Newly identified diversity of Dinomycetaceae (Rhizophydiales, Chytridiomycota), a family of fungal parasites of marine dinoflagellates.

We identified two new parasite species of Chytridiomycota isolated during blooms of the dinoflagellate Alexandrium minutum in the coastal Mediterranean Sea. Light and electron microscopy together with molecular characterization of the nuclear 18S, ITS, and 28S rDNA regions led to their identification as two new species, Dinomyces gilberthii and Paradinomyces evelyniae, both belonging to the family Dinomycetaceae, order Rhizophydiales. Dinomyces gilberthii differs from the previously described D. arenysensis by the presence of discharge papillae and the development of a drop-shaped sporangium. Paradinomyces evelyniae differs from the previously described P. triforaminorum by the prominent lipid globule present in early sporangia and by the pointed end producing a rhizoid. The two chytrids differed in their geographical distribution. Dinomyces gilberthii was detected in several Mediterranean habitats, including harbours and beaches, and was particularly prevalent during summer dinoflagellate blooms. Its widespread occurrence in coastal ecosystems suggested a high level of adaptability to this environment. Paradinomyces evelyniae had a more restricted distribution in the coastal-marine environment, occurring in harbour sediments and only occasionally in the water column during winter and early spring. Paradinomyces evelyniae has previously been detected in the Baltic Sea, suggesting that its distribution encompasses contrasting coastal environments, although its presence is rare.

To the Origin of Fungi: Analysis of MFS Transporters of First Assembled Aphelidium Genome Highlights Dissimilarity of Osmotrophic Abilities between Aphelida and Fungi.

Aphelids are a holomycotan group, represented exclusively by parasitoids infecting algae. They form a sister lineage to Fungi in the phylogenetic tree and represent a key group for reconstruction of the evolution of Holomycota and for analysis of the origin of Fungi. The newly assembled genome of Aphelidium insullamus (Holomycota, Aphelida) with a total length of 18.9 Mb, 7820 protein-coding genes and a GC percentage of 52.05% was obtained by a hybrid assembly based on Oxford Nanopore long reads and Illumina paired reads. In order to trace the origin and the evolution of fungal osmotrophy and its presence or absence in Aphelida, we analyzed the set of main fungal transmembrane transporters, which are proteins of the Major Facilitator superfamily (MFS), in the predicted aphelid proteomes. This search has shown an absence of a specific fungal protein family Drug:H+ antiporters-2 (DAH-2) and specific fungal orthologs of the sugar porters (SP) family, and the presence of common opisthokont's orthologs of the SP family in four aphelid genomes. The repertoire of SP orthologs in aphelids turned out to be less diverse than in free-living opisthokonts, and one of the most limited among opisthokonts. We argue that aphelids do not show signs of similarity with fungi in terms of their osmotrophic abilities, despite the sister relationships of these groups. Moreover, the osmotrophic abilities of aphelids appear to be reduced in comparison with free-living unicellular opisthokonts. Therefore, we assume that the evolution of fungi-specific traits began after the separation of fungal and aphelid lineages, and there are no essential reasons to consider aphelids as a prototype of the fungal ancestor.

Vegetative cell fusion and a new stage in the life cycle of the Aphelida (Opisthosporidia).

The aphelids, intracellular parasitoids of algae, represent a large cluster of species sister to Fungi in molecular phylogenetic trees. Sharing a common ancestor with Fungi, they are very important in terms of evolution of these groups of Holomycota. Aphelid life cycle being superficially similar to that of Chytridiomycetes is understudied. We have found in the aphelids a new stage-big multiflagellar and amoeboid cells, formed from a plasmodium that has two sorts of nuclei after trophic stage fusion. The families of protein-coding genes involved in the vegetative cell fusion in Opisthokonta were also discussed.

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.].

The new chytridiomycete Paradinomyces triforaminorum gen. et sp. nov. co-occurs with other parasitoids during a Kryptoperidinium foliaceum (Dinophyceae) bloom in the Baltic Sea.

A new chytrid genus and species was isolated and cultured from samples obtained in the Baltic Sea during a dinoflagellate bloom event. This species is characterized by having a spherical sporangium without papillae and zoospores of 2-3 µm in diameter that are released through 3 discharge pores. Molecular phylogeny based on ribosomal operon showed its sister position to the Dinomyces cluster in Rhizophydiales. Zoospores lack fenestrated cisternae but contain a paracrystalline inclusion, found in a Rhizophydiales representative for the first time. Additionally, the kinetid features are uncommon for Rhizophydiales and only observed in Dinomyces representatives so far. These morphological features and its phylogenetic relationships justify the description of the new genus and speciesParadinomyces triforaminorum gen. nov. sp. nov. belonging to the family Dinomycetaceae. The chytrid was detected during a high-biomass bloom of the dinoflagellate Kryptoperidinium foliaceum. Laboratory experiments suggest this species is highly specific and demonstrate the impact it can have on HAB development. The chytrid co-occurred with three other parasites belonging to Chytridiomycota (Fungi) and Perkinsea (Alveolata), highlighting that parasitic interactions are common during HABs in brackish and marine systems, and these multiple parasites compete for similar hosts.

Microbial predators form a new supergroup of eukaryotes.

Molecular phylogenetics of microbial eukaryotes has reshaped the tree of life by establishing broad taxonomic divisions, termed supergroups, that supersede the traditional kingdoms of animals, fungi and plants, and encompass a much greater breadth of eukaryotic diversity1. The vast majority of newly discovered species fall into a small number of known supergroups. Recently, however, a handful of species with no clear relationship to other supergroups have been described2-4, raising questions about the nature and degree of undiscovered diversity, and exposing the limitations of strictly molecular-based exploration. Here we report ten previously undescribed strains of microbial predators isolated through culture that collectively form a diverse new supergroup of eukaryotes, termed Provora. The Provora supergroup is genetically, morphologically and behaviourally distinct from other eukaryotes, and comprises two divergent clades of predators-Nebulidia and Nibbleridia-that are superficially similar to each other, but differ fundamentally in ultrastructure, behaviour and gene content. These predators are globally distributed in marine and freshwater environments, but are numerically rare and have consequently been overlooked by molecular-diversity surveys. In the age of high-throughput analyses, investigation of eukaryotic diversity through culture remains indispensable for the discovery of rare but ecologically and evolutionarily important eukaryotes.

Genomic analysis reveals cryptic diversity in aphelids and sheds light on the emergence of Fungi.

Over the past decade, molecular phylogenetics has reshaped our understanding of the fungal tree of life by unraveling a hitherto elusive diversity of the protistan relatives of Fungi. Aphelida constitutes one of these novel deep branches that precede the emergence of osmotrophic fungal lifestyle and hold particular significance as the pathogens of algae. Here, we obtain and analyze the genomes of aphelid species Amoeboaphelidium protococcarum and Amoeboaphelidium occidentale. Genomic data unmask the vast divergence between these species, hidden behind their morphological similarity, and reveal hybrid genomes with a complex evolutionary history in two strains of A. protococcarum. We confirm the proposed sister relationship between Aphelida and Fungi using phylogenomic analysis and chart the reduction of characteristic proteins involved in phagocytic activity in the evolution of Holomycota. Annotation of aphelid genomes demonstrates the retention of actin nucleation-promoting complexes associated with phagocytosis and amoeboid motility and also reveals a conspicuous expansion of receptor-like protein kinases, uncharacteristic of fungal lineages. We find that aphelids possess multiple carbohydrate-processing enzymes that are involved in fungal cell wall synthesis but do not display rich complements of algal cell-wall-processing enzymes, suggesting an independent origin of fungal plant-degrading capabilities. Aphelid genomes show that the emergence of Fungi from phagotrophic ancestors relied on a common cell wall synthetic machinery but required a different set of proteins for digestion and interaction with the environment.

On the origin of TSAR: morphology, diversity and phylogeny of Telonemia.

Telonemia is a poorly known major phylum of flagellated eukaryotes with a unique combination of morphological traits. Phylogenomics recently revealed the phylogenetic position of telonemids as sister to SAR, one of the largest groups of eukaryotes, comprising Stramenopiles, Alveolata and Rhizaria. Due to this key evolutionary position, investigations of telonemids are of critical importance for elucidating the origin and diversification of an astounding diversity of eukaryotic forms and life strategies. To date, however, only two species have been morphologically characterized from Telonemia, which do not represent this genetically very diverse group. In this study, we established cultures for six new telonemid strains, including the description of five new species and a new genus. We used these cultures to update the phylogeny of Telonemia and provide a detailed morphological and ultrastructural investigation. Our data elucidate the origin of TSAR from flagellates with complex morphology and reconstruction of the ancestral structure of stramenopiles, alveolates and rhizarians, and their main synapomorphic characters. Since telonemids are a common component of aquatic environments, the features of their feeding, behaviour and ecological preferences observed in clonal cultures and the results of global metabarcoding analysis contribute to a deeper understanding of organization of microbial food webs.

Microscopical Studies on Ministeria vibrans Tong, 1997 (Filasterea) Highlight the Cytoskeletal Structure of the Common Ancestor of Filasterea, Metazoa and Choanoflagellata.

Ministeria vibrans (Filasterea) is a tiny amoeboid species described by Tong in 1997. It has been sporadically found in different habitats, and cultured strains were established. M. vibrans is well characterised by molecular phylogeny but until now was not ultrastructurally investigated in detail. Here, we provide the ultrastructure for this species based on a new strain isolated from oxygen-depleted water of the Baltic Sea. A thin vibrating flagellum could be observed but no vibrating movement of the cell body and no stalk. Our first ultrastructural study of a filasterean taxon revealed radial microvilli supported by bundles of microfilaments. Two centrioles located in the nuclear pit can migrate to the cell periphery and transform into the kinetid: the centriole orthogonal to the kinetosome with a fibrillar root and a basal foot that initiates microtubules. Microvilli in Ministeria suggest their presence in the common ancestor of Filasterea and Choanoflagellata. The kinetid structure of Ministeria is similar to that of the choanocytes of the most deep-branching sponges, differing essentially from the kinetid of choanoflagellates. Thus, kinetid and microvilli of Ministeria illustrate features of the common ancestor of three holozoan groups: Filasterea, Metazoa and Choanoflagellata.

Kinetid Structure of Aphelidium and Paraphelidium (Aphelida) Suggests the Features of the Common Ancestor of Fungi and Opisthosporidia.

The aphelids (phylum Aphelida) are phagotrophic parasitoids of algae and represent the most basal branch in superphylum Opisthosporidia, which contains the Microsporidia, Rozellosporidia and Aphelida. Being the closest group to traditional fungi, the aphelids should have ancestral features of both phyla. As in chytrids and other zoosporic fungi, the structure of zoospores is the most informative and important morphological feature for the phylogeny and taxonomy of aphelids. Though a general zoospore description exists for some aphelid species, their flagellar apparatus (kinetid) structure, which contains pivotal taxonomic and phylogenetic characters, has not been studied. Here we represent the kinetid structure in two genera, Aphelidium and Paraphelidium, and demonstrate independent reduction in the kinetid in each genus. The kinetid-mitochondrion connection found in Aphelidium and Paraphelidium is rare for opisthokonts in general, but present in the most basal branches of Fungi and Opisthosporidia. We suggest, therefore, that this connection represents an ancestral character for both these phyla.

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.

Global transcriptome analysis of the aphelid Paraphelidium tribonemae supports the phagotrophic origin of fungi.

Aphelids are little-known phagotrophic parasites of algae whose life cycle and morphology resemble those of the parasitic rozellids (Cryptomycota, Rozellomycota). In previous phylogenetic analyses of RNA polymerase and rRNA genes, aphelids, rozellids and Microsporidia (parasites of animals) formed a clade, named Opisthosporidia, which appeared as the sister group to Fungi. However, the statistical support for the Opisthosporidia was always moderate. Here, we generated full life-cycle transcriptome data for the aphelid species Paraphelidium tribonemae. In-depth multi-gene phylogenomic analyses using several protein datasets place this aphelid as the closest relative of fungi to the exclusion of rozellids and Microsporidia. In contrast with the comparatively reduced Rozella allomycis genome, we infer a rich, free-living-like aphelid proteome, with a metabolism similar to fungi, including cellulases likely involved in algal cell-wall penetration and enzymes involved in chitin biosynthesis. Our results suggest that fungi evolved from complex aphelid-like ancestors that lost phagotrophy and became osmotrophic.

Morphology, biology and phylogeny of Phalansterium arcticum sp. n. (Amoebozoa, Variosea), isolated from ancient Arctic permafrost.

A new species, Phalansterium arcticum sp. n., was isolated from an 8580-year-old Arctic permafrost layer. This organism typically lives as a sedentary uniflagellated cell enclosed in a thin flexible mucilaginous sheath, but can form naked swimming cells and amoeboid cells with eruptive pseudopodia accompanied with the formation of short, filopodia-like projections. In an SSU rDNA phylogenetic tree, it robustly groups with other species of this genus. Along with a description of the species, we also add new details to the description of the cell division of Phalansterium and the feeding process in this organism.

The Chytrid-like Parasites of Algae Amoeboradix gromovi gen. et sp. nov. and Sanchytrium tribonematis Belong to a New Fungal Lineage.

Fungi encompass, in addition to classically well-studied lineages, an ever-expanding diversity of poorly known lineages including zoosporic chytrid-like parasites. Here, we formally describe Amoeboradix gromovi gen. et sp. nov. comprising a set of closely related strains of chytrid-like parasites of the yellow-green alga Tribonema gayanum unusually endowed with amoeboid zoospores. Morphological and ultrastructural features of A. gromovi observed by light and transmission electron microscopy recall previous descriptions of Rhizophydium anatropum. A. gromovi exhibits one of the longest kinetosomes known in eukaryotes, composed of microtubular singlets or doublets. To carry out molecular phylogenetic analysis and validate the identification of different life cycle stages, we amplified 18S rRNA genes from three A. gromovi strains infecting T. gayanum cultures, single sporangia and single zoospores. Molecular phylogenetic analyses of 18S+28S rRNA concatenated genes of the type strain revealed that A. gromovi is closely related to the recently described species Sanchytrium tribonematis, another parasite of Tribonema that had been tentatively classified within Monoblepharidomycetes. However, our phylogenetic analysis with an extended taxon sampling did not show any particular affinity of Amoeboradix and Sanchytrium with described fungal taxa. Therefore, Amoeboradix gromovi and Sanchytrium tribonematis likely represent a new divergent taxon that remains incertae sedis within Fungi.

Monoblepharidomycetes diversity includes new parasitic and saprotrophic species with highly intronized rDNA.

The Monoblepharidomycetes is the sister class to the Chytridiomycetes in the phylum Chytridiomycota. The six known genera have thalli that are either monocentric and without rhizoids or produce hyphae with an independent evolutionary origin from the hyphae of higher fungi. On the basis of morphological characters and phylogenetic evidence from the small and large subunits of nuclear ribosomal RNA, we established two new genera, Sanchytrium and Telasphaerula, each with a single species. We re-analyzed intergeneric relationships within the monoblephs, and established two new families. The new genera significantly expand the known morphological and ecological diversity of the Monoblepharidomycetes by adding a monocentric, epibiotic, algal parasitic species and a rhizomycelial, saprotrophic species. Based on the presence of environmental sequences related to Sanchytrium strains, the Monoblepharidomycetes contain previously unsuspected diversity. The ribosomal DNA of the new genera contains an unusually high density of group I introns. We found 20 intron insertion positions including six that are new for rRNA genes (S1053, L803, L829, L961, L1844, and L2281).

Molecular Phylogeny of Paraphelidium letcheri sp. nov. (Aphelida, Opisthosporidia).

Aphelids remain poorly known parasitoids of algae and have recently raised considerable interest due to their phylogenetic position at the base of Holomycota. Together with Cryptomycota (Rozellosporidia) and Microsporidia, they have been recently re-classified as the Opisthosporidia, which constitutes the sister group to the fungi within the Holomycota. Molecular environmental studies have revealed a huge diversity of aphelids, but only four genera have been described: Aphelidium, Amoeboaphelidium, Paraphelidium, and Pseudaphelidium. Here, we describe the life cycle of a new representative of Aphelida, Paraphelidium letcheri sp. nov., and provide the 18S rRNA gene sequence for this species. Molecular phylogenetic analysis indicates that P. letcheri is sister to Paraphelidium tribonemae and together they form a monophyletic cluster which is distantly related to both, Aphelidium, with flagellated zoospores, and Amoebaphelidium, with amoeboid zoospores.

Morphological and Genetic Diversity of Opisthosporidia: New Aphelid Paraphelidium tribonemae gen. et sp. nov.

Aphelids are a poorly known group of parasitoids of algae that have raised considerable interest due to their pivotal phylogenetic position. Together with Cryptomycota and the highly derived Microsporidia, they have been recently re-classified as the Opisthosporidia, which constitute the sister group to the fungi within the Holomycota. Despite their huge diversity, as revealed by molecular environmental studies, and their phylogenetic interest, only three genera have been described (Aphelidium, Amoeboaphelidium, and Pseudaphelidium), from which 18S rRNA gene sequences exist only for Amoeboaphelidium and Aphelidium species. Here, we describe the life cycle and ultrastructure of a new representative of Aphelida, Paraphelidium tribonemae gen. et sp. nov., and provide the first 18S rRNA gene sequence obtained for this genus. Molecular phylogenetic analysis indicates that Paraphelidium is distantly related to both Aphelidium and Amoebaphelidium, highlighting the wide genetic diversity of aphelids. Paraphelidium tribonemae has amoeboflagellate zoospores containing a lipid-microbody complex, dictyosomes, and mitochondria with rhomboid cristae, which are also present in trophonts and plasmodia. The amoeboid trophont uses pseudopodia to feed from the host cytoplasm. Although genetically distinct, the genus Paraphelidium is morphologically indistinguishable from other aphelid genera and has zoospores able to produce lamellipodia with subfilopodia like those of Amoeboaphelidium.

Heterokont Predator Develorapax marinus gen. et sp. nov. - A Model of the Ochrophyte Ancestor.

Heterotrophic lineages of Heterokonta (or stramenopiles), in contrast to a single monophyletic group of autotrophs, Ochrophyta, form several clades that independently branch off the heterokont stem lineage. The nearest neighbors of Ochrophyta in the phylogenetic tree appear to be almost exclusively bacterivorous, whereas the hypothesis of plastid acquisition by the ancestors of the ochrophyte lineage suggests an ability to engulf eukaryotic alga. In line with this hypothesis, the heterotrophic predator at the base of the ochrophyte lineage may be regarded as a model for the ochrophyte ancestor. Here, we present a new genus and species of marine free-living heterotrophic heterokont Develorapax marinus, which falls into an isolated heterokont cluster, along with the marine flagellate Developayella elegans, and is able to engulf eukaryotic cells. Together with environmental sequences D. marinus and D. elegans form a class-level clade Developea nom. nov. represented by species adapted to different environmental conditions and with a wide geographical distribution. The position of Developea among Heterokonta in large-scale phylogenetic tree is discussed. We propose that members of the Developea clade represent a model for transition from bacterivory to a predatory feeding mode by selection for larger prey. Presumably, such transition in the grazing strategy is possible in the presence of bacterial biofilms or aggregates expected in eutrophic environment, and has likely occurred in the ochrophyte ancestor.

Flagellar apparatus structure of choanoflagellates.

Phylum choanoflagellata is the nearest unicellular neighbor of metazoa at the phylogenetic tree. They are single celled or form the colonies, can be presented by naked cells or live in theca or lorica, but in all cases they have a flagellum surrounded by microvilli of the collar. They have rather uniform and peculiar flagellar apparatus structure with flagellar basal body (FB) producing a flagellum, and non-flagellar basal body (NFB) lying orthogonal to the FB. Long flagellar transition zone contains a unique structure among eukaryotes, the central filament, which connects central microtubules to the transversal plate. Both basal bodies are composed of triplets and interconnected with fibrillar bridge. They also contain the internal arc-shaped connectives between the triplets. The FB has prominent transitional fibers similar to those of chytrid zoospores and choanocytes of sponges, and a radial microtubular root system. The ring-shaped microtubule organizing center (MTOC) produces radial root microtubules, but in some species a MTOC is represented by separate foci. The NFB has a narrow fibrillar root directed towards the Golgi apparatus in association with membrane-bounded sac. Prior to cell division, the basal bodies replicate and migrate to poles of elongated nucleus. The basal bodies serve as MTOCs for the spindle microtubules during nuclear division by semiopen orthomitosis.