Discocelia Plataet Sp. n., a Small Incertae Sedis Cercozoan Flagellate

Cercozoa = Filosa (Rhizaria, SAR) is one of the largest rhizarian subgroups and consists of a diverse assemblage of amoeboid and flagellated protists. They are ecologically significant in microbial food webs, widely diverse, and even abundant in soils and deep marine sediments according to environmental sequencing. In spite of this, the cercozoan phylogeny remains poorly resolved by SSU rRNA gene analysis, and omics data are available for only a few well-characterized species. Here, we have sequenced the transcriptomes of three new gliding monadofilosan strains: the glissomonad RAM19S6, the marimonad CRO19P5, and the discocelid GT001. Because of its unusual morphology, we performed a thorough morphological characterization of the strain GT001 using light and electron microscopy and described a new species, Discocelia plataet sp. n. Transmission electron microscopy and expansion microscopy revealed the structure of the flagellar apparatus, allowing us to identify cercozoan microtubular root homologies and supplement our knowledge of the discocelid cell structure with new details. Unique features of the new species are the absence of body tip and velum tip, discoidal mitochondrial cristae, and presence of an acronema on the posterior flagellum. We discuss the phylogenetic position of the three strains within Monadofilosa and the evolutionary context of the order Discocelida.