Convergent Evolution of Hydrogenosomes from Mitochondria by Gene Transfer and Loss.

Hydrogenosomes are H2-producing mitochondrial homologs found in some anaerobic microbial eukaryotes that provide a rare intracellular niche for H2-utilizing endosymbiotic archaea. Among ciliates, anaerobic and aerobic lineages are interspersed, demonstrating that the switch to an anaerobic lifestyle with hydrogenosomes has occurred repeatedly and independently. To investigate the molecular details of this transition, we generated genomic and transcriptomic data sets from anaerobic ciliates representing three distinct lineages. Our data demonstrate that hydrogenosomes have evolved from ancestral mitochondria in each case and reveal different degrees of independent mitochondrial genome and proteome reductive evolution, including the first example of complete mitochondrial genome loss in ciliates. Intriguingly, the FeFe-hydrogenase used for generating H2 has a unique domain structure among eukaryotes and appears to have been present, potentially through a single lateral gene transfer from an unknown donor, in the common aerobic ancestor of all three lineages. The early acquisition and retention of FeFe-hydrogenase helps to explain the facility whereby mitochondrial function can be so radically modified within this diverse and ecologically important group of microbial eukaryotes.

An efficient single-cell transcriptomics workflow for microbial eukaryotes benchmarked on Giardia intestinalis cells.

BACKGROUND: Most diversity in the eukaryotic tree of life is represented by microbial eukaryotes, which is a polyphyletic group also referred to as protists. Among the protists, currently sequenced genomes and transcriptomes give a biased view of the actual diversity. This biased view is partly caused by the scientific community, which has prioritized certain microbes of biomedical and agricultural importance. Additionally, some protists remain difficult to maintain in cultures, which further influences what has been studied. It is now possible to bypass the time-consuming process of cultivation and directly analyze the gene content of single protist cells. Single-cell genomics was used in the first experiments where individual protists cells were genomically explored. Unfortunately, single-cell genomics for protists is often associated with low genome recovery and the assembly process can be complicated because of repetitive intergenic regions. Sequencing repetitive sequences can be avoided if single-cell transcriptomics is used, which only targets the part of the genome that is transcribed. RESULTS: In this study we test different modifications of Smart-seq2, a single-cell RNA sequencing protocol originally developed for mammalian cells, to establish a robust and more cost-efficient workflow for protists. The diplomonad Giardia intestinalis was used in all experiments and the available genome for this species allowed us to benchmark our results. We could observe increased transcript recovery when freeze-thaw cycles were added as an extra step to the Smart-seq2 protocol. Further we reduced the reaction volume and purified the amplified cDNA with alternative beads to test different cost-reducing changes of Smart-seq2. Neither improved the procedure, and reducing the volumes by half led to significantly fewer genes detected. We also added a 5' biotin modification to our primers and reduced the concentration of oligo-dT, to potentially reduce generation of artifacts. Except adding freeze-thaw cycles and reducing the volume, no other modifications lead to a significant change in gene detection. Therefore, we suggest adding freeze-thaw cycles to Smart-seq2 when working with protists and further consider our other modification described to improve cost and time-efficiency. CONCLUSIONS: The presented single-cell RNA sequencing workflow represents an efficient method to explore the diversity and cell biology of individual protist cells.

RNA Sequencing of Stentor Cell Fragments Reveals Transcriptional Changes during Cellular Regeneration.

While ciliates of the genus Stentor are known for their ability to regenerate when their cells are damaged or even fragmented, the physical and molecular mechanisms underlying this process are poorly understood. To identify genes involved in the regenerative capability of Stentor cells, RNA sequencing of individual Stentor polymorphus cell fragments was performed. After splitting a cell over the anterior-posterior axis, the posterior fragment has to regenerate the oral apparatus, while the anterior part needs to regenerate the hold fast. Altogether, differential expression analysis of both posterior and anterior S. polymorphus cell fragments for four different post-split time points revealed over 10,000 upregulated genes throughout the regeneration process. Among these, genes involved in cell signaling, microtubule-based movement, and cell cycle regulation seemed to be particularly important during cellular regeneration. We identified roughly nine times as many upregulated genes in regenerating S. polymorphus posterior fragments as compared to anterior fragments, indicating that regeneration of the anterior oral apparatus is a complex process that involves many genes. Our analyses identified several expanded groups of genes, such as dual-specific tyrosine-(Y)-phosphorylation-regulated kinases and MORN domain-containing proteins that seemingly act as key regulators of cellular regeneration. In agreement with earlier morphological and cell biological studies [1, 2], our differential expression analyses indicate that cellular regeneration and vegetative division share many similarities.

Molecular Investigation of the Ciliate Spirostomum semivirescens, with First Transcriptome and New Geographical Records.

The ciliate Spirostomum semivirescens is a large freshwater protist densely packed with endosymbiotic algae and capable of building a protective coating from surrounding particles. The species has been rarely recorded and it lacks any molecular investigations. We obtained such data from S. semivirescens isolated in the UK and Sweden. Using single-cell RNA sequencing of isolates from both countries, the transcriptome of S. semivirescens was generated. A phylogenetic analysis identified S. semivirescens as a close relative to S. minus. Additionally, rRNA sequence analysis of the green algal endosymbiont revealed that it is closely related to Chlorella vulgaris. Along with the molecular species identification, an analysis of the ciliates' stop codons was carried out, which revealed a relationship where TGA stop codon frequency decreased with increasing gene expression levels. The observed codon bias suggests that S. semivirescens could be in an early stage of reassigning the TGA stop codon. Analysis of the transcriptome indicates that S. semivirescens potentially uses rhodoquinol-dependent fumarate reduction to respire in the oxygen-depleted habitats where it lives. The data also shows that despite large geographical distances (over 1,600km) between the sampling sites investigated, a morphologically-identical species can share an exact molecular signature, suggesting that some ciliate species, even those over 1mm in size, could have a global biogeographical distribution.