Deep RNA sequencing reveals the smallest known mitochondrial micro exon in animals: The placozoan cox1 single base pair exon

Beauty in the beast - Placozoan biodiversity explored through molluscan predator genomics

The marine animal phylum Placozoa is characterized by a poorly explored cryptic biodiversity combined with very limited knowledge of their ecology. While placozoans are typically found as part of the epibenthos of coastal waters, known placozoan predators, namely small, shell-less sea slugs belonging to the family Rhodopidae (Mollusca: Gastropoda: Heterobranchia), inhabit the interstitium of seafloor sediment. In order to gain further insights into this predator-prey relationship and to expand our understanding of placozoan ecological niches, we screened publicly available whole-body metagenomic data from two rhodopid specimens collected from coastal sediments. Our analysis not only revealed the signatures of three previously unknown placozoan lineages in these sea slug samples but also enabled the assembly of three complete and two partial mitochondrial chromosomes belonging to four previously described placozoan genera, substantially extending the picture of placozoan biodiversity. Our findings further refine the molecular phylogeny of the Placozoa, corroborate the recently established taxonomic ranks in this phylum, and provide molecular support that known placozoan clades should be referred to as genera. We finally discuss the main finding of our study - the presence of placozoans in the sea floor sediment interstitium - in the context of their ecological, biological, and natural history implications.

Mitochondrially mediated RNA interference, a retrograde signaling system affecting nuclear gene expression

Several functional classes of short noncoding RNAs are involved in manifold regulatory processes in eukaryotes, including, among the best characterized, miRNAs. One of the most intriguing regulatory networks in the eukaryotic cell is the mito-nuclear crosstalk: recently, miRNA-like elements of mitochondrial origin, called smithRNAs, were detected in a bivalve species, Ruditapes philippinarum. These RNA molecules originate in the organelle but were shown in vivo to regulate nuclear genes. Since miRNA genes evolve easily de novo with respect to protein-coding genes, in the present work we estimate the probability with which a newly arisen smithRNA finds a suitable target in the nuclear transcriptome. Simulations with transcriptomes of 12 bivalve species suggest that this probability is high and not species specific: one in a hundred million (1 × 10-8) if five mismatches between the smithRNA and the 3' mRNA are allowed, yet many more are allowed in animals. We propose that novel smithRNAs may easily evolve as exaptation of the pre-existing mitochondrial RNAs. In turn, the ability of evolving novel smithRNAs may have played a pivotal role in mito-nuclear interactions during animal evolution, including the intriguing possibility of acting as speciation trigger.

Refining Mitochondrial Intron Classification With ERPIN: Identification Based on Conservation of Sequence Plus Secondary Structure Motifs

Mitochondrial genomes—in particular those of fungi—often encode genes with a large number of Group I and Group II introns that are conserved at both the sequence and the RNA structure level. They provide a rich resource for the investigation of intron and gene structure, self- and protein-guided splicing mechanisms, and intron evolution. Yet, the degree of sequence conservation of introns is limited, and the primary sequence differs considerably among the distinct intron sub-groups. It makes intron identification, classification, structural modeling, and the inference of gene models a most challenging and error-prone task—frequently passed on to an “expert” for manual intervention. To reduce the need for manual curation of intron structures and mitochondrial gene models, computational methods using ERPIN sequence profiles were initially developed in 2007. Here we present a refinement of search models and alignments using the now abundant publicly available fungal mtDNA sequences. In addition, we have tested in how far members of the originally proposed sub-groups are clearly distinguished and validated by our computational approach. We confirm clearly distinct mitochondrial Group I sub-groups IA1, IA3, IB3, IC1, IC2, and ID. Yet, IB1, IB2, and IB4 ERPIN models are overlapping substantially in predictions, and are therefore combined and reported as IB. We have further explored the conversion of our ERPIN profiles into covariance models (CM). Current limitations and prospects of the CM approach will be discussed.

Mitochondrial Genomic Landscape: A Portrait of the Mitochondrial Genome 40 Years after the First Complete Sequence

Notwithstanding the initial claims of general conservation, mitochondrial genomes are a largely heterogeneous set of organellar chromosomes which displays a bewildering diversity in terms of structure, architecture, gene content, and functionality. The mitochondrial genome is typically described as a single chromosome, yet many examples of multipartite genomes have been found (for example, among sponges and diplonemeans); the mitochondrial genome is typically depicted as circular, yet many linear genomes are known (for example, among jellyfish, alveolates, and apicomplexans); the chromosome is normally said to be “small”, yet there is a huge variation between the smallest and the largest known genomes (found, for example, in ctenophores and vascular plants, respectively); even the gene content is highly unconserved, ranging from the 13 oxidative phosphorylation-related enzymatic subunits encoded by animal mitochondria to the wider set of mitochondrial genes found in jakobids. In the present paper, we compile and describe a large database of 27,873 mitochondrial genomes currently available in GenBank, encompassing the whole eukaryotic domain. We discuss the major features of mitochondrial molecular diversity, with special reference to nucleotide composition and compositional biases; moreover, the database is made publicly available for future analyses on the MoZoo Lab GitHub page.

Recent and Ongoing Horizontal Transfer of Mitochondrial Introns Between Two Fungal Tree Pathogens

Two recently introduced fungal plant pathogens (Ceratocystis lukuohia and Ceratocystis huliohia) are responsible for Rapid ‘ōhi‘a Death (ROD) in Hawai‘i. Despite being sexually incompatible, the two pathogens often co-occur in diseased ‘ōhi‘a sapwood, where genetic interaction is possible. We sequenced and annotated 33 mitochondrial genomes of the two pathogens and related species, and investigated 35 total Ceratocystis mitogenomes. Ten mtDNA regions [one group I intron, seven group II introns, and two autonomous homing endonuclease (HE) genes] were heterogeneously present in C. lukuohia mitogenomes, which were otherwise identical. Molecular surveys with specific primers showed that the 10 regions had uneven geographic distribution amongst populations of C. lukuohia. Conversely, identical orthologs of each region were present in every studied isolate of C. huliohia regardless of geographical origin. Close relatives of C. lukuohia lacked or, rarely, had few and dissimilar orthologs of the 10 regions, whereas most relatives of C. huliohia had identical or nearly identical orthologs. Each region included or worked in tandem with HE genes or reverse transcriptase/maturases that could facilitate interspecific horizontal transfers from intron-minus to intron-plus alleles. These results suggest that the 10 regions originated in C. huliohia and are actively moving to populations of C. lukuohia, perhaps through transient cytoplasmic contact of hyphal tips (anastomosis) in the wound surface of ‘ōhi‘a trees. Such contact would allow for the transfer of mitochondria followed by mitochondrial fusion or cytoplasmic exchange of intron intermediaries, which suggests that further genomic interaction may also exist between the two pathogens.

HERMES: An improved method to test mitochondrial genome molecular synapomorphies among clades

Mitochondrial chromosomes have diversified among eukaryotes and many different architectures and features are now acknowledged for this genome. Here we present the improved HERMES index, which can measure and quantify the amount of molecular change experienced by mitochondrial genomes. We test the improved approach with ten molecular phylogenetic studies based on complete mitochondrial genomes, representing six bilaterian Phyla. In most cases, HERMES analysis spotted out clades or single species with peculiar molecular synapomorphies, allowing to identify phylogenetic and ecological patterns. The software presented herein handles linear, circular, and multi-chromosome genomes, thus widening the HERMES scope to the complete eukaryotic domain.

Mitochondrial Genome Evolution of Placozoans: Gene Rearrangements and Repeat Expansions

Placozoans, nonbilaterian animals with the simplest known metazoan bauplan, are currently classified into 20 haplotypes belonging to three genera, Polyplacotoma, Trichoplax, and Hoilungia. The latter two comprise two and five clades, respectively. In Trichoplax and Hoilungia, previous studies on six haplotypes belonging to four different clades have shown that their mtDNAs are circular chromosomes of 32–43 kb in size, which encode 12 protein-coding genes, 24 tRNAs, and two rRNAs. These mitochondrial genomes (mitogenomes) also show unique features rarely seen in other metazoans, including open reading frames (ORFs) of unknown function, and group I and II introns. Here, we report seven new mitogenomes, covering the five previously described haplotypes H2, H17, H19, H9, and H11, as well as two new haplotypes, H23 (clade III) and H24 (clade VII). The overall gene content is shared between all placozoan mitochondrial genomes, but genome sizes, gene orders, and several exon–intron boundaries vary among clades. Phylogenomic analyses strongly support a tree topology different from previous 16S rRNA analyses, with clade VI as the sister group to all other Hoilungia clades. We found small inverted repeats in all 13 mitochondrial genomes of the Trichoplax and Hoilungia genera and evaluated their distribution patterns among haplotypes. Because Polyplacotoma mediterranea (H0), the sister to the remaining haplotypes, has a small mitochondrial genome with few small inverted repeats and ORFs, we hypothesized that the proliferation of inverted repeats and ORFs substantially contributed to the observed increase in the size and GC content of the Trichoplax and Hoilungia mitochondrial genomes.

Variation in protein gene and intron content among land plant mitogenomes

The functional content of the mitochondrial genome (mitogenome) is highly diverse across eukaryotes. Among land plants, our understanding of the variation in mitochondrial gene and intron content is improving from concerted efforts to densely sample mitogenomes from diverse land plants. Here I review the current state of knowledge regarding the diversity in content of protein genes and introns in the mitogenomes of all major land plant lineages. Mitochondrial protein gene content is largely conserved among mosses and liverworts, but it varies substantially among and within other land plant lineages due to convergent losses of genes encoding ribosomal proteins and, to a lesser extent, genes for proteins involved in cytochrome c maturation and oxidative phosphorylation. Mitochondrial intron content is fairly stable within each major land plant lineage, but highly variable among lineages, resulting from occasional gains and many convergent losses over time. Trans-splicing has evolved dozens of times in various vascular plant lineages, particularly those with relatively higher rates of mitogenomic rearrangement. Across eukaryotes, mitochondrial protein gene and intron content has been shaped massive convergent evolution.

Gene fragmentation and RNA editing without borders: eccentric mitochondrial genomes of diplonemids

Diplonemids are highly abundant heterotrophic marine protists. Previous studies showed that their strikingly bloated mitochondrial genome is unique because of systematic gene fragmentation and manifold RNA editing. Here we report a comparative study of mitochondrial genome architecture, gene structure and RNA editing of six recently isolated, phylogenetically diverse diplonemid species. Mitochondrial gene fragmentation and modes of RNA editing, which include cytidine-to-uridine (C-to-U) and adenosine-to-inosine (A-to-I) substitutions and 3′ uridine additions (U-appendage), are conserved across diplonemids. Yet as we show here, all these features have been pushed to their extremes in the Hemistasiidae lineage. For example, Namystynia karyoxenos has its genes fragmented into more than twice as many modules than other diplonemids, with modules as short as four nucleotides. Furthermore, we detected in this group multiple A-appendage and guanosine-to-adenosine (G-to-A) substitution editing events not observed before in diplonemids and found very rarely elsewhere. With >1,000 sites, C-to-U and A-to-I editing in Namystynia is nearly 10 times more frequent than in other diplonemids. The editing density of 12% in coding regions makes Namystynia’s the most extensively edited transcriptome described so far. Diplonemid mitochondrial genome architecture, gene structure and post-transcriptional processes display such high complexity that they challenge all other currently known systems.

Polyplacotoma mediterranea is a new ramified placozoan species

The enigmatic phylum Placozoa is harboring an unknown number of cryptic species and has become a challenge for modern systematics. Only recently, a second species has been described [1], while the presence of more than a hundred additional species has been suggested [2]. The original placozoan species Trichoplax adhaerens[3], the second species Hoilungia hongkongensis[1] and all yet undescribed species are morphologically indistinguishable (i.e. no species diagnostic characters are available [4]). Here, we report on a new placozoan species, Polyplacotoma mediterranea gen. nov., spec. nov., which differs from other placozoans in its completely different morphological habitus, including long polytomous body branches and a maximum body length of more than 10 mm. Polyplacotoma mediterranea also necessitates a different view of placozoan mitochondrial genetics. P. mediterranea harbors a highly compact mitochondrial genome with overlapping mitochondrial tRNA and protein coding genes. Furthermore, the new species lacks typical placozoan features, including the cox1 micro exon and cox1 barcode intron. As phylogenetic analyses suggest a sister group relationship of P. mediterranea to all other placozoans, this new species may also be relevant for studies addressing the relationships at the base of the metazoan tree of life.

Putative group I introns in the eukaryote nuclear internal transcribed spacers

Group I introns are mobile genetic elements that interrupt genes encoding proteins and RNAs. In the rRNA operon, introns can insert in the small subunit (SSU) and large subunit (LSU) of a wide variety of protists and various prokaryotes, but they were never found in the ITS region. In this study, unusually long ITS regions of fungi and closely related unicellular organisms (Polychytrium aggregatum, Mitosporidium daphniae, Amoeboaphelidium occidentale and Nuclearia simplex) were analysed. While the insertion of repeats is responsible for long ITS in other eukaryotes, the increased size of the sequences analysed herein seems rather due to the presence of introns in ITS-1 or ITS-2. The identified insertions can be folded in secondary structures according to group I intron models, and they cluster within introns in conserved core-based phylogeny. In addition, for Mitosporidium, Amoeboaphelidium and Nuclearia, more conventional ITS-2 structures can be deduced once spacer introns are removed. Sequences of five shark species were also analysed for their structure and included in phylogeny because of unpublished work reporting introns in their ITS, obtaining congruent results. Overall, the data presented herein indicate that spacer regions may contain introns.

Trichoplax genomes reveal profound admixture and suggest stable wild populations without bisexual reproduction

The phylum Placozoa officially consists of only a single described species, Trichoplax adhaerens, although several lineages can be separated by molecular markers, geographical distributions and environmental demands. The placozoan 16S haplotype H2 (Trichoplax sp. H2) is the most robust and cosmopolitan lineage of placozoans found to date. In this study, its genome was found to be distinct but highly related to the Trichoplax adhaerens reference genome, for remarkably unique reasons. The pattern of variation and allele distribution between the two lineages suggests that both originate from a single interbreeding event in the wild, dating back at least several decades ago, and both seem not to have engaged in sexual reproduction since. We conclude that populations of certain placozoan haplotypes remain stable for long periods without bisexual reproduction. Furthermore, allelic variation within and between the two Trichoplax lineages indicates that successful bisexual reproduction between related placozoan lineages might serve to either counter accumulated negative somatic mutations or to cope with changing environmental conditions. On the other hand, enrichment of neutral or beneficial somatic mutations by vegetative reproduction, combined with rare sexual reproduction, could instantaneously boost genetic variation, generating novel ecotypes and eventually species.

Comparative genomics and the nature of placozoan species

Placozoans are a phylum of nonbilaterian marine animals currently represented by a single described species, Trichoplax adhaerens, Schulze 1883. Placozoans arguably show the simplest animal morphology, which is identical among isolates collected worldwide, despite an apparently sizeable genetic diversity within the phylum. Here, we use a comparative genomics approach for a deeper appreciation of the structure and causes of the deeply diverging lineages in the Placozoa. We generated a high-quality draft genome of the genetic lineage H13 isolated from Hong Kong and compared it to the distantly related T. adhaerens. We uncovered substantial structural differences between the two genomes that point to a deep genomic separation and provide support that adaptation by gene duplication is likely a crucial mechanism in placozoan speciation. We further provide genetic evidence for reproductively isolated species and suggest a genus-level difference of H13 to T. adhaerens, justifying the designation of H13 as a new species, Hoilungia hongkongensis nov. gen., nov. spec., now the second described placozoan species and the first in a new genus. Our multilevel comparative genomics approach is, therefore, likely to prove valuable for species distinctions in other cryptic microscopic animal groups that lack diagnostic morphological characters, such as some nematodes, copepods, rotifers, or mites.