Cultured Asgard Archaea Shed Light on Eukaryogenesis.

The first cultured Asgard archaeon lives in metabolic symbiosis with hydrogen-scavenging microbes. Its full-genome analysis authenticates the existence of Asgard archaea, previously only known from metagenome-assembled genomes, confirms their closer phylogenetic relatedness to eukaryotes and reinforces the idea that the eukaryotic cell evolved from an integrated archaeal-bacterial syntrophic consortium.

Coevolution of reproducers and replicators at the origin of life and the conditions for the origin of genomes.

There are two fundamentally distinct but inextricably linked types of biological evolutionary units, reproducers and replicators. Reproducers are cells and organelles that reproduce via various forms of division and maintain the physical continuity of compartments and their content. Replicators are genetic elements (GE), including genomes of cellular organisms and various autonomous elements, that both cooperate with reproducers and rely on the latter for replication. All known cells and organisms comprise a union between replicators and reproducers. We explore a model in which cells emerged via symbiosis between primordial "metabolic" reproducers (protocells) which evolved, on short time scales, via a primitive form of selection and random drift, and mutualist replicators. Mathematical modeling identifies the conditions, under which GE-carrying protocells can outcompete GE-less ones, taking into account that, from the earliest stages of evolution, replicators split into mutualists and parasites. Analysis of the model shows that, for the GE-containing protocells to win the competition and to be fixed in evolution, it is essential that the birth-death process of the GE is coordinated with the rate of protocell division. At the early stages of evolution, random, high-variance cell division is advantageous compared with symmetrical division because the former provides for the emergence of protocells containing only mutualists, preventing takeover by parasites. These findings illuminate the likely order of key events on the evolutionary route from protocells to cells that involved the origin of genomes, symmetrical cell division, and antiparasite defense systems.

Phylogenomics Supports the Monophyly of Aphelids and Fungi and Identifies New Molecular Synapomorphies.

The supergroup Holomycota, composed of Fungi and several related lineages of unicellular organisms (Nucleariida, Rozellida, Microsporidia, and Aphelida), represents one of the major branches in the phylogeny of eukaryotes. Nevertheless, except for the well-established position of Nucleariida as the first holomycotan branch to diverge, the relationships among the other lineages have so far remained unresolved largely owing to the lack of molecular data for some groups. This was notably the case aphelids, a poorly known group of endobiotic phagotrophic protists that feed on algae with cellulose walls. The first molecular phylogenies including aphelids supported their sister relationship with Rozellida and Microsporidia which, collectively, formed a new group called Opisthosporidia (the "Opisthosporidia hypothesis"). However, recent phylogenomic analyses including massive sequence data from two aphelid genera, Paraphelidium and Amoeboaphelidium, suggested that the aphelids are sister to fungi (the "Aphelida $+$ Fungi hypothesis"). Should this position be confirmed, aphelids would be key to understanding the early evolution of Holomycota and the origin of Fungi. Here, we carry out phylogenomic analyses with an expanded taxonomic sampling for aphelids after sequencing the transcriptomes of two species of the genus Aphelidium (Aphelidium insulamus and Aphelidium tribonematis) in order to test these competing hypotheses. Our new phylogenomic analyses including species from the three known aphelid genera strongly rejected the Opisthosporidia hypothesis. Furthermore, comparative genomic analyses further supported the Aphelida $+$ Fungi hypothesis via the identification of 19 orthologous genes exclusively shared by these two lineages. Seven of them originated from ancient horizontal gene transfer events predating the aphelid-fungal split and the remaining 12 likely evolved de novo, constituting additional molecular synapomorphies for this clade. Ancestral trait reconstruction based on our well-resolved phylogeny of Holomycota suggests that the progenitor of both fungi and rozellids, was aphelid-like, having an amoeboflagellate state and likely preying endobiotically on cellulose-containing, cell-walled organisms. Two lineages, which we propose to call Phytophagea and Opisthophagea, evolved from this ancestor. Phytophagea, grouping aphelids and classical fungi, mainly specialized in endobiotic predation of algal cells. Fungi emerged from this lineage after losing phagotrophy in favor of osmotrophy. Opisthophagea, grouping rozellids and Microsporidia, became parasites, mostly of chitin-containing hosts. This lineage entered a progressive reductive process that resulted in a unique lifestyle, especially in the highly derived Microsporidia. [Aphelida, fungi, Holomycota, horizontal gene transfer, phylogenomics, synapomorphy.].

Morphological and molecular characterization of a new member of the phylum Rhodelphidia.

Rhodelphidia is a recently discovered phylum within the supergroup Archaeplastida, comprising only two known representatives (Rhodelphis marinus and Rhodelphis limneticus). Despite its close phylogenetic relatedness to red algae, Rhodelphidia differ markedly by being nonphotosynthetic eukaryotrophic flagellates with gene- and intron-rich genomes. Here, we describe a new freshwater Rhodelphidia species, Rhodelphis mylnikovi sp. n., strain Rhod-M. It shows clear morphological differences with the two other Rhodelphis species, including larger cell body size, presence of two contractile vacuoles, short and blunt pseudopodia, absence of cysts, and tendency to cannibalism. 18S rRNA-based phylogenetic analysis placed it sister to the freshwater species R. limneticus.

Refurbishing the marine parasitoid order Pirsoniales with newly (re)described marine and freshwater free-living predators.

Pirsoniales is a stramenopile order composed of marine parasitoids of diatoms with unique life cycle. Until recently, a single genus, Pirsonia, uniting six species, was known. The recent identification of new free-living eukaryotrophic Pirsoniales Pirsonia chemainus, Feodosia pseudopoda, and Koktebelia satura changed our understanding of this group as exclusively parasitic. However, their cell ultrastructure and feeding preferences were not fully studied due to the death of the cultures. In this study, we re-isolated some of these Pirsoniales and established six new strains exhibiting predatory behavior, including a first freshwater representative. This allowed us to describe five new genera and species, as well as to emend the diagnosis of the order Pirsoniales. The 18S rRNA gene phylogenetic analysis revealed the position of new strains within Pirsoniales and their relationships with parasitoid relatives and environmental sequence lineages. Feeding experiments on novel Pirsoniales strains using diverse algal prey showed that they were not able to form trophosomes and auxosomes. The ability of cell aggregation in Pirsoniales was observed for the first time. One of the studied strains contained intracellular gammaproteobacteria distantly related to Coxiella. Ultrastructural analyses revealed a more complex cytoskeleton structure in Pirsoniales than previously thought and supported the monophyly of Bigyromonadea and Pseudofungi.

Expanding the molecular and morphological diversity of Apusomonadida, a deep-branching group of gliding bacterivorous protists.

Apusomonads are cosmopolitan bacterivorous biflagellate protists usually gliding on freshwater and marine sediment or wet soils. These nanoflagellates form a sister lineage to opisthokonts and may have retained ancestral features helpful to understanding the early evolution of this large supergroup. Although molecular environmental analyses indicate that apusomonads are genetically diverse, few species have been described. Here, we morphologically characterize 11 new apusomonad strains. Based on molecular phylogenetic analyses of the rRNA gene operon, we describe four new strains of the known species Multimonas media, Podomonas capensis, Apusomonas proboscidea, and Apusomonas australiensis, and rename Thecamonas oxoniensis as Mylnikovia oxoniensis n. gen., n. comb. Additionally, we describe four new genera and six new species: Catacumbia lutetiensis n. gen. n. sp., Cavaliersmithia chaoae n. gen. n. sp., Singekia montserratensis n. gen. n. sp., Singekia franciliensis n. gen. n. sp., Karpovia croatica n. gen. n. sp., and Chelonemonas dolani n. sp. Our comparative analysis suggests that apusomonad ancestor was a fusiform biflagellate with a dorsal pellicle, a plastic ventral surface, and a sleeve covering the anterior flagellum, that thrived in marine, possibly oxygen-poor, environments. It likely had a complex cell cycle with dormant and multiple fission stages, and sex. Our results extend known apusomonad diversity, allow updating their taxonomy, and provide elements to understand early eukaryotic evolution.

Molecular and morphological characterization of four new ancyromonad genera and proposal for an updated taxonomy of the Ancyromonadida.

Ancyromonads are small biflagellated protists with a bean-shaped morphology. They are cosmopolitan in marine, freshwater, and soil environments, where they attach to surfaces while feeding on bacteria. These poorly known grazers stand out by their uncertain phylogenetic position in the tree of eukaryotes, forming a deep-branching "orphan" lineage that is considered key to a better understanding of the early evolution of eukaryotes. Despite their ecological and evolutionary interest, only limited knowledge exists about their true diversity. Here, we aimed to characterize ancyromonads better by integrating environmental surveys with behavioral observation and description of cell morphology, for which sample isolation and culturing are indispensable. We studied 18 ancyromonad strains, including 14 new isolates and seven new species. We described three new and genetically divergent genera: Caraotamonas, Nyramonas, and Olneymonas, together encompassing four species. The remaining three new species belong to the already-known genera Fabomonas and Ancyromonas. We also raised Striomonas, formerly a subgenus of Nutomonas, to full genus status, on morphological and phylogenetic grounds. We studied the morphology of diverse ancyromonads under light and electron microscopy and carried out molecular phylogenetic analyses, also including 18S rRNA gene sequences from several environmental surveys. Based on these analyses, we have updated the taxonomy of Ancyromonadida.

Planktonic microbial communities from microbialite-bearing lakes sampled along a salinity-alkalinity gradient.

Continental freshwater systems are particularly vulnerable to environmental variation. Climate change-induced desertification and the anthropogenic exploitation of hydric resources result in the progressive evaporation and salinization of inland water bodies in many areas of the globe. However, how this process impacts microbial communities and their activities in biogeochemical cycles is poorly known. Here, we take a space-for-time substitution approach and characterize the prokaryotic and eukaryotic microbial communities of two planktonic cell-size fractions (0.2-5 µm and 5-30 µm) from lakes of diverse trophic levels sampled along a salinity-alkalinity gradient located in the Trans-Mexican Volcanic Belt (TMVB). We applied a 16S/18S rRNA gene metabarcoding strategy to determine the microbial community composition of 54 samples from 12 different lakes, from the low-salinity lake Zirahuén to the hypersaline residual ponds of Rincón de Parangueo. Except for systems at both extremes of the salinity gradient, most lakes along the evaporation trend bear actively forming microbialites, which harbor microbial communities clearly distinct from those of plankton. Several lakes were sampled in winter and late spring and the crater lakes Alchichica and Atexcac were sampled across the water column. Physicochemical parameters related to salinity-alkalinity were the most influential drivers of microbial community structure whereas trophic status, depth, or season were less important. Our results suggest that climate change and anthropogenic-induced hydric deficit could significantly affect microbial communities, potentially altering ecosystem functioning.

Ancient Adaptive Lateral Gene Transfers in the Symbiotic Opalina-Blastocystis Stramenopile Lineage.

Lateral gene transfer is a very common process in bacterial and archaeal evolution, playing an important role in the adaptation to new environments. In eukaryotes, its role and frequency remain highly debated, although recent research supports that gene transfer from bacteria to diverse eukaryotes may be much more common than previously appreciated. However, most of this research focused on animals and the true phylogenetic and functional impact of bacterial genes in less-studied microbial eukaryotic groups remains largely unknown. Here, we have analyzed transcriptome data from the deep-branching stramenopile Opalinidae, common members of frog gut microbiomes, and distantly related to the well-known genus Blastocystis. Phylogenetic analyses suggest the early acquisition of several bacterial genes in a common ancestor of both lineages. Those lateral gene transfers most likely facilitated the adaptation of the free-living ancestor of the Opalinidae-Blastocystis symbiotic group to new niches in the oxygen-depleted animal gut environment.

Core microbial communities of lacustrine microbialites sampled along an alkalinity gradient.

Microbialites are usually carbonate-rich sedimentary rocks formed by the interplay of phylogenetically and metabolically complex microbial communities with their physicochemical environment. Yet, the biotic and abiotic determinants of microbialite formation remain poorly constrained. Here, we analysed the structure of prokaryotic and eukaryotic communities associated with microbialites occurring in several crater lakes of the Trans-Mexican volcanic belt along an alkalinity gradient. Microbialite size and community structure correlated with lake physicochemical parameters, notably alkalinity. Although microbial community composition varied across lake microbialites, major taxa-associated functions appeared quite stable with both, oxygenic and anoxygenic photosynthesis and, to less extent, sulphate reduction, as major putative carbonatogenic processes. Despite interlake microbialite community differences, we identified a microbial core of 247 operational taxonomic units conserved across lake microbialites, suggesting a prominent ecological role in microbialite formation. This core mostly encompassed Cyanobacteria and their typical associated taxa (Bacteroidetes, Planctomycetes) and diverse anoxygenic photosynthetic bacteria, notably Chloroflexi, Alphaproteobacteria (Rhodobacteriales, Rhodospirilalles), Gammaproteobacteria (Chromatiaceae) and minor proportions of Chlorobi. The conserved core represented up to 40% (relative abundance) of the total community in lakes Alchichica and Atexcac, displaying the highest alkalinities and the most conspicuous microbialites. Core microbialite communities associated with carbonatogenesis might be relevant for inorganic carbon sequestration purposes.

Archaeal overdominance close to life-limiting conditions in geothermally influenced hypersaline lakes at the Danakil Depression, Ethiopia.

The Dallol protovolcanic area on the Danakil Depression (Afar region, Ethiopia) exhibits unique hydrothermal manifestations in hypersaline context, yielding varied polyextreme physicochemical conditions. Previous studies identified a wide archaeal diversity in less extreme brines but failed to identify microorganisms thriving in either high-chaotropicity, low-water-activity brines or hyperacidic-hypersaline Na-Fe-rich brines. Recently, we accessed several small lakes under intense degassing activity adjacent to the Round Mountain, west to the Dallol dome [Western Canyon Lakes (WCL); WCL1-5]. They exhibited intermediate parameter combinations (pH ~ 5, 34%-41% (weight/volume) NaCl-dominated salts with relatively high levels of chaotropic Mg-Ca salts) that should allow to better constrain life limits. These lakes were overwhelmingly dominated by Archaea, encompassing up to 99% of prokaryotic 16S rRNA gene amplicon sequences in metabarcoding studies. The majority belonged to Halobacteriota and Nanohaloarchaeota, the latter representing up to half of prokaryotic sequences. Optical and epifluorescence microscopy showed active cells in natural samples and diverse morphotypes in enrichment cultures. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy revealed tiny cells (200-300 nm diameter) epibiotically associated with somewhat larger cells (0.6-1 µm) but also the presence of silica-dominated precipitates of similar size and shape, highlighting the difficulty of distinguishing microbes from mineral biomorphs in this kind of low-biomass systems.

Environmental drivers of plankton protist communities along latitudinal and vertical gradients in the oldest and deepest freshwater lake.

Identifying which abiotic and biotic factors determine microbial community assembly is crucial to understand ecological processes and predict how communities will respond to environmental change. While global surveys aim at addressing this question in the world's oceans, equivalent studies in large freshwater systems are virtually lacking. Being the oldest, deepest and most voluminous freshwater lake on Earth, Lake Baikal offers a unique opportunity to test the effect of horizontal versus vertical gradients in community structure. Here, we characterized the structure of planktonic microbial eukaryotic communities (0.2-30 µm cell size) along a North-South latitudinal gradient (~600 km) from samples collected in coastal and pelagic waters and from surface to the deepest zones (5-1400 m) using an 18S rRNA gene metabarcoding approach. Our results show complex and diverse protist communities dominated by alveolates (ciliates and dinoflagellates), ochrophytes and holomycotan lineages, with cryptophytes, haptophytes, katablepharids and telonemids in moderate abundance and many low-frequency lineages, including several typical marine members, such as diplonemids, syndinians and radiolarians. Depth had a strong significant effect on protist community stratification. By contrast, the effect of the latitudinal gradient was marginal and no significant difference was observed between coastal and surface open water communities. Co-occurrence network analyses showed that epipelagic communities were significantly more interconnected than communities from the dark water column and suggest specific biotic interactions between autotrophic, heterotrophic and parasitic lineages that influence protist community structure. Since climate change is rapidly affecting Siberia and Lake Baikal, our comprehensive protist survey constitutes a useful reference to monitor ongoing community shifts.

Small freshwater ecosystems with dissimilar microbial communities exhibit similar temporal patterns.

Despite small freshwater ecosystems being biodiversity reservoirs and contributing significantly to greenhouse fluxes, their microbial communities remain largely understudied. Yet, microorganisms intervene in biogeochemical cycling and impact water quality. Because of their small size, these ecosystems are in principle more sensitive to disturbances, seasonal variation and pluri-annual climate change. However, how microbial community composition varies over space and time, and whether archaeal, bacterial and microbial eukaryote communities behave similarly remain unanswered. Here, we aim to unravel the composition and intra/interannual temporal dynamic patterns for archaea, bacteria and microbial eukaryotes in a set of small freshwater ecosystems. We monitored archaeal and bacterial community composition during 24 consecutive months in four ponds and one brook from northwestern France by 16S rRNA gene amplicon sequencing (microbial eukaryotes were previously investigated for the same systems). Unexpectedly for oxic environments, bacterial Candidate Phyla Radiation (CPR) were highly diverse and locally abundant. Our results suggest that microbial community structure is mainly driven by environmental conditions acting over space (ecosystems) and time (seasons). A low proportion of operational taxonomic units (OTUs) (<1%) was shared by the five ecosystems despite their geographical proximity (2-9 km away), making microbial communities almost unique in each ecosystem and highlighting the strong selective influence of local environmental conditions. Marked and similar seasonality patterns were observed for archaea, bacteria and microbial eukaryotes in all ecosystems despite strong turnovers of rare OTUs. Over the 2-year survey, microbial community composition varied despite relatively stable environmental parameters. This suggests that biotic associations play an important role in interannual community assembly.

Re-evaluation of Amphidiniopsis (Dinophyceae) Morphogroups Based On Phylogenetic Relationships, and Description of Three New Sand-dwelling Species From the NW Mediterranean.

Amphidiniopsis is a diverse genus of thecate heterotrophic dinoflagellates within the benthic, sand-dwelling species, with more than 20 currently described. Although molecular information about members of this genus is still scarce, morphological heterogeneity suggests the genus is paraphyletic. We investigated the diversity of Amphidiniopsis species in the NW Mediterranean Sea by morphological and molecular approaches, which led to the description of three new species, A. bulla sp. nov., A. erinacea sp. nov., and A. selene sp. nov. Phylogenetic reconstructions based on SSU and LSU rDNA sequences obtained from individual cells and the observed morphological characters confirm, as previously suggested, the paraphyly of the genus and the existence of at least four phylogenetic subgroups, instead of the three main subgroups defined to date. We also morphologically characterized Herdmania litoralis, suggesting the existence of more than one species belonging to this monotypic genus. Herdmania is a sister taxon to Amphidiniopsis, both morphologically and phylogenetically, and given the paraphyly of the latter, it should be considered a member of the newly termed Amphidiniopsis genus complex. The finding of the three new species highlights that the Mediterranean harbors distinctive, sand-dwelling dinoflagellates and needs further investigations of its unexplored diversity.

Secondary Plastids of Euglenids and Chlorarachniophytes Function with a Mix of Genes of Red and Green Algal Ancestry.

Endosymbiosis has been common all along eukaryotic evolution, providing opportunities for genomic and organellar innovation. Plastids are a prominent example. After the primary endosymbiosis of the cyanobacterial plastid ancestor, photosynthesis spread in many eukaryotic lineages via secondary endosymbioses involving red or green algal endosymbionts and diverse heterotrophic hosts. However, the number of secondary endosymbioses and how they occurred remain poorly understood. In particular, contrasting patterns of endosymbiotic gene transfer have been detected and subjected to various interpretations. In this context, accurate detection of endosymbiotic gene transfers is essential to avoid wrong evolutionary conclusions. We have assembled a strictly selected set of markers that provides robust phylogenomic evidence suggesting that nuclear genes involved in the function and maintenance of green secondary plastids in chlorarachniophytes and euglenids have unexpected mixed red and green algal origins. This mixed ancestry contrasts with the clear red algal origin of most nuclear genes carrying similar functions in secondary algae with red plastids.

Horizontal and endosymbiotic gene transfer in early plastid evolution.

Plastids evolved from a cyanobacterium that was engulfed by a heterotrophic eukaryotic host and became a stable organelle. Some of the resulting eukaryotic algae entered into a number of secondary endosymbioses with diverse eukaryotic hosts. These events had major consequences on the evolution and diversification of life on Earth. Although almost all plastid diversity derives from a single endosymbiotic event, the analysis of nuclear genomes of plastid-bearing lineages has revealed a mosaic origin of plastid-related genes. In addition to cyanobacterial genes, plastids recruited for their functioning eukaryotic proteins encoded by the host nucleus and also bacterial proteins of noncyanobacterial origin. Therefore, plastid proteins and plastid-localised metabolic pathways evolved by tinkering and using gene toolkits from different sources. This mixed heritage seems especially complex in secondary algae containing green plastids, the acquisition of which appears to have been facilitated by many previous acquisitions of red algal genes (the 'red carpet hypothesis').

Time series are critical to understand microbial plankton diversity and ecology.

How diverse are marine planktonic protist communities? How much seasonality do they exhibit? For a very long time, these two old and challenging questions in the field of plankton ecology could be addressed only for large-size protist species, based on cell counting under the microscope. The recent application of molecular techniques, notably massive marker-gene amplicon sequencing approaches (metabarcoding), has allowed investigating with unprecedented level of resolution the small-sized (<20 µm) planktonic eukaryotes too. An amazing diversity of these tiny organisms has been unveiled but details about their temporal dynamics remain much more elusive. In a From the Cover article in this issue of Molecular Ecology, Giner et al. (2019) introduce a new Recurrence Index (RI) to specifically look for seasonality in time-series metabarcoding data. They inspected the temporal dynamics of all operational taxonomic units (OTUs) in a rich sequence data set of pico- and nanoplanktonic eukaryotes in samples collected monthly during 10 years. Although most OTUs did not show seasonality, some abundant ones did, which explains why some averaging methods can find seasonality at the less detailed level of whole planktonic communities. Not surprisingly, the very complex small-sized eukaryotic plankton communities are composed of organisms with miscellaneous temporal dynamics.

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

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.

New Member of Gromochytriales (Chytridiomycetes)-Apiochytrium granulosporum nov. gen. et sp.

Molecular phylogenetic analysis of 18S rRNA gene sequences of nearly any species of Chytridiomycota has typically challenged traditional classification and triggered taxonomic revision. This has often led to the establishment of new taxa which, normally, appears well supported by zoospore ultrastructure, which provides diagnostic characters. To construct a meaningful and comprehensive classification of Chytridiomycota, the combination of molecular phylogenies and morphological studies of traditionally defined chytrid species is needed. In this work, we have studied morphological and ultrastructural features based on light and transmission electron microscopy as well as molecular phylogenetic analysis of a parasite (strain X-124 CCPP ZIN RAS) morphologically similar to Rhizophydium granulosporum living on the yellow-green alga Tribonema gayanum. Phylogenetic analysis of the 18S rRNA gene sequence of this strain supports that it represents a new genus and species affiliated to the recently established order Gromochytriales. The ultrastructure of X-124 confirms its phylogenetic position sister to Gromochytrium and serves as the basis for the description of the new genus and species Apiochytrium granulosporum. The 18S rRNA gene of A. granulosporum contains a S943 group I intron that carries a homing endonuclease pseudogene.

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

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 Parvularia atlantis gen. et sp. nov. (formerly Nuclearia sp. ATCC 50694), and compare its features with those of other nucleariid genera. Parvularia 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. P. atlantis 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 Nuclearia species. Likewise, 18S rRNA-based phylogenetic analyses show that P. atlantis represents a distinct lineage within the nucleariids.