a higher proportion of cells with food-vacuoles than reported earlier. To further study if Dinophysis needs constant refill of new plastids from the environment, a new method combining flow-cytometry and quantitative real-time PCR was developed to compare the levels of nuclear and plastid DNA in different phases of the cell-cycle. Results showed that plastid acquisition in Dinophysis was uncoupled with the cell-cycle, which is different than the pattern seen in microalgal species with permanent plastids. Furthermore, when quantitative real-time PCR combined with flow-cytometry was used to follow D. caudata cultures during a 65 days starvation/feeding experiment, the cells first went through a steady decrease in plastid DNA during starvation. In contrast, after feeding on the ciliate Myrionecta rubra, plastid DNA in starved cells increased 7-fold, thereby directly revealing the kleptoplastic behavior. The main conclusion from this thesis is that Dinophysis cells are actively taking up kleptoplastids from the ciliates on which they feed, and that kleptoplasty is an important key to understand Dinophysis ecology. Part of this thesis work has also been dedicated to the application and optimization of new methods, and it shows how quantitative real-time PCR, flow cytometry and molecular methods in different combinations can be used as powerful tools for the study of plankton ecology.