<italic>Parvularia atlantis</italic> gen. et sp. nov., a Nucleariid Filose Amoeba (Holomycota, Opisthokonta).

Abstract: The opisthokonts constitute a eukaryotic supergroup divided into two main clades: the holozoans, which include animals and their unicellular relatives, and the holomycotans, which include fungi, opisthosporidians, and nucleariids. Nucleariids are phagotrophic filose amoebae that phenotypically resemble more their distant holozoan cousins than their holomycotan phylogenetic relatives. Despite their evolutionary interest, the diversity and internal phylogenetic relationships within the nucleariids remain poorly studied. Here, we formally describe and characterize by molecular phylogeny and microscopy observations <italic>Parvularia atlantis</italic> gen. et sp. nov. (formerly <italic>Nuclearia</italic> sp. ATCC 50694), and compare its features with those of other nucleariid genera. <italic>Parvularia</italic> is an amoebal genus characterized by radiating knobbed and branching filopodia. It exhibits prominent vacuoles observable under light microscopy, a cyst‐like stage, and completely lacks cilia. <italic>P. atlantis</italic> possesses one or two nuclei with a central nucleolus, and mitochondria with flat or discoid cristae. These morphological features, although typical of nucleariids, represent a combination of characters different to those of any other described <italic>Nuclearia</italic> species. Likewise, 18S rRNA‐based phylogenetic analyses show that <italic>P. atlantis</italic> represents a distinct lineage within the nucleariids. [ABSTRACT FROM AUTHOR]