Morphological and Genetic Diversity of Opisthosporidia: New Aphelid Paraphelidium tribonemae gen. et sp. nov.

Abstract

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.

Molecular Phylogeny of Paraphelidium letcheri sp. nov. (Aphelida, Opisthosporidia)

“…This trait was not observed in Aphelidium species, which seem to produce filopodia straight from the cell body and never from lamellipodia (Karpov et al. ,b). The amoeboid appearance of zoospores has been described in all aphelids, so we can distinguish the genera by the morphology of this stage.…”

“…The aphelid strain X‐129 can be attributed with certainty to the genus Paraphelidium based on its life cycle and type of flagellated zoospore, which is able to produce a lamellipodium with subfilopodia (Karpov et al. ,b). The 18S rRNA gene phylogeny strongly supports this affiliation as strain X‐129 branches close to P. tribonemae (Fig.…”

“…Consequently, zoospore morphology is used to distinguish aphelid genera from each other: Amoeboaphelidium has amoeboid zoospores with a highly reduced flagellum, a pseudocilium, thus, they cannot swim; Aphelidium and Paraphelidium have flagellated zoospores able to produce filopodia and lamellipodia; and the marine Pseudaphelidium produces amoeboid zoospores that encyst immediately after being released and subsequently release flagellated zoospores from these cysts (Gromov ; Karpov et al. , , ; Letcher et al. , ; Schweikert and Schnepf ).…”

“…) and three strains of the recently described genus and species Paraphelidium tribonemae (Karpov et al. ). Recently, one species, Aphelidium tribonemae, has found its place in molecular phylogenetic analyses as sister to A .…”

Molecular Phylogeny and Ultrastructure of Aphelidium desmodesmi, a New Species in Aphelida (Opisthosporidia)

“…Within Aphelida are four genera as well as a number of environmental sequences (Karpov et al. , , ,c). The genera Aphelidium (Zopf ) emend Gromov (with six species, including the one described herein), Amoeboaphelidium (Scherffel ) emend Gromov ; emend Karpov 2014 (with five species), and Paraphelidium Karpov, Moriera & López‐García 2016 (with two species) occur in freshwater habitats, while the monotypic Pseudaphelidium Schweikert and Schnepf (, ) occurs in marine environments.…”