Ultrastructure of a novel tube-forming, intracellular parasite of dinoflagellates: Parvilucifera prorocentri sp. nov. (Alveolata, Myzozoa).

We have characterized the intracellular development and ultrastructure of a novel parasite that infected the marine benthic dinoflagellate Prorocentrum fukuyoi. The parasite possessed a combination of features described for perkinsids and syndineans, and also possessed novel characters associated with its parasitic life cycle. Reniform zoospores, about 4 microm long, possessed a transverse flagellum, alveoli, a refractile body, a mitochondrion with tubular cristae, a syndinean-like nucleus with condensed chromatin, micronemes, bipartite trichocysts with square profiles (absent in perkinsids) and oblong microbodies. Like Parvilucifera, the zoospores also possessed a shorter posterior flagellum, a heteromorphic pair of central microtubules in the anterior axoneme and a reduced pseudoconoid positioned directly above an orthogonal pair of basal bodies. Early developmental stages consisted of a sporangium about 5-15 microm in diam that contained spherical bodies and amorphous spaces. The undifferentiated sporangium increased to about 20-25 microm in diam before being enveloped by a wall with a convoluted mid-layer. The sporangium differentiated into an unordered mass of zoospores that escaped from the cyst through a pronounced germ tube about 4-5 microm in diam and 10-15 microm long. Weakly developed germ tubes have been described in Perkinsus but are absent altogether in Parvilucifera and syndineans. Comparison of these data with other myzozoans led us to classify the parasite as Parvilucifera prorocentri sp. nov., Myzozoa. Although we were hesitant to erect a new genus name in the absence of molecular sequence data, our ultrastructural data strongly indicated that this parasite is most closely related to perkinsids and syndineans, and represents an intriguing candidate for the cellular identity of a major subclade of Group I alveolates.