Mitochondrion-to-nucleus communication mediated by RNA export: a survey of potential mechanisms and players across eukaryotes.

The nucleus interacts with the other organelles to perform essential functions of the eukaryotic cell. Mitochondria have their own genome and communicate back to the nucleus in what is known as mitochondrial retrograde response. Information is transferred to the nucleus in many ways, leading to wide-ranging changes in nuclear gene expression and culminating with changes in metabolic, regulatory or stress-related pathways. RNAs are emerging molecules involved in this signalling. RNAs encode precise information and are involved in highly target-specific signalling, through a wide range of processes known as RNA interference. RNA-mediated mitochondrial retrograde response requires these molecules to exit the mitochondrion, a process that is still mostly unknown. We suggest that the proteins/complexes translocases of the inner membrane, polynucleotide phosphorylase, mitochondrial permeability transition pore, and the subunits of oxidative phosphorylation complexes may be responsible for RNA export.

The Evolution and Characterization of the RNA Interference Pathways in Lophotrochozoa.

In animals, three main RNA interference mechanisms have been described so far, which respectively maturate three types of small noncoding RNAs (sncRNAs): miRNAs, piRNAs, and endo-siRNAs. The diversification of these mechanisms is deeply linked with the evolution of the Argonaute gene superfamily since each type of sncRNA is typically loaded by a specific Argonaute homolog. Moreover, other protein families play pivotal roles in the maturation of sncRNAs, like the DICER ribonuclease family, whose DICER1 and DICER2 paralogs maturate respectively miRNAs and endo-siRNAs. Within Metazoa, the distribution of these families has been only studied in major groups, and there are very few data for clades like Lophotrochozoa. Thus, we here inferred the evolutionary history of the animal Argonaute and DICER families including 43 lophotrochozoan species. Phylogenetic analyses along with newly sequenced sncRNA libraries suggested that in all Trochozoa, the proteins related to the endo-siRNA pathway have been lost, a part of them in some phyla (i.e. Nemertea, Bryozoa, Entoprocta), while all of them in all the others. On the contrary, early diverging phyla, Platyhelminthes and Syndermata, showed a complete endo-siRNA pathway. On the other hand, miRNAs were revealed the most conserved and ubiquitous mechanism of the metazoan RNA interference machinery, confirming their pivotal role in animal cell regulation.

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.

Multi-tissue RNA-Seq Analysis and Long-read-based Genome Assembly Reveal Complex Sex-specific Gene Regulation and Molecular Evolution in the Manila Clam.

The molecular factors and gene regulation involved in sex determination and gonad differentiation in bivalve molluscs are unknown. It has been suggested that doubly uniparental inheritance (DUI) of mitochondria may be involved in these processes in species such as the ubiquitous and commercially relevant Manila clam, Ruditapes philippinarum. We present the first long-read-based de novo genome assembly of a Manila clam, and a RNA-Seq multi-tissue analysis of 15 females and 15 males. The highly contiguous genome assembly was used as reference to investigate gene expression, alternative splicing, sequence evolution, tissue-specific co-expression networks, and sexual contrasting SNPs. Differential expression (DE) and differential splicing (DS) analyses revealed sex-specific transcriptional regulation in gonads, but not in somatic tissues. Co-expression networks revealed complex gene regulation in gonads, and genes in gonad-associated modules showed high tissue specificity. However, male gonad-associated modules showed contrasting patterns of sequence evolution and tissue specificity. One gene set was related to the structural organization of male gametes and presented slow sequence evolution but high pleiotropy, whereas another gene set was enriched in reproduction-related processes and characterized by fast sequence evolution and tissue specificity. Sexual contrasting SNPs were found in genes overrepresented in mitochondrial-related functions, providing new candidates for investigating the relationship between mitochondria and sex in DUI species. Together, these results increase our understanding of the role of DE, DS, and sequence evolution of sex-specific genes in an understudied taxon. We also provide resourceful genomic data for studies regarding sex diagnosis and breeding in bivalves.

Mito-nuclear coevolution and phylogenetic artifacts: the case of bivalve mollusks.

Mito-nuclear phylogenetic discordance in Bivalvia is well known. In particular, the monophyly of Amarsipobranchia (Heterodonta + Pteriomorphia), retrieved from mitochondrial markers, contrasts with the monophyly of Heteroconchia (Heterodonta + Palaeoheterodonta), retrieved from nuclear markers. However, since oxidative phosphorylation nuclear markers support the Amarsipobranchia hypothesis instead of the Heteroconchia one, interacting subunits of the mitochondrial complexes ought to share the same phylogenetic signal notwithstanding the genomic source, which is different from the signal obtained from other nuclear markers. This may be a clue of coevolution between nuclear and mitochondrial genes. In this work we inferred the phylogenetic signal from mitochondrial and nuclear oxidative phosphorylation markers exploiting different phylogenetic approaches and added two more datasets for comparison: genes of the glycolytic pathway and genes related to the biogenesis of regulative small noncoding RNAs. All trees inferred from mitochondrial and nuclear subunits of the mitochondrial complexes support the monophyly of Amarsipobranchia, regardless of the phylogenetic pipeline. However, not every single marker agrees with this topology: this is clearly visible in nuclear subunits that do not directly interact with the mitochondrial counterparts. Overall, our data support the hypothesis of a coevolution between nuclear and mitochondrial genes for the oxidative phosphorylation. Moreover, we suggest a relationship between mitochondrial topology and different nucleotide composition between clades, which could be associated to the highly variable gene arrangement in Bivalvia.

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.

Clues of in vivo nuclear gene regulation by mitochondrial short non-coding RNAs.

Gene expression involves multiple processes, from transcription to translation to the mature, functional peptide, and it is regulated at multiple levels. Small RNA molecules are known to bind RNA messengers affecting their fate in the cytoplasm (a process generically termed 'RNA interference'). Such small regulatory RNAs are well-known to be originated from the nuclear genome, while the role of mitochondrial genome in RNA interference was largely overlooked. However, evidence is growing that mitochondrial DNA does provide the cell a source of interfering RNAs. Small mitochondrial highly transcribed RNAs (smithRNAs) have been proposed to be transcribed from the mitochondrion and predicted to regulate nuclear genes. Here, for the first time, we show in vivo clues of the activity of two smithRNAs in the Manila clam, Ruditapes philippinarum. Moreover, we show that smithRNAs are present and can be annotated in representatives of the three main bilaterian lineages; in some cases, they were already described and assigned to a small RNA category (e.g., piRNAs) given their biogenesis, while in other cases their biogenesis remains unclear. If mitochondria may affect nuclear gene expression through RNA interference, this opens a plethora of new possibilities for them to interact with the nucleus and makes metazoan mitochondrial DNA a much more complex genome than previously thought.

Natural Heteroplasmy and Mitochondrial Inheritance in Bivalve Molluscs.

Heteroplasmy is the presence of more than one type of mitochondrial genome within an individual, a condition commonly reported as unfavorable and affecting mitonuclear interactions. So far, no study has investigated heteroplasmy at protein level, and whether it occurs within tissues, cells, or even organelles. The only known evolutionarily stable and natural heteroplasmic system in Metazoa is the Doubly Uniparental Inheritance (DUI)-reported so far in ∼100 bivalve species-in which two mitochondrial lineages are present: one transmitted through eggs (F-type) and the other through sperm (M-type). Because of such segregation, mitochondrial oxidative phosphorylation proteins reach a high amino acid sequence divergence (up to 52%) between the two lineages in the same species. Natural heteroplasmy coupled with high sequence divergence between F- and M-type proteins provides a unique opportunity to study their expression and assess the level and extent of heteroplasmy. Here, for the first time, we immunolocalized F- and M-type variants of three mitochondrially-encoded proteins in the DUI species Ruditapes philippinarum, in germline and somatic tissues at different developmental stages. We found heteroplasmy at organelle level in undifferentiated germ cells of both sexes, and in male soma, whereas gametes were homoplasmic: eggs for the F-type and sperm for the M-type. Thus, during gametogenesis, only the sex-specific mitochondrial variant is maintained, likely due to a process of meiotic drive. We examine the implications of our results for DUI proposing a revised model, and we discuss interactions of mitochondria with germ plasm and their role in germline development. Molecular and phylogenetic evidence suggests that DUI evolved from the common Strictly Maternal Inheritance, so the two systems likely share the same underlying molecular mechanism, making DUI a useful system for studying mitochondrial biology.

The Economic Burden and Impact on Quality of Life of Herpes Zoster and Postherpetic Neuralgia in Individuals Aged 50 Years or Older in Italy.

BACKGROUND: To estimate the health care resource utilization, costs, and impact on quality of life (QoL) of herpes zoster (HZ) and postherpetic neuralgia (PHN) in adults aged ≥50 years in Italy. METHODS: This was a prospective, observational, multicenter, community physician-based surveillance study (NCT01772160) in Italy. Health-related QoL data were collected using the EuroQoL-5 Dimension (EQ-5D) and Zoster Brief Pain Inventory (ZBPI) questionnaires. Both questionnaires were assessed at days 0 (HZ rash onset), 15, 30, 60, and 90 for all patients, and monthly thereafter for patients who developed PHN. Resource utilization was recorded for 3 months post-HZ onset and 9 months for PHN patients. Costs from both payer and societal perspectives were estimated and were composed of direct medical costs (general practitioner/specialist visits, procedures, hospitalizations, medications), work loss by patient/caregiver, and transport costs. RESULTS: A total of 391 patients with HZ were included, of whom 40 developed PHN. The mean ZBPI worst pain score was 5.7 at day 0, reducing to 2.6 at day 30 and 0.7 by day 90. Patients with PHN had a mean worst pain score of 5.7 at day 90. We estimated an overall disutility associated with HZ of 0.134. The mean cost per HZ patient from a payer/societal perspective was €153/€298, respectively, and the mean cost per HZ patients who developed PHN was €176/€426, respectively. CONCLUSIONS: HZ is associated with impaired QoL and substantial health care resource use, highlighting the need for preventive strategies. This could reduce the disease burden for the patient and health care system. CLINICALTRIALSGOV STUDY REGISTRY: NCT01772160.

Comparative Transcriptomics in Two Bivalve Species Offers Different Perspectives on the Evolution of Sex-Biased Genes.

Comparative genomics has become a central tool for evolutionary biology, and a better knowledge of understudied taxa represents the foundation for future work. In this study, we characterized the transcriptome of male and female mature gonads in the European clam Ruditapes decussatus, compared with that in the Manila clam Ruditapes philippinarum providing, for the first time in bivalves, information about transcription dynamics and sequence evolution of sex-biased genes. In both the species, we found a relatively low number of sex-biased genes (1,284, corresponding to 41.3% of the orthologous genes between the two species), probably due to the absence of sexual dimorphism, and the transcriptional bias is maintained in only 33% of the orthologs. The dN/dS is generally low, indicating purifying selection, with genes where the female-biased transcription is maintained between the two species showing a significantly higher dN/dS. Genes involved in embryo development, cell proliferation, and maintenance of genome stability show a faster sequence evolution. Finally, we report a lack of clear correlation between transcription level and evolutionary rate in these species, in contrast with studies that reported a negative correlation. We discuss such discrepancy and call into question some methodological approaches and rationales generally used in this type of comparative studies.

Genome Survey of the Freshwater Mussel Venustaconcha ellipsiformis (Bivalvia: Unionida) Using a Hybrid De Novo Assembly Approach.

Freshwater mussels (Bivalvia: Unionida) serve an important role as aquatic ecosystem engineers but are one of the most critically imperilled groups of animals. Here, we used a combination of sequencing strategies to assemble and annotate a draft genome of Venustaconcha ellipsiformis, which will serve as a valuable genomic resource given the ecological value and unique "doubly uniparental inheritance" mode of mitochondrial DNA transmission of freshwater mussels. The genome described here was obtained by combining high-coverage short reads (65× genome coverage of Illumina paired-end and 11× genome coverage of mate-pairs sequences) with low-coverage Pacific Biosciences long reads (0.3× genome coverage). Briefly, the final scaffold assembly accounted for a total size of 1.54 Gb (366,926 scaffolds, N50 = 6.5 kb, with 2.3% of "N" nucleotides), representing 86% of the predicted genome size of 1.80 Gb, while over one third of the genome (37.5%) consisted of repeated elements and >85% of the core eukaryotic genes were recovered. Given the repeated genetic bottlenecks of V. ellipsiformis populations as a result of glaciations events, heterozygosity was also found to be remarkably low (0.6%), in contrast to most other sequenced bivalve species. Finally, we reassembled the full mitochondrial genome and found six polymorphic sites with respect to the previously published reference. This resource opens the way to comparative genomics studies to identify genes related to the unique adaptations of freshwater mussels and their distinctive mitochondrial inheritance mechanism.

Lose it or keep it: (how bivalves can provide) insights into mitochondrial inheritance mechanisms.

The strictly maternal inheritance (SMI) is a pattern of mitochondrial inheritance observed across the whole animal kingdom. However, some interesting exceptions are known for the class Bivalvia, in which several species show an unusual pattern called doubly uniparental inheritance (DUI) whose outcome is a heteroplasmic pool of mtDNA in males. Even if DUI has been studied for long, its molecular basis has not been established yet. The aim of this work is to select classes of proteins known to be involved in the maintenance of SMI and to compare their features in two clam species differing for their mitochondrial inheritance mechanism, that is, the SMI species Ruditapes decussatus and the DUI species Ruditapes philippinarum. Data have been obtained from the transcriptomes of male and female ripe gonads of both species. Our analysis focused on nucleases and polymerases, ubiquitination and ubiquitin-like modifier pathways, and proteins involved in autophagy and mitophagy. For each protein group of interest, transcription bias (male or female), annotation, and mitochondrial targeting (when appropriate) were assessed. We did not find evidence supporting a role of nucleases/polymerases or autophagic machinery in the enforcement of SMI in R. decussatus. On the other hand, ubiquitinating enzymes with the expected features have been retrieved, providing us with two alternative testable models for mitochondrial inheritance mechanisms at the molecular level.

Germ cell line during the seasonal sexual rest of clams: finding niches of cells for gonad renewal.

Reconstitution and renewal of tissues are key topics in developmental biology. In this brief work, we analyzed the wintry spent phase of the reproductive cycle in the Manila clam Ruditapes philippinarum (Bivalvia, Veneridae) in order to study the gonad rebuilding that in this species occurs at the beginning of the warmer months. We labeled VASA homolog protein-a germ cell marker-and compared the histological observations of the spent phase with those of the previously analyzed gametogenic phase. In R. philippinarum, during the reproductive season, most of the body mass is represented by sack-like structures (acini) full of developing gametes. In that period, VASA-stained cells are present at the basal pole of the gut epithelium, in the connective tissue, and around the acini. We here show that during the spent phase large portions of the intestine lack such cell type, except for some areas showing a few faintly VASA-stained cells. Cells with similar nuclear morphology are present among loosely organized cells of connective tissue, sometimes as single units, sometimes in small groups, rarely partially organized in primordial gonadic structures. These observations match the findings of RNA-targeting studies that during the spent phase identified the source of bivalve germ cells within the connective tissue in the form of quiescent units and add new information on the possible maintenance of VASA-stained, multipotent cells among the batiprismatic cells of the intestine during the whole life span of these bivalves.

The complete mitochondrial genome of the grooved carpet shell, Ruditapes decussatus (Bivalvia, Veneridae).

Despite the large number of animal complete mitochondrial genomes currently available in public databases, knowledge about mitochondrial genomics in invertebrates is uneven. This paper reports, for the first time, the complete mitochondrial genome of the grooved carpet shell, Ruditapes decussatus, also known as the European clam. Ruditapes decussatus is morphologically and ecologically similar to the Manila clam Ruditapes philippinarum, which has been recently introduced for aquaculture in the very same habitats of Ruditapes decussatus, and that is replacing the native species. Currently the production of the European clam is almost insignificant, nonetheless it is considered a high value product, and therefore it is an economically important species, especially in Portugal, Spain and Italy. In this work we: (i) assembled Ruditapes decussatus mitochondrial genome from RNA-Seq data, and validated it by Sanger sequencing; (ii) analyzed and characterized the Ruditapes decussatus mitochondrial genome, comparing its features with those of other venerid bivalves; (iii) assessed mitochondrial sequence polymorphism (SP) and copy number variation (CNV) of tandem repeats across 26 samples. Despite using high-throughput approaches we did not find evidence for the presence of two sex-linked mitochondrial genomes, typical of the doubly uniparental inheritance of mitochondria, a phenomenon known in ∼100 bivalve species. According to our analyses, Ruditapes decussatus is more genetically similar to species of the Genus Paphia than to the congeneric Ruditapes philippinarum, a finding that bolsters the already-proposed need of a taxonomic revision. We also found a quite low genetic variability across the examined samples, with few SPs and little variability of the sequences flanking the control region (Largest Unassigned Regions (LURs). Strikingly, although we found low nucleotide variability along the entire mitochondrial genome, we observed high levels of length polymorphism in the LUR due to CNV of tandem repeats, and even a LUR length heteroplasmy in two samples. It is not clear if the lack of genetic variability in the mitochondrial genome of Ruditapes decussatus is a cause or an effect of the ongoing replacement of Ruditapes decussatus with the invasive Ruditapes philippinarum, and more analyses, especially on nuclear sequences, are required to assess this point.

SmithRNAs: Could Mitochondria "Bend" Nuclear Regulation?

Typically, animal mitochondria have very compact genomes, with few short intergenic regions, and no introns. Hence, it may seem that there is little space for unknown functions in mitochondrial DNA (mtDNA). However, mtDNA can also operate through RNA interference, as small non coding RNAs (sncRNAs) produced by mtDNA have already been proposed for humans. We sequenced sncRNA libraries from isolated mitochondria of Ruditapes philippinarum (Mollusca Bivalvia) gonads, a species with doubly uniparental inheritance of mitochondria, and identified several putative sncRNAs of mitochondrial origin. Some sncRNAs are transcribed by intergenic regions that form stable stem-hairpin structures, which makes them good miRNA-like candidates. We decided to name them small mitochondrial highly-transcribed RNAs (smithRNAs). Many concurrent data support that we have recovered sncRNAs of mitochondrial origin that might be involved in gonad formation and able to affect nuclear gene expression. This possibility has been never suggested before. If mtDNA can affect nuclear gene expression through RNA interference, this opens a plethora of new possibilities for it to interact with the nucleus, and makes metazoan mtDNA a much more complex genome than previously thought.

VASA expression suggests shared germ line dynamics in bivalve molluscs.

Germ line segregation can occur during embryogenesis or after embryogenesis completion, with multipotent cells able to give rise to both germ and somatic cells in the developing juvenile or even in adulthood. These undifferentiated cells, in some animals, are self-renewing stem cells. In all these cell lineages, the same set of genes, among which vasa, appears to be expressed. We traced VASA expression during the peculiar gonad rebuilding of bivalves to verify its presence from undifferentiated germ cells to mature gametes in an animal taxon in which the mechanism of germ line establishment is still under investigation. We utilized antibodies produced against VASPH, VASA homolog of Ruditapes philippinarum (Subclass Heterodonta), to compare the known expression pattern of R. philippinarum to two species of the Subclass Pteriomorphia, Anadara kagoshimensis and Crassostrea gigas, and another species of the Subclass Heterodonta, Mya arenaria. The immunohistological data obtained support a conserved mechanism of proliferation of "primordial stem cells" among the simple columnar epithelium of the gut, as well as in the connective tissue, contributing to the seasonal gonad reconstitution. Given the taxonomic separation of the analyzed species, we suggest that the process could be shared in bivalve molluscs. The presence of germ cell precursors in the gut epithelium appears to be a feature in common with model organisms, such as mouse, fruit fly, and human. Thus, the comparative study of germ line establishment can add details on bivalve development, but can also help to clarify the role that VASA plays during germ cell specification.

Burrowers from the Past: Mitochondrial Signatures of Ordovician Bivalve Infaunalization.

Bivalves and gastropods are the two largest classes of extant molluscs. Despite sharing a huge number of features, they do not share a key ecological one: gastropods are essentially epibenthic, although most bivalves are infaunal. However, this is not the ancestral bivalve condition; Cambrian forms were surface crawlers and only during the Ordovician a fundamental infaunalization process took place, leading to bivalves as we currently know them. This major ecological shift is linked to the exposure to a different redox environoments (hypoxic or anoxic) and with the Lower Devonian oxygenation event. We investigated selective signatures on bivalve and gastropod mitochondrial genomes with respect to a time calibrated mitochondrial phylogeny by means of dN/dS ratios. We were able to detect 1) a major signal of directional selection between the Ordovician and the Lower Devonian for bivalve mitochondrial Complex I, and 2) an overall higher directional selective pressure on bivalve Complex V with respect to gastropods. These and other minor dN/dS patterns and timings are discussed, showing that the Ordovician infaunalization event left heavy traces in bivalve mitochondrial genomes.

Pursuing the quest for better understanding the taxonomic distribution of the system of doubly uniparental inheritance of mtDNA.

There is only one exception to strict maternal inheritance of mitochondrial DNA (mtDNA) in the animal kingdom: a system named doubly uniparental inheritance (DUI), which is found in several bivalve species. Why and how such a radically different system of mitochondrial transmission evolved in bivalve remains obscure. Obtaining a more complete taxonomic distribution of DUI in the Bivalvia may help to better understand its origin and function. In this study we provide evidence for the presence of sex-linked heteroplasmy (thus the possible presence of DUI) in two bivalve species, i.e., the nuculanoid Yoldia hyperborea(Gould, 1841)and the veneroid Scrobicularia plana(Da Costa,1778), increasing the number of families in which DUI has been found by two. An update on the taxonomic distribution of DUI in the Bivalvia is also presented.

Comparative Large-Scale Mitogenomics Evidences Clade-Specific Evolutionary Trends in Mitochondrial DNAs of Bivalvia.

Despite the figure of complete bivalve mitochondrial genomes keeps growing, an assessment of the general features of these genomes in a phylogenetic framework is still lacking, despite the fact that bivalve mitochondrial genomes are unusual under different aspects. In this work, we constructed a dataset of one hundred mitochondrial genomes of bivalves to perform the first systematic comparative mitogenomic analysis, developing a phylogenetic background to scaffold the evolutionary history of the class' mitochondrial genomes. Highly conserved domains were identified in all protein coding genes; however, four genes (namely, atp6, nad2, nad4L, and nad6) were found to be very divergent for many respects, notwithstanding the overall purifying selection working on those genomes. Moreover, the atp8 gene was newly annotated in 20 mitochondrial genomes, where it was previously declared as lacking or only signaled. Supernumerary mitochondrial proteins were compared, but it was possible to find homologies only among strictly related species. The rearrangement rate on the molecule is too high to be used as a phylogenetic marker, but here we demonstrate for the first time in mollusks that there is correlation between rearrangement rates and evolutionary rates. We also developed a new index (HERMES) to estimate the amount of mitochondrial evolution. Many genomic features are phylogenetically congruent and this allowed us to highlight three main phases in bivalve history: the origin, the branching of palaeoheterodonts, and the second radiation leading to the present-day biodiversity.

The Complete Female- and Male-Transmitted Mitochondrial Genome of Meretrix lamarckii.

Bivalve mitochondrial genomes show many uncommon features, like additional genes, high rates of gene rearrangement, high A-T content. Moreover, Doubly Uniparental Inheritance (DUI) is a distinctive inheritance mechanism allowing some bivalves to maintain and transmit two separate sex-linked mitochondrial genomes. Many bivalve mitochondrial features, such as gene extensions or additional ORFs, have been proposed to be related to DUI but, up to now, this topic is far from being understood. Several species are known to show this unusual organelle inheritance but, being widespread only among Unionidae and Mytilidae, DUI distribution is unclear. We sequenced and characterized the complete female- (F) and male-transmitted (M) mitochondrial genomes of Meretrix lamarckii, which, in fact, is the second species of the family Veneridae where DUI has been demonstrated so far. The two mitochondrial genomes are comparable in length and show roughly the same gene content and order, except for three additional tRNAs found in the M one. The two sex-linked genomes show an average nucleotide divergence of 16%. A 100-aminoacid insertion in M. lamarckii M-cox2 gene was found; moreover, additional ORFs have been found in both F and M Long Unassigned Regions of M. lamarckii. Even if no direct involvement in DUI process has been demonstrated so far, the finding of cox2 insertions and supernumerary ORFs in M. lamarckii both strengthens this hypothesis and widens the taxonomical distribution of such unusual features. Finally, the analysis of inter-sex genetic variability shows that DUI species form two separate clusters, namely Unionidae and Mytilidae+Veneridae; this dichotomy is probably due to different DUI regimes acting on separate taxa.