More than two billion years ago, life consisted entirely of microorganisms living in the oceans, marine sediments and hydrothermal vents. Within these diverse microbial ecosystems evolved a particular lineage of archaea: the Asgard archaea. Recent phylogenomic studies have revealed that their modern descendants are the closest known relatives of eukaryotes. These discoveries have made it possible to reconstruct one of the most significant events in the history of life: the establishment of a stable symbiotic association between an Asgard archaeon and an α-proteobacterium, giving rise to the first eukaryotic cell. From this unlikely alliance emerged animals, plants and fungi, together with an extraordinary diversity of unicellular organisms. The eukaryotic cell represents one of evolution’s most remarkable innovations. It contains a nucleus that protects its DNA, mitochondria that provide the energy required for cellular functions and, in plants and algae, chloroplasts that capture sunlight through photosynthesis. Behind this sophisticated cellular organisation lies an unexpected story: that of a composite cell formed through the lasting integration of once-independent microorganisms. This first endosymbiotic event did not merely give rise to eukaryotes. Subsequent endosymbiotic events, including the acquisition of chloroplasts and later endosymbioses in several algal lineages, further transformed the history of life. Endosymbiosis is now recognised as one of the major drivers of evolutionary innovation, underpinning many of the major innovations that shaped eukaryotic evolution.

 

1. The eukaryotic cell is a chimera

EukaryotesSingle-cell or multicellular organisms whose cells have a nucleus and organelles (endoplasmic reticulum, Golgi apparatus, various plastids, mitochondria, etc.) delimited by membranes. Eukaryotes are, along with bacteria and archaea, one of the three groups of living organisms. correspond to multicellular organisms (animals, plants, fungi) and some unicellular organisms (protozoa, for example). The main characteristic of the eukaryotic cell (Figure 1) is the existence of a nucleus (in prokaryotes, the genome is only very rarely surrounded by a membrane) surrounded by a cytoplasm containing many organelles, such as mitochondriaOrganelles of the cytoplasm of eukaryotic cells (plants, algae, animals). As the site of cellular respiration, mitochondria convert the energy of organic molecules from digestion (glucose) into energy that can be directly used by the cell (ATP) during the “Krebs cycle”. This reaction requires the presence of oxygen and releases CO₂, so it plays an essential role in the carbon cycle. Mitochondria originate from a prokaryotic organism (α-proteobacteria) integrated into eukaryotic protocells 2 billion years ago. (where respiration, present in all eukaryotic cells, takes place) and chloroplastsOrganelles of the cytoplasm of photosynthetic eukaryotic cells (plants, algae). As a site of photosynthesis, chloroplasts produce O₂ oxygen and play an essential role in the carbon cycle: they use light energy to fix CO₂ and synthesize organic matter. They are thus responsible for the autotrophy of plants. Chloroplasts are the result of the endosymbiosis of a photosynthetic prokaryote (cyanobacterium type) in a eukaryotic cell about 1.5 billion years ago. (site for photosynthesisBioenergetic process that allows plants, algae and some bacteria to synthesize organic matter from atmospheric CO₂ by using sunlight. Solar energy is used to oxidize water and reduce carbon dioxide in order to synthesize organic substances (carbohydrates). The oxidation of water leads to the formation of O₂ oxygen found in the atmosphere. Photosynthesis is the basis of autotrophy, it is the result of the integrated functioning of the chloroplast within the cell., in plants in the broad sense, terrestrial plants and algae). These organelles are frequently displaced or reorganized by the cytoskeleton that triggers intracellular mobility (Figure 1).

The eukaryotic nucleus is delimited by a double membrane called the nuclear envelope (Figure 1). It contains the nuclear genome characteristic of the eukaryotic cell, i.e. the genetic material of an individual encoded in its DNA (deoxyribonucleic acid). It is usually this genome that is referred to when the genome of a eukaryote is mentioned. However, the eukaryotic cell also contains non-nuclear genomes within the organelles:

• The mitochondrial genome, within the mitochondrial matrix (Figure 1);
• The chloroplastic genome, within the chloroplast stroma (e.g. plants or algae).

The DNA constituting these three genomes is not organized in the same way. In the nucleus, the genome is distributed over several linear DNA molecules, and organized into well-differentiated chromosomes. DNA contains all coding sequences (transcribed into messenger RNA, mRNA, and translated into proteins) and non-coding sequences (not transcribed, or transcribed into RNA, but not translated). The three-dimensional configuration of the nuclear genome has a functional importance: the winding (or “condensation”) of DNA on itself and around proteins, the histonesBasic proteins associating with DNA to form the basic structure of chromatin. Histones play an important role in DNA packaging and folding, allowing a large amount of genetic information to be packaged in the tiny nucleus of a cell. Mitochondrial or chloroplastic DNA do not have the same organization: it is generally circular, rarely linear (plant mitochondria), generally without intron, and is not associated with histone proteins.

ProkaryoticMicroorganisms (usually unicellular) with a simple cellular structure, no nucleus, and almost never internal compartmentalization (the only exception being thylakoids in cyanobacteria). Two of the three groups that make up living organisms are prokaryotes: Archaea and Bacteria.-type cells (Bacteria and ArchaeaSingle-celled prokaryotic microorganisms living in particular in extreme environments (anaerobic, high salinity, very hot…). Phylogenetic research by Carl Woese and George E. Fox (1977) differentiated between archaea and other prokaryotic organisms (bacteria). Currently, living organisms are considered to consist of three groups: Archaea, Bacteria and Eukaryota.), do not have a nucleus and their DNA is circular (or -in some rare cases- linear) and organized like that of chloroplasts or mitochondria. In this way, DNA replication, transcription and translation directly take place into the cytoplasm. It should be noted, however, that Archaea are only superficially similar to Bacteria in their cellular aspect: their metabolism differs greatly, and the mechanisms and proteins involved in the replication, transcription and translation processes have similar characteristics to those of eukaryotes. Finally, prokaryotes -in general- do not have internal compartments and, if present, they are less complex (cyanobacteria are an example of an exception). Above all, compartments, when they exist, are not mobile in the cell: the cytoskeleton, which is beginning to be discovered, does not move the cellular components within it.

Table 1. Comparison of eukaryotic and prokaryotic cells

Table 1 compares the properties of prokaryotic and eukaryotic cells (with their mitochondria and possibly their chloroplasts). It shows that mitochondria and chloroplasts have many characteristics in common with those of prokaryotic cells. Beyond the structure of DNA, eukaryotic cell organelles are formed from pre-existing organelles, dividing by fission to multiply, like a bacteria. Similarly, they have the same protein synthesis machinery (free 70S ribosomesA huge complex composed of RNAs and ribosomal proteins that allows the translation of mRNAs into proteins. Common to all cells (prokaryotes and eukaryotes), the ribosome varies according to the organisms: 80S ribosome in eukaryotes and 70S ribosome in prokaryotes and cellular organelles (mitochondria, chloroplast). in the matrix or stroma) while in the cytoplasm of the eukaryotic cell, this machinery consists of 80S ribosomes, sometimes fixed on the membranes of the endoplasmic reticulumMembrane network of the eukaryotic cell cytoplasm, essential for cellular metabolism (lipid and protein synthesis, calcium storage). Associated with ribosomes, it is the place of synthesis of proteins secreted outside the cell and, on the other hand, proteins and lipids constituting the membranes of cellular organelles (Golgi apparatus, lysosomes, mitochondria, nucleus, ribosomes, vesicles…).. Finally, bacteria also have the metabolism of mitochondria (i.e. respiration) and, in some peculiar cases, of chloroplasts (i.e. photosynthesis). On the other hand, the eukaryotic cell is distinguished by the existence of an active protein network, the cytoskeleton, a self-organized system capable of mobility, which positions and displaces the organelles in the cell. Such a protein network is static, or even absent, in prokaryotes, and poorly developed in mitochondria and chloroplasts.

The analysis of genomic sequences, made possible by DNA sequencing, has revealed much about the evolutionary history of living organisms, particularly their relationships — their phylogeny (see ‘What is biodiversity?’ and ‘Inheritance or convergence…’). Phylogenetic analysis of maize’s nuclear genome, as well as its mitochondrial and chloroplast genomes, allows this plant to be placed on the tree of life (Figure 2) [1]. The mitochondria and chloroplasts of Zea mays (maize) illustrate their respective endosymbiotic origins (α-proteobacterial and cyanobacterial). The nuclear genome of eukaryotes is of Asgardian origin.

These properties show that the eukaryotic cell is a chimera: it combines components specific to itself (the nucleus) with organelles possessing typically prokaryotic properties (chloroplasts, mitochondria).

The distinction between prokaryotes and eukaryotes was proposed in 1925 by the Pasteur Institute scientist Edouard Chatton (who named these two cell types) [2], although it was not widely recognised until the 1950s and 1960s. The chimeric nature of eukaryotic cells had already been suspected at the turn of the 19^th to the 20^thcentury. Whilst the botanist Andreas Schimper was the first, in 1883, to suggest that photosynthetic organisms result from the combination of distinct organisms; it was the Russian biologist Konstantin Mereschkowsky who provided the first solid evidence showing that certain cells arise from the union of two different cell types (endosymbiosis). In his 1905 article [3], Mereschkowsky puts forward the idea that chloroplasts are a form of cyanobacteria which, early in evolution, established a symbiosis with a heterotrophic host, and that the autotrophy of plants is entirely inherited from cyanobacteria.

Mereschkowsky had not addressed the origin of mitochondria; this was left to the French microbiologist Paul Portier, who wrote in a 1918 text [4], that ‘all living beings, all animals (…), all plants (…) are constituted by the association of two different beings. Every living cell contains structures that cytologists call ‘mitochondria’. In my view, these organelles are nothing more than symbiotic bacteria, symbiotes.’ These observations attracted little attention, and the theory fell out of favour, as it proved impossible to culture plastids and mitochondria — evidence, in the 19^th century, was deemed essential to establish a bacterial nature [5]. It was not until the advent of new methods for studying the cell — electron microscopy, biochemistry, molecular biology — that the endosymbiotic theory was revived, around 1970, by the American microbiologist Lynn Margulis [6].

However, the discovery that eukaryotes evolved from a specific lineage of archaea (Asgard) has revolutionised the paradigm separating prokaryotes and eukaryotes (See La révolution des archées Asgard).

2. How did the eukaryotic cell arise?

The emergence of the eukaryotic cell, around 1.5 to 2 billion years ago, represents one of the most significant evolutionary transitions in the history of life. Occurring nearly a billion years after the emergence of the first prokaryotes, it enabled the development of unprecedented cellular complexity (nucleus, organelles, dynamic cytoskeleton) and paved the way for multicellularity. Palaeontological, bioenergetic , structural and, above all, phylogenomic approaches now converge on a clear scenario, illustrated in Figures 3 and 4.

2.1. The host cell: an Asgard archaeon

Phylogenomic analyses have radically revised our understanding of the tree of life (Figure 3). Eukaryotes no longer constitute an independent domain, but are descended from a group of Asgard archaea (phylum Promethearchaeota), more specifically closely related to the Heimdallarchaeia [7],[8] (see Focus: The Asgard Revolution) (Figure 3). These microorganisms possess numerous ‘eukaryotic signature genes’ (ESP) encoding proteins involved in the cytoskeleton, membrane trafficking, ubiquitination and membrane remodelling.

Recent reconstructions of the last common ancestor of eukaryotes (LECA; see Focus on LUCA, LECA and the common ancestors of the tree of life) show that the vast majority of conserved genes of non-mitochondrial origin derive from this Asgard lineage, with a dominant contribution in most cellular functional systems (replication, transcription, translation, cytoskeleton, endomembranes). LECA was already a complex cell, possessing a nucleus, a dynamic cytoskeleton , a well-developed endomembrane system, phagocytic capabilities and, in all likelihood, sexual reproduction (meiosis) [9],[10].

2.2. The symbiotic scenario: metabolism and molecular mechanisms of H₂ syntrophy

The most widely supported scenario (Figure 4) [11] is based on a metabolic symbiosis (syntrophy) between an H₂-dependent Asgard archaeon and a facultative anaerobic α-proteobacterium [5].

Molecular mechanisms of H₂ syntrophy:

In anaerobic environments, many fermentative bacteria produce molecular hydrogen (H₂) as a by-product. However, these reactions become thermodynamically unfavourable as the H₂ concentration increases. H₂-dependent archaea (methanogens or acetogens) consume this hydrogen to reduce CO₂ to methane or acetate, thereby maintaining a very low concentration of H₂. This ‘interception’ makes bacterial fermentations exergonic and benefits both partners [5],[12],[13].

In Asgards, this syntrophic metabolism involves [FeFe] or [NiFe] hydrogenases and interspecific electron transfer systems (possibly via nanotubes or direct membrane contacts). The host archaeon, likely acetogenic or methanogenic, was heavily dependent on this exogenous H₂. The ancestral α-proteobacterium was capable of H₂-producing fermentation under anaerobic conditions and respiration in the presence of oxygen. This flexibility enabled an increasingly close association, progressing from extracellular syntrophy to endosymbiosis [11].

Internalisation probably followed the Entangle-Engulf-Endogenise (E³) model (see Figure 4) observed in Candidatus Prometheoarchaeum syntrophicum [14] : the archaeon used its membrane protrusions to gradually encircle the bacterial partner before internalising it. Once inside, the bacterium evolved into a mitochondrion, developing an electron transport chain and efficient oxidative phosphorylation (producing up to 18 times more ATP per molecule of glucose compared with fermentation).

Unlike chloroplasts, which were acquired on several occasions, the mitochondrion appears to have arisen only once in the history of life: this single acquisition constitutes the founding event from which the entire current eukaryotic lineage descends.

2.3. Gene transfers: a massive and continuous process

This integration was accompanied by massive transfers of genes from the bacterium to the host’s nuclear genome (see Figure 4). Nearly 99 per cent of the original genes of the alpha-proteobacterial ancestor have been lost or transferred via:

• Relocation of bacterial DNA fragments into the nucleus;
• Acquisition of eukaryotic regulatory sequences and a transit peptide;
• Loss of the redundant copy of the organelle [15]

These transfers involved not only metabolic genes but also genes involved in mitochondrial replication, transcription and translation. They considerably enriched the nuclear genome, creating a veritable genetic chimera [16], whilst making the organelle dependent on the nucleus for the majority of its proteins. This process continued over hundreds of millions of years and constitutes a major example of evolution by endosymbiosis.

2.4. The debate on oxygen and chronology

Phylogenetic reconstructions point to a common H₂-dependent anaerobic ancestor [5, 13]. The ancestral α-proteobacterium was itself facultatively anaerobic. However, recent research shows that some Asgard could tolerate low concentrations of oxygen. The ancestral α-proteobacterium was itself facultatively anaerobic. The mitochondrion would therefore not have been solely a response to the global increase in atmospheric O₂, but rather an energy amplifier that enabled this oxygen to be utilised efficiently where it was locally available.

Some of the eukaryotic complexity may have emerged before the acquisition of mitochondria by the Asgard host, with the mitochondria primarily serving to stabilise and amplify these capabilities. The debate over the precise chronology remains open.

2.5 And what about the nucleus?

The question of the origin of the eukaryotic cell is also linked to that of the nucleus. The establishment of a new membrane system—the nuclear membrane—in the host following the acquisition of mitochondria could be due to the aggregation of membrane vesicles composed of bacterial lipids. This separation between the nucleus and the cytoplasm may have been a response to the need, following massive gene transfers, to separate RNA splicing from translation. It was then selective pressure that would have led to the establishment of this compartmentalisation [5].

The eukaryotic cell is therefore the result of a profound chimerisation: an Asgard archaeon providing the informational architecture and part of the cytoskeleton, an α-proteobacterium contributing oxidative metabolism via the mitochondrion through an initial H₂ syntrophy, and massive gene transfers fusing the two lineages [5],[11]. This association overcame the energy limitations of prokaryotes and enabled the emergence of unprecedented cellular complexity.

3. The endosymbiotic origin of the chloroplast

During phagocytosis, which is observed in white blood cells and many protozoa (Figure 5), the ingested cells are usually digested immediately — as is the case with prey. However, they are sometimes harboured for a prolonged period: these are referred to as endosymbionts. An organelle therefore results from the internalisation, via phagocytosis, of a prokaryote within a eukaryote, without the prokaryote being digested (Figure 6). This is the case with the chloroplasts of terrestrial plants, but also with red and green algae, which are closely related to them [17],[18] .

During phagocytosis, the plasma membrane invaginates around the prey and isolates it within endocytic vesicles, where it is digested: these vesicles fuse with others, the lysosomes, which contain digestive enzymes. By analogy, it was long assumed that the outer membrane of organelles originated from this endocytic membrane. The situation is probably more complex (Figure 5): the prokaryotes that gave rise to chloroplasts and mitochondria are Gram-negative bacteria, characterised by a double peripheral membrane. However, the outer membrane of chloroplasts, which is in contact with the cytosol, contains characteristic glycolipids found in cyanobacteria [12],[19],[20]. It is therefore possible that the endocytic membrane disappeared during the integration of the prokaryote. Indeed, this is what is currently observed in Elysia chlorotica (see Focus), a marine mollusc that grazes on algae, digests some of their cells but retains functional chloroplasts in the cytoplasm of certain cells for several months — a phenomenon known as kleptoplasty. It was long thought that this persistence was explained by a transfer of genes from the alga to the slug’s genome, which would thus have acquired the ability to produce the proteins necessary for maintaining the chloroplasts. However, complete genome sequencing of Elysia chlorotica, which is more rigorous than the initial analyses, has found no trace of such a transfer in its DNA [21]. The remarkable longevity of these ‘stolen’ chloroplasts therefore remains largely unexplained — a good example of a compelling hypothesis being revised as data accumulates.

Primary and secondary endosymbiosis
Over the course of evolution, several endosymbioses have occurred independently, giving rise to a variety of organisms. In a primary endosymbiosis, the eukaryotic cell incorporates a living prokaryote. Thus, the chloroplasts of the green lineage — red algae, green algae and land plants — originated from primary endosymbioses involving a cyanobacterium. In some eukaryotes, mitochondria have adapted to anaerobic environments without ever disappearing: they have given rise to specialised mitochondria, known as hydrogenosomes, which carry out H₂-producing fermentation (for example, in certain ciliates) [22], as well as small organelles involved solely in biosynthesis for the host cell, known as mitosomes [19].

Secondary endosymbiosis repeats the process: a eukaryote that already possesses an endosymbiont in turn enters into endosymbiosis with another eukaryote (Figure 6). This is the origin of plastids with more than two membranes found in certain plant groups, for example the internalisation of a green alga in Euglena, or the independent internalisation of a red alga in brown algae. Tertiary endosymbiosis events, which are rarer, have also been described. Each has given rise to a new evolutionary lineage [23],[24].

4. The integration of the prokaryote within the eukaryotic cell

All these lineages share a marked genetic reduction in the endosymbionts. Compared with free-living proteobacteria such as Escherichia coli, mitochondria have lost 99 per cent of their genes. At the extreme end of the spectrum, hydrogenosomes and mitosomes no longer have a genome at all! The plastids of the green lineage show a reduction of around 95 per cent compared with free-living cyanobacteria: the number of genes has fallen from several thousand to around a hundred or two in chloroplasts — or even to zero in the reduced plastids of the parasitic plant Rafflesia, as confirmed by a complete sequencing of its genome.

This reduction is primarily explained by the loss of genes necessary for free-living and for certain metabolic functions that have become redundant. Thus, as in all Gram-negative bacteria, a layer of peptidoglycan is found between the two membranes of cyanobacterial , which is essential for maintaining their structure in their natural, low-osmolarity environment. Once incorporated into the host cell, the prokaryote finds itself in a cytoplasm whose osmolarity is close to that of its own internal environment. The peptidoglycan layer then becomes unnecessary, and the genes that produced it have been lost in the chloroplasts — except in glaucophytes.

Although the genome of the organelles has regressed, their repertoire of proteins (the proteome), where known, remains similar to that of free-living bacteria: proteins performing new functions have compensated for these losses. Their coding has been taken over by the host’s nuclear genome: genes from the nucleus are translated into proteins in the cytosol, which are then directed to the organelle via a transit peptide. This relocalisation is absolutely essential for the integration of the prokaryote within the host cell. The targeting machinery responsible for these transfers represents a convergent innovation in plastids and mitochondria, clearly illustrating the new functions associated with intracellular life. These systems, which import proteins synthesised in the cytosol across the two membranes of mitochondria and chloroplasts, comprise proteins of complex evolutionary origin: some prokaryotic, others eukaryotic, encoded sometimes within the organelle, sometimes in the nucleus. Together, they ensure the recognition of the protein, its unfolding and subsequent import — the protein must remain unfolded to cross the membranes —, prior to the cleavage of the targeting peptide and its localisation within the functional compartment [25].

What is the origin of these nuclear genes that encode functions intended for organelles? There are two (Figure 7) [26]. In some cases, genes already present in the nucleus have replaced the organelle’s own genes: their products have acquired the ability to be directed towards the organelle. This acquisition may have created redundancy whenever a gene already encoded the same function within the organelle — a redundancy that allowed the corresponding organelle gene to be lost without causing any harm (Figure 7a) [25].

Other cases involve the transfer of genes from the organelle to the nucleus, in two main stages (Figure 7b). First, a DNA fragment encoding an organelle protein is relocated and then integrated into the nuclear genome. This sequence can only be expressed if mutations adapt it to the nuclear genetic code and enable it to acquire regulatory sequences for transcription. It must also acquire the transit peptide pre-sequence, which will ensure that the mature protein is directed to the organelle. This results in genetic redundancy: either copy may be lost without causing harm. The loss of function and/or the disappearance of the organelle copy then seals the transfer (Figure 7) [25].

The transfer of DNA fragments from organelles to the nucleus is not uncommon: large blocks of organelle DNA are inserted into the genome of certain plants, and may subsequently become active. Nearly 10 per cent of the nuclear genes in Arabidopsis thaliana thus originate from transfers from the plastid, often followed by duplications [27]. It is not known how the endosymbiont’s DNA manages to integrate into the host’s genome, but it is thought that this occurs during the breakdown of damaged or ageing organelles, which accidentally release DNA fragments into the host’s cytoplasm, which are then randomly integrated into the nuclear DNA.

The cytoplasmic genomes of organelles are subject to conflicting selective pressures: some favour their regression, such as the need to co-express certain genes, whilst others, on the contrary, favour the persistence of particular genes. ly, this is the case with selection for a small genome size, which accelerates the multiplication of organelles and improves their transmission to daughter cells, notably favouring the transfer of genes to the nucleus. The nucleus thus accumulates genetic potential from different lineages, coexisting with it within the cell [21]. Thus, whilst endosymbiosis reduces the genomes of the endosymbionts, it enriches that of the host nucleus, contributing to its genetic diversification and strengthening the endosymbiotic association. Endosymbiosis therefore blends evolutionary lineages, both through structural interlocking and through genetic chimerisation of the nucleus.

Finally, the vertical transmission of the endosymbiont, from one generation to the next, is essential to the sustainability of endosymbiosis. Plastids must divide before the host cell and be distributed equally between the two daughter cells: if they divide too quickly, they would gain an advantage over the host; if they divide too slowly, they could disappear. Coordinating cell division with that of the symbiont has therefore been essential to the success of endosymbiosis. Most of the proteins involved in chloroplast division originate from the cellular machinery of cyanobacteria, but a few appear to be of eukaryotic origin; all are encoded in the nucleus, enabling the host to retain control over chloroplast division.

5. Is symbiosis the driving force behind evolution?

In conclusion, the extremely diverse symbiotic relationships that led to the formation of the eukaryotic cell [1, 9] are at the root of the development of eukaryotic biodiversity throughout evolution. Endosymbiosis gives rise to new evolutionary lineages, and the extreme diversity of organisms resulting from chloroplast endosymbiosis is a good example of this (Figure 8) [24]. Moreover, the process is not static: evolution continues to make use of it! Even today, certain single-celled algae, cryptophytes and heterokonts (Figure 8), whose four-membraned plastid derives from a secondary endosymbiosis, live in symbiosis within the cytoplasm of dinoflagellates that have lost their own plastids — three successive, superimposed endosymbioses!

Far more than a biological curiosity, symbiosis is one of the most powerful drivers of biological evolution. It rapidly creates chimeric organisms, capable of giving rise to new lineages. By bringing partners together, it facilitates massive gene transfers that render the genomes themselves chimeric: the nuclear genome thus contains eukaryotic genes, but also genes of bacterial origin, derived from the mitochondria or plastids with which it coexists. Such events may explain some of the major evolutionary leaps that punctuate the history of life, giving rise to today’s major lineages and the biological diversity we know.

Thus, whilst renewing the Darwinian vision of evolution through descent with modification — where one species gives rise to two — the mechanisms of endosymbiosis remind us that two species, once free and distinct, can also merge into a single one. Human beings themselves can be seen as an extremely integrated symbiotic community, comprising eukaryotic cytoplasm and mitochondria, but also the archaea and bacteria that inhabit their gut…

 

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References and notes

Cover Image : [Source Photo © Jacques Joyard]

[1] Figure 2 is created using data from the following references : Lang T. et al. (2000) Autophagy and the cvt pathway both depend on AUT9. J Bacteriol 182, 2125-2133. doi: 10.1128/JB.182.8.2125-2133.2000 ; Spang, A., Saw, J. H., Jørgensen, S. L., Zaremba-Niedzwiedzka, K., Martijn, J., Lind, A. E., … Ettema, T. J. G. (2015). Complex archaea that bridge the gap between prokaryotes and eukaryotes. Nature, 521(7551), 173–179. https://doi.org/10.1038/nature14447 ; Zaremba-Niedzwiedzka, K., Caceres, E. F., Saw, J. H., Bäckström, D., Juzokaite, L., Vancaester, E., … Ettema, T. J. G. (2017). Asgard archaea illuminate the origin of eukaryotic cellular complexity. Nature, 541(7637), 353–358. https://doi.org/10.1038/nature21031 ; Imachi, H., Nobu, M. K., Nakahara, N., Morono, Y., Ogawara, M., Takaki, Y., … Takai, K. (2020). Isolation of an archaeon at the prokaryote–eukaryote interface. Nature, 577(7791), 519–525. https://doi.org/10.1038/s41586-019-1916-6

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[21] Bhattacharya D., Pelletreau K.N., Price D.C., Sarver K.E., Rumpho M.E. (2013) Genome analysis of Elysia chlorotica egg DNA provides no evidence for horizontal gene transfer into the germ line of this kleptoplastic mollusc. Mol. Biol. Evol. 30, 1843–1852; see also Maeda T. et al. (2021) Chloroplast acquisition without gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus. eLife 10, e60176.

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[27] Jarvis P. (2004) Organellar Proteomics: Chloroplasts in the Spotlight. Current Biology 14, R317–9. http://www.cell.com/current-biology/references/S0960-9822%2804%2900231-3

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