The Ultrastructure of Sanchytrium tribonematis (Sanchytriaceae, Fungi incertae sedis) Confirms its Close Relationship to Amoeboradix

Abstract

Fungi encompass, in addition to classically well‐studied lineages, an ever‐expanding diversity of poorly known lineages that include, among others, zoosporic chytrid‐like parasites. According to recent phylogenetic analysis based on 18S + 28S rRNA concatenated genes two unusual chytrid‐like fungi Amoeboradix gromovi and Sanchytrium tribonematis form a monophyletic group, the family Sanchytriaceae, which represents a new divergent taxon that remains incertae sedis within Fungi. Zoospores of Amoeboradix gromovi contain one of the longest kinetosomes known in eukaryotic cells, which are composed of microtubular singlets or doublets. However, the ultrastructure of S. tribonematis, the type species of the genus had not been yet studied. Here, we provide the results of TEM investigations of zoospores and sporangia from two strains of S. tribonematis. The two strains are endowed with unusual features. Like in A. gromovi, amoeboid zoospores of S. tribonematis contain a long kinetosome composed of microtubular singlets, and the two orthogonal centrioles in their sporangia have nine microtubular singlets with an internal ring. The morphological and ultrastructural features of S. tribonematis are now included in the improved taxonomic diagnosis for this species.

Phylogenomics of a new fungal phylum reveals multiple waves of reductive evolution across Holomycota

“…To assess if sanchytrid metabolic capabilities are as reduced as suggested by their small genome sizes, we inferred their metabolic potential in comparison with other major fungal clades as well as other opisthokonts and amoebozoa as outgroup (43 species). Only half of the predicted sanchytrid proteins could be functionally annotated using EggNOG 72 (3,838 for A. gromovi ; 4,772 for S. tribonematis ), probably due to the fact that, being fast-evolving parasites 35,36,45 , many genes have evolved beyond recognition by annotation programs 73,74 . However, low annotation proportions are common in holomycotans, including fast-evolving parasites (e.g.…”

“…The number and timing of flagellum losses along the holomycotan branch remains to be solidly established. The flagellum is completely absent in nucleariids 12,51 but is found in representatives of all other major holomycotan clades, including rozellids 87 , aphelids 10 , and various canonical fungal groups, namely chytrids 78 , Blastocladiomycota 88 , Olpidium 21,26 , and sanchytrids 35,36,45 , although the latter are atypical. Sanchytrid amoeboid zoospores have never been observed swimming, but glide on solid surfaces via two types of pseudopods: thin filopodia growing in all directions and a broad hyaline pseudopodium at the anterior end.…”

“…Although several few-genes phylogenies suggested a relationship with Zoopagomycota 26,31,34 , its phylogenetic position remains unclear. Another interesting lineage of uncertain position is the sanchytrids, a group of chytrid-like parasites of algae represented by the genera Amoeboradix and Sanchytrium , which exhibit a highly reduced flagellum with an extremely long kinetosome 35,36 . Determining the phylogenetic position of these two zoosporic lineages remains decisive to infer the history of flagellum losses and the transition to hyphal-based multicellularity.…”

A flagellate-to-amoeboid switch in the closest living relatives of animals

“…Consistent with an ancient origin of all three phenotypes, amoeboid, flagellate, and amoeboflagellate cells are all broadly distributed in opisthokonts (animals, fungi, and their relatives; Figure 6M ). Interestingly, amoeboflagellate phenotypes have recently been described in several species occupying key phylogenetic positions, including in the sister group of choanozoans (the filastereans [ Tikhonenkov et al, 2020 ; Hehenberger et al, 2017 ]), in early-branching fungi ( Karpov et al, 2019 ; Karpov et al, 2018 ; Fritz-Laylin et al, 2017b ; Galindo et al, 2020 ), in the two closest known relatives of opisthokonts (apusomonads [ Cavalier-Smith and Chao, 2010 ] and breviates [ Minge et al, 2009 ]), and in early-branching amoebozoans ( Ptáčková et al, 2013 ). This phylogenetic distribution is consistent with an ancient origin and broad conservation of the amoeboflagellate phenotype in opisthokonts ( Cavalier-Smith and Chao, 2003 ).…”

A flagellate-to-amoeboid switch in the closest living relatives of animals

“…Consistent with an ancient origin of all three phenotypes, amoeboid, flagellate and amoeboflagellate cells are all broadly distributed in opisthokonts (animals, fungi, and their relatives). Interestingly, amoeboflagellate phenotypes have recently been described in several species occupying key phylogenetic positions, including in the sister-group of choanozoans (the filastereans) 37,38 , in early-branching fungi 39,68,69 , in the two closest known relatives of opisthokonts (apusomonads 70 and breviates 71 ), and in early-branching amoebozoans 72 . This is consistent with an ancient origin and broad conservation of the amoeboflagellate phenotype in opisthokonts 73 .…”