Macronuclear genome sequence of the ciliate Tetrahymena thermophila, a model eukaryote

Importin-9 and a TPR domain protein MpH drive periodic patterning of ciliary arrays in Tetrahymena

We explored how the number of structures is determined in an intracellular organelle series. In Tetrahymena, the oral apparatus contains three diagonal ciliary rows: M1, M2, and M3. During development, the M rows emerge by sequential segmentation of a group of basal bodies, starting with the longest and most anterior M1 and ending with the shortest and most posterior M3. The mpD-1 and mpH-1 alleles increase and decrease the number of M rows, respectively. We identify MpH as a TPR protein and MpD as an importin-9. Both proteins localize to the M rows and form concentration gradients. MpH is a row elongation factor whose loss shortens all M rows and often prevents the formation of M3. MpD limits row initiation after the emergence of M2. MpD could be a part of a negative feedback loop that limits row initiation when M1 assembly is properly advanced. We conclude that the forming oral apparatus has properties of a semi-autonomous intracellular developmental field.

Tetrahymena thermophila glutathione-S-transferase superfamily: an eco-paralogs gene network differentially responding to various environmental abiotic stressors and an update on this gene family in ciliates

Glutathione S-transferases constitute a superfamily of enzymes involved mainly, but not exclusively, in the detoxification of xenobiotic compounds that are considered environmental pollutants. In this work, an updated analysis of putative cytosolic glutathione S-transferases (cGST) from ciliate protozoa is performed although this analysis is mainly focused on Tetrahymena thermophila. Among ciliates, the genus Tetrahymena has the highest number (58 on average) of cGST genes. As in mammals, the Mu class of cGST is present in all analyzed ciliates and is the majority class in Tetrahymena species. After an analysis of the occurrence of GST domains in T. thermophila, out of the 54 GSTs previously considered to be Mu class, six of them have been discarded as they do not have recognizable GST domains. In addition, there is one GST species-specific and another GST-EF1G (elongation factor 1 gamma). A structural analysis of T. thermophila GSTs has shown a wide variety of β-sheets/α-helix patterns, one of the most abundant being the canonical thioredoxin-folding pattern. Within the categories of bZIP and C4 zinc finger transcription factors, potential binding sites for c-Jun and c-Fos are abundant (32% as average), along with GATA-1 (71% average) in the T. thermophila GST gene promoters. The alignment of all MAPEG (Membrane Associated Proteins involved in Eicosanoid and Glutathione metabolism) GST protein sequences from Tetrahymena species shows that this family is divided into two well-defined clans. The phylogenetic analysis of T. thermophila GSTs has shown that a cluster of 19 Mu-class GST genes are phylogenetic predecessors of members from the omega, theta and zeta classes. This means that the current GST phylogenetic model needs to be modified. Sixteen T. thermophila GST genes, together with two clusters including three genes each with very high identity, have been selected for qRT-PCR analysis under stress from eleven different environmental stressors. This analysis has revealed that there are GST genes that respond selectively and/or differentially to each stressor, independently of the GST class to which it belongs. Most of them respond to the two more toxic metal(loid)s used (Cd or As).

Decoding the Nature of the Peritrich Stalk: A Distinctive Organelle in a Large Group of Ciliated Unicellular Eukaryotes

Ciliates represent a diverse assemblage of ancient single-celled eukaryotes characterized by diverse morphological features. Among certain sessilid peritrich ciliates, an exceptional morphological structure known as the stalk has been documented since the pioneering work of Antonie van Leeuwenhoek in the 17th century. This study conducts a comparative genomic analysis of three sessile peritrich species-Epistylis sp., Vorticella campanula, and Zoothamnium arbuscula-and two free-swimming species, Tetrahymena thermophila and Paramecium tetraurelia, within the class Oligohymenophorea. We find that carbohydrate-related components are consistently associated with diverse stalk substructures. Evidence suggests that the branched stalks of colonial E. hentscheli are supported by chitin-based ring-like structures. Through proteomic analysis of the Epistylis stalk, we found peritrich-specific genes, including coiled-coil domain-containing (CCDC) proteins and epidermal growth factor-like (EGF-like) proteins, as key stalk components. CCDC proteins are part of the stalk sheath, and their N-glycosylation may enhance adhesion between the cell body and stalk through lectin interactions. This study sheds light on the genetic innovations behind the stalk in peritrichs, which support their sessile and colonial lifestyles, and identifies peritrich-specific CCDC proteins as potential targets for disrupting the attachment of sessilids to aquaculture animals, addressing issues related to epibiotic burden.

Organismal complexity strongly correlates with the number of protein families and domains

In the pregenomic era, scientists were puzzled by the observation that haploid genome size (the C-value) did not correlate well with organismal complexity. This phenomenon, called the "C-value paradox," is mostly explained by the fact that protein-coding genes occupy only a small fraction of eukaryotic genomes. When the first genome sequences became available, scientists were even more surprised by the fact that the number of genes (G-value) was also a poor predictor of complexity, which gave rise to the "G-value paradox." The proposed explanations usually invoke mechanisms that increase the information content of each individual gene (protein-protein interactions, intrinsic disorder, posttranslational modifications, alternative splicing, etc.). Less attention has been paid to mechanisms that increase the amount of genetic material but do not increase (or not to the same extent) the amount of information encoded in the genome, such as gene duplication and domain shuffling. Proteins belonging to the same family and/or sharing the same domains often carry out similar or even redundant functions. We thus hypothesized that an organism's number of different protein families and domains should be suitable predictors of organismal complexity. In agreement with our hypothesis, we observed that the number of protein families, clans, domains, and motifs increases from simple to progressively more complex organisms. In addition, these metrics correlate with the number of cell types better than and independently of the number of protein-coding genes and several previously proposed predictors of organismal complexity. Our observations have the potential to represent a resolution to the G-value paradox.

Dual modes of DNA N6-methyladenine maintenance by distinct methyltransferase complexes

Stable inheritance of DNA N6-methyladenine (6mA) is crucial for its biological functions in eukaryotes. Here, we identify two distinct methyltransferase (MTase) complexes, both sharing the catalytic subunit AMT1, but featuring AMT6 and AMT7 as their unique components, respectively. While the two complexes are jointly responsible for 6mA maintenance methylation, they exhibit distinct enzymology, DNA/chromatin affinity, genomic distribution, and knockout phenotypes. AMT7 complex, featuring high MTase activity and processivity, is connected to transcription-associated epigenetic marks, including H2A.Z and H3K4me3, and is required for the bulk of maintenance methylation. In contrast, AMT6 complex, with reduced activity and processivity, is recruited by PCNA to initiate maintenance methylation immediately after DNA replication. These two complexes coordinate in maintenance methylation. By integrating signals from both replication and transcription, this mechanism ensures the faithful and efficient transmission of 6mA as an epigenetic mark in eukaryotes.

Comprehensive genome annotation of the model ciliate Tetrahymena thermophila by in-depth epigenetic and transcriptomic profiling

The ciliate Tetrahymena thermophila is a well-established unicellular model eukaryote, contributing significantly to foundational biological discoveries. Despite its acknowledged importance, current studies on Tetrahymena biology face challenges due to gene annotation inaccuracy, particularly the notable absence of untranslated regions (UTRs). To comprehensively annotate the Tetrahymena macronuclear genome, we collected extensive transcriptomic data spanning various cell stages. To ascertain transcript orientation and transcription start/end sites, we incorporated data on epigenetic marks displaying enrichment towards the 5' end of gene bodies, including H3 lysine 4 tri-methylation (H3K4me3), histone variant H2A.Z, nucleosome positioning and N6-methyldeoxyadenine (6mA). Cap-seq data was subsequently applied to validate the accuracy of identified transcription start sites. Additionally, we integrated Nanopore direct RNA sequencing (DRS), strand-specific RNA sequencing (RNA-seq) and assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) data. Using a newly developed bioinformatic pipeline, coupled with manual curation and experimental validation, our work yielded substantial improvements to the current gene models, including the addition of 2,481 new genes, updates to 23,936 existing genes, and the incorporation of 8,339 alternatively spliced isoforms. Furthermore, novel UTR information was annotated for 26,687 high-confidence genes. Intriguingly, 20% of protein-coding genes were identified to have natural antisense transcripts characterized by high diversity in alternative splicing, thus offering insights into understanding transcriptional regulation. Our work will enhance the utility of Tetrahymena as a robust genetic toolkit for advancing biological research, and provides a promising framework for genome annotation in other eukaryotes.

Tetrahymena predation drives adaptive evolution of Salmonella by disrupting O-antigen biosynthesis and upregulating transcriptional regulator csgD

Protozoan predation is increasingly understood to be one of the main environmental factors driving bacterial virulence evolution and adaptation strategies. In this study, we reveal the adaptive evolution of Salmonella Enteritidis in phenotypic and genomic traits after passage through Tetrahymena thermophila. We identified a beneficial and fixed mutation that occurs at the coding region of rfbP, encoding the undecaprenyl-phosphate galactose phosphotransferase, and demonstrated that almost all observed phenotypic changes caused by selection pressure, including enhanced biofilm formation and reduced bacterial motility, are related to the early termination of RfbP protein translation. This mutation blocks the lipopolysaccharide O-antigen synthesis and leads to upregulation of the transcriptional factor csgD, which plays a central role in regulating Salmonella adaptation to the adverse environment. Our findings underscore the selective pressure from Tetrahymena as a pivotal driver of adaptive evolution in Salmonella, elucidating the nexus between adaptation to protozoan predation and augmented environmental persistence. This investigation advances our understanding of the ecological role of protozoan predation in the natural selection of bacterial communities.

Genes and proteins expressed at different life cycle stages in the model protist Euplotes vannus revealed by both transcriptomic and proteomic approaches

Sexual reproduction first appeared in unicellular protists and has continued to be an essential biological process in almost all eukaryotes. Ciliated protists, which contain both germline and somatic genomes within a single cell, have evolved a special form of sexual reproduction called conjugation that involves mitosis, meiosis, fertilization, nuclear differentiation, genome rearrangement, and the development of unique cellular structures. The molecular basis and mechanisms of conjugation vary dramatically among ciliates, and many details of the process and its regulation are still largely unknown. In order to better comprehend these processes and mechanisms from an evolutionary perspective, this study provides the first comprehensive overview of the transcriptome and proteome profiles during the entire life cycle of the newly-established marine model ciliate Euplotes vannus. Transcriptome analyses from 14 life cycle stages (three vegetative stages and 11 sexual stages) revealed over 26,000 genes that are specifically expressed at different stages, many of which are related to DNA replication, transcription, translation, mitosis, meiosis, nuclear differentiation, and/or genome rearrangement. Quantitative proteomic analyses identified 338 proteins with homologs associated with conjugation and/or somatic nuclear development in other ciliates, including dicer-like proteins, Hsp90 proteins, RNA polymerase II and transcription elongation factors, ribosomal-associated proteins, and ubiquitin-related proteins. Four of these homologs belong to the PIWI family, each with different expression patterns identified and confirmed by RT-qPCR, which may function in small RNA-mediated genome rearrangement. Proteins involved in the nonhomologous end-joining pathway are induced early during meiosis and accumulate in the developing new somatic nucleus, where more than 80% of the germline sequences are eliminated from the somatic genome. A number of new candidate genes and proteins likely to play roles in conjugation and its related genome rearrangements have also been revealed. The gene expression profiles reported here will be valuable resources for further studies of the origin and evolution of sexual reproduction in this new model species.

The 'Janus A' gene encodes a polo-kinase whose loss creates a dorsal/ventral intracellular homeosis in the ciliate, Tetrahymena

Genetic studies on the protist, Tetrahymena thermophila provide a glimpse into the unexpectedly rich world of intracellular patterning that unfolds within the ciliate cell cortex. Ciliate pattern studies provide a useful counterpoint to animal models of pattern formation in that the unicellular model draws attention away from fields of cells (or nuclei) as the principal players in the metazoan pattern paradigm, focusing instead on fields of ciliated basal bodies serving as sources of positional information. In this study, we identify JANA, a Polo kinase of Tetrahymena, that serves as an important factor driving global, circumferential pattern. Loss of function of JanA results in global, mirror-duplication of ventral organelles on the dorsal surface: a kind of intracellular homeosis that has been named the 'janus' phenotype. Gain of function (over-expression) reduces or even eliminates cortical organelles within the ventral 'hemi-cell'. GFP-tagging reveals that JanA decorates basal bodies predominantly within the left-dorsal hemi-cell. These results led us to propose a model in which the default state of cortical patterning is a mirror-image assemblage of cortical organelles including oral apparatus, contractile vacuole pores and cytoproct. JanA normally suppresses organelle assembly in the dorsal hemi-cellular cortex, resulting in a simple, ventral assemblage of these organelles, a 'half-pattern' as it were. PLK inhibitors produce a janus phenocopy, but reveal other unanticipated roles for PLK activities involving more local patterning events that control organelle dimensions and organization. We discuss results in light of metazoan studies in which PLK activity links cell cycle control to intracellular symmetry breaking.

The phenotypic and demographic response to the combination of copper and thermal stressors strongly varies within the ciliate species, Tetrahymena thermophila

Copper pollution can alter biological and trophic functions. Organisms can utilise different tolerance strategies, including accumulation mechanisms (intracellular vacuoles, external chelation, etc.) to maintain themselves in copper-polluted environments. Accumulation mechanisms can influence the expression of other phenotypic traits, allowing organisms to deal with copper stress. Whether copper effects on accumulation strategies interact with other environmental stressors such as temperature and how this may differ within species are still unsolved questions. Here, we tested experimentally whether the combined effect of copper and temperature modulates traits linked to demography, morphology, movement and accumulation in six strains of the ciliate Tetrahymena thermophila. We also explored whether copper accumulation might modulate environmental copper concentration effects on phenotypic and demographic traits. Results showed high intraspecific variability in the phenotypic and demographic response to copper, with interactive effects between temperature and copper. In addition, they suggested an attenuation effect of copper accumulation on the sensitivity of traits to copper, but with great variation between strains, temperatures and copper concentrations. Diversity of responses among strains and their thermal dependencies pleads for the integration of intraspecific variability and multiple stressors approaches in ecotoxicological studies, thus improving the reliability of assessments of the effects of pollutants on biodiversity.

Nuclear dualism without extensive DNA elimination in the ciliate Loxodes magnus

Most eukaryotes have one nucleus and nuclear genome per cell. Ciliates have instead evolved distinct nuclei that coexist in each cell: a silent germline vs. transcriptionally active somatic nuclei. In the best-studied model species, both nuclei can divide asexually, but only germline nuclei undergo meiosis and karyogamy during sex. Thereafter, thousands of DNA segments, called internally eliminated sequences (IESs), are excised from copies of the germline genomes to produce the streamlined somatic genome. In Loxodes, however, somatic nuclei cannot divide but instead develop from germline copies even during asexual cell division, which would incur a huge overhead cost if genome editing was required. Here, we purified and sequenced both genomes in Loxodes magnus to see whether their nondividing somatic nuclei are associated with differences in genome architecture. Unlike in other ciliates studied to date, we did not find canonical germline-limited IESs, implying Loxodes does not extensively edit its genomes. Instead, both genomes appear large and equivalent, replete with retrotransposons and repetitive sequences, unlike the compact, gene-rich somatic genomes of other ciliates. Two other hallmarks of nuclear development in ciliates-domesticated DDE-family transposases and editing-associated small RNAs-were also not found. Thus, among the ciliates, Loxodes genomes most resemble those of conventional eukaryotes. Nonetheless, base modifications, histone marks, and nucleosome positioning of vegetative Loxodes nuclei are consistent with functional differentiation between actively transcribed somatic vs. inactive germline nuclei. Given their phylogenetic position, it is likely that editing was present in the ancestral ciliate but secondarily lost in the Loxodes lineage.

Complex evolutionary patterns within the tubulin gene family of ciliates, unicellular eukaryotes with diverse microtubular structures

BACKGROUND

Tubulins are major components of the eukaryotic cytoskeletons that are crucial in many cellular processes. Ciliated protists comprise one of the oldest eukaryotic lineages possessing cilia over their cell surface and assembling many diverse microtubular structures. As such, ciliates are excellent model organisms to clarify the origin and evolution of tubulins in the early stages of eukaryote evolution. Nonetheless, the evolutionary history of the tubulin subfamilies within and among ciliate classes is unclear.

RESULTS

We analyzed the evolutionary pattern of ciliate tubulin gene family based on genomes/transcriptomes of 60 species covering 10 ciliate classes. Results showed: (1) Six tubulin subfamilies (α_Tub, β_Tub, γ_Tub, δ_Tub, ε_Tub, and ζ_Tub) originated from the last eukaryotic common ancestor (LECA) were observed within ciliates. Among them, α_Tub, β_Tub, and γ_Tub were present in all ciliate species, while δ_Tub, ε_Tub, and ζ_Tub might be independently lost in some species. (2) The evolutionary history of the tubulin subfamilies varied. Evolutionary history of ciliate γ_Tub, δ_Tub, ε_Tub, and ζ_Tub showed a certain degree of consistency with the phylogeny of species after the divergence of ciliate classes, while the evolutionary history of ciliate α_Tub and β_Tub varied among different classes. (3) Ciliate α- and β-tubulin isoforms could be classified into an "ancestral group" present in LECA and a "divergent group" containing only ciliate sequences. Alveolata-specific expansion events probably occurred within the "ancestral group" of α_Tub and β_Tub. The "divergent group" might be important for ciliate morphological differentiation and wide environmental adaptability. (4) Expansion events of the tubulin gene family appeared to be consistent with whole genome duplication (WGD) events in some degree. More Paramecium-specific tubulin expansions were detected than Tetrahymena-specific ones. Compared to other Paramecium species, the Paramecium aurelia complex underwent a more recent WGD which might have experienced more tubulin expansion events.

CONCLUSIONS

Evolutionary history among different tubulin gene subfamilies seemed to vary within ciliated protists. And the complex evolutionary patterns of tubulins among different ciliate classes might drive functional diversification. Our investigation provided meaningful information for understanding the evolution of tubulin gene family in the early stages of eukaryote evolution.

Extreme mitochondrial reduction in a novel group of free-living metamonads

Metamonads are a diverse group of heterotrophic microbial eukaryotes adapted to living in hypoxic environments. All metamonads but one harbour metabolically altered 'mitochondrion-related organelles' (MROs) with reduced functions, however the degree of reduction varies. Here, we generate high-quality draft genomes, transcriptomes, and predicted proteomes for five recently discovered free-living metamonads. Phylogenomic analyses placed these organisms in a group we name the 'BaSk' (Barthelonids+Skoliomonads) clade, a deeply branching sister group to the Fornicata, a phylum that includes parasitic and free-living flagellates. Bioinformatic analyses of gene models shows that these organisms are predicted to have extremely reduced MRO proteomes in comparison to other free-living metamonads. Loss of the mitochondrial iron-sulfur cluster assembly system in some organisms in this group appears to be linked to the acquisition in their common ancestral lineage of a SUF-like minimal system Fe/S cluster pathway by lateral gene transfer. One of the isolates, Skoliomonas litria, appears to have lost all other known MRO pathways. No proteins were confidently assigned to the predicted MRO proteome of this organism suggesting that the organelle has been lost. The extreme mitochondrial reduction observed within this free-living anaerobic protistan clade demonstrates that mitochondrial functions may be completely lost even in free-living organisms.

Correlated responses to selection across diverse environments during experimental evolution of Tetrahymena thermophila

Correlated responses to selection have long been observed and studied; however, it remains unclear when they will arise, and in what direction. To contribute to a growing understanding of correlated responses to selection, we used experimental evolution of the ciliate Tetrahymena thermophila to study direct and correlated responses in a variety of different environmental conditions. One experiment focused on adaptation to two different temperatures and the correlated responses across temperatures. Another experiment used inhibitory concentrations of a variety of compounds to test direct and correlated responses to selection. We found that all populations adapted to the environments in which they evolved. We also found many cases of correlated evolution across environments; few conditions resulted in trade-offs and many resulted in a positive correlated response. Surprisingly, in many instances, the correlated response was of a larger magnitude than the direct response. We find that ancestral fitness predicts the extent of adaptation, consistent with diminishing returns epistasis. Unexpectedly, we also find that this pattern of diminishing returns holds across environments regardless of the environment in which evolution occurs. We also found that the correlated response is asymmetric across environments, that is, the fitness of a population evolved in one environment and assayed in a second was inversely related to the fitness of a population evolved in the second environment and assayed in the first. These results support the notion that positive correlated responses to selection across environments are frequent, and worth further study.

Revealing gene expression heterogeneity in a clonal population of Tetrahymena thermophila through single-cell RNA sequencing

We performed single-cell RNA sequencing (scRNA-seq) on a population of 5,000 Tetrahymena thermophila, using the 10x Genomics 3' gene expression analysis, to investigate gene expression variability within this clonal population. Initially, we estimated the 3'-untranslated regions (3' UTRs), which were absent in existing annotation files but are crucial for the 10x Genomics 3' gene expression analysis, using the peaks2utr method. This allowed us to create a modified annotation file, which was then utilized in our scRNA-seq analysis. Our analysis revealed significant gene expression variability within the population, even after removing the effect of cell phase-related features. This variability predominantly appeared in six distinct clusters. Through gene ontology and KEGG pathway enrichment analyses, we identified that these were primarily associated with ribosomal proteins, proteins specific to mitochondria, proteins involved in peroxisome-specific carbon metabolism, cytoskeletal proteins, motor proteins, and immobilized antigens.

Integrative analyses on the ciliates Colpoda illuminate the life history evolution of soil microorganisms

Microorganisms play a central role in sustaining soil ecosystems and agriculture, and these functions are usually associated with their complex life history. Yet, the regulation and evolution of life history have remained enigmatic and poorly understood, especially in protozoa, the third most abundant group of organisms in the soil. Here, we explore the life history of a cosmopolitan species-Colpoda steinii. Our analysis has yielded a high-quality macronuclear genome for C. steinii, with size of 155 Mbp and 37,123 protein-coding genes, as well as mean intron length of ~93 bp, longer than most other studied ciliates. Notably, we identify two possible whole-genome duplication events in C. steinii, which may account for its genome being about twice the size of C. inflata's, another co-existing species. We further resolve the gene expression profiles in diverse life stages of C. steinii, which are also corroborated in C. inflata. During the resting cyst stage, genes associated with cell death and vacuole formation are upregulated, and translation-related genes are downregulated. While the translation-related genes are upregulated during the excystment of resting cysts. Reproductive cysts exhibit a significant reduction in cell adhesion. We also demonstrate that most genes expressed in specific life stages are under strong purifying selection. This study offers a deeper understanding of the life history evolution that underpins the extraordinary success and ecological functions of microorganisms in soil ecosystems.IMPORTANCEColpoda species, as a prominent group among the most widely distributed and abundant soil microorganisms, play a crucial role in sustaining soil ecosystems and promoting plant growth. This investigation reveals their exceptional macronuclear genomic features, including significantly large genome size, long introns, and numerous gene duplications. The gene expression profiles and the specific biological functions associated with the transitions between various life stages are also elucidated. The vast majority of genes linked to life stage transitions are subject to strong purifying selection, as inferred from multiple natural strains newly isolated and deeply sequenced. This substantiates the enduring and conservative nature of Colpoda's life history, which has persisted throughout the extensive evolutionary history of these highly successful protozoa in soil. These findings shed light on the evolutionary dynamics of microbial eukaryotes in the ever-fluctuating soil environments. This integrative research represents a significant advancement in understanding the life histories of these understudied single-celled eukaryotes.

Use of E-64 cysteine protease inhibitor for the recombinant protein production in Tetrahymena thermophila

Tetrahymena thermophila is an alternative organism for recombinant protein production. However, the production efficiency in T. thermophila is quite low mainly due to the rich cysteine proteases. In this study, we studied whether supplementation of the E-64 inhibitor to T. thermophila cultures increases the recombinant protein production efficiency without any toxic side effects. Our study showed that supplementation of E-64 had no lethal effects on T. thermophila cells in flask culture at 30 °C and 38 °C. In vitro protease activity analysis using secretome as protease enzyme source from E-64-supplemented cell cultures showed a reduced protein substrate degradation using bovine serum albumin, rituximab, and milk lactoglobulin proteins. E-64 also prevented proteolysis of the recombinantly produced and secreted TtmCherry2-sfGFP fusion protein at some level. This reduced inhibitory effect of E-64 could be due to genetic compensation of the inhibited proteases. As a result, the 5 µM concentration of E-64 was found to be a non-toxic protease inhibitory supplement to improve extracellular recombinant protein production efficiency in T. thermophila. This study suggests that the use of E-64 may increase the efficiency of extracellular recombinant protein production by continuously reducing extracellular cysteine protease activity during cultivation.

Cellular Response of Adapted and Non-Adapted Tetrahymena thermophila Strains to Europium Eu(III) Compounds

Europium is one of the most reactive lanthanides and humans use it in many different applications, but we still know little about its potential toxicity and cellular response to its exposure. Two strains of the eukaryotic microorganism model Tetrahymena thermophila were adapted to high concentrations of two Eu(III) compounds (EuCl3 or Eu2O3) and compared to a control strain and cultures treated with both compounds. In this ciliate, EuCl3 is more toxic than Eu2O3. LC50 values show that this microorganism is more resistant to these Eu(III) compounds than other microorganisms. Oxidative stress originated mainly by Eu2O3 is minimized by overexpression of genes encoding important antioxidant enzymes. The overexpression of metallothionein genes under treatment with Eu(III) compounds supports the possibility that this lanthanide may interact with the -SH groups of the cysteine residues from metallothioneins and/or displace essential cations of these proteins during their homeostatic function. Both lipid metabolism (lipid droplets fusing with europium-containing vacuoles) and autophagy are involved in the cellular response to europium stress. Bioaccumulation, together with a possible biomineralization to europium phosphate, seems to be the main mechanism of Eu(III) detoxification in these cells.

Genome assembly of a symbiotic balantidia (Balantidium ctenopharyngodoni) in fish hindgut

Balantidium ctenopharyngodoni is identified as the sole ciliate species that exclusively resides within the hindgut of grass carp with high prevalence and intensity. In this study, the successful cultivation of B. ctenopharyngodoni enabled us to collect enough cells for genome sequencing. Consequently, we acquired a high-quality genome assembly spanning 68.66 Mb, encompassing a total of 22,334 nanochromosomes. Furthermore, we predicted 29,348 protein-coding genes, and 95.5% of them was supported by the RNA-seq data. The trend of GC content in the subtelomeric regions of single-gene chromosomes was similar to other ciliates containing nanochromosomes. A large number of genes encoding carbohydrate-binding modules with affinities for starch and peptidoglycans was identified. The identification of mitochondrion-related organelles (MROs) within genome indicates its well-suited adaptation to the anaerobic conditions in the hindgut environment. In summary, our results will offer resources for understanding the genetic basis and molecular adaptations of balantidia to hindgut of herbivorous fish.

The genome-wide meiotic recombination landscape in ciliates and its implications for crossover regulation and genome evolution

Meiotic recombination is essential for sexual reproduction and its regulation has been extensively studied in many taxa. However, genome-wide recombination landscape has not been reported in ciliates and it remains unknown how it is affected by the unique features of ciliates: the synaptonemal complex (SC)-independent meiosis and the nuclear dimorphism. Here, we show the recombination landscape in the model ciliate Tetrahymena thermophila by analyzing single-nucleotide polymorphism datasets from 38 hybrid progeny. We detect 1021 crossover (CO) events (35.8 per meiosis), corresponding to an overall CO rate of 9.9 cM/Mb. However, gene conversion by non-crossover is rare (1.03 per meiosis) and not biased towards G or C alleles. Consistent with the reported roles of SC in CO interference, we find no obvious sign of CO interference. CO tends to occur within germ-soma common genomic regions and many of the 44 identified CO hotspots localize at the centromeric or subtelomeric regions. Gene ontology analyses show that CO hotspots are strongly associated with genes responding to environmental changes. We discuss these results with respect to how nuclear dimorphism has potentially driven the formation of the observed recombination landscape to facilitate environmental adaptation and the sharing of machinery among meiotic and somatic recombination.

MicroRNAs in Tetrahymena thermophila: An epigenetic regulatory mechanism in the response to cadmium stress

Among the epigenetic mechanisms based on non-coding RNA are microRNAs (miRNAs) that are involved in the post-transcriptional regulation of mRNAs. In many organisms, the expression of genes involved in the cellular response to biotic or abiotic stress depends on the regulation, generally inhibitory, performed by miRNAs. For the first time in the eukaryotic microorganism (ciliate-model) Tetrahymena thermophila, miRNAs involved in the post-transcriptional regulation of transcripts linked to the response to cadmium have been isolated and analyzed. Forty de novo miRNAs (we named tte-miRNAs) have been isolated from control and Cd-treated populations (1 or 24 h exposures). An exhaustive comparative analysis of the features of these mature tte-miRNAs and their precursor sequences (pre-tte-miRNAs) confirms that they are true miRNAs. In addition to the three types of miRNA isoforms previously described in other organisms, two new types are also described among the tte-miRNAs studied. A certain percentage of the pre-tte-miRNA sequences are in introns from genes with many introns, and have been defined as 5', 3'-tailed mirtrons. A qRT-PCR analysis of selected tte-miRNAs together with some of their targets has validated them. Cd is one of the most toxic metals for the cell, which must defend itself against its toxicity by various mechanisms, such as expulsion by membrane pumps, chelation by metallothioneins, among others. Like other toxic metals, Cd also causes a well-known series of cellular effects such as intense proteotoxicity. Many of the targets that are regulated by the tte-miRNAs are transcripts encoding proteins that fit into these defense mechanisms and toxic metal effects.

Global and local functions of the Fused kinase ortholog CdaH in intracellular patterning in Tetrahymena

Ciliates assemble numerous microtubular structures into complex cortical patterns. During ciliate division, the pattern is duplicated by intracellular segmentation that produces a tandem of daughter cells. In Tetrahymena thermophila, the induction and positioning of the division boundary involves two mutually antagonistic factors: posterior CdaA (cyclin E) and anterior CdaI (Hippo kinase). Here, we characterized the related cdaH-1 allele, which confers a pleiotropic patterning phenotype including an absence of the division boundary and an anterior-posterior mispositioning of the new oral apparatus. CdaH is a Fused or Stk36 kinase ortholog that localizes to multiple sites that correlate with the effects of its loss, including the division boundary and the new oral apparatus. CdaH acts downstream of CdaA to induce the division boundary and drives asymmetric cytokinesis at the tip of the posterior daughter. CdaH both maintains the anterior-posterior position of the new oral apparatus and interacts with CdaI to pattern ciliary rows within the oral apparatus. Thus, CdaH acts at multiple scales, from induction and positioning of structures on the cell-wide polarity axis to local organelle-level patterning.

Label-free proteomic comparison reveals ciliary and nonciliary phenotypes of IFT-A mutants

DIFFRAC is a powerful method for systematically comparing proteome content and organization between samples in a high-throughput manner. By subjecting control and experimental protein extracts to native chromatography and quantifying the contents of each fraction using mass spectrometry, it enables the quantitative detection of alterations to protein complexes and abundances. Here, we applied DIFFRAC to investigate the consequences of genetic loss of Ift122, a subunit of the intraflagellar transport-A (IFT-A) protein complex that plays a vital role in the formation and function of cilia and flagella, on the proteome of Tetrahymena thermophila. A single DIFFRAC experiment was sufficient to detect changes in protein behavior that mirrored known effects of IFT-A loss and revealed new biology. We uncovered several novel IFT-A-regulated proteins, which we validated through live imaging in Xenopus multiciliated cells, shedding new light on both the ciliary and non-ciliary functions of IFT-A. Our findings underscore the robustness of DIFFRAC for revealing proteomic changes in response to genetic or biochemical perturbation.

Genome-wide identification and in-silico analysis of papain-family cysteine protease encoding genes in Tetrahymena thermophila

Tetrahymena thermophila is a promising host for recombinant protein production, but its utilization in biotechnology is mostly limited due to the presence of intracellular and extracellular papain-family cysteine proteases (PFCPs). In this study, we employed bioinformatics approaches to investigate the T. thermophila PFCP genes and their encoded proteases (TtPFCPs), the most prominent protease family in the genome. Results from the multiple sequence alignment, protein modeling, and conserved motif analyses revealed that all TtPFCPs showed considerably high homology with mammalian cysteine cathepsins and contained conserved amino acid motifs. The total of 121 TtPFCP-encoding genes, 14 of which were classified as non-peptidase homologs, were found. Remaining 107 true TtPFCPs were divided into four distinct subgroups depending on their homology with mammalian lysosomal cathepsins: cathepsin L-like (TtCATLs), cathepsin B-like (TtCATBs), cathepsin C-like (TtCATCs), and cathepsin X-like (TtCATXs) PFCPs. The majority of true TtPFCPs (96 out of the total) were in TtCATL-like peptidase subgroup. Both phylogenetic and chromosomal localization analyses of TtPFCPs supported the hypothesis that TtPFCPs likely evolved through tandem gene duplication events and predominantly accumulated on micronuclear chromosome 5. Additionally, more than half of the identified TtPFCP genes are expressed in considerably low quantities compared to the rest of the TtPFCP genes, which are expressed at a higher level. However, their expression patterns fluctuate based on the stage of the life cycle. In conclusion, this study provides the first comprehensive in-silico analysis of TtPFCP genes and encoded proteases. The results would help designing an effective strategy for protease knockout mutant cell lines to discover biological function and to improve the recombinant protein production in T. thermophila.

Somatic genome architecture and molecular evolution are decoupled in "young" linage-specific gene families in ciliates

The evolution of lineage-specific gene families remains poorly studied across the eukaryotic tree of life, with most analyses focusing on the recent evolution of de novo genes in model species. Here we explore the origins of lineage-specific genes in ciliates, a ~1 billion year old clade of microeukaryotes that are defined by their division of somatic and germline functions into distinct nuclei. Previous analyses on conserved gene families have shown the effect of ciliates' unusual genome architecture on gene family evolution: extensive genome processing-the generation of thousands of gene-sized somatic chromosomes from canonical germline chromosomes-is associated with larger and more diverse gene families. To further study the relationship between ciliate genome architecture and gene family evolution, we analyzed lineage specific gene families from a set of 46 transcriptomes and 12 genomes representing x species from eight ciliate classes. We assess how the evolution lineage-specific gene families occurs among four groups of ciliates: extensive fragmenters with gene-size somatic chromosomes, non-extensive fragmenters with "large'' multi-gene somatic chromosomes, Heterotrichea with highly polyploid somatic genomes and Karyorelictea with 'paradiploid' somatic genomes. Our analyses demonstrate that: 1) most lineage-specific gene families are found at shallow taxonomic scales; 2) extensive genome processing (i.e., gene unscrambling) during development likely influences the size and number of young lineage-specific gene families; and 3) the influence of somatic genome architecture on molecular evolution is increasingly apparent in older gene families. Altogether, these data highlight the influences of genome architecture on the evolution of lineage-specific gene families in eukaryotes.

Towards high-throughput parallel imaging and single-cell transcriptomics of microbial eukaryotic plankton

Single-cell transcriptomics has the potential to provide novel insights into poorly studied microbial eukaryotes. Although several such technologies are available and benchmarked on mammalian cells, few have been tested on protists. Here, we applied a microarray single-cell sequencing (MASC-seq) technology, that generates microscope images of cells in parallel with capturing their transcriptomes, on three species representing important plankton groups with different cell structures; the ciliate Tetrahymena thermophila, the diatom Phaeodactylum tricornutum, and the dinoflagellate Heterocapsa sp. Both the cell fixation and permeabilization steps were adjusted. For the ciliate and dinoflagellate, the number of transcripts of microarray spots with single cells were significantly higher than for background spots, and the overall expression patterns were correlated with that of bulk RNA, while for the much smaller diatom cells, it was not possible to separate single-cell transcripts from background. The MASC-seq method holds promise for investigating "microbial dark matter", although further optimizations are necessary to increase the signal-to-noise ratio.

The macronuclear genomic landscape within Tetrahymena thermophila

The extent of intraspecific genomic variation is key to understanding species evolutionary history, including recent adaptive shifts. Intraspecific genomic variation remains poorly explored in eukaryotic micro-organisms, especially in the nuclear dimorphic ciliates, despite their fundamental role as laboratory model systems and their ecological importance in many ecosystems. We sequenced the macronuclear genome of 22 laboratory strains of the oligohymenophoran Tetrahymena thermophila, a model species in both cellular biology and evolutionary ecology. We explored polymorphisms at the junctions of programmed eliminated sequences, and reveal their utility to barcode very closely related cells. As for other species of the genus Tetrahymena, we confirm micronuclear centromeres as gene diversification centres in T. thermophila, but also reveal a two-speed evolution in these regions. In the rest of the genome, we highlight recent diversification of genes coding for extracellular proteins and cell adhesion. We discuss all these findings in relation to this ciliate's ecology and cellular characteristics.

MarFERReT, an open-source, version-controlled reference library of marine microbial eukaryote functional genes

Metatranscriptomics generates large volumes of sequence data about transcribed genes in natural environments. Taxonomic annotation of these datasets depends on availability of curated reference sequences. For marine microbial eukaryotes, current reference libraries are limited by gaps in sequenced organism diversity and barriers to updating libraries with new sequence data, resulting in taxonomic annotation of about half of eukaryotic environmental transcripts. Here, we introduce Marine Functional EukaRyotic Reference Taxa (MarFERReT), a marine microbial eukaryotic sequence library designed for use with taxonomic annotation of eukaryotic metatranscriptomes. We gathered 902 publicly accessible marine eukaryote genomes and transcriptomes and assessed their sequence quality and cross-contamination issues, selecting 800 validated entries for inclusion in MarFERReT. Version 1.1 of MarFERReT contains reference sequences from 800 marine eukaryotic genomes and transcriptomes, covering 453 species- and strain-level taxa, totaling nearly 28 million protein sequences with associated NCBI and PR2 Taxonomy identifiers and Pfam functional annotations. The MarFERReT project repository hosts containerized build scripts, documentation on installation and use case examples, and information on new versions of MarFERReT.

Membrane-anchored calpains - hidden regulators of growth and development beyond plants?

Calpains are modulatory proteases that modify diverse cellular substrates and play essential roles in eukaryots. The best studied are animal cytosolic calpains. Here, we focus on enigmatic membrane-anchored calpains, their structural and functional features as well as phylogenetic distribution. Based on domain composition, we identified four types of membrane-anchored calpains. Type 1 and 2 show broad phylogenetic distribution among unicellular protists and streptophytes suggesting their ancient evolutionary origin. Type 3 and 4 diversified early and are present in brown algae and oomycetes. The plant DEK1 protein is the only representative of membrane-anchored calpains that has been functionally studied. Here, we present up to date knowledge about its structural features, putative regulation, posttranslational modifications, and biological role. Finally, we discuss potential model organisms and available tools for functional studies of membrane-anchored calpains with yet unknown biological role. Mechanistic understanding of membrane-anchored calpains may provide important insights into fundamental principles of cell polarization, cell fate control, and morphogenesis beyond plants.

Mismatch Repair Protein Msh6Tt Is Necessary for Nuclear Division and Gametogenesis in Tetrahymena thermophila

DNA mismatch repair (MMR) improves replication accuracy by up to three orders of magnitude. The MutS protein in E. coli or its eukaryotic homolog, the MutSα (Msh2-Msh6) complex, recognizes base mismatches and initiates the mismatch repair mechanism. Msh6 is an essential protein for assembling the heterodimeric complex. However, the function of the Msh6 subunit remains elusive. Tetrahymena undergoes multiple DNA replication and nuclear division processes, including mitosis, amitosis, and meiosis. Here, we found that Msh6Tt localized in the macronucleus (MAC) and the micronucleus (MIC) during the vegetative growth stage and starvation. During the conjugation stage, Msh6Tt only localized in MICs and newly developing MACs. MSH6Tt knockout led to aberrant nuclear division during vegetative growth. The MSH6TtKO mutants were resistant to treatment with the DNA alkylating agent methyl methanesulfonate (MMS) compared to wild type cells. MSH6Tt knockout affected micronuclear meiosis and gametogenesis during the conjugation stage. Furthermore, Msh6Tt interacted with Msh2Tt and MMR-independent factors. Downregulation of MSH2Tt expression affected the stability of Msh6Tt. In addition, MSH6Tt knockout led to the upregulated expression of several MSH6Tt homologs at different developmental stages. Msh6Tt is involved in macronuclear amitosis, micronuclear mitosis, micronuclear meiosis, and gametogenesis in Tetrahymena.

Structure and dynamics of the contractile vacuole complex in Tetrahymena thermophila

The contractile vacuole complex (CVC) is a dynamic and morphologically complex membrane organelle, comprising a large vesicle (bladder) linked with a tubular reticulum (spongiome). CVCs provide key osmoregulatory roles across diverse eukaryotic lineages, but probing the mechanisms underlying their structure and function is hampered by the limited tools available for in vivo analysis. In the experimentally tractable ciliate Tetrahymena thermophila, we describe four proteins that, as endogenously tagged constructs, localize specifically to distinct CVC zones. The DOPEY homolog Dop1p and the CORVET subunit Vps8Dp localize both to the bladder and spongiome but with different local distributions that are sensitive to osmotic perturbation, whereas the lipid scramblase Scr7p colocalizes with Vps8Dp. The H+-ATPase subunit Vma4 is spongiome specific. The live imaging permitted by these probes revealed dynamics at multiple scales including rapid exchange of CVC-localized and soluble protein pools versus lateral diffusion in the spongiome, spongiome extension and branching, and CVC formation during mitosis. Although the association with DOP1 and VPS8D implicate the CVC in endosomal trafficking, both the bladder and spongiome might be isolated from bulk endocytic input.

Identification of a non-canonical ciliate nuclear genetic code where UAA and UAG code for different amino acids

The genetic code is one of the most highly conserved features across life. Only a few lineages have deviated from the "universal" genetic code. Amongst the few variants of the genetic code reported to date, the codons UAA and UAG virtually always have the same translation, suggesting that their evolution is coupled. Here, we report the genome and transcriptome sequencing of a novel uncultured ciliate, belonging to the Oligohymenophorea class, where the translation of the UAA and UAG stop codons have changed to specify different amino acids. Genomic and transcriptomic analyses revealed that UAA has been reassigned to encode lysine, while UAG has been reassigned to encode glutamic acid. We identified multiple suppressor tRNA genes with anticodons complementary to the reassigned codons. We show that the retained UGA stop codon is enriched in the 3'UTR immediately downstream of the coding region of genes, suggesting that there is functional drive to maintain tandem stop codons. Using a phylogenomics approach, we reconstructed the ciliate phylogeny and mapped genetic code changes, highlighting the remarkable number of independent genetic code changes within the Ciliophora group of protists. According to our knowledge, this is the first report of a genetic code variant where UAA and UAG encode different amino acids.

Recombinant production of hormonally active human insulin from pre-proinsulin by Tetrahymena thermophila

Alternative cell factories, such as the unicellular ciliate eukaryotic Tetrahymena thermophila, may be required for the production of protein therapeutics that are challenging to produce in conventional expression systems. T. thermophila (Tt) can secrete proteins with the post-translational modifications necessary for their function in humans. In this study, we tested if T. thermophila could process the human pre-proinsulin to produce hormonally active human insulin (hINS) with correct modifications. Flask and bioreactor culture of T. thermophila were used to produce the recombinant Tt-hINS either with or without an affinity tag from a codon-adapted pre-proinsulin sequence. Our results indicate that T. thermophila can produce a 6 kDa Tt-hINS monomer with the appropriate disulfide bonds after removal of the human insulin signal sequence or endogenous phospholipase A signal sequence, and the C-peptide of the human insulin. Additionally, Tt-hINS can form 12 kDa dimeric, 24 kDa tetrameric, and 36 kDa hexameric complexes. Tt-hINS-sfGFP fusion protein was localized to the vesicles within the cytoplasm and was secreted extracellularly. Assessing the affinity-purified Tt-hINS activity using the in vivo T. thermophila extracellular glucose drop assay, we observed that Tt-hINS induced a significant reduction (approximately 21 %) in extracellular glucose levels, indicative of its functional insulin activity. Our results demonstrate that T. thermophila is a promising candidate for the pharmaceutical and biotechnology industries as a host organism for the production of human protein drugs.

Effects of six pyrimidine analogs on the growth of Tetrahymena thermophila and their implications in pyrimidine metabolism

Tetrahymena are ciliated protists that have been used to study the effects of toxic chemicals, including anticancer drugs. In this study, we tested the inhibitory effects of six pyrimidine analogs (5-fluorouracil, floxuridine, 5'-deoxy-5-fluorouridine, 5-fluorouridine, gemcitabine, and cytarabine) on wild-type CU428 and conditional mutant NP1 Tetrahymena thermophila at room temperature and the restrictive temperature (37°C) where NP1 does not form the oral apparatus. We found that phagocytosis was not required for pyrimidine analog entry and that all tested pyrimidine analogs inhibited growth except for cytarabine. IC50 values did not significantly differ between CU428 and NP1 for the same analog at either room temperature or 37°C. To investigate the mechanism of inhibition, we used two pyrimidine bases (uracil and thymine) and three nucleosides (uridine, thymidine, and 5-methyluridine) to determine whether the inhibitory effects from the pyrimidine analogs were reversible. We found that the inhibitory effects from 5-fluorouracil could be reversed by uracil and thymine, from floxuridine could be reversed by thymidine, and from 5'-deoxy-5-fluorouridine could be reversed by uracil. None of the tested nucleobases or nucleosides could reverse the inhibitory effects of gemcitabine or 5-fluorouridine. Our results suggest that the five pyrimidine analogs act on different sites to inhibit T. thermophila growth and that nucleobases and nucleosides are metabolized differently in Tetrahymena.

Cross-kingdom analyses of transmembrane protein kinases show their functional diversity and distinct origins in protists

Transmembrane kinases (TMKs) are important mediators of cellular signaling cascades. The kinase domains of most metazoan and plant TMKs belong to the serine/threonine/tyrosine kinase (S/T/Y-kinase) superfamily. They share a common origin with prokaryotic kinases and have diversified into distinct subfamilies. Diverse members of the eukaryotic crown radiation such as amoebae, ciliates, and red and brown algae (grouped here under the umbrella term "protists") have long diverged from higher eukaryotes since their ancient common ancestry, making them ideal organisms for studying TMK evolution. Here, we developed an accurate and high-throughput pipeline to predict TMKomes in cellular organisms. Cross-kingdom analyses revealed distinct features of TMKomes in each grouping. Two-transmembrane histidine kinases constitute the main TMKomes of bacteria, while metazoans, plants, and most protists have a large proportion of single-pass TM S/T/Y-kinases. Phylogenetic analyses classified most protist S/T/Y-kinases into three clades, with clades II and III specifically expanded in amoebae and oomycetes, respectively. In contrast, clade I kinases were widespread in all protists examined here, and likely shared a common origin with other eukaryotic S/T/Y-kinases. Functional annotation further showed that most non-kinase domains were grouping-specific, suggesting that their recombination with the more conserved kinase domains led to the divergence of S/T/Y-kinases. However, we also found that protist leucine-rich repeat (LRR)- and G-protein-coupled receptor (GPCR)-type TMKs shared similar sensory domain architectures with respective plant and animal TMKs, despite that they belong to distinct kinase subfamilies. Collectively, our study revealed the functional diversity of TMKomes and the distinct origins of S/T/Y-kinases in protists.

Codon Usage Bias Analysis in Macronuclear Genomes of Ciliated Protozoa

Ciliated protozoa (ciliates) are unicellular eukaryotes, several of which are important model organisms for molecular biology research. Analyses of codon usage bias (CUB) of the macronuclear (MAC) genome of ciliates can promote a better understanding of the genetic mode and evolutionary history of these organisms and help optimize codons to improve gene editing efficiency in model ciliates. In this study, the following indices were calculated: the guanine-cytosine (GC) content, the frequency of the nucleotides at the third position of codons (T3, C3, A3, G3), the effective number of codons (ENc), GC content at the 3rd position of synonymous codons (GC3s), and the relative synonymous codon usage (RSCU). Parity rule 2 plot analysis, Neutrality plot analysis, ENc plot analysis, and correlation analysis were employed to explore the main influencing factors of CUB. The results showed that the GC content in the MAC genomes of each of 21 ciliate species, the genomes of which were relatively complete, was lower than 50%, and the base compositions of GC and GC3s were markedly distinct. Synonymous codon analysis revealed that the codons in most of the 21 ciliates ended with A or T and four codons were the general putative optimal codons. Collectively, our results indicated that most of the ciliates investigated preferred using the codons with anof AT-ending and that codon usage bias was affected by gene mutation and natural selection.

The Tetrahymena bcd1 mutant implicates endosome trafficking in ciliate, cortical pattern formation

Ciliates, such as Tetrahymena thermophila, evolved complex mechanisms to determine both the location and dimensions of cortical organelles such as the oral apparatus (OA: involved in phagocytosis), cytoproct (Cyp: for eliminating wastes), and contractile vacuole pores (CVPs: involved in water expulsion). Mutations have been recovered in Tetrahymena that affect both the localization of such organelles along anterior-posterior and circumferential body axes and their dimensions. Here we describe BCD1, a ciliate pattern gene that encodes a conserved Beige-BEACH domain-containing protein a with possible protein kinase A (PKA)-anchoring activity. Similar proteins have been implicated in endosome trafficking and are linked to human Chediak-Higashi syndrome and autism. Mutations in the BCD1 gene broaden cortical organelle domains as they assemble during predivision development. The Bcd1 protein localizes to membrane pockets at the base of every cilium that are active in endocytosis. PKA activity has been shown to promote endocytosis in other organisms, so we blocked clathrin-mediated endocytosis (using "dynasore") and inhibited PKA (using H89). In both cases, treatment produced partial phenocopies of the bcd1 pattern mutant. This study supports a model in which the dimensions of diverse cortical organelle assembly-platforms may be determined by regulated balance between constitutive exocytic delivery and PKA-regulated endocytic retrieval of organelle materials and determinants.

Single-cell transcriptome reveals cell division-regulated hub genes in the unicellular eukaryote Paramecium

The transition from growth to division during the cell cycle encompasses numerous conserved processes such as large-scale DNA replication and protein synthesis. In ciliate cells, asexual cell division is accompanied by additional cellular changes including amitotic nuclear division, extensive ciliogenesis, and trichocyst replication. However, the molecular mechanisms underlying these processes remain elusive. In this study, we present single-cell gene expression profiles of Paramecium cf. multimicronucleatum cells undergoing cell division. Our results reveal that the most up-regulated genes in dividing cells compared to growing cells are associated with 1) cell cycle signaling pathways including transcription, DNA replication, chromosome segregation and protein degradation; 2) microtubule proteins and tubulin glycylases which are essential for ciliogenesis, nuclei separation and structural differentiation signaling; and 3) trichocyst matrix proteins involved in trichocyst synthesis and reproduction. Furthermore, weighted gene co-expression network analysis identified hub genes that may play crucial roles during cell division. Our findings provide insights into cell cycle regulators, microtubules and trichocyst matrix proteins that may exert influence on this process in ciliates.

Omics of an Enigmatic Marine Amoeba Uncovers Unprecedented Gene Trafficking from Giant Viruses and Provides Insights into Its Complex Life Cycle

Amoebozoa include lineages of diverse ecology, behavior, and morphology. They are assumed to encompass members with the largest genome sizes of all living things, yet genomic studies in the group are limited. Trichosphaerium, a polymorphic, multinucleate, marine amoeba with a complicated life cycle, has puzzled experts for over a century. In an effort to explore the genomic diversity and investigate extraordinary behavior observed among the Amoebozoa, we used integrated omics approaches to study this enigmatic marine amoeba. Omics data, including single-cell transcriptomics and cytological data, demonstrate that Trichosphaerium sp. possesses the complete meiosis toolkit genes. These genes are expressed in life stages of the amoeba including medium and large cells. The life cycle of Trichosphaerium sp. involves asexual processes via binary fission and multiple fragmentation of giant cells, as well as sexual-like processes involving genes implicated in sexual reproduction and polyploidization. These findings are in stark contrast to a life cycle previously reported for this amoeba. Despite the extreme morphological plasticity observed in Trichosphaerium, our genomic data showed that populations maintain a species-level intragenomic variation. A draft genome of Trichosphaerium indicates elevated lateral gene transfer (LGT) from bacteria and giant viruses. Gene trafficking in Trichosphaerium is the highest within Amoebozoa and among the highest in microbial eukaryotes.

iGDP: An integrated genome decontamination pipeline for wild ciliated microeukaryotes

Ciliates are a large group of ubiquitous and highly diverse single-celled eukaryotes that play an essential role in the functioning of microbial food webs. However, their genomic diversity is far from clear due to the need to develop cultivation methods for most species, so most research is based on wild organisms that almost invariably contain contaminants. Here we establish an integrated Genome Decontamination Pipeline (iGDP) that combines homology search, telomere reads-assisted and clustering approaches to filter contaminated ciliate genome assemblies from wild specimens. We benchmarked the performance of iGDP using genomic data from a contaminated ciliate culture and the results showed that iGDP could recall 91.9% of the target sequences with 96.9% precision. We also used a synthetic dataset to offer guidelines for the application of iGDP in the removal of various groups of contaminants. Compared with several popular metagenome binning tools, iGDP could show better performance. To further validate the effectiveness of iGDP on real-world data, we applied it to decontaminate genome assemblies of three wild ciliate specimens and obtained their genomes with high quality comparable to that of previously well-studied model ciliate genomes. It is anticipated that the newly generated genomes and the established iGDP method will be valuable community resources for detailed studies on ciliate biodiversity, phylogeny, ecology and evolution. The pipeline (https://github.com/GWang2022/iGDP) can be implemented automatically to reduce manual filtering and classification and may be further developed to apply to other microeukaryotes.

Label-free proteomic comparison reveals ciliary and non-ciliary phenotypes of IFT-A mutants

DIFFRAC is a powerful method for systematically comparing proteome content and organization between samples in a high-throughput manner. By subjecting control and experimental protein extracts to native chromatography and quantifying the contents of each fraction using mass spectrometry, it enables the quantitative detection of alterations to protein complexes and abundances. Here, we applied DIFFRAC to investigate the consequences of genetic loss of Ift122, a subunit of the intraflagellar transport-A (IFT-A) protein complex that plays a vital role in the formation and function of cilia and flagella, on the proteome of Tetrahymena thermophila . A single DIFFRAC experiment was sufficient to detect changes in protein behavior that mirrored known effects of IFT-A loss and revealed new biology. We uncovered several novel IFT-A-regulated proteins, which we validated through live imaging in Xenopus multiciliated cells, shedding new light on both the ciliary and non-ciliary functions of IFT-A. Our findings underscore the robustness of DIFFRAC for revealing proteomic changes in response to genetic or biochemical perturbation.

Developmentally Programmed Switches in DNA Replication: Gene Amplification and Genome-Wide Endoreplication in Tetrahymena

Locus-specific gene amplification and genome-wide endoreplication generate the elevated copy number of ribosomal DNA (rDNA, 9000 C) and non-rDNA (90 C) chromosomes in the developing macronucleus of Tetrahymena thermophila. Subsequently, all macronuclear chromosomes replicate once per cell cycle during vegetative growth. Here, we describe an unanticipated, programmed switch in the regulation of replication initiation in the rDNA minichromosome. Early in development, the 21 kb rDNA minichromosome is preferentially amplified from 2 C to ~800 C from well-defined origins, concurrent with genome-wide endoreplication (2 C to 8-16 C) in starved mating Tetrahymena (endoreplication (ER) Phase 1). Upon refeeding, rDNA and non-rDNA chromosomes achieve their final copy number through resumption of just the endoreplication program (ER Phase 2). Unconventional rDNA replication intermediates are generated primarily during ER phase 2, consistent with delocalized replication initiation and possible formation of persistent RNA-DNA hybrids. Origin usage and replication fork elongation are affected in non-rDNA chromosomes as well. Despite the developmentally programmed 10-fold reduction in the ubiquitous eukaryotic initiator, the Origin Recognition Complex (ORC), active initiation sites are more closely spaced in ER phases 1 and 2 compared to vegetative growing cells. We propose that initiation site selection is relaxed in endoreplicating macronuclear chromosomes and may be less dependent on ORC.

Snf2 Proteins Are Required to Generate Gamete Pronuclei in Tetrahymena thermophila

During sexual reproduction/conjugation of the ciliate Tetrahymena thermophila, the germinal micronucleus undergoes meiosis resulting in four haploid micronuclei (hMICs). All hMICs undergo post-meiotic DNA double-strand break (PM-DSB) formation, cleaving their genome. DNA lesions are subsequently repaired in only one ‘selected’ hMIC, which eventually produces gametic pronuclei. DNA repair in the selected hMIC involves chromatin remodeling by switching from the heterochromatic to the euchromatic state of its genome. Here, we demonstrate that, among the 15 Tetrahymena Snf2 family proteins, a core of the ATP-dependent chromatin remodeling complex in Tetrahymena, the germline nucleus specific Iswi in Tetrahymena IswiGTt and Rad5Tt is crucial for the generation of gametic pronuclei. In either gene knockout, the selected hMIC which shows euchromatin markers such as lysine-acetylated histone H3 does not appear, but all hMICs in which markers for DNA lesions persist are degraded, indicating that both IswiGTt and Rad5Tt have important roles in repairing PM-DSB DNA lesions and remodeling chromatin for the euchromatic state in the selected hMIC.

Ciliate Microtubule Diversities: Insights from the EFBTU3 Tubulin in the Antarctic Ciliate Euplotes focardii

Protozoans of the Phylum Ciliophora (ciliates) assemble many diverse microtubular structures in a single cell throughout the life cycle, a feature that made them useful models to study microtubule complexity and the role of tubulin isotypes. In the Antarctic ciliate Euplotes focardii we identified five β-tubulin isotypes by genome sequencing, named EFBTU1, EFBTU2, EFBTU3, EFBTU4 and EFBTU5. By using polyclonal antibodies directed against EFBTU2/EFBTU1 and EFBTU3, we show that the former isotypes appear to be involved in the formation of all microtubular structures and are particularly abundant in cilia, whereas the latter specifically localizes at the bases of cilia. By RNA interference (RNAi) technology, we silenced the EFBTU3 gene and provided evidence that this isotype has a relevant role in cilia regeneration upon deciliation and in cell division. These results support the long-standing concept that tubulin isotypes possess functional specificity in building diverse microtubular structures.

Comparative genomics reveals insight into the evolutionary origin of massively scrambled genomes

Ciliates are microbial eukaryotes that undergo extensive programmed genome rearrangement, a natural genome editing process that converts long germline chromosomes into smaller gene-rich somatic chromosomes. Three well-studied ciliates include Oxytricha trifallax, Tetrahymena thermophila, and Paramecium tetraurelia, but only the Oxytricha lineage has a massively scrambled genome, whose assembly during development requires hundreds of thousands of precisely programmed DNA joining events, representing the most complex genome dynamics of any known organism. Here we study the emergence of such complex genomes by examining the origin and evolution of discontinuous and scrambled genes in the Oxytricha lineage. This study compares six genomes from three species, the germline and somatic genomes for Euplotes woodruffi, Tetmemena sp., and the model ciliate O. trifallax. We sequenced, assembled, and annotated the germline and somatic genomes of E. woodruffi, which provides an outgroup, and the germline genome of Tetmemena sp. We find that the germline genome of Tetmemena is as massively scrambled and interrupted as Oxytricha's: 13.6% of its gene loci require programmed translocations and/or inversions, with some genes requiring hundreds of precise gene editing events during development. This study revealed that the earlier diverged spirotrich, E. woodruffi, also has a scrambled genome, but only roughly half as many loci (7.3%) are scrambled. Furthermore, its scrambled genes are less complex, together supporting the position of Euplotes as a possible evolutionary intermediate in this lineage, in the process of accumulating complex evolutionary genome rearrangements, all of which require extensive repair to assemble functional coding regions. Comparative analysis also reveals that scrambled loci are often associated with local duplications, supporting a gradual model for the origin of complex, scrambled genomes via many small events of DNA duplication and decay.

A practical reference for studying meiosis in the model ciliate Tetrahymena thermophila

Meiosis is a critical cell division program that produces haploid gametes for sexual reproduction. Abnormalities in meiosis are often causes of infertility and birth defects (e.g., Down syndrome). Most organisms use a highly specialized zipper-like protein complex, the synaptonemal complex (SC), to guide and stabilize pairing of homologous chromosomes in meiosis. Although the SC is critical for meiosis in many eukaryotes, there are organisms that perform meiosis without a functional SC. However, such SC-less meiosis is poorly characterized. To understand the features of SC-less meiosis and its adaptive significance, the ciliated protozoan Tetrahymena was selected as a model. Meiosis research in Tetrahymena has revealed intriguing aspects of the regulatory programs utilized in its SC-less meiosis, yet additional efforts are needed for obtaining an in-depth comprehension of mechanisms that are associated with the absence of SC. Here, aiming at promoting a wider application of Tetrahymena for meiosis research, we introduce basic concepts and core techniques for studying meiosis in Tetrahymena and then suggest future directions for expanding the current Tetrahymena meiosis research toolbox. These methodologies could be adopted for dissecting meiosis in poorly characterized ciliates that might reveal novel features. Such data will hopefully provide insights into the function of the SC and the evolution of meiosis from a unique perspective.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-022-00149-8.

A novel Tetrahymena thermophila sterol C-22 desaturase belongs to the Fatty Acid Hydroxylase/Desaturase superfamily

Sterols in eukaryotic cells play important roles in modulating membrane fluidity and in cell signaling and trafficking. During evolution, a combination of gene losses and acquisitions gave rise to an extraordinary diversity of sterols in different organisms. The sterol C-22 desaturase identified in plants and fungi as a cytochrome P-450 monooxygenase evolved from the first eukaryotic cytochrome P450 and was lost in many lineages. Although the ciliate Tetrahymena thermophila desaturates sterols at the C-22 position, no cytochrome P-450 orthologs are present in the genome. Here, we aim to identify the genes responsible for the desaturation as well as their probable origin. We used gene knockout and yeast heterologous expression approaches to identify two putative genes, retrieved from a previous transcriptomic analysis, as sterol C-22 desaturases. Furthermore, we demonstrate using bioinformatics and evolutionary analyses that both genes encode a novel type of sterol C-22 desaturase that belongs to the large fatty acid hydroxylase/desaturase superfamily and the genes originated by genetic duplication prior to functional diversification. These results stress the widespread existence of nonhomologous isofunctional enzymes among different lineages of the tree of life as well as the suitability for the use of T. thermophila as a valuable model to investigate the evolutionary process of large enzyme families.

Transcriptomic Differences between Free-Living and Parasitic Chilodonella uncinata (Alveolata, Ciliophora)

Chilodonella uncinata is a facultatively parasitic ciliate, which can opportunistically parasitize on fish gills and fins, and sometimes even cause host mortality. Previous molecular studies of C. uncinata mainly focused on genetic diversity and molecular evolution. There are currently no transcriptome reports studying differences between free-living and parasitic C. uncinata. We addressed this by sequencing transcriptomes of these two C. uncinata lifestyle types using Smart-seq2 and Illumina HiSeq technologies. In total, 1040 differentially expressed genes (DEGs) were identified. Compared with the free-living type, 494 genes of the parasitic type were downregulated and 546 genes were upregulated. These DEGs were identified through BLAST with NCBI-nr, Swiss-Port, and Pfam databases and then annotated by GO enrichment and KEGG pathway analysis. The results showed that parasitism-related genes such as heat shock proteins (HSPs), actin I, and leishmanolysin were significantly upregulated in parasitic C. uncinata. The ciliary-related dynein heavy chain also had a higher expression in parasitic C. uncinata. Furthermore, there were significant differences in the amino acid metabolism, fatty acid metabolism, lipid metabolism, and TCA cycle. This study increases the volume of molecular data available for C. uncinata and contributes to our understanding of the mechanisms underlying the transition from a free-living to a parasitic lifestyle.

Identification of novel, functional, long noncoding RNAs involved in programmed, large-scale genome rearrangements

Noncoding RNAs (ncRNAs) make up to ∼98% percent of the transcriptome of a given organism. In recent years, one relatively new class of ncRNAs, long noncoding RNAs (lncRNAs), were shown to be more than mere by-products of gene expression and regulation. The unicellular eukaryote Paramecium tetraurelia is a member of the ciliate phylum, an extremely heterogeneous group of organisms found in most bodies of water across the globe. A hallmark of ciliate genetics is nuclear dimorphism and programmed elimination of transposons and transposon-derived DNA elements, the latter of which is essential for the maintenance of the somatic genome. Paramecium and ciliates in general harbor a plethora of different ncRNA species, some of which drive the process of large-scale genome rearrangements, including DNA elimination, during sexual development. Here, we identify and validate the first known functional lncRNAs in ciliates to date. Using deep-sequencing and subsequent bioinformatic processing and experimental validation, we show that Paramecium expresses at least 15 lncRNAs. These candidates were predicted by a highly conservative pipeline, and informatic analyses hint at differential expression during development. Depletion of two lncRNAs, lnc1 and lnc15, resulted in clear phenotypes, decreased survival, morphological impairment, and a global effect on DNA elimination.

Microtubules in Microorganisms: How Tubulin Isotypes Contribute to Diverse Cytoskeletal Functions

The cellular functions of the microtubule (MT) cytoskeleton range from relatively simple to amazingly complex. Assembled from tubulin, a heterodimeric protein with α- and β-tubulin subunits, microtubules are long, hollow cylindrical filaments with inherent polarity. They are intrinsically dynamic polymers that utilize GTP binding by tubulin, and subsequent hydrolysis, to drive spontaneous assembly and disassembly. Early studies indicated that cellular MTs are composed of multiple variants, or isotypes, of α- and β-tubulins, and that these multi-isotype polymers are further diversified by a range of posttranslational modifications (PTMs) to tubulin. These findings support the multi-tubulin hypothesis whereby individual, or combinations of tubulin isotypes possess unique properties needed to support diverse MT structures and/or cellular processes. Beginning 40 years ago researchers have sought to address this hypothesis, and the role of tubulin isotypes, by exploiting experimentally accessible, genetically tractable and functionally conserved model systems. Among these systems, important insights have been gained from eukaryotic microbial models. In this review, we illustrate how using microorganisms yielded among the earliest evidence that tubulin isotypes harbor distinct properties, as well as recent insights as to how they facilitate specific cellular processes. Ongoing and future research in microorganisms will likely continue to reveal basic mechanisms for how tubulin isotypes facilitate MT functions, along with valuable perspectives on how they mediate the range of conserved and diverse processes observed across eukaryotic microbes.