Mitochondrial genomes of the ciliates

Highly Conserved Elements and Chromosome Structure Evolution in Mitochondrial Genomes in Ciliates

Recent phylogenetic analyses are incorporating ultraconserved elements (UCEs) and highly conserved elements (HCEs). Models of evolution of the genome structure and HCEs initially faced considerable algorithmic challenges, which gave rise to (often unnatural) constraints on these models, even for conceptually simple tasks such as the calculation of distance between two structures or the identification of UCEs. In our recent works, these constraints have been addressed with fast and efficient solutions with no constraints on the underlying models. These approaches have led us to an unexpected result: for some organelles and taxa, the genome structure and HCE set, despite themselves containing relatively little information, still adequately resolve the evolution of species. We also used the HCE identification to search for promoters and regulatory elements that characterize the functional evolution of the genome.

[Review of mtDNA in molecular evolution studies]

Mitochondria are old organelles found in most eukaryotic cells. Due to its rapid mutation ratio, mitochondrial DNA (mtDNA) has been widely used as a DNA marker in molecular studies and has long been suggested to undergo neutral evolution or purifying selection. Mitochondria produces 95% of the adenosine triphosphate (ATP) needed for locomotion, and heat for thermoregulation. Recent studies had found that mitochondria play critical roles in energy metabolism, and proved that functional constraints acting on mitochondria, due to energy metabolism and/or thermoregulation, influence the evolution of mtDNA. This review summarizes mitochondrial genome composition, evolution, and its applications in molecular evolution studies (reconstruction of species phylogenesis, the relationship between biological energy metabolism and mtDNA evolution, and the mtDNA codon reassignment influences the adaptation in different creatures).

Mitochondrial genome deletions and minicircles are common in lice (Insecta: Phthiraptera)

Background

The gene composition, gene order and structure of the mitochondrial genome are remarkably stable across bilaterian animals. Lice (Insecta: Phthiraptera) are a major exception to this genomic stability in that the canonical single chromosome with 37 genes found in almost all other bilaterians has been lost in multiple lineages in favour of multiple, minicircular chromosomes with less than 37 genes on each chromosome.

Results

Minicircular mt genomes are found in six of the ten louse species examined to date and three types of minicircles were identified: heteroplasmic minicircles which coexist with full sized mt genomes (type 1); multigene chromosomes with short, simple control regions, we infer that the genome consists of several such chromosomes (type 2); and multiple, single to three gene chromosomes with large, complex control regions (type 3). Mapping minicircle types onto a phylogenetic tree of lice fails to show a pattern of their occurrence consistent with an evolutionary series of minicircle types. Analysis of the nuclear-encoded, mitochondrially-targetted genes inferred from the body louse, Pediculus, suggests that the loss of mitochondrial single-stranded binding protein (mtSSB) may be responsible for the presence of minicircles in at least species with the most derived type 3 minicircles (Pediculus, Damalinia).

Conclusions

Minicircular mt genomes are common in lice and appear to have arisen multiple times within the group. Life history adaptive explanations which attribute minicircular mt genomes in lice to the adoption of blood-feeding in the Anoplura are not supported by this expanded data set as minicircles are found in multiple non-blood feeding louse groups but are not found in the blood-feeding genus Heterodoxus. In contrast, a mechanist explanation based on the loss of mtSSB suggests that minicircles may be selectively favoured due to the incapacity of the mt replisome to synthesize long replicative products without mtSSB and thus the loss of this gene lead to the formation of minicircles in lice.

An SSU rDNA barcoding approach to the diversity of marine interstitial cercozoans, including descriptions of four novel genera and nine novel species

Environmental DNA surveys have revealed a great deal of hidden diversity within the Cercozoa. An investigation into the biodiversity of heterotrophic flagellates in marine benthic habitats of British Columbia, Canada, demonstrated the presence of several undescribed taxa with morphological features that resemble the cercozoan genera Cryothecomonas and Protaspis. Nine novel species of marine interstitial cercozoans are described that are distributed into five genera, four of which are new. Phylogenetic analyses of small subunit rDNA sequences derived from two uncultured isolates of Protaspis obliqua and nine novel cercozoan species (within four novel genera) provided organismal anchors that helped establish the cellular identities of several different environmental sequence clades. These data, however, also showed that the rarity of distinctive morphological features in cryomonads, and other groups of cercozoans, makes the identification and systematics of the group very difficult. Therefore, a DNA barcoding approach was applied as a diagnostic tool for species delimitation that used a 618 bp region at the 5' end of the SSU rDNA sequence. Nucleotide sequence analysis of this region showed high intergeneric sequence divergences of about 7% and very low intraspecific sequence divergences of 0-0.5%; phylogenetic analyses inferred from this barcoding region showed very similar tree topologies to those inferred from the full-length sequence of the gene. Overall, this study indicated that the 618 bp barcoding region of SSU rDNA sequences is a useful molecular signature for understanding the biodiversity and interrelationships of marine benthic cercozoans.

Barcoding ciliates: a comprehensive study of 75 isolates of the genus Tetrahymena

The mitochondrial cytochrome-coxidase subunit 1 (cox1) gene has been proposed as a DNA barcode to identify animal species. To test the applicability of thecox1gene in identifying ciliates, 75 isolates of the genusTetrahymenaand three non-Tetrahymenaciliates that are close relatives ofTetrahymena,Colpidium campylum,Colpidium colpodaandGlaucoma chattoni, were selected. All tetrahymenines of unproblematic species could be identified to the species level using 689 bp of thecox1sequence, with about 11 % interspecific sequence divergence. Intraspecific isolates ofTetrahymena borealis,Tetrahymena lwoffi,Tetrahymena patulaandTetrahymena thermophilacould be identified by theircox1sequences, showing <0.65 % intraspecific sequence divergence. In addition, isolates of these species were clustered together on acox1neighbour-joining (NJ) tree. However, strains identified asTetrahymena pyriformisandTetrahymena tropicalisshowed high intraspecific sequence divergence values of 5.01 and 9.07 %, respectively, and did not cluster together on acox1NJ tree. This may indicate the presence of cryptic species. The mean interspecific sequence divergence ofTetrahymenawas about 11 times greater than the mean intraspecific sequence divergence, and this increased to 58 times when all isolates of species with high intraspecific sequence divergence were excluded. This result is similar to DNA barcoding studies on animals, indicating that congeneric sequence divergences are an order of magnitude greater than conspecific sequence divergences. Our analysis also demonstrated low sequence divergences of <1.0 % between some isolates ofT. pyriformisandTetrahymena setosaon the one hand and some isolates ofTetrahymena furgasoniandT. lwoffion the other, suggesting that the latter species in each pair is a junior synonym of the former. Overall, our study demonstrates the feasibility of using the mitochondrialcox1gene as a taxonomic marker for ‘barcoding’ and identifyingTetrahymenaspecies and some other ciliated protists.

The highly reduced and fragmented mitochondrial genome of the early-branching dinoflagellate Oxyrrhis marina shares characteristics with both apicomplexan and dinoflagellate mitochondrial genomes

The mitochondrial genome and the expression of the genes within it have evolved to be highly unusual in several lineages. Within alveolates, apicomplexans and dinoflagellates share the most reduced mitochondrial gene content on record, but differ from one another in organisation and function. To clarify how these characteristics originated, we examined mitochondrial genome form and expression in a key lineage that arose close to the divergence of apicomplexans and dinoflagellates, Oxyrrhis marina. We show that Oxyrrhis is a basal member of the dinoflagellate lineage whose mitochondrial genome has some unique characteristics while sharing others with apicomplexans or dinoflagellates. Specifically, Oxyrrhis has the smallest gene complement known, with several rRNA fragments and only two protein coding genes, cox1 and a cob-cox3 fusion. The genome appears to be highly fragmented, like that of dinoflagellates, but genes are frequently arranged as tandem copies, reminiscent of the repeating nature of the Plasmodium genome. In dinoflagellates and Oxyrrhis, genes are found in many arrangements, but the Oxyrrhis genome appears to be more structured, since neighbouring genes or gene fragments are invariably the same: cox1 and the cob-cox3 fusion were never found on the same genomic fragment. Analysing hundreds of cDNAs for both genes and circularized mRNAs from cob-cox3 showed that neither uses canonical start or stop codons, although a UAA terminator is created in the cob-cox3 fusion mRNA by post-transcriptional oligoadenylation. mRNAs from both genes also use a novel 5' oligo(U) cap. Extensive RNA editing is characteristic of dinoflagellates, but we find no editing in Oxyrrhis. Overall, the combination of characteristics found in the Oxyrrhis genome allows us to plot the sequence of many events that led to the extreme organisation of apicomplexan and dinoflalgellate mitochondrial genomes.

Fragmentation of mitochondrial large subunit rRNA in the dinoflagellate Alexandrium catenella and the evolution of rRNA structure in alveolate mitochondria

Extensive investigations on apicomplexan mitochondria, such as those of Plasmodium falciparum, revealed that ribosomal RNAs (rRNAs) are fragmented into multiple short pieces. In this study, we isolated three mitochondrial large subunit rRNA (mtLSU rRNA) fragments from the dinoflagellate Alexandrium catenella. A piece of mtLSU rRNA that possesses high sequence similarity to the P. falciparum LSU rRNA E fragment was identified in a 1.7-kbp mitochondrial (mt) DNA clone. We further confirmed that the A. catenella "E-like" fragment is indeed transcriptionally active and that the transcript could form appropriate RNA secondary structures. In addition, we identified expression of two additional rRNA fragments with sequence similarities to P. falciparum F and G fragments. Notably, the 1.7-kbp mt DNA clone contains only one of the three rRNA fragments identified in this study, suggesting that the rRNA fragments are separately encoded in the A. catenella mt genome. Given the sister relationship between apicomplexa and dinoflagellates in eukaryote phylogeny, it is most parsimonious to assume that the mt rRNA fragmentation was established prior to the separation of the two protist groups. However, current sequence data on dinoflagellate mitochondria are insufficient to reject the alternative scenario, in which the rRNA fragmentation evolved independently in apicomplexan and dinoflagellate mitochondria.

Mitochondria of Protists

Over the past several decades, our knowledge of the origin and evolution of mitochondria has been greatly advanced by determination of complete mitochondrial genome sequences. Among the most informative mitochondrial genomes have been those of protists (primarily unicellular eukaryotes), some of which harbor the most gene-rich and most eubacteria-like mitochondrial DNAs (mtDNAs) known. Comparison of mtDNA sequence data has provided insights into the radically diverse trends in mitochondrial genome evolution exhibited by different phylogenetically coherent groupings of eukaryotes, and has allowed us to pinpoint specific protist relatives of the multicellular eukaryotic lineages (animals, plants, and fungi). This comparative genomics approach has also revealed unique and fascinating aspects of mitochondrial gene expression, highlighting the mitochondrion as an evolutionary playground par excellence.

Widespread and extensive editing of mitochondrial mRNAS in dinoflagellates

We report evidence of extensive substitutional editing of mitochondrial mRNAs in the dinoflagellate species Pfiesteria piscicida, Prorocentrum minimum and Crypthecodinium cohnii, based on a comparison of genomic and corresponding cDNA sequences determined for two mitochondrial DNA-encoded genes, cox1 (cytochrome oxidase subunit 1) and cob (apocytochrome b). In the cox1 mRNA, we identify 72 substitutions at 40 sites in 39 codons, whereas in cob mRNA, we infer 86 editing events at 51 sites in 48 codons. Editing, which takes place in distinct clusters, changes approximately 2% of the total sequence, occurs predominantly at first and second positions of codons, and involves mostly (but not exclusively) A-->G (47%), U-->C (23%) and C-->U (17%) substitutions. In all but four of the 158 cases, editing changes the identity of the specified amino acid. At 21 (cox1) and 26 (cob) sites, the same nucleotide change is observed at the same position in at least two of the species investigated. At about one-third of the sites, editing results in an amino acid change that increases similarity between the dinoflagellate Cox1 and Cob sequences and their homologs in other organisms; presumably editing at these sites is of particular functional significance. Overall, about half of the editing events either maintain or increase similarity between the dinoflagellate protein sequences and their non-dinoflagellate homologs, while a further one-third of the alterations are "dinoflagellate-specific" (i.e. they involve a change to an amino acid residue selectively conserved in at least two of the dinoflagellate species at a given position). The nature, pattern and phylogenetic distribution of the inferred edits implies either that more than one type of previously described editing process operates on a given transcript in dinoflagellate mitochondria, or that a mechanistically unique type of mitochondrial mRNA editing has evolved within the dinoflagellate lineage.

Phage N15 telomere resolution. Target requirements for recognition and processing by the protelomerase

The Escherichia coli prophage N15 exists as a linear DNA molecule with covalently closed ends. Purified N15 protelomerase TelN is the only protein required to convert circular DNA substrates to the linear form with hairpin termini. Within the center of the telomerase occupancy site tos, the target for TelN is the 56-bp telRL consisting of the central 22-bp palindrome telO and two 14-bp flanking inverted sequence repetitions. DNase I footprinting of TelN-telRL complexes shows a segment of approximately 50 bp protected by TelN. Surface plasmon resonance studies demonstrate that this extended footprint is caused by two TelN molecules bound to telRL. Stable TelN-target DNA complexes are achieved with telRL; however, the additional sequences of tos stabilize the TelN-target complexes. TelO alone is not sufficient for specific stable complex formation. However, processing can occur, i.e. generation of the linear covalently closed DNA. Within the context of telRL, sequences of telO are involved in specific TelN-telRL complex formation, in processing itself, and/or in recognition of the processing site. The sequence of the central (CG)(3) within telO that is part of a 14-bp stretch proposed to have Z-DNA conformation is essential for processing but not for formation of specific TelN-telRL complexes. The concerted action of both TelN molecules at the target site is the basis for telomere resolution. Capturing of reaction intermediates demonstrates that TelN binds covalently to the 3'-phosphoryl of the cleaved strands.

Genomic sequence and analysis of the atypical temperate bacteriophage N15

N15 is a temperate bacteriophage that forms stable lysogens in Escherichia coli. While its virion is morphologically very similar to phage lambda and its close relatives, it is unusual in that the prophage form replicates autonomously as a linear DNA molecule with closed hairpin telomeres. Here, we describe the genomic architecture of N15, and its global pattern of gene expression, which reveal that N15 contains several plasmid-derived genes that are expressed in N15 lysogens. The tel site, at which processing occurs to form the prophage ends is close to the center of the genome in a similar location to that occupied by the attachment site, attP, in lambda and its relatives and defines the boundary between the left and right arms. The left arm contains a long cluster of structural genes that are closely related to those of the lambda-like phages, but also includes homologs of umuD', which encodes a DNA polymerase accessory protein, and the plasmid partition genes, sopA and sopB. The right arm likewise contains a mixture of apparently phage- and plasmid-derived genes including genes encoding plasmid replication functions, a phage repressor, a transcription antitermination system, as well as phage host cell lysis genes and two putative DNA methylases. The unique structure of the N15 genome suggests that the large global population of bacteriophages may exhibit a much greater diversity of genomic architectures than was previously recognized.

Complete sequence of the mitochondrial genome of Tetrahymena pyriformis and comparison with Paramecium aurelia mitochondrial DNA

We report the complete nucleotide sequence of the Tetrahymena pyriformis mitochondrial genome and a comparison of its gene content and organization with that of Paramecium aurelia mtDNA. T. pyriformis mtDNA is a linear molecule of 47,172 bp (78.7 % A+T) excluding telomeric sequences (identical tandem repeats of 31 bp at each end of the genome). In addition to genes encoding the previously described bipartite small and large subunit rRNAs, the T. pyriformis mitochondrial genome contains 21 protein-coding genes that are clearly homologous to genes of defined function in other mtDNAs, including one (yejR) that specifies a component of a cytochrome c biogenesis pathway. As well, T. pyriformis mtDNA contains 22 open reading frames of unknown function larger than 60 codons, potentially specifying proteins ranging in size from 74 to 1386 amino acid residues. A total of 13 of these open reading frames ("ciliate-specific") are found in P. aurelia mtDNA, whereas the remaining nine appear to be unique to T. pyriformis; however, of the latter, five are positionally equivalent and of similar size in the two ciliate mitochondrial genomes, suggesting they may also be homologous, even though this is not evident from sequence comparisons. Only eight tRNA genes encoding seven distinct tRNAs are found in T. pyriformis mtDNA, formally confirming a long-standing proposal that most T. pyriformis mitochondrial tRNAs are nucleus-encoded species imported from the cytosol. Atypical features of mitochondrial gene organization and expression in T. pyriformis mtDNA include split and rearranged large subunit rRNA genes, as well as a split nad1 gene (encoding subunit 1 of NADH dehydrogenase of respiratory complex I) whose two segments are located on and transcribed from opposite strands, as is also the case in P. aurelia. Gene content and arrangement are very similar in T. pyriformis and P. aurelia mtDNAs, the two differing by a limited number of duplication, inversion and rearrangement events. Phylogenetic analyses using concatenated sequences of several mtDNA-encoded proteins provide high bootstrap support for the monophyly of alveolates (ciliates, dinoflagellates and apicomplexans) and slime molds.

Mitochondrial genome evolution and the origin of eukaryotes

Recent results from ancestral (minimally derived) protists testify to the tremendous diversity of the mitochondrial genome in various eukaryotic lineages, but also reinforce the view that mitochondria, descendants of an endosymbiotic alpha-Proteobacterium, arose only once in evolution. The serial endosymbiosis theory, currently the most popular hypothesis to explain the origin of mitochondria, postulates the capture of an alpha-proteobacterial endosymbiont by a nucleus-containing eukaryotic host resembling extant amitochondriate protists. New sequence data have challenged this scenario, instead raising the possibility that the origin of the mitochondrion was coincident with, and contributed substantially to, the origin of the nuclear genome of the eukaryotic cell. Defining more precisely the alpha-proteobacterial ancestry of the mitochondrial genome, and the contribution of the endosymbiotic event to the nuclear genome, will be essential for a full understanding of the origin and evolution of the eukaryotic cell as a whole.

Mitochondrial evolution

The serial endosymbiosis theory is a favored model for explaining the origin of mitochondria, a defining event in the evolution of eukaryotic cells. As usually described, this theory posits that mitochondria are the direct descendants of a bacterial endosymbiont that became established at an early stage in a nucleus-containing (but amitochondriate) host cell. Gene sequence data strongly support a monophyletic origin of the mitochondrion from a eubacterial ancestor shared with a subgroup of the alpha-Proteobacteria. However, recent studies of unicellular eukaryotes (protists), some of them little known, have provided insights that challenge the traditional serial endosymbiosis-based view of how the eukaryotic cell and its mitochondrion came to be. These data indicate that the mitochondrion arose in a common ancestor of all extant eukaryotes and raise the possibility that this organelle originated at essentially the same time as the nuclear component of the eukaryotic cell rather than in a separate, subsequent event.

Genome structure and gene content in protist mitochondrial DNAs

Although the collection of completely sequenced mitochondrial genomes is expanding rapidly, only recently has a phylogenetically broad representation of mtDNA sequences from protists (mostly unicellular eukaryotes) become available. This review surveys the 23 complete protist mtDNA sequences that have been determined to date, commenting on such aspects as mitochondrial genome structure, gene content, ribosomal RNA, introns, transfer RNAs and the genetic code and phylogenetic implications. We also illustrate the utility of a comparative genomics approach to gene identification by providing evidence that orfB in plant and protist mtDNAs is the homolog of atp8 , the gene in animal and fungal mtDNA that encodes subunit 8 of the F0portion of mitochondrial ATP synthase. Although several protist mtDNAs, like those of animals and most fungi, are seen to be highly derived, others appear to be have retained a number of features of the ancestral, proto-mitochondrial genome. Some of these ancestral features are also shared with plant mtDNA, although the latter have evidently expanded considerably in size, if not in gene content, in the course of evolution. Comparative analysis of protist mtDNAs is providing a new perspective on mtDNA evolution: how the original mitochondrial genome was organized, what genes it contained, and in what ways it must have changed in different eukaryotic phyla.

The cytochrome oxidase subunit 1 gene (cox1) from the dinoflagellate, Crypthecodinium cohnii

To date, no genes have been characterized from dinoflagellate mitochondrial DNA. Here we present the complete sequence of the gene (cox1) encoding subunit 1 of cytochrome c oxidase in the dinoflagellate, Crypthecodinium cohnii. Analysis of nucleotide and deduced amino acid sequences predicts a protein of 523 amino acids that is translated using universal initiation, stop and tryptophan codons. COX1 amino acid identity and phylogenetic tree analyses strongly support a close evolutionary relationship between dinoflagellates and apicomplexans; however, inclusion of the ciliates in this clade is less well supported, a result likely due to the highly derived nature of ciliate COX1 sequences.

Primary sequence and post-transcriptional modification pattern of an unusual mitochondrial tRNA(Met) from Tetrahymena pyriformis

In a previous investigation of the rDNA region in Tetrahymena pyriformis mitochondrial DNA, we identified a putative tRNA(Met) gene [Heinonen et al. (1987) J. Biol. Chem. 262, 2879-2887]. On the basis of Northern hybridization analyses, we suggested that this gene is expressed, even though the resulting tRNA would be unusually small and have an atypical dihydrouridine stem-loop domain. We report here the complete nucleotide sequence and post-transcriptional modification pattern of this tRNA(Met), confirming its predicted primary structure and supporting the view that this structurally aberrant species functions in translation in T. pyriformis mitochondria.

A conserved heptamer motif for ribosomal RNA transcription termination in animal mitochondria

A search of sequence data bases for a tridecamer transcription termination signal, previously described in human mtDNA as being responsible for the accumulation of mitochondrial ribosomal RNAs (rRNAs) in excess over the rest of mitochondrial genes, has revealed that this termination signal occurs in equivalent positions in a wide variety of organisms from protozoa to mammals. Due to the compact organization of the mtDNA, the tridecamer motif usually appears as part of the 3' adjacent gene sequence. Because in phylogenetically widely separated organisms the mitochondrial genome has experienced many rearrangements, it is interesting that its occurrence near the 3' end of the large rRNA is independent of the adjacent gene. The tridecamer sequence has diverged in phylogenetically widely separated organisms. Nevertheless, a well-conserved heptamer--TGGCAGA, the mitochondrial rRNA termination box--can be defined. Although extending the experimental evidence of its role as a transcription termination signal in humans will be of great interest, its evolutionary conservation strongly suggests that mitochondrial rRNA transcription termination could be a widely conserved mechanism in animals. Furthermore, the conservation of a homologous tridecamer motif in one of the last 3' secondary loops of nonmitochondrial 23S-like rRNAs suggests that the role of the sequence has changed during mitochondrial evolution.