Amoebophrya is a syndinian parasite that kills harmful bloom forming algae. Previously uncharacterized ultrastructural aspects of infection and development were elucidated. The biflagellate dinospore has two mitochondria, electron-dense bodies, striated strips, trichocysts, and a nucleus with peripherally condensed chromatin. After finding an Akashiwo sanguinea host and adhering to its surface, the parasite penetrates the host surface, apparently using a microfilament based motility and electron-dense bodies within a microtubular basket in the process of parasitophorous vacuole membrane formation. After entering the host nucleus, possibly by a similar mechanism used to enter the host cell, the parasite cytosol expanded substantially prior to mitosis. From 12-36 hours mitochondria were inconspicuous but present. Chromatin condensation was variable. By 36 hours post-infection, parasites had multiple nuclei, a microtubule-supported cytopharynx, and were beginning to form a fully internal mastigocoel. By 48 hours, the characteristic "beehive" appearance was apparent with flagella projecting into a fully developed mastigocoel. The cytoplasm contained trichocysts, elongated mitochondria, and nuclei with peripherally condensed chromatin. Although Amoebophrya lacks an apical complex, its electron-dense bodies show functional similarities to apicomplexan rhoptries. Its lack of permanently condensed chromosomes, but compact dinospore chromatin, supports the idea that dinoflagellate permanently condensed chromosomes may be a remnant of a parasitic ancestor with a compact dispersal stage.
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