Evolutionary relationship of Rickettsiae and mitochondria.

Phylogenetic data support an origin of mitochondria from the alpha-proteobacterial order Rickettsiales. This high-rank taxon comprises exceptionally obligate intracellular endosymbionts of eukaryotic cells, and includes family Rickettsiaceae and a group of microorganisms termed Rickettsia-like endosymbionts (RLEs). Most detailed phylogenetic analyses of small subunit rRNA and chaperonin 60 sequences consistently show the RLEs to have emerged before Rickettsiaceae and mitochondria sister clades. These data suggest that the origin of mitochondria and Rickettsiae has been preceded by the long-term mutualistic relationship of an intracellular bacterium with a pro-eukaryote, in which an invader has lost many dispensable genes, yet evolved carrier proteins to exchange respiration-derived ATP for host metabolites as envisaged in classic endosymbiont theory.     

Suggested mitochondrial ancestry of nonmitochondrial ATP/ADP carrier

One of the major evolutionary events that transformed an endosymbiotic bacterium into a mitochondrion was the acquisition of the ATP/ADP carrier (AAC) in order to supply the host with respiration-derived ATP. Along with the mitochondrial carrier, an unrelated carrier is known, which is characteristic of intracellular chlamydiae, plastids, parasitic intracellular eukaryote Encephalitozoon cuniculi, and the genus Rickettsia of obligate endosymbiotic α-proteobacteria. This nonmitochondrial carrier was recently described in rickettsia-like endosymbionts (RLE), a group of obligate intracellular bacteria classified with the order Rickettsiales, which have diverged after free-living α-proteobacteria but before sister groups of the Rickettsiaceae assemblage (true rickettsiae) and mitochondria. Published controversial phylogenetic data on nonmitochondrial AAC were re-analyzed in the present work, using both DNA and protein sequences and various methods including Bayesian analysis. The data presented are consistent with the classic endosymbiont theory for the origin of mitochondria and suggest that even the last but one common ancestor of rickettsiae and organelles was an endosymbiotic bacterium, in which AAC first originated.     

Suggested mitochondrial ancestry of non-mitochondrial ATP/ADP

One of the major evolutionary events that transformed endosymbiotic bacterium into mitochondrion was an acquisition of ATP/ADP carrier in order to supply the host with respiration-derived ATP. Along with mitochondrial carrier, unrelated carrier is known which is characteristic of intracellular chlamydiae, plastids, parasitic intracellular eukaryote Encephalitozoon cuniculi, and the genus Rickettsia of obligate endosymbiotic alpha-Proteobacteria. This non-mitochondrial ATP/ADP carrier was recently described in rickettsia-like endosymbionts - a group of obligate intracellular bacteria, classified with the order Rickettsiales, which have diverged after free-living alpha-Proteobacteria but before sister groups of the Rickettsiaceae assemblage (true rickettsiae) and mitochondria. Published controversial phylogenetic data on the non-mitochondrial carrier were reanalysed in the present work using both DNA and protein sequences, and various methods including Bayesian analysis. The data presented are consistent with classic endosymbiont theory for the origin of mitochondria and also suggest that even last but one common ancestor of rickettsiae and organelles may have been an endosymbiotic bacterium in which ATP/ADP carrier has first originated.     

Single-cell genomics of a rare environmental alphaproteobacterium provides unique insights into Rickettsiaceae evolution

The bacterial family Rickettsiaceae includes a group of well-known etiological agents of many human and vertebrate diseases, including epidemic typhus-causing pathogen Rickettsia prowazekii. Owing to their medical relevance, rickettsiae have attracted a great deal of attention and their host-pathogen interactions have been thoroughly investigated. All known members display obligate intracellular lifestyles, and the best-studied genera, Rickettsia and Orientia, include species that are hosted by terrestrial arthropods. Their obligate intracellular lifestyle and host adaptation is reflected in the small size of their genomes, a general feature shared with all other families of the Rickettsiales. Yet, despite that the Rickettsiaceae and other Rickettsiales families have been extensively studied for decades, many details of the origin and evolution of their obligate host-association remain elusive. Here we report the discovery and single-cell sequencing of ‘Candidatus Arcanobacter lacustris'', a rare environmental alphaproteobacterium that was sampled from Damariscotta Lake that represents a deeply rooting sister lineage of the Rickettsiaceae. Intriguingly, phylogenomic and comparative analysis of the partial ‘Candidatus Arcanobacter lacustris'' genome revealed the presence chemotaxis genes and vertically inherited flagellar genes, a novelty in sequenced Rickettsiaceae, as well as several host-associated features. This finding suggests that the ancestor of the Rickettsiaceae might have had a facultative intracellular lifestyle. Our study underlines the efficacy of single-cell genomics for studying microbial diversity and evolution in general, and for rare microbial cells in particular.     

A robust species tree for the alphaproteobacteria

The branching order and coherence of the alphaproteobacterial orders have not been well established, and not all studies have agreed that mitochondria arose from within the Rickettsiales. A species tree for 72 alphaproteobacteria was produced from a concatenation of alignments for 104 well-behaved protein families. Coherence was upheld for four of the five orders with current standing that were represented here by more than one species. However, the family Hyphomonadaceae was split from the other Rhodobacterales, forming an expanded group with Caulobacterales that also included Parvularcula. The three earliest-branching alphaproteobacterial orders were the Rickettsiales, followed by the Rhodospirillales and then the Sphingomonadales. The principal uncertainty is whether the expanded Caulobacterales group is more closely associated with the Rhodobacterales or the Rhizobiales. The mitochondrial branch was placed within the Rickettsiales as a sister to the combined Anaplasmataceae and Rickettsiaceae, all subtended by the Pelagibacter branch. Pelagibacter genes will serve as useful additions to the bacterial outgroup in future evolutionary studies of mitochondrial genes, including those that have transferred to the eukaryotic nucleus.     

First detection of endosymbiotic bacteria in biting midges Culicoides pulicaris and Culicoides punctatus,important Palaearctic vectors of bluetongue virus

Heritable bacteria have been highlighted as important components of vector biology, acting as required symbionts with an anabolic role, altering competence for disease transmission, and affecting patterns of gene flow by altering cross compatibility. In this paper, we tested eight U.K. species of Culicoides (Diptera: Ceratopogonidae) midge for the presence of five genera of endosymbiotic bacteria: Cardinium (Bacteroidales: Bacteroidaceae); Wolbachia (Rickettsiales: Rickettsiaceae); Spiroplasma (Entomoplasmatales: Spiroplasmataceae); Arsenophonus (Enterobacteriales: Enterobacteriaceae), and Rickettsia (Rickettsiales: Rickettsiaceae). Cardinium spp. were detected in both sexes of Culicoides pulicaris and Culicoides punctatus, two known vectors of bluetongue virus. Cardinium spp. were not detected in any other species, including the Culicoides obsoletus group, the main vector of bluetongue and Schmallenberg viruses in northern Europe. The other endosymbionts were not detected in any Culicoides species. The Cardinium strain detected in U.K. Culicoides species is very closely related to the Candidatus Cardinium hertigii group C, previously identified in Culicoides species in Asia. Further, we infer that the symbiont is not a sex ratio distorter and shows geographic variation in prevalence within a species. Despite its detection in several species of Culicoides that vector arboviruses worldwide, the absence of Cardinium in the C. obsoletus group suggests that infections of these symbionts may not be necessary to the arboviral vector competence of biting midges.     

Common evolutionary origin of mitochondrial and rickettsial respiratory chains

Comprehensive phylogenetic analysis of the subunits of respiratory chain was carried out using a variety of mitochondrial and bacterial sequences including those from all unfinished alpha-proteobacterial genomes known to date. Maximum likelihood, neighbor-joining, and maximum parsimony consensus trees, based on four proton-translocating complexes, placed mitochondria as a sister group to the order Rickettsiales of obligate endosymbiotic bacteria to the exclusion of free-living alpha-proteobacteria. Thus, phylogenetic relationship of most eukaryotic respiratory enzymes conforms to canonical pattern of mitochondrial ancestry, prior established in analyses of ribosomal RNAs, which are encoded by residual mitochondrial genomes. These data suggest that mitochondria may have derived from a reduced intracellular bacterium and that respiration may be the only evolutionary novelty brought into eukaryotes by mitochondrial endosymbiont.     

Occurrence of Chaperonin 60 and Chaperonin 10 in primary and secondary bacterial symbionts of aphids: Implications for the evolution of an endosymbiotic system in aphids

Summary All aphids harbor symbiotrophic prokaryotes (primary symbionts) in a specialized-abdominal cell, the bacteriocyte. Chaperonin 60 (Cpn60, symbionin) and chaperonin 10 (Cpn10), which are high and low molecular weight heatshock proteins, were sought in tissues of more than 60 aphid species. The endosymbionts were compared immunologically and histologically. It was demonstrated that (1) there are two types of aphids in terms of the endosymbiotic system: some with only primary symbionts and others with, in addition, secondary symbionts; (2) the primary symbionts of various aphids are quite similar in morphology whereas the secondary symbionts vary; and (3) irrespective of the aphid species, Cpn60 is abundant in both the primary and secondary symbionts, while Cpn10 is abundant in the secondary symbionts but present in small amounts in the primary ones. Based on these results, we suggest that the primary symbionts have been derived from a prokaryote that was acquired by the common ancestor of aphids whereas the secondary symbionts have been acquired by various aphids independently after divergence of the aphid species. In addition, we point out the possibility that the prokaryotes under intracellular conditions have been subject to some common evolutionary pressures, and as a result, have come to resemble cell organelles.     

Phylogenetic Analysis of the Complete Genome Sequence of <Emphasis Type="Italic">Encephalitozoon cuniculi</Emphasis> Supports the Fungal Origin of Microsporidia and Reveals a High Frequency of Fast-Evolving Genes

Microsporidia are unicellular eukaryotes living as obligate intracellular parasites. Lacking mitochondria, they were initially considered as having diverged before the endosymbiosis at the origin of mitochondria. That microsporidia were primitively amitochondriate was first questioned by the discovery of microsporidial sequences homologous to genes encoding mitochondrial proteins and then refuted by the identification of remnants of mitochondria in their cytoplasm. Various molecular phylogenies also cast doubt on the early divergence of microsporidia, these organisms forming a monophyletic group with or within the fungi. The 2001 proteins putatively encoded by the complete genome of Encephalitozoon cuniculi provided powerful data to test this hypothesis. Phylogenetic analysis of 99 proteins selected as adequate phylogenetic markers indicated that the E. cuniculi sequences having the lowest evolutionary rates preferentially clustered with fungal sequences or, more rarely, with both animal and fungal sequences. Because sequences with low evolutionary rates are less sensitive to the long-branch attraction artifact, we concluded that microsporidia are evolutionarily related to fungi. This analysis also allowed comparing the accuracy of several phylogenetic algorithms for a fast-evolving lineage with real rather than simulated sequences.This article contains online supplementary material.Reviewing Editor: Dr. Wen-Hsiung LiSupplementary material is available at     

The SAR11 group of alpha-proteobacteria is not related to the origin of mitochondria

Although free living, members of the successful SAR11 group of marine alpha-proteobacteria contain a very small and A+T rich genome, two features that are typical of mitochondria and related obligate intracellular parasites such as the Rickettsiales. Previous phylogenetic analyses have suggested that Candidatus Pelagibacter ubique, the first cultured member of this group, is related to the Rickettsiales+mitochondria clade whereas others disagree with this conclusion. In order to determine the evolutionary position of the SAR11 group and its relationship to the origin of mitochondria, we have performed phylogenetic analyses on the concatenation of 24 proteins from 5 mitochondria and 71 proteobacteria. Our results support that SAR11 group is not the sistergroup of the Rickettsiales+mitochondria clade and confirm that the position of this group in the alpha-proteobacterial tree is strongly affected by tree reconstruction artefacts due to compositional bias. As a consequence, genome reduction and bias toward a high A+T content may have evolved independently in the SAR11 species, which points to a different direction in the quest for the closest relatives to mitochondria and Rickettsiales. In addition, our analyses raise doubts about the monophyly of the newly proposed Pelagibacteraceae family.     

Evolution of the chaperonin families (HSP60, HSP 10 and TCP-1) of proteins and the origin of eukaryotic cells

The members of the 10 kDa and 60 kDa heat-shock chaperonin proteins (Hsp10 and Hsp60 or Cpn10 and Cpn60), which form an operon in bacteria, are present in all eubacteria and eukaryotic ceil organelles such as mitochondria and chloroplasts. In archaebacteria and eukaryotic cell cytosol, no close homologues of Hsp10 or Hsp60 have been identified. However, these species (or ceil compartments) contain the Tcp-1 family of proteins (distant homologues of Hsp60). Phylogenetic analysis based on global alignments of Hsp60 and Hsp10 sequences presented here provide some evidence regarding the evolution of mitochondria from a member of the α-subdivision of Gram-negative bacteria and chloroplasts from cyanobacterial species, respectively. This inference is strengthened by the presence of sequence signatures that are uniquely shared between Hsp60 homologues from α-purple bacteria and mitochondria on one hand, and the chloroplasts and cyanobacterial hsp60s on the other. Within the α-purple subdivision, species such as Rickettsia and Ehrlichia, which live intracellularly within eukaryotic cells, are indicated to be the closest relatives of mitochondrial Homologues, In the Hsp60 evolutionary tree, rooted using the Tcp-1 homologue, the order of branching of the major groups was as follows: Gram-positive bacteria — cyanobacteria and chloroplasts — chlamydiae and spirochaetes —β and γ-Gram-negative purple bacteria —α-purple bacteria — mitochondria. A similar branching order was observed independently in the Hsp10 tree. Multiple Hsp60 homologues, when present in a group of species, were found to be clustered together in the trees, indicating that they evolved by independent gene-duplication events. This review also considers in detail the evolutionary relationship between Hsp50 and Tcp-1 families of proteins based on two different models (viz. archaebacterial and chimeric) for the origin of eukaryotic cell nucleus. Some predictions of the chimeric model are also discussed.     

Chalcone synthase-like gene in the liverwort, <Emphasis Type="Italic">Marchantia paleacea</Emphasis> var. <Emphasis Type="Italic">diptera</Emphasis>

A chalcone synthase (CHS)-like gene, MpCHSLK1, was isolated from liverwort, Marchantia paleacea var. diptera. Phylogenetic analysis revealed that MpCHSLK1 is closely related to stilbene synthase of the whisk fern, Psilotum nudum. Southern blot analysis using an MpCHSLK1 probe revealed that the gene belongs to a small gene family. Northern blot analysis indicated that CHS-like genes were expressed in either the mother plants or photoautotrophic cells. In photoautotrophic cells, the CHS-like genes were expressed light-dependently, and this expression was completely inhibited by the photosynthetic electron transport inhibitor, DCMU.Abbreviations CHS Chalcone synthase - DCMU 3-(3,4-Dichlorophenyl)-1-1-dimethylurea - POR Protochlorophyllide oxidoreductase - STS Stilbene synthase     

Bacterial genome chimaerism and the origin of mitochondria

Many studies have sought to determine the origin and evolution of mitochondria. Although the Alphaproteobacteria are thought to be the closest relatives of the mitochondrial progenitor, there is dispute as to what its particular sister group is. Some have argued that mitochondria originated from ancestors of the order Rickettsiales, or more specifically of the Rickettsiaceae family, while others believe that ancestors of the family Rhodospirillaceae are also equally likely the progenitors. To resolve some of these disputes, sequence similarity searches and phylogenetic analyses were performed against mitochondria-related proteins in Saccharomyces cerevisiae. The 86 common matches of 5 Alphaproteobacteria (Rickettsia prowazekii, Rhodospirillum rubrum, Rhodopseudomonas palustris, Rhodobacter sphaeroides, and Ochrobactrum anthropi) to yeast mitochondrial proteins were distributed fairly evenly among the 5 species when sorted by highest identity or score. Moreover, exploratory phylogenetic analyses revealed that among these common matches, 44.19% (38) had branched most closely with O. anthropi, while only 34.88% (30) corresponded with Rickettsia prowazekii. More detailed phylogenetic analyses with additional Alphaproteobacteria and including genes from the mitochondria of Reclinomonas americana found matches of mitochondrial genes to those of members of the Rickettsiaceae, Anaplasmataceae, and Rhodospirillaceae families. The results support the idea that notable bacterial genome chimaerism has occurred en route to the formation of mitochondria.     

The <Emphasis Type="Italic">Chlamydomonas</Emphasis> genome reveals its secrets: chaperone genes and the potential roles of their gene products in the chloroplast

The first draft of the Chlamydomonas nuclear genome was searched for genes potentially encoding members of the five major chaperone families, Hsp100/Clp, Hsp90, Hsp70, Hsp60, the small heat shock proteins, and the Hsp70 and Cpn60 co-chaperones GrpE and Cpn10/20, respectively. This search yielded 34 potential (co-)chaperone genes, among them those 8 that have been reported earlier inChlamydomonas. These 34 genes encode all the (co-)chaperones that have been expected for the different compartments and organelles from genome searches in Arabidopsis, where 74 genes have been described to encode basically the same set of (co-)chaperones. Genome data from Arabidopsis and Chlamydomonas on the five major chaperone families are compared and discussed, with particular emphasis on chloroplast chaperones.     

The parasitic bacterium Wolbachia and the origin of the eukaryotic cell

The acquisition of endosymbiotic alphaproteobacteria that gave rise to mitochondria was one of the key events in the origin of eukaryotic cell. To reconstruct this process, it is important to analyze relationships that developed later between eukaryotes and other alphaproteobacteria. Wolbachia pipientis, a bacterium that inhabits cells of numerous terrestrial invertebrates and exerts diverse effects on its hosts, is used as a model. Although Wolbachia is similar to mitochondria in many important features (basic metabolism, small molecule membrane transport, envelope structure, etc.), their relationships with the nucleocytoplasm are different. Mitochondria import most of their required proteins from the nucleocytoplasm and are controlled by the nucleocytoplasmic regulatory systems. On the contrary, Wolbachia exports its proteins into the host’s cytoplasm, thus causing dramatic aberrations in the ontogeny and reproduction of the host. This difference may be due to the fact that most of the protomitochondrial genes had been transferred into the central (nuclear) genome at the early stages of the development of the endosymbiotic system, while Wolbachia genes were not transferred into the nucleus. This fits well with the previously suggested hypothesis that there was a period of rapid lateral gene transfer in the evolution of proto-eukaryotes; the acquisition of mitochondria took place during this period. Later, eukaryotes, and especially metazoans, developed powerful mechanisms for prevention of lateral gene transfer. Therefore, the genes of the newly acquired endosymbionts cannot be transferred into the central genome, and the endosymbionts retain the capacity for selfish evolution.     

The Evolutionary History of <Emphasis Type="Italic">Shigella</Emphasis> and Enteroinvasive <Emphasis Type="Italic">Escherichia coli</Emphasis> Revised

In Shigella and enteroinvasive Escherichia coli (EIEC), the etiologic agents of shigellosis in humans, the determinants responsible for entry of bacteria into and dissemination within epithelial cells are encoded by a virulence plasmid. To understand the evolution of the association between the virulence plasmid and the chromosome, we performed a phylogenetic analysis using the sequences of four chromosomal genes (trpA, trpB, pabB, and putP) and three virulence plasmid genes (ipaB, ipaD, and icsA) of a collection of 51 Shigella and EIEC strains. The phylogenetic tree derived from chromosomal genes showed a typical star phylogeny, indicating a fast diversification of Shigella and EIEC groups. Phylogenetic groups obtained from the chromosomal and plasmidic genes were similar, suggesting that the virulence plasmid and the chromosome share similar evolutionary histories. The few incongruences between the trees could be attributed to exchanges of fragments of different plasmids and not to the transfer of an entire plasmid. This indicates that the virulence plasmid was not transferred between the different Shigella and EIEC groups. These data support a model of evolution in which the acquisition of the virulence plasmid in an ancestral E. coli strain preceded the diversification by radiation of all Shigella and EIEC groups, which led to highly diversified but highly specialized pathogenic groups.     

Isolation and characterization of a cDNA encoding mitochondrial chaperonin 10 from Arabidopsis thaliana by functional complementation of an Escherichia coli groES mutant

Chaperonin (Cpn) is one of the molecular chaperones. Cpn10 is a co-factor of Cpn60, which regulates Cpn60-mediated protein folding. It is known that Cpn10 is located in mitochondria and chloroplasts in plant cells. The Escherichia coli homologue of Cpn10 is called GroES. A cDNA for the Cpn10 homologue was isolated from Arabidopsis thaliana by functional complementation of the E. coli groES mutant. The cDNA was 647 bp long and encoded a polypeptide of 98 amino acids. The deduced amino acid sequence showed approximately 50% identity to mammalian mitochondrial Cpn10s and 30% identity to GroES. A Northern blot analysis revealed that the mRNA for the Cpn10 homologue was expressed uniformly in various organs and was markedly induced by heat-shock treatment. The Cpn10 homologue was constitutively expressed in transgenic tobaccos. Immunogold and immunoblot analyses following the subcellular fractionation of leaves from transgenic tobaccos revealed that the Cpn10 homologue was localized in mitochondria and accumulated at a high level in transgenic tobaccos.     

Phylogenomic evidence for a myxococcal contribution to the mitochondrial fatty acid beta-oxidation

Background

The origin of eukaryotes remains a fundamental question in evolutionary biology. Although it is clear that eukaryotic genomes are a chimeric combination of genes of eubacterial and archaebacterial ancestry, the specific ancestry of most eubacterial genes is still unknown. The growing availability of microbial genomes offers the possibility of analyzing the ancestry of eukaryotic genomes and testing previous hypotheses on their origins.

Methodology/Principal Findings

Here, we have applied a phylogenomic analysis to investigate a possible contribution of the Myxococcales to the first eukaryotes. We conducted a conservative pipeline with homologous sequence searches against a genomic sampling of 40 eukaryotic and 357 prokaryotic genomes. The phylogenetic reconstruction showed that several eukaryotic proteins traced to Myxococcales. Most of these proteins were associated with mitochondrial lipid intermediate pathways, particularly enzymes generating reducing equivalents with pivotal roles in fatty acid β-oxidation metabolism. Our data suggest that myxococcal species with the ability to oxidize fatty acids transferred several genes to eubacteria that eventually gave rise to the mitochondrial ancestor. Later, the eukaryotic nucleocytoplasmic lineage acquired those metabolic genes through endosymbiotic gene transfer.

Conclusions/Significance

Our results support a prokaryotic origin, different from α-proteobacteria, for several mitochondrial genes. Our data reinforce a fluid prokaryotic chromosome model in which the mitochondrion appears to be an important entry point for myxococcal genes to enter eukaryotes.     

The evolutionary origin of red algae as deduced from the nuclear genes encoding cytosolic and chloroplast glyceraldehyde-3-phosphate dehydrogenases from Chondrus crispus

Algae are a heterogeneous group of photosynthetic eukaryotes traditionally separated into three major subdivisions: rhodophytes, chlorophytes, and chromophytes. The evolutionary origin of rhodophytes or red algae and their links to other photosynthetic and nonphotosynthetic eukaryotes have been a matter of much controversy and speculation. Here we present the first cDNAs of nuclear protein genes from red algae: Those encoding cytosolic and chloroplast glyceraldehyde-3-phosphate dehydrogenases (GAPDH) from Chondrus crispus. A phylogenetic analysis including GAPDH gene sequences from a number of eukaryotic taxa, cyanobacteria, and purple bacteria suggests that chloroplasts and rhodoplasts together form a monophyletic group of cyanobacterial descent and that rhodophytes separated from chlorophytes at about the same time as animals and fungi. The composite GAPDH tree further demonstrates that chloroplast and cytosolic GAPDH genes are closely related to their homologs in cyanobacteria and purple bacteria, respectively, the presumptive ancestors of chloroplasts and mitochondria, thereby firmly establishing the endosymbiotic origin of these nuclear genes and their fixation in eukaryotic cells before the rhodophyte/chlorophyte separation. The present data are in conflict with phylogenetic inferences based on plastid-encoded rbcL sequences supporting a polyphyletic origin of rhodoplasts and chloroplasts. Comparison of rbcL to GAPDH phylogenies suggests that rbcL trees may be misleading because they are composed of branches representing ancient duplicated (paralogous) genes. Correspondence to: R. Cerff     

Endosymbiosis,cell evolution,and speciation

In 1905, the Russian biologist C. Mereschkowsky postulated that plastids (e.g., chloroplasts) are the evolutionary descendants of endosymbiotic cyanobacteria-like organisms. In 1927, I. Wallin explicitly postulated that mitochondria likewise evolved from once free-living bacteria. Here, we summarize the history of these endosymbiotic concepts to their modern-day derivative, the “serial endosymbiosis theory”, which collectively expound on the origin of eukaryotic cell organelles (plastids, mitochondria) and subsequent endosymbiotic events. Additionally, we review recent hypotheses about the origin of the nucleus. Model systems for the study of “endosymbiosis in action” are also described, and the hypothesis that symbiogenesis may contribute to the generation of new species is critically assessed with special reference to the secondary and tertiary endosymbiosis (macroevolution) of unicellular eukaryotic algae.