The aphelids (phylum Aphelida) are phagotrophic parasitoids of algae and represent the most basal branch in superphylum Opisthosporidia, which contains the Microsporidia, Rozellosporidia and Aphelida. Being the closest group to traditional fungi, the aphelids should have ancestral features of both phyla. As in chytrids and other zoosporic fungi, the structure of zoospores is the most informative and important morphological feature for the phylogeny and taxonomy of aphelids. Though a general zoospore description exists for some aphelid species, their flagellar apparatus (kinetid) structure, which contains pivotal taxonomic and phylogenetic characters, has not been studied. Here we represent the kinetid structure in two genera, Aphelidium and Paraphelidium, and demonstrate independent reduction in the kinetid in each genus. The kinetid-mitochondrion connection found in Aphelidium and Paraphelidium is rare for opisthokonts in general, but present in the most basal branches of Fungi and Opisthosporidia. We suggest, therefore, that this connection represents an ancestral character for both these phyla.
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