Molecular phylogeny and ultrastructure of Aphelidium aff. melosirae (Aphelida, Opisthosporidia)

Aphelids are a poorly known group of parasitoids of algae that have raised considerable interest due to their pivotal phylogenetic position. Together with Cryptomycota and the highly derived Microsporidia, they have been recently re-classified as Opisthosporidia, being the sister group to fungi. Despite their huge diversity, as revealed by molecular environmental studies, and their phylogenetic interest, only three genera have been described (Aphelidium, Amoeboaphelidium, and Pseudaphelidium), from which 18S rRNA gene sequences exist only for Amoeboaphelidium species. Here, we describe the life cycle and ultrastructure of Aphelidium aff. melosirae, and provide the first 18S rRNA gene sequence obtained for this genus. Molecular phylogeny analysis indicates that Aphelidium is very distantly related to Amoebaphelidium, highlighting the wide genetic diversity of the aphelids. The parasitoid encysts and penetrates the host alga, Tribonema gayanum through an infection tube. Cyst germination leads to a young trophont that phagocytes the algal cell content and progressively develops a plasmodium, which becomes a zoospore-producing sporangium. Aphelidium aff. melosirae has amoeboflagellate zoospores, tubular/lamellar mitochondrial cristae, a metazoan type of centrosome, and closed orthomitosis with an intranuclear spindle. These features together with trophont phagocytosis distinguish Aphelidium from fungi and support the erection of the new superphylum Opisthosporidia as sister to fungi.