Sexual reproduction and clonality in eukaryotes are mostly seen as exclusive, the latter being rather exceptional. This view might be biased by focusing almost exclusively on metazoans. We analyze and discuss reproduction in the context of extant eukaryotic diversity, paying special attention to protists. We present results of phylogenetically extended searches for homologs of two proteins functioning in cell and nuclear fusion, respectively (HAP2 and GEX1), providing indirect evidence for these processes in several eukaryotic lineages where sex has not been observed yet. We argue that (i) the debate on the relative significance of sex and clonality in eukaryotes is confounded by not appropriately distinguishing multicellular and unicellular organisms; (ii) eukaryotic sex is extremely widespread and already present in the last eukaryotic common ancestor; and (iii) the general mode of existence of eukaryotes is best described by clonally propagating cell lines with episodic sex triggered by external or internal clues. However, important questions concern the relative longevity of true clonal species (i.e., species not able to return to sexual procreation anymore). Long-lived clonal species seem strikingly rare. We analyze their properties in the light of meiotic sex development from existing prokaryotic repair mechanisms. Based on these considerations, we speculate that eukaryotic sex likely developed as a cellular survival strategy, possibly in the context of internal reactive oxygen species stress generated by a (proto) mitochondrion. Thus, in the context of the symbiogenic model of eukaryotic origin, sex might directly result from the very evolutionary mode by which eukaryotic cells arose.

• Keywords
• eukaryotes, evolution, protists, reactive oxygen species, sex,
• MeSH
• Eukaryotic Cells physiology   MeSH
• Cell Fusion MeSH
• Genome MeSH
• Meiosis MeSH
• Mitochondria metabolism   MeSH
• Molecular Sequence Data MeSH
• Reactive Oxygen Species metabolism   MeSH
• Reproduction * MeSH
• Sequence Analysis, DNA MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Reactive Oxygen Species MeSH

There are a variety of complex metabolic processes ongoing simultaneously in the single, large mitochondrion of Trypanosoma brucei. Understanding the organellar environment and dynamics of mitochondrial proteins requires quantitative measurement in vivo. In this study, we have validated a method for immobilizing both procyclic stage (PS) and bloodstream stage (BS) T. brucei brucei with a high level of cell viability over several hours and verified its suitability for undertaking fluorescence recovery after photobleaching (FRAP), with mitochondrion-targeted yellow fluorescent protein (YFP). Next, we used this method for comparative analysis of the translational diffusion of mitochondrial RNA-binding protein 1 (MRP1) in the BS and in T. b. evansi. The latter flagellate is like petite mutant Saccharomyces cerevisiae because it lacks organelle-encoded nucleic acids. FRAP measurement of YFP-tagged MRP1 in both cell lines illuminated from a new perspective how the absence or presence of RNA affects proteins involved in mitochondrial RNA metabolism. This work represents the first attempt to examine this process in live trypanosomes.

• MeSH
• Mitochondrial Proteins genetics   MeSH
• Mutation MeSH
• RNA-Binding Proteins genetics   metabolism   MeSH
• Protozoan Proteins genetics   metabolism   MeSH
• RNA Interference MeSH
• RNA, Mitochondrial MeSH
• RNA genetics   MeSH
• Saccharomyces cerevisiae genetics   MeSH
• Trypanosoma brucei brucei genetics   MeSH
• Cell Survival genetics   MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• gBP21 protein, Trypanosoma brucei MeSH Browser
• Mitochondrial Proteins MeSH
• RNA-Binding Proteins MeSH
• Protozoan Proteins MeSH
• RNA, Mitochondrial MeSH
• RNA MeSH

Letm1 is a conserved protein in eukaryotes bearing energized mitochondria. Hemizygous deletion of its gene has been implicated in symptoms of the human disease Wolf-Hirschhorn syndrome. Studies almost exclusively performed in opisthokonts have attributed several roles to Letm1, including maintaining mitochondrial morphology, mediating either calcium or potassium/proton antiport, and facilitating mitochondrial translation. We address the ancestral function of Letm1 in the highly diverged protist and significant pathogen, Trypanosoma brucei. We demonstrate that Letm1 is involved in maintaining mitochondrial volume via potassium/proton exchange across the inner membrane. This role is essential in the vector-dwelling procyclic and mammal-infecting bloodstream stages as well as in Trypanosoma brucei evansi, a form of the latter stage lacking an organellar genome. In the pathogenic bloodstream stage, the mitochondrion consumes ATP to maintain an energized state, whereas that of T. brucei evansi also lacks a conventional proton-driven membrane potential. Thus, Letm1 performs its function in different physiological states, suggesting that ion homeostasis is among the few characterized essential pathways of the mitochondrion at this T. brucei life stage. Interestingly, Letm1 depletion in the procyclic stage can be complemented by exogenous expression of its human counterpart, highlighting the conservation of protein function between highly divergent species. Furthermore, although mitochondrial translation is affected upon Letm1 ablation, it is an indirect consequence of K(+) accumulation in the matrix.

• Keywords
• Bioenergetics, Letm1, Mitochondria, Potassium Transport, Translation, Trypanosome,
• MeSH
• Anti-Bacterial Agents pharmacology   MeSH
• Potassium metabolism   MeSH
• Phenotype MeSH
• Homeostasis MeSH
• Cations MeSH
• Humans MeSH
• Membrane Potential, Mitochondrial MeSH
• Mitochondrial Proteins chemistry   metabolism   physiology   MeSH
• Mitochondria metabolism   MeSH
• Protein Biosynthesis MeSH
• Protozoan Proteins chemistry   metabolism   physiology   MeSH
• Flow Cytometry methods   MeSH
• RNA Interference MeSH
• Genetic Complementation Test MeSH
• Trypanosoma brucei brucei metabolism   MeSH
• Gene Silencing MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Anti-Bacterial Agents MeSH
• Potassium MeSH
• Cations MeSH
• Mitochondrial Proteins MeSH
• Protozoan Proteins MeSH