<i>Nucleophaga amutiana</i> , sp. nov.—a novel intranuclear parasite of amoebae from the Far East of Russia expands the diversity and biogeography of microsporidia-like organisms

The genus <i>Nucleophaga</i> comprises poorly studied intranuclear parasites that infect amoebae. Currently classified within the phylum Rozellomycota, this genus belongs among numerous lineages with unresolved taxonomic positions, primarily identified through metagenomic studies. Three species of <i>Nucleophaga</i> were described at the morphological and molecular levels-<i>N. amoebae, N. terricolae</i>, and <i>N. striatae</i>, all isolated in Europe. Here, we report the discovery of a fourth species of the genus, isolated from the Far East of Russia, infecting nuclei of <i>Thecamoeba</i> sp. A detailed light microscopic study revealed several remarkable morphological features of this organism. For the first time, the division of <i>Nucleophaga</i> plasmodium was illustrated. Experimental infections demonstrated that <i>Thecamoeba quadrilineata</i> and <i>T. foliovenanda</i> can support the development of the parasite, whereas <i>Nucleophaga</i> showed abnormal development in <i>T. onigiri</i> and was unable to survive in <i>T. vumurta</i>. Phylogenetic analysis, based on the nuc 18S rRNA gene sequences, placed the new isolate as a distinct lineage within the genus <i>Nucleophaga</i>. Based on its molecular characteristics, the studied isolate was described as a new species, <i>Nucleophaga amutiana</i>.