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.
Parasitism, aptly defined as one of the ‘living-together’ strategies (Trager, 1986), presents a dynamic system in which the parasite and its host are under evolutionary pressure to evolve new and ...specific adaptations, thus enabling the coexistence of the two closely interacting partners. Microsporidia are very frequently encountered obligatory intracellular protistan parasites that can infect both animals and some protists and are a consummate example of various aspects of the ‘living-together’ strategy. Microsporidia, relatives of fungi in the superkingdom Opisthokonta, belong to the relatively small group of parasites for which the host cell cytoplasm is the site of both reproduction and maturation. The structural and physiological reduction of their vegetative stage, together with the manipulation of host cell physiology, enables microsporidia to live in the cytosolic environment for most of their life cycle in a way resembling endocytobionts. The ability to form structurally complex spores and the invention and assembly of a unique injection mechanism enable microsporidia to disperse within host tissues and between host organisms, resulting in long-lasting infections. Microsporidia have adapted their genomes to the intracellular way of life, evolved strategies how to obtain nutrients directly from the host and how to manipulate not only the infected cells, but also the hosts themselves. The enormous variability of host organisms and their tissues provide microsporidian parasites a virtually limitless terrain for diversification and ecological expansion. This review attempts to present a general overview of microsporidia, emphasising some less known and/or more recently discovered facets of their biology.
Though widespread, RNA editing is rare, except in endosymbiotic organelles. A combination of higher mutation rates, relaxation of energetic constraints, and high genetic drift is found within ...plastids and mitochondria and is conducive for evolution and expansion of editing processes, possibly starting as repair mechanisms. To illustrate this, we present an exhaustive phylogenetic overview of editing types.
Trypanosomes and leishmanias are widely known parasites of humans. However, they are just two out of several phylogenetic lineages that constitute the family Trypanosomatidae. Although dixeny – the ...ability to infect two hosts – is a derived trait of vertebrate-infecting parasites, the majority of trypanosomatids are monoxenous. Like their common ancestor, the monoxenous Trypanosomatidae are mostly parasites or commensals of insects. This review covers recent advances in the study of insect trypanosomatids, highlighting their diversity as well as genetic, morphological and biochemical complexity, which, until recently, was underappreciated. The investigation of insect trypanosomatids is providing an important foundation for understanding the origin and evolution of parasitism, including colonization of vertebrates and the appearance of human pathogens.
Dixenous trypanosomatids, such as the human Trypanosoma parasites, infect both insects and vertebrates. Yet phylogenetic analyses have revealed that these are the exception, and that insect-infecting monoxenous lineages are both abundant and diverse.
Globally, over 10% of true bugs and flies are infected with monoxenous trypanosomatids, whereas other insect groups are infected much less frequently. Some trypanosomatids are confined to a single host species, whereas others parasitize a wide spectrum of hosts.
Many trypanosomatids are themselves infected with viruses and bacteria that have been acquired from insects, terrestrial invertebrates, and fungi. At least two lineages contain bacteria; these endosymbiotic events occurred independently and evolved differently.
Genomes and transcriptomes of monoxenous trypanosomatids will bring new insight into the origins of parasitism and how trypanosomes and leishmanias evolved their capacity to infect humans and other vertebrates.
When trying to reconstruct the evolutionary trajectories during early eukaryogenesis, one is struck by clear differences in the developments of two organelles of endosymbiotic origin: the ...mitochondrion and the chloroplast. From a symbiogenic perspective, eukaryotic development can be interpreted as a process in which many of the defining eukaryotic characteristics arose as a result of mutual adaptions of both prokaryotes (an archaeon and a bacterium) involved. This implies that many steps during the bacterium-to-mitochondrion transition trajectory occurred in an intense period of dramatic and rapid changes. In contrast, the subsequent cyanobacterium-to-chloroplast development in a specific eukaryotic subgroup, leading to the photosynthetic lineages, occurred in a full-fledged eukaryote. The commonalities and differences in the two trajectories shed an interesting light on early, and ongoing, eukaryotic evolutionary driving forces, especially endogenous reactive oxygen species (ROS) formation. Differences between organellar ribosomes, changes to the electron transport chain (ETC) components, and mitochondrial codon reassignments in nonplant mitochondria can be understood when mitochondrial ROS formation, e.g., during high energy consumption in heterotrophs, is taken into account.
The early eukaryotic evolution was deeply influenced by the acquisition of two endosymbiotic organelles - the mitochondrion and the chloroplast. Here we discuss the possibly important role of reactive oxygen species in these processes.
Apicomplexa are known to contain greatly reduced organellar genomes. Their mitochondrial genome carries only three protein-coding genes, and their plastid genome is reduced to a 35-kb-long circle. ...The discovery of coral-endosymbiotic algae
Chromera velia
and
Vitrella brassicaformis
, which share a common ancestry with Apicomplexa, provided an opportunity to study possibly ancestral forms of organellar genomes, a unique glimpse into the evolutionary history of apicomplexan parasites. The structurally similar mitochondrial genomes of
Chromera
and
Vitrella
differ in gene content, which is reflected in the composition of their respiratory chains. Thus,
Chromera
lacks respiratory complexes I and III, whereas
Vitrella
and apicomplexan parasites are missing only complex I. Plastid genomes differ substantially between these algae, particularly in structure: The
Chromera
plastid genome is a linear, 120-kb molecule with large and divergent genes, whereas the plastid genome of
Vitrella
is a highly compact circle that is only 85 kb long but nonetheless contains more genes than that of
Chromera
. It appears that organellar genomes have already been reduced in free-living phototrophic ancestors of apicomplexan parasites, and such reduction is not associated with parasitism.
The mitochondria contain their own genome derived from an alphaproteobacterial endosymbiont. From thousands of protein-coding genes originally encoded by their ancestor, only between 1 and about 70 ...are encoded on extant mitochondrial genomes (mitogenomes). Thanks to a dramatically increasing number of sequenced and annotated mitogenomes a coherent picture of why some genes were lost, or relocated to the nucleus, is emerging. In this review, we describe the characteristics of mitochondria-to-nucleus gene transfer and the resulting varied content of mitogenomes across eukaryotes. We introduce a 'burst-upon-drift' model to best explain nuclear-mitochondrial population genetics with flares of transfer due to genetic drift.
Most of the genetic, cellular, and biochemical diversity of life rests within single-celled organisms — the prokaryotes (bacteria and archaea) and microbial eukaryotes (protists). Very close ...interactions, or symbioses, between protists and prokaryotes are ubiquitous, ecologically significant, and date back at least two billion years ago to the origin of mitochondria. However, most of our knowledge about the evolution and functions of eukaryotic symbioses comes from the study of animal hosts, which represent only a small subset of eukaryotic diversity. Here, we take a broad view of bacterial and archaeal symbioses with protist hosts, focusing on their evolution, ecology, and cell biology, and also explore what functions (if any) the symbionts provide to their hosts. With the immense diversity of protist symbioses starting to come into focus, we can now begin to see how these systems will impact symbiosis theory more broadly.
Symbiosis is a major source of evolutionary innovation, but our understanding of it mainly comes from animal systems. Husnik et al. review symbioses involving protists, the microbial cells that represent most of eukaryotic phylogenetic diversity, and examine which aspects of symbiosis are conserved across diverse hosts and which are more context-dependent.
MRP1/2 is a heteromeric protein complex that functions in the trypanosomatid mitochondrion as part of the RNA editing machinery, which facilitates multiple targeted insertions and deletions of ...uridines. MRP1/2 was shown to interact with MRB8170, which initiates RNA editing by marking pre-edited mRNAs, while TbRGG2 is required for its efficient progression on pan-edited mRNAs. Both MRP1/2 and TbRGG2 are capable of modulating RNA-RNA interactions in vitro. As determined by using iCLIP and RIP-qPCR, RNAs bound to MRP1/2 are characterized and compared with those associated with MRB8170 and TbRGG2. We provide evidence that MRP1 and MRB8170 have correlated binding and similar RNA crosslinking peak profiles over minimally and never-edited mRNAs. Our results suggest that MRP1 assists MRB8170 in RNA editing on minimally edited mRNAs.
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