Does ribosomal DNA get out of the micronuclear chromosome in Paramecium tetraurelia by means of a rolling circle?

Using digital PCR to predict ciliate abundance from ribosomal RNA gene copy numbers

Ciliates play a key role in most ecosystems. Their abundance in natural samples is crucial for answering many ecological questions. Traditional methods of quantifying individual species, which rely on microscopy, are often labour-intensive, time-consuming and can be highly biassed. As a result, we investigated the potential of digital polymerase chain reaction (dPCR) for quantifying ciliates. A significant challenge in this process is the high variation in the copy number of the taxonomic marker gene (ribosomal RNA [rRNA]). We first quantified the rRNA gene copy numbers (GCN) of the model ciliate, Paramecium tetraurelia, during different stages of the cell cycle and growth phases. The per-cell rRNA GCN varied between approximately 11,000 and 130,000, averaging around 50,000 copies per cell. Despite these variations in per-cell rRNA GCN, we found a highly significant correlation between GCN and cell numbers. This is likely due to the coexistence of different cellular stages in an uncontrolled (environmental) ciliate population. Thanks to the high sensitivity of dPCR, we were able to detect the target gene in a sample that contained only a single cell. The dPCR approach presented here is a valuable addition to the molecular toolbox in protistan ecology. It may guide future studies in quantifying and monitoring the abundance of targeted (even rare) ciliates in natural samples.

Exogenous RNAi mechanisms contribute to transcriptome adaptation by phased siRNA clusters in Paramecium

Extensive research has characterized distinct exogenous RNAi pathways interfering in gene expression during vegetative growth of the unicellular model ciliate Paramecium. However, role of RNAi in endogenous transcriptome regulation, and environmental adaptation is unknown. Here, we describe the first genome-wide profiling of endogenous sRNAs in context of different transcriptomic states (serotypes). We developed a pipeline to identify, and characterize 2602 siRNA producing clusters (SRCs). Our data show no evidence that SRCs produce miRNAs, and in contrast to other species, no preference for strand specificity of siRNAs. Interestingly, most SRCs overlap coding genes and a separate group show siRNA phasing along the entire open reading frame, suggesting that the mRNA transcript serves as a source for siRNAs. Integrative analysis of siRNA abundance and gene expression levels revealed surprisingly that mRNA and siRNA show negative as well as positive associations. Two RNA-dependent RNA Polymerase mutants, RDR1 and RDR2, show a drastic loss of siRNAs especially in phased SRCs accompanied with increased mRNA levels. Importantly, most SRCs depend on both RDRs, reminiscent to primary siRNAs in the RNAi against exogenous RNA, indicating mechanistic overlaps between exogenous and endogenous RNAi contributing to flexible transcriptome adaptation.

Automated analysis of small RNA datasets with RAPID

Understanding the role of short-interfering RNA (siRNA) in diverse biological processes is of current interest and often approached through small RNA sequencing. However, analysis of these datasets is difficult due to the complexity of biological RNA processing pathways, which differ between species. Several properties like strand specificity, length distribution, and distribution of soft-clipped bases are few parameters known to guide researchers in understanding the role of siRNAs. We present RAPID, a generic eukaryotic siRNA analysis pipeline, which captures information inherent in the datasets and automatically produces numerous visualizations as user-friendly HTML reports, covering multiple categories required for siRNA analysis. RAPID also facilitates an automated comparison of multiple datasets, with one of the normalization techniques dedicated for siRNA knockdown analysis, and integrates differential expression analysis using DESeq2.

First molecular characterization of Balantioides coli (Malmsten, 1857) isolates maintained in vitro culture and from feces of captive animals, Rio de Janeiro, Brazil

Ciliate protozoa of the genus Balantioides can parasitize a variety of animals. The morphology of the evolutionary forms of the parasite and the host species affected have long been the only characteristics used to taxonomically identify the species of these protozoa, but these variables are not very precise. To confirm species identity, molecular biology tools are currently used. In this context, this study aimed to analyze protozoan isolates maintained in culture medium and from fecal samples from captive animals in Rio de Janeiro, Brazil, by means of molecular tools. Forty isolates maintained in Pavlova modified medium (30 were isolated from feces of pigs and 10 from feces of cynomolgus macaques) were analyzed. In addition, 34 fecal samples (8 from pigs, 8 from cynomolgus macaques and 18 from rhesus macaques) containing Balantioides coli-like cysts were analyzed. All samples were subjected to DNA extraction and the polymerase chain reaction (PCR) to amplify the fragment ITS1 - 5.8s rRNA - ITS2, and the PCR products were purified and sequenced. All samples (100%) presented sequences that were grouped in the Balantioides coli cluster. The type A0 variant predominated. These sequences were 96% to 99% identical to those deposited in GenBank, including a B. coli sequence that had been obtained from human fecal material in Bolivia. It seems that the culturing system did not select variants, because this variant was also seen in the amplified sequences of fecal samples containing cysts. The isolate sequences in the cultures showed few ambiguities and substitutions, thus generating reliable chromatograms. This was the first study to identify B. coli in captive animals in Brazil, through molecular biology. In addition, it was the first to evaluate a large panel of isolates of the parasite through culturing.

Paramecium aurelia revisited

The species Paramecium aurelia sensu latu, containing 15 sexually isolated subspecies (syngens), is the classic example of a sibling species complex in the ciliates. Using DNA sequence comparison, it is now possible to see whether this example parallels other studied sibling species complexes. We sequenced the internal transcribed spacer (ITS) region of the nuclear ribosomal cistron for 13 of the syngens plus two other Paramecium species and several Tetrahymena spp. Using available spirotrich sequences of the internal transcribed spacer 2 (ITS2), we established the RNA transcript folding pattern for ciliates. Ciliates exhibit the two highly conserved helices in their RNA transcript folding pattern in common with other eukaryotes, despite their unusual nuclear behavior and their presumed low copy number of micronuclear ribosomal repeats. Consequently, the set of 111-116 ITS2 nucleotide positions that are relatively conserved in evolution can be derived and used for comparative analysis. Mating behavior (i.e. gamete agglutination and fusion) is the character showing greatest correlation with the degree of ITS2 evolution in the P. aurelia complex, as also found in other eukaryotes. The degree of change in the ITS2 relatively conserved sequences found among the sibling species of P. aurelia is the same degree as found among the sibling species of the Drosophila melanogaster-mauritania-sechellia-simulans-yakuba species complex. The relatively conserved subregion of ITS2, determined from transcript secondary structure, is a tool for identifying the level of the biological species in the absence of knowledge of sexual compatibility in both micro- and macro-eukaryote species complexes.