How mitochondrial cristae illuminate the important role of oxygen during eukaryogenesis.

Inner membranes of mitochondria are extensively folded, forming cristae. The observed overall correlation between efficient eukaryotic ATP generation and the area of internal mitochondrial inner membranes both in unicellular organisms and metazoan tissues seems to explain why they evolved. However, the crucial use of molecular oxygen (O2) as final acceptor of the electron transport chain is still not sufficiently appreciated. O2 was an essential prerequisite for cristae development during early eukaryogenesis and could be the factor allowing cristae retention upon loss of mitochondrial ATP generation. Here I analyze illuminating bacterial and unicellular eukaryotic examples. I also discuss formative influences of intracellular O2 consumption on the evolution of the last eukaryotic common ancestor (LECA). These considerations bring about an explanation for the many genes coming from other organisms than the archaeon and bacterium merging at the start of eukaryogenesis.

The contours of evolution: In defence of Darwin's tree of life paradigm.

Both the concept of a Darwinian tree of life (TOL) and the possibility of its accurate reconstruction have been much criticized. Criticisms mostly revolve around the extensive occurrence of lateral gene transfer (LGT), instances of uptake of complete organisms to become organelles (with the associated subsequent gene transfer to the nucleus), as well as the implications of more subtle aspects of the biological species concept. Here we argue that none of these criticisms are sufficient to abandon the valuable TOL concept and the biological realities it captures. Especially important is the need to conceptually distinguish between organismal trees and gene trees, which necessitates incorporating insights of widely occurring LGT into modern evolutionary theory. We demonstrate that all criticisms, while based on important new findings, do not invalidate the TOL. After considering the implications of these new insights, we find that the contours of evolution are best represented by a TOL.

Mitochondrial genomes revisited: why do different lineages retain different genes?

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.

Trypanosomes as a magnifying glass for cell and molecular biology.

The African trypanosome, Trypanosoma brucei, has developed into a flexible and robust experimental model for molecular and cellular parasitology, allowing us to better combat these and related parasites that cause worldwide suffering. Diminishing case numbers, due to efficient public health efforts, and recent development of new drug treatments have reduced the need for continued study of T. brucei in a disease context. However, we argue that this pathogen has been instrumental in revolutionary discoveries that have widely informed molecular and cellular biology and justifies continuing research as an experimental model. Ongoing work continues to contribute towards greater understanding of both diversified and conserved biological features. We discuss multiple examples where trypanosomes pushed the boundaries of cell biology and hope to inspire researchers to continue exploring these remarkable protists as tools for magnifying the inner workings of cells.

Lessons from the deep: mechanisms behind diversification of eukaryotic protein complexes.

Genetic variation is the major mechanism behind adaptation and evolutionary change. As most proteins operate through interactions with other proteins, changes in protein complex composition and subunit sequence provide potentially new functions. Comparative genomics can reveal expansions, losses and sequence divergence within protein-coding genes, but in silico analysis cannot detect subunit substitutions or replacements of entire protein complexes. Insights into these fundamental evolutionary processes require broad and extensive comparative analyses, from both in silico and experimental evidence. Here, we combine data from both approaches and consider the gamut of possible protein complex compositional changes that arise during evolution, citing examples of complete conservation to partial and total replacement by functional analogues. We focus in part on complexes in trypanosomes as they represent one of the better studied non-animal/non-fungal lineages, but extend insights across the eukaryotes by extensive comparative genomic analysis. We argue that gene loss plays an important role in diversification of protein complexes and hence enhancement of eukaryotic diversity.

How mitochondria showcase evolutionary mechanisms and the importance of oxygen.

Darwinian evolution can be simply stated: natural selection of inherited variations increasing differential reproduction. However, formulated thus, links with biochemistry, cell biology, ecology, and population dynamics remain unclear. To understand interactive contributions of chance and selection, higher levels of biological organization (e.g., endosymbiosis), complexities of competing selection forces, and emerging biological novelties (such as eukaryotes or meiotic sex), we must analyze actual examples. Focusing on mitochondria, I will illuminate how biology makes sense of life's evolution, and the concepts involved. First, looking at the bacterium - mitochondrion transition: merging with an archaeon, it lost its independence, but played a decisive role in eukaryogenesis, as an extremely efficient aerobic ATP generator and internal ROS source. Second, surveying later mitochondrion adaptations and diversifications illustrates concepts such as constructive neutral evolution, dynamic interactions between endosymbionts and hosts, the contingency of life histories, and metabolic reprogramming. Without oxygen, mitochondria disappear; with (intermittent) oxygen diversification occurs in highly complex ways, especially upon (temporary) phototrophic substrate supply. These expositions show the Darwinian model to be a highly fruitful paradigm.

Neisseria meningitidis Sibling Small Regulatory RNAs Connect Metabolism with Colonization by Controlling Propionate Use.

Neisseria meningitidis (meningococcus) colonizes the human nasopharynx, primarily as a commensal, but sporadically causing septicemia and meningitis. During colonization and invasion, it encounters different niches with specific nutrient compositions. Small noncoding RNAs (sRNAs) are used to fine-tune expression of genes, allowing adaptation to their physiological differences. We have previously characterized sRNAs (Neisseria metabolic switch regulators [NmsRs]) controlling switches between cataplerotic and anaplerotic metabolism. Here, we extend the NmsR regulon by studying methylcitrate lyase (PrpF) and propionate kinase (AckA-1) involved in the methylcitrate cycle and serine hydroxymethyltransferase (GlyA) and 3-hydroxyacid dehydrogenase (MmsB) involved in protein degradation. These proteins were previously shown to be dysregulated in a ΔnmsRs strain. Levels of transcription of target genes and NmsRs were assessed by reverse transcriptase quantitative PCR (RT-qPCR). We also used a novel gene reporter system in which the 5' untranslated region (5' UTR) of the target gene is fused to mcherry to study NmsRs-target gene interaction in the meningococcus. Under nutrient-rich conditions, NmsRs downregulate expression of PrpF and AckA-1 by direct interaction with the 5' UTR of their mRNA. Overexpression of NmsRs impaired growth under nutrient-limiting growth conditions with pyruvate and propionic acid as the only carbon sources. Our data strongly suggest that NmsRs downregulate propionate metabolism by lowering methylcitrate enzyme activity under nutrient-rich conditions. Under nutrient-poor conditions, NmsRs are downregulated, increasing propionate metabolism, resulting in higher tricarboxylic acid (TCA) activities. IMPORTANCE Neisseria meningitidis colonizes the human nasopharynx, forming a reservoir for the sporadic occurrence of epidemic invasive meningococcal disease like septicemia and meningitis. Propionic acid generated by other bacteria that coinhabit the human nasopharynx can be utilized by meningococci for replication in this environment. Here, we showed that sibling small RNAs, designated NmsRs, riboregulate propionic acid utilization by meningococci and, thus, colonization. Under conditions mimicking the nasopharyngeal environment, NmsRs are downregulated. This leads to the conversion of propionic acid to pyruvate and succinate, resulting in higher tricarboxylic acid cycle activity, allowing colonization of the nasopharynx. NmsRs link metabolic state with colonization, which is a crucial step on the trajectory to invasive meningococcal disease.

How germline genes promote malignancy in cancer cells.

Cancers often express hundreds of genes otherwise specific to germ cells, the germline/cancer (GC) genes. Here, we present and discuss the hypothesis that activation of a "germline program" promotes cancer cell malignancy. We do so by proposing four hallmark processes of the germline: meiosis, epigenetic plasticity, migration, and metabolic plasticity. Together, these hallmarks enable replicative immortality of germ cells as well as cancer cells. Especially meiotic genes are frequently expressed in cancer, implying that genes unique to meiosis may play a role in oncogenesis. Because GC genes are not expressed in healthy somatic tissues, they form an appealing source of specific treatment targets with limited side effects besides infertility. Although it is still unclear why germ cell specific genes are so abundantly expressed in cancer, from our hypothesis it follows that the germline's reproductive program is intrinsic to cancer development.

Molecular characteristics of the multi-functional FAO enzyme ACAD9 illustrate the importance of FADH2 /NADH ratios for mitochondrial ROS formation.

A decade ago I postulated that ROS formation in mitochondria was influenced by different FADH2 /NADH (F/N) ratios of catabolic substrates. Thus, fatty acid oxidation (FAO) would give higher ROS formation than glucose oxidation. Both the emergence of peroxisomes and neurons not using FAO, could be explained thus. ROS formation in NADH:ubiquinone oxidoreductase (Complex I) comes about by reverse electron transport (RET) due to high QH2 levels, and scarcity of its electron-acceptor (Q) during FAO. The then new, unexpected, finding of an FAO enzyme, ACAD9, being involved in complex I biogenesis, hinted at connections in line with the hypothesis. Recent findings about ACAD9's role in regulation of respiration fit with predictions the model makes: cementing connections between ROS production and F/N ratios. I describe how ACAD9 might be central to reversing the oxidative damage in complex I resulting from FAO. This seems to involve two distinct, but intimately connected, ACAD9 characteristics: (i) its upregulation of complex I biogenesis, and (ii) releasing FADH2 , with possible conversion into FMN, the crucial prosthetic group of complex I. Also see the video abstract here: https://youtu.be/N7AT_HBNumg.

Eukaryotic cellular intricacies shape mitochondrial proteomic complexity.

Mitochondria have been fundamental to the eco-physiological success of eukaryotes since the last eukaryotic common ancestor (LECA). They contribute essential functions to eukaryotic cells, above and beyond classical respiration. Mitochondria interact with, and complement, metabolic pathways occurring in other organelles, notably diversifying the chloroplast metabolism of photosynthetic organisms. Here, we integrate existing literature to investigate how mitochondrial metabolism varies across the landscape of eukaryotic evolution. We illustrate the mitochondrial remodelling and proteomic changes undergone in conjunction with major evolutionary transitions. We explore how the mitochondrial complexity of the LECA has been remodelled in specific groups to support subsequent evolutionary transitions, such as the acquisition of chloroplasts in photosynthetic species and the emergence of multicellularity. We highlight the versatile and crucial roles played by mitochondria during eukaryotic evolution, extending from its huge contribution to the development of the LECA itself to the dynamic evolution of individual eukaryote groups, reflecting both their current ecologies and evolutionary histories.

Zombie ideas about early endosymbiosis: Which entry mechanisms gave us the "endo" in different endosymbionts?

Recently, a review regarding the mechanics and evolution of mitochondrial fission appeared in Nature. Surprisingly, it stated authoritatively that the mitochondrial outer membrane, in contrast with the inner membrane of bacterial descent, was acquired from the host, presumably during uptake. However, it has been known for quite some time that this membrane was also derived from the Gram-negative, alpha-proteobacterium related precursor of present-day mitochondria. The zombie idea of the host membrane still surrounding the endosymbiont is not only wrong, but more importantly, might hamper the proper conception of possible scenarios of eukaryogenesis. Why? Because it steers the imagination not only with regard to possible uptake mechanisms, but also regarding what went on before. Here I critically discuss both the evidence for the continuity of the bacterial outer membrane, the reasons for the persistence of the erroneous host membrane hypothesis and the wider implications of these misconceptions for the ideas regarding events occurring during the first steps towards the evolution of the eukaryotes and later major eukaryotic differentiations. I will also highlight some of the latest insights regarding different instances of endosymbiont evolution.