ABSTRACT. This revision of the classification of unicellular eukaryotes updates that of Levine et al. (1980) for the protozoa and expands it to include other protists. Whereas the previous revision was primarily to incorporate the results of ultrastructural studies, this revision incorporates results from both ultrastructural research since 1980 and molecular phylogenetic studies. We propose a scheme that is based on nameless ranked systematics. The vocabulary of the taxonomy is updated, particularly to clarify the naming of groups that have been repositioned. We recognize six clusters of eukaryotes that may represent the basic groupings similar to traditional ''kingdoms.'' The multicellular lineages emerged from within monophyletic protist lineages: animals and fungi from Opisthokonta, plants from Archaeplastida, and brown algae from Stramenopiles.

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Using flow cytometric analysis of fluorescence, we measured the genome sizes of 18 cultured ''freeliving'' species and 29 Symbiodinium spp. isolates cultured from stony corals, gorgonians, anemones, jellyfish, and giant clams. Genome size directly correlated with cell size, as documented previously for most eukaryotic cell lines. Among the smallest of dinoflagellates, Symbiodinium spp. (6-15 mm) possessed the lowest DNA content that we measured (1.5-4.8 pg . cell À 1 ). Bloom-forming or potentially harmful species in the genera Alexandrium, Karenia, Pfiesteria, and Prorocentrum possessed genomes approximately 2 to 50 times larger in size. A phylogenetic analysis indicated that genome/cell size has apparently increased and decreased repeatedly during the evolution of dinoflagellates. In contrast, genome sizes were relatively consistent across distantly and closely related Symbiodinium spp. This may be the product of intracellular host habitats imposing strong selective pressures that have restricted symbiont size.