Eukaryotes Acquired Mitochondria Later Than Expected

Late acquisition of mitochondria by a host with chimaeric prokaryotic ancestry

Authors:

Pittis et al

Abstract:

The origin of eukaryotes stands as a major conundrum in biology1. Current evidence indicates that the last eukaryotic common ancestor already possessed many eukaryotic hallmarks, including a complex subcellular organization1, 2, 3. In addition, the lack of evolutionary intermediates challenges the elucidation of the relative order of emergence of eukaryotic traits. Mitochondria are ubiquitous organelles derived from an alphaproteobacterial endosymbiont4. Different hypotheses disagree on whether mitochondria were acquired early or late during eukaryogenesis5. Similarly, the nature and complexity of the receiving host are debated, with models ranging from a simple prokaryotic host to an already complex proto-eukaryote1, 3, 6, 7. Most competing scenarios can be roughly grouped into either mito-early, which consider the driving force of eukaryogenesis to be mitochondrial endosymbiosis into a simple host, or mito-late, which postulate that a significant complexity predated mitochondrial endosymbiosis3. Here we provide evidence for late mitochondrial endosymbiosis. We use phylogenomics to directly test whether proto-mitochondrial proteins were acquired earlier or later than other proteins of the last eukaryotic common ancestor. We find that last eukaryotic common ancestor protein families of alphaproteobacterial ancestry and of mitochondrial localization show the shortest phylogenetic distances to their closest prokaryotic relatives, compared with proteins of different prokaryotic origin or cellular localization. Altogether, our results shed new light on a long-standing question and provide compelling support for the late acquisition of mitochondria into a host that already had a proteome of chimaeric phylogenetic origin. We argue that mitochondrial endosymbiosis was one of the ultimate steps in eukaryogenesis and that it provided the definitive selective advantage to mitochondria-bearing eukaryotes over less complex forms.

A Single Cell Organism With an *EYE* Developed From Endosymbiotic Components

Eye-like ocelloids are built from different endosymbiotically acquired components

Authors:

Gavelis et al

Abstract:

Multicellularity is often considered a prerequisite for morphological complexity, as seen in the camera-type eyes found in several groups of animals. A notable exception exists in single-celled eukaryotes called dinoflagellates, some of which have an eye-like ‘ocelloid’ consisting of subcellular analogues to a cornea, lens, iris, and retina. These planktonic cells are uncultivated and rarely encountered in environmental samples, obscuring the function and evolutionary origin of the ocelloid. Here we show, using a combination of electron microscopy, tomography, isolated-organelle genomics, and single-cell genomics, that ocelloids are built from pre-existing organelles, including a cornea-like layer made of mitochondria and a retinal body made of anastomosing plastids. We find that the retinal body forms the central core of a network of peridinin-type plastids, which in dinoflagellates and their relatives originated through an ancient endosymbiosis with a red alga. As such, the ocelloid is a chimaeric structure, incorporating organelles with different endosymbiotic histories. The anatomical complexity of single-celled organisms may be limited by the components available for differentiation, but the ocelloid shows that pre-existing organelles can be assembled into a structure so complex that it was initially mistaken for a multicellular eye. Although mitochondria and plastids are acknowledged chiefly for their metabolic roles, they can also be building blocks for greater structural complexity.

Morphological Evidence Mitochondria are Endosymbiotic α-proteobacterium

Ancient Homology of the Mitochondrial Contact Site and Cristae Organizing System Points to an Endosymbiotic Origin of Mitochondrial Cristae

Authors:

Muñoz-Gómez et al

Abstract:

Mitochondria are eukaryotic organelles that originated from an endosymbiotic α-proteobacterium. As an adaptation to maximize ATP production through oxidative phosphorylation, mitochondria contain inner membrane invaginations called cristae. Recent work has characterized a multi-protein complex in yeast and animal mitochondria called MICOS (mitochondrial contact site and cristae organizing system), responsible for the determination and maintenance of cristae [ 1–4 ]. However, the origin and evolution of these characteristic mitochondrial features remain obscure. We therefore conducted a comprehensive search for MICOS components across the major groups that encompass eukaryotic diversity to determine the extent of conservation of this complex. We detected homologs for the majority of MICOS components among opisthokonts (the group containing animals and fungi), but only Mic60 and Mic10 were consistently identified outside this group. The conservation of Mic60 and Mic10 in eukaryotes is consistent with their central role in MICOS function [ 5–7 ], indicating that the basic mechanism for cristae determination arose early in evolution and has remained relatively unchanged. We found that eukaryotes with ultrastructurally simplified anaerobic mitochondria that lack cristae have also lost MICOS. We then searched for a prokaryotic MICOS and identified a homolog of Mic60 present only in α-proteobacteria, providing evidence for the endosymbiotic origin of mitochondrial cristae. Our study clarifies the origins of mitochondrial cristae and their subsequent evolutionary history, provides evidence for a general mechanism of cristae formation and maintenance in eukaryotes, and points to a new potential factor involved in membrane differentiation in prokaryotes.

Archaea Relative to Modern Eukaryotes Found

In a new study, published in Nature this week, a research team led from Uppsala University in Sweden presents the discovery of a new microbe that represents a missing link in the evolution of complex life. The study provides a new understanding of how, billions of years ago, the complex cell types that comprise plants, fungi, but also animals and humans, evolved from simple microbes.

Cells are the basic building blocks of all life on our planet. Yet, whereas the cells of bacteria and other microbes are small and simple, all visible life, including us humans, is generally made up of large and complex cell types. The origin of these complex cell types has long been a mystery to the scientific community, but now researchers from Uppsala University in Sweden have discovered a new group of microorganisms that represents a missing link in the evolutionary transition from simple to complex cells.

In the 1970s, the acclaimed biologist Carl Woese discovered a completely new group of microorganisms, the Archaea, and showed that these represented a separate branch in the Tree of Life -- a finding that stunned the scientific community at the time. Despite that archaeal cells were simple and small like bacteria, researchers found that Archaea were more closely related to organisms with complex cell types, a group collectively known as 'eukaryotes'. This observation has puzzled scientists for decades: How could the complex cell types from eukaryotes have emerged from the simple cells of Archaea?

In this weeks' edition of Nature, researchers from Uppsala University in Sweden, along with collaborators from the universities in Bergen (Norway) and Vienna (Austria) report the discovery of a new group of Archaea, the Lokiarchaeota (or 'Loki' for short), and identify it to be a missing link in the origin of eukaryotes.

"The puzzle of the origin of the eukaryotic cell is extremely complicated, as many pieces are still missing. We hoped that Loki would reveal a few more pieces of the puzzle, but when we obtained the first results, we couldn't believe our eyes. The data simply looked spectacular", says Thijs Ettema at the Department of Cell and Molecular Biology, Uppsala University, who lead the scientific team that carried out the study.

"By studying its genome, we found that Loki represents an intermediate form in-between the simple cells of microbes, and the complex cell types of eukaryotes", says Thijs Ettema.

When Loki was placed in the Tree of Life, this idea was confirmed.

link.

Using Ancient Dynamin Segments to Understand Mitochondrial Endosymbiosis During the Statherian PaleoProtoerozoic

Ancient dynamin segments capture early stages of host–mitochondrial integration

Authors:

Purkanti et al

Abstract:

Eukaryotic cells use dynamins—mechano-chemical GTPases—to drive the division of endosymbiotic organelles. Here we probe early steps of mitochondrial and chloroplast endosymbiosis by tracing the evolution of dynamins. We develop a parsimony-based phylogenetic method for protein sequence reconstruction, with deep time resolution. Using this, we demonstrate that dynamins diversify through the punctuated transformation of sequence segments on the scale of secondary-structural elements. We find examples of segments that have remained essentially unchanged from the 1.8-billion-y-old last eukaryotic common ancestor to the present day. Stitching these together, we reconstruct three ancestral dynamins: The first is nearly identical to the ubiquitous mitochondrial division dynamins of extant eukaryotes, the second is partially preserved in the myxovirus-resistance-like dynamins of metazoans, and the third gives rise to the cytokinetic dynamins of amoebozoans and plants and to chloroplast division dynamins. The reconstructed sequences, combined with evolutionary models and published functional data, suggest that the ancestral mitochondrial division dynamin also mediated vesicle scission. This bifunctional protein duplicated into specialized mitochondrial and vesicle variants at least three independent times—in alveolates, green algae, and the ancestor of fungi and metazoans—accompanied by the loss of the ancient prokaryotic mitochondrial division protein FtsZ. Remarkably, many extant species that retain FtsZ also retain the predicted ancestral bifunctional dynamin. The mitochondrial division apparatus of such organisms, including amoebozoans, red algae, and stramenopiles, seems preserved in a near-primordial form.