The aphelids are a small group of intracellular parasitoids of common species of eukaryotic phytoplankton with three known genera Aphelidium, Amoeboaphelidium, and Pseudaphelidium, and 10 valid species, which form along with related environmental sequences a very diversified group. The phyla Microsporidia and Cryptomycota, and the class Aphelidea have recently been considered to be a deep branch of the Holomycota lineage forming the so called the ARM-clade which is sister to the fungi. In this review we reorganize the taxonomy of ARM-clade, and establish a new superphylum the Opisthosporidia with three phyla: Aphelida phyl. nov., Cryptomycota and Microsporidia. We discuss here all aspects of aphelid investigations: history of our knowledge, life cycle peculiarities, the morphology (including the ultrastructure), molecular phylogeny, ecology, and provide a taxonomic revision of the phylum supplied with a list of species. We compare the aphelids with their nearest relatives, the species of Rozella, and improve the diagnosis of the phylum Cryptomycota.

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 the Opisthosporidia, which constitute the sister group to the fungi within the Holomycota. 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 and Aphelidium species. Here, we describe the life cycle and ultrastructure of a new representative of Aphelida, Paraphelidium tribonemae gen. et sp.nov., and provide the first 18S rRNA gene sequence obtained for this genus. Molecular phylogenetic analysis indicates that Paraphelidium is distantly related to both Aphelidium and Amoebaphelidium, highlighting the wide genetic diversity of aphelids. Paraphelidium tribonemae has amoeboflagellate zoospores, rhomboid mitochondrial cristae in zoospores, trophonts and plasmodia; lipid-microbody complex and dictyosomes. The amoeboid trophont uses pseudopodia to feed from the host cytoplasm. Though genetically distinct, the genus Paraphelidium is morphologically indistinguishable from other aphelid genera and has zoospores able to produce lamellipodia with subfilopodia like those of Amoeboaphelidium.

Fungi encompass, in addition to classically well-studied lineages, an ever-expanding diversity of poorly known lineages including zoosporic chytrid-like parasites. Here, we formally describe Amoeboradix gromovi gen. et sp. nov. comprising a set of closely related strains of chytrid-like parasites of the yellow-green alga Tribonema gayanum unusually endowed with amoeboid zoospores. Morphological and ultrastructural features of A. gromovi observed by light and transmission electron microscopy recall previous descriptions of Rhizophydium anatropum. A. gromovi exhibits one of the longest kinetosomes known in eukaryotes, composed of microtubular singlets or doublets. To carry out molecular phylogenetic analysis and validate the identification of different life cycle stages, we amplified 18S rRNA genes from three A. gromovi strains infecting T. gayanum cultures, single sporangia and single zoospores. Molecular phylogenetic analyses of 18S+28S rRNA concatenated genes of the type strain revealed that A. gromovi is closely related to the recently described species Sanchytrium tribonematis, another parasite of Tribonema that had been tentatively classified within Monoblepharidomycetes. However, our phylogenetic analysis with an extended taxon sampling did not show any particular affinity of Amoeboradix and Sanchytrium with described fungal taxa. Therefore, Amoeboradix gromovi and Sanchytrium tribonematis likely represent a new divergent taxon that remains incertae sedis within Fungi.

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.

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During the last decade several new orders were established in the class Chytridiomycetes on the basis of zoospore ultrastructure and molecular phylogeny. Here we present the ultrastructure and molecular phylogeny of strain x-51 CALU – a parasite of the alga Tribonema gayanum, originally described as Rhizophydium sp. based on light microscopy. Detailed investigation revealed that the zoospore ultrastructure of this strain has unique characters not found in any order of Chytridiomycetes: posterior ribosomal core unbounded by the endoplasmic reticulum and detached from the nucleus or microbody-lipid complex, and kinetosome composed of microtubular doublets. An isolated phylogenetic position of x-51 is further confirmed by the analysis of 18S and 28S rRNA sequences, and motivates the description of a new genus and species Gromochytrium mamkaevae. The sister position of G. mamkaevae branch relative to Mesochytrium and a cluster of environmental sequences, as well as the ultrastructural differences between Gromochytrium and Mesochytrium zoospores prompted us to establish two new orders: Gromochytriales and Mesochytriales.

Aphelids remain poorly known parasitoids of algae and have recently raised considerable interest due to their phylogenetic position at the base of Holomycota. Together with Cryptomycota (Rozellosporidia) and Microsporidia, they have been recently re-classified as the Opisthosporidia, which constitutes the sister group to the fungi within the Holomycota. Molecular environmental studies have revealed a huge diversity of aphelids, but only four genera have been described: Aphelidium, Amoeboaphelidium, Paraphelidium, and Pseudaphelidium. Here, we describe the life cycle of a new representative of Aphelida, Paraphelidium letcheri sp. nov., and provide the 18S rRNA gene sequence for this species. Molecular phylogenetic analysis indicates that P. letcheri is sister to Paraphelidium tribonemae and together they form a monophyletic cluster which is distantly related to both, Aphelidium, with flagellated zoospores, and Amoebaphelidium, with amoeboid zoospores

Aphelids remain poorly known parasitoids of algae but have raised considerable interest due to their phylogenetic position at the base of Holomycota. Together with Cryptomycota and Microsporidia, they have been recently reclassified in the Opisthosporidia, which constitutes the sister group to the fungi within the Holomycota. Molecular environmental studies have revealed a diversity of aphelids, but only four genera have been described: Aphelidium, Amoeboaphelidium, Paraphelidium and Pseudaphelidium. Here, we describe the life cycle of a known representative of Aphelida, Aphelidium tribonemae, and provide the first 18S rRNA gene sequence obtained for this species. Molecular phylogenetic analysis indicates that Aphelidium tribonemae is sister to A. aff. melosirae and both form a monophyletic cluster, which is distantly related to both Paraphelidium with flagellated zoospores, and Amoebaphelidium with amoeboid zoospores.