Molecular population genetics of male and female mitochondrial genomes in subarctic Mytilus trossulus

Polymorphism of CaCO3 and the variability of elemental composition of the calcareous skeletons secreted by invertebrates along the salinity gradient of the Baltic Sea

Biomineralization is of great importance in ecosystem functioning and for the use of carbonate skeleton as environmental proxies. Skeletal formation is controlled to different degrees by environmental parameters and biological mechanisms. While salinity is one of the most important factors affecting ecological processes and ocean physiochemistry, the goal of this investigation was to identify how salinity influences the mineral type and the concentrations of chemical elements in the whole skeleton of invertebrates from the Baltic Sea. In this model system, the surface salinity decreases from marine values (27.2) to almost fresh water (6.1). The selected organisms, mussels (Mytilus spp.), bryozoans (Einhornia crustulenta, Cribrilina cryptooecium, Cryptosula pallasiana, Electra pilosa, Escharella immersa), barnacles (Amphibalanus improvisus, Semibalanus balanoides), and polychaetes (Spirorbis tridentatus), precipitated skeleton composed of calcite and aragonite, most likely as a result of various interacting environmental and biological factors. The concentrations of all elements in bulk skeleton were highly variable between species from the same location, underlining the role of the biological mechanisms in skeletal formation. The concentration of Ca, Mg, Sr, and Na increased in the bulk skeleton of stenohaline organisms with increasing salinity, while in the bulk skeleton of euryhaline species, only the concentration of Na increased with increasing salinity. The concentrations of Mn, Ba, Cu, Pb, Y, V, Cd, and U in the skeleton of euryhaline species generally decreased at higher salinities, most likely reflecting the lower bioavailability of elements at higher salinity. However, the concentrations of elements in the skeleton of stenohaline organisms were highly variable with no clear salinity impact. This study suggests that, although the composition of skeleton of calcifying organisms along the salinity gradient of the Baltic Sea is to a large extent affected by biological mechanisms, it also reflects the responses to environmental conditions.

Health Status of Mytilus chilensis from Intensive Culture Areas in Chile Assessed by Molecular, Microbiological, and Histological Analyses

Shellfish farming is a relevant economic activity in Chile, where the inner sea in Chiloé island concentrates 99% of the production of the mussel Mytilus chilensis. This area is characterized by the presence of numerous human activities, which could harm the quality of seawater. Additionally, the presence of potentially pathogenic microorganisms can influence the health status of mussels, which must be constantly monitored. To have a clear viewpoint of the health status of M. chilensis and to study its potential as a host species for exotic diseases, microbiological, molecular, and histological analyses were performed. This study was carried out in October 2018, where M. chilensis gut were studied for: presence of food-borne bacteria (Vibrio parahaemolyticus, Escherichia coli, Salmonella spp.), exotic bacteria (“Candidatus Xenohaliotis californiensis”), viruses (abalone and Ostreid herpes virus), and protozoa (Marteilia spp., Perkinsus spp. and Bonamia spp.). Additionally, 18S rDNA metabarcoding and histology analyses were included to have a complete evaluation of the health status of M. chilensis. Overall, despite the presence of risk factors, abnormal mortality rates were not reported during the monitoring period and the histological examination did not reveal significant lesions. Pathogens of mandatory notification to World Organization for Animal Health (OIE) and the Chilean National Fisheries and Aquaculture Service (SERNAPESCA) were not detected, which confirms that M. chilensis have a good health status, highlighting the importance of an integrated vision of different disciplines to ensure the sustainability of this important mussel industry in Chile.

Relaxed selection on male mitochondrial genes in DUI bivalves eases the need for mitonuclear coevolution

Mitonuclear coevolution is an important prerequisite for efficient energy production in eukaryotes. However, many bivalve taxa experience doubly uniparental inheritance (DUI) and have sex-specific mitochondrial (mt) genomes, providing a challenge for mitonuclear coevolution. We examined possible mechanisms to reconcile mitonuclear coevolution with DUI. No nuclear-encoded, sex-specific OXPHOS paralogs were found in the DUI clam Ruditapes philippinarum, refuting OXPHOS paralogy as a solution in this species. It is also unlikely that mt changes causing disruption of nuclear interactions are strongly selected against because sex-specific mt-residues or those under positive selection in M mt genes were not depleted for contacting nuclear-encoded residues. However, M genomes showed consistently higher d_N /d_S ratios compared to putatively ancestral F genomes in all mt OXPHOS genes and across all DUI species. Further analyses indicated that this was consistently due to relaxed, not positive selection on M vs. F mt OXPHOS genes. Similarly, selection was relaxed on the F genome of DUI species compared to species with strict maternal inheritance. Coupled with recent physiological and molecular evolution studies, we suggest that relaxed selection on M mt function limits the need to maintain mitonuclear interactions in M genomes compared to F genomes. We discuss our findings with regard to OXPHOS function and the origin of DUI.

Combined threats to native smooth-shelled mussels (genus Mytilus) in Australia: bioinvasions and hybridization

Human-mediated pressures, including bioinvasions, threaten the biotas of every continent. Hybridization and introgression between invasive and native species may result in loss of genetic integrity of native taxa but, in many cases, these events are hard to detect because the invader is impossible to tell apart from the native taxon. The problem of cryptic invasive taxa and its importance for biodiversity protection have been underestimated, because of the limited number of studies of broadly distributed taxa using sensitive nuclear DNA markers. We employed a panel of 51 single nucleotide polymorphism (SNP) markers to examine genetic interactions between Australian native smooth-shelled mussels, Mytilus planulatus, and invasive and cryptic Northern Hemisphere M. galloprovincialis along 4400 km of coastline from the Pacific to the Indian Ocean. Overall, 20.8% of mussels from ten sites were native species. The centre of distribution of M. planulatus is in south-eastern Australia, in particular in Tasmania. We suggest that ongoing spatial and temporal monitoring of Tasmanian sites is required to test for the presence of M. galloprovincialis and its possible further spread, and that hatchery production of M. planulatus for farming and reseeding into the wild may help reduce the likelihood of its loss.

Semimytilus algosus: first known hermaphroditic mussel with doubly uniparental inheritance of mitochondrial DNA

Doubly uniparental inheritance (DUI) of mitochondrial DNA is a rare phenomenon occurring in some freshwater and marine bivalves and is usually characterized by the mitochondrial heteroplasmy of male individuals. Previous research on freshwater Unionida mussels showed that hermaphroditic species do not have DUI even if their closest gonochoristic counterparts do. No records showing DUI in a hermaphrodite have ever been reported. Here we show for the first time that the hermaphroditic mussel Semimytilus algosus (Mytilida), very likely has DUI, based on the complete sequences of both mitochondrial DNAs and the distribution of mtDNA types between male and female gonads. The two mitogenomes show considerable divergence (34.7%). The presumably paternal M type mitogenome dominated the male gonads of most studied mussels, while remaining at very low or undetectable levels in the female gonads of the same individuals. If indeed DUI can function in the context of simultaneous hermaphroditism, a change of paradigm regarding its involvement in sex determination is needed. It is apparently associated with gonadal differentiation rather than with sex determination in bivalves.

Trans-Atlantic Distribution and Introgression as Inferred from Single Nucleotide Polymorphism: Mussels Mytilus and Environmental Factors

Large-scale climate changes influence the geographic distribution of biodiversity. Many taxa have been reported to extend or reduce their geographic range, move poleward or displace other species. However, for closely related species that can hybridize in the natural environment, displacement is not the only effect of changes of environmental variables. Another option is subtler, hidden expansion, which can be found using genetic methods only. The marine blue mussels Mytilus are known to change their geographic distribution despite being sessile animals. In addition to natural dissemination at larval phase—enhanced by intentional or accidental introductions and rafting—they can spread through hybridization and introgression with local congeners, which can create mixed populations sustaining in environmental conditions that are marginal for pure taxa. The Mytilus species have a wide distribution in coastal regions of the Northern and Southern Hemisphere. In this study, we investigated the inter-regional genetic differentiation of the Mytilus species complex at 53 locations in the North Atlantic and adjacent Arctic waters and linked this genetic variability to key local environmental drivers. Of seventy-nine candidate single nucleotide polymorphisms (SNPs), all samples were successfully genotyped with a subset of 54 SNPs. There was a clear interregional separation of Mytilus species. However, all three Mytilus species hybridized in the contact area and created hybrid zones with mixed populations. Boosted regression trees (BRT) models showed that inter-regional variability was important in many allele models but did not prevail over variability in local environmental factors. Local environmental variables described over 40% of variability in about 30% of the allele frequencies of Mytilus spp. For the 30% of alleles, variability in their frequencies was only weakly coupled with local environmental conditions. For most studied alleles the linkages between environmental drivers and the genetic variability of Mytilus spp. were random in respect to “coding” and “non-coding” regions. An analysis of the subset of data involving functional genes only showed that two SNPs at Hsp70 and ATPase genes correlated with environmental variables. Total predictive ability of the highest performing models (r² between 0.550 and 0.801) were for alleles that discriminated most effectively M. trossulus from M. edulis and M. galloprovincialis, whereas the best performing allele model (BM101A) did the best at discriminating M. galloprovincialis from M. edulis and M. trossulus. Among the local environmental variables, salinity, water temperature, ice cover and chlorophyll a concentration were by far the greatest predictors, but their predictive performance varied among different allele models. In most cases changes in the allele frequencies along these environmental gradients were abrupt and occurred at a very narrow range of environmental variables. In general, regions of change in allele frequencies for M. trossulus occurred at 8–11 psu, 0–10 °C, 60%–70% of ice cover and 0–2 mg m⁻³ of chlorophyll a, M. edulis at 8–11 and 30–35 psu, 10–14 °C and 60%–70% of ice cover and for M. galloprovincialis at 30–35 psu, 14–20 °C.

Cryptic diversity in smooth-shelled mussels on Southern Ocean islands: connectivity, hybridisation and a marine invasion

Background

Large numbers of endemic species inhabit subantarctic continental coasts and islands that are characterised by highly variable environmental conditions. Southern hemisphere populations of taxa that are morphologically similar to northern counterparts have traditionally been considered to be extensions of such Northern hemisphere taxa, and may not exhibit differentiation amongst geographically isolated populations in the Southern Ocean. Smooth-shelled blue mussels of the genus Mytilus that exhibit an anti-tropical distribution are a model group to study phylogeography, speciation and hybridisation in the sea, and contribute to the theory and practice of marine biosecurity.

Methods

We used a single nucleotide polymorphism (SNPs) panel that has the ability to accurately identify reference Northern and Southern hemisphere Mytilus taxa to test for evolutionary differentiation amongst native Southern Ocean island populations.

Results

Native mussels from the Falkland Islands and the Kerguelen Islands exhibited greatest affinity to native M. platensis d’Orbigny 1846 from the Atlantic coast of South America. The major Southern Ocean current flow from west to east is likely to explain the spreading of M. platensis to remote offshore islands, as adults via the process of rafting or perhaps directly as larvae. SNPs variation revealed that mussels from Tasmania were native and clearly differentiated from all other blue mussel groups in the Southern and Northern hemispheres. The native mussels M. planulatus from Tasmania and from mainland New Zealand (NZ), and tentatively M. aoteanus from the two NZ Southern Ocean offshore island groups (the Auckland Islands and Campbell Island), formed a distinct M. galloprovincialis–like Southern hemisphere group with closest affinity to Northern hemisphere M. galloprovincialis from the Mediterranean Sea. In all cases, the SNPs revealed evidence of hybridisation between two or more distinct taxa. The invasive Northern hemisphere M. galloprovincialis was identified only in Tasmania, amongst native mussels of a distinct Australian M. planulatus lineage.

Conclusion

Overall, our results reveal that Southern hemisphere island mussels have mixed genome ancestry and are native, not introduced by human activities. The preservation of distinct evolutionary lineages of Southern hemisphere species needs to be an ongoing focus of conservation efforts, given that population sizes on some of the remote offshore oceanic islands will be small and may be more easily adversely affected by invasion and subsequent hybridisation and introgression than larger populations elsewhere.

Electronic supplementary material

The online version of this article (10.1186/s12983-019-0332-y) contains supplementary material, which is available to authorized users.

Mantle transcriptome sequencing of Mytilus spp. and identification of putative biomineralization genes

In molluscs, the shell secreted by mantle tissue during the biomineralization process is the first barrier against predators and mechanical damage. Changing environmental conditions, such as ocean acidification, influence shell strength and thus protection of the soft body within. Mussels are marine bivalves with important commercial and ecological value worldwide. Despite this importance, the proteins involved in the biomineralization and pigmentation processes in Mytilus spp. remain unclear, as does taxonomy of Mytilus taxa, though there have been many molecular studies. To further understanding in these areas, this study aimed to characterize and compare mantle transcriptomes of four mussel taxa using next generation sequencing. Mussels representing four taxa, were collected from several localities and RNA from mantle tissue was extracted. RNA sequences obtained were assembled, annotated and potential molecular markers, including simple sequence repeats (SSRs) and single nucleotide polymorphisms (SNPs) were identified. Candidate contigs putatively related to biomineralization and pigmentation processes were then selected and several transcripts were chosen for phylogenetic analyses from the Bivalvia class. Transcriptome comparisons between Mytilus taxa, including gene ontology (GO) enrichment analysis and orthologues identification were performed. Of assembled contigs, 46.57%, 37.28% and 17.53% were annotated using NCBI NR, GO and Kyoto Encyclopedia of Genes and Genomes databases, respectively. Potential SSRs (483) and SNPs (1,497) were identified. Results presented a total of 1,292 contigs putatively involved in biomineralization and melanogenesis. Phylogenetic analyses of α-carbonic anhydrase, chitinase and tyrosinase revealed complex evolutionary history and diversity of these genes, which may be a result of duplication events or adaptation to different environments in mussels and other bivalves. Enrichment analyses revealed GO terms associated with pH and thermal response in Mytilus edulis from the North Sea and M. galloprovincialis from the Mediterranean Sea. The phylogenetic analysis within the genus Mytilus revealed M. californianus and M. coruscus to be genetically more distant from the other taxa: M. trossulus, M. edulis, M. chilensis and M. galloprovincialis. This work represents the first mantle transcriptome comparison between Mytilus taxa and provides contigs putatively involved in biomineralization.

DNA barcodes of Antipode marine invertebrates in Bay of Biscay and Gulf of Lion ports suggest new biofouling challenges

Marine biological invasions threaten global biodiversity nowadays. In this article, we have studied fouling communities from 10 port areas of south Bay of Biscay (Atlantic Ocean) and Gulf of Lion (Mediterranean Sea). A total of 834 individuals were genetically barcoded and corresponded to 95 different species. A total of 76 native species 8 genera and 1 family were identified, 58 from the Bay of Biscay and 23 from the Gulf of Lion. Furthermore, 19 species were identified as non-indigenous or cryptogenic (18 from the Bay of Biscay and 4 from the Gulf of Lion). We found a high proportion of Antipode non-indigenous species (NIS) that represented the 19.3% of all sampled individuals and the 54.21% of NIS specimens of this study. A framework for inference of donor regions based on a phylogenetic screening of genetic sequences was proposed as a proof of concept and tested, as well as models for the relationship between NIS introductions, maritime imports and distance to NIS native range and inferred donor areas. Consistent generalized linear models (GLM) with positive association between NIS genetic diversity and distance, not with maritime growth weight imports, strongly suggest that distant NIS could pose higher invasion risk than closer species. Selection for wider tolerance ranges during the long travel -direct or stepwise, as well as environmental similarity between donor and receiving regions, may explain these results.

Mitogenomics of Perumytilus purpuratus (Bivalvia: Mytilidae) and its implications for doubly uniparental inheritance of mitochondria

Animal mitochondria are usually inherited through the maternal lineage. The exceptional system allowing fathers to transmit their mitochondria to the offspring exists in some bivalves. Its taxonomic spread is poorly understood and new mitogenomic data are needed to fill the gap. Here, we present for the first time the two divergent mitogenomes from Chilean mussel Perumytilus purpuratus. The existence of these sex-specific mitogenomes confirms that this species has the doubly uniparental inheritance (DUI) of mitochondria. The genetic distance between the two mitochondrial lineages in P. purpuratus is not only much bigger than in the Mytilus edulis species complex but also greater than the distance observed in Musculista senhousia, the only other DUI-positive member of the Mytilidae family for which both complete mitochondrial genomes were published to date. One additional, long ORF (open reading frame) is present exclusively in the maternal mitogenome of P. purpuratus. This ORF evolves under purifying selection, and will likely be a target for future DUI research.

Single nucleotide polymorphisms in native South American Atlantic coast populations of smooth shelled mussels: hybridization with invasive European Mytilus galloprovincialis

BACKGROUND

Throughout the world, harvesting of mussels Mytilus spp. is based on the exploitation of natural populations and aquaculture. Aquaculture activities include transfers of spat and live adult mussels between various geographic locations, which may result in large-scale changes in the world distribution of Mytilus taxa. Mytilus taxa are morphologically similar and difficult to distinguish. In spite of much research on taxonomy, evolution and geographic distribution, the native Mytilus taxa of the Southern Hemisphere are poorly understood. Recently, single nucleotide polymorphisms (SNPs) have been used to clarify the taxonomic status of populations of smooth shelled mussels from the Pacific coast of South America. In this paper, we used a set of SNPs to characterize, for the first time, populations of smooth shelled mussels Mytilus from the Atlantic coast of South America.

RESULTS

Mytilus spp. samples were collected from eastern South America. Six reference samples from the Northern Hemisphere were used: Mytilus edulis from USA and Northern Ireland, Mytilus trossulus from Canada, and Mytilus galloprovincialis from Spain and Italy. Two other reference samples from the Southern Hemisphere were included: M. galloprovincialis from New Zealand and Mytilus chilensis from Chile. Fifty-five SNPs were successfully genotyped, of which 51 were polymorphic. Population genetic analyses using the STRUCTURE program revealed the clustering of eight populations from Argentina (Mytilus platensis) and the clustering of the sample from Ushuaia with M. chilensis from Chile. All individuals in the Puerto Madryn (Argentina) sample were identified as M. platensis × M. galloprovincialis F2 (88.89%) hybrids, except one that was classified as Mediterranean M. galloprovincialis. No F1 hybrids were observed.

CONCLUSIONS

We demonstrate that M. platensis (or Mytilus edulis platensis) and M. chilensis are distinct native taxa in South America, which indicates that the evolutionary histories of Mytilus taxa along the Atlantic and Pacific coasts differ. M. platensis is endangered by hybridization with M. galloprovincialis that was introduced from Europe into the Puerto Madryn area in Argentina, presumably by accidental introduction via ship traffic. We confirm the occurrence of a native M. chilensis population in southern Argentina on the coast of Patagonia.

Gender-Associated Mitochondrial DNA Heteroplasmy in Somatic Tissues of the Endangered Freshwater Mussel Unio crassus (Bivalvia: Unionidae): Implications for Sex Identification and Phylogeographical Studies

Some bivalve species possess two independent mitochondrial DNA lineages: maternally (F-type) and paternally (M-type) inherited. This phenomenon is called doubly uniparental inheritance. It is generally agreed that F-type mtDNA is typically present in female somatic and gonadal tissues as well as in male somatic tissues, whereas the M-type mtDNA occurs only in male germ line and gonadal tissue. In the present study, the mtDNA heteroplasmy (for both F and M genomes) in male somatic tissues of Unio crassus (Philipsson, 1788), species threatened with extinction, has been confirmed. Taking advantage from the presence of Mcox1 marker only in male somatic tissues, we developed a new method of sex identification in this endangered species, using nondestructive tissue sampling. Probability of correct sex identification was estimated at 97.5%. The present study is the first report on gender-associated mitochondrial DNA heteroplasmy in male somatic tissues of thick-shelled river mussel and first approach to U. crassus sex identification at molecular level. Our study also confirmed the utility of paternally inherited Mcox1 gene fragment as a complementary molecular tool for resolving phylogeographical relationships among populations of thick-shelled river mussel.

Disruption of doubly uniparental inheritance of mitochondrial DNA associated with hybridization area of European Mytilus edulis and Mytilus trossulus in Norway

Doubly uniparental inheritance of mitochondria (DUI) is best known in the blue mussel Mytilus. Under this model, two types of mitochondrial DNA exist: female type (F), transmitted from females to offspring of both genders, and male type (M), transmitted exclusively from males to sons. The mitogenomes are usually highly divergent, but an occasional replacement of a typical M genome by a particular F genome has been postulated to explain reduction of this divergence. Disruption of the DUI model has been reported in hybridization areas. Here, we present a new case of DUI disruption in a hybrid M. trossulus/M. edulis population from the North Sea (Norway). No M haplotypes derived from M. trossulus were identified in this population. Typical M haplotypes derived from M. edulis (ME) were rare. Two F-type haplogroups were found: one derived from M. edulis (FE) and the second derived from M. trossulus (FT). Many haplotypes from the FT group were recombinants, with the male CR sequence coming from the M. trossulus genome (FT1 haplogroup) in contrast to M. edulis CR as in the Baltic. FT1 haplotypes were abundant in the studied population, including homoplasmic females. However, males significantly more often carried these haplotypes; therefore, male heteroplasmy involved the original FE and recombinant FT, indicating that the FT genome undergoes masculinization. Structural similarity of FT1 CR with previously reported, masculinized Baltic haplotypes, which were derived from FE/ME recombination, provides further evidence that CR M-F recombination is a prerequisite for masculinization, also in the context of native M. trossulus mtDNA.

The complete maternal and paternal mitochondrial genomes of Unio crassus: Mitochondrial molecular clock and the overconfidence of molecular dating

The availability of a rapidly growing number of complete mitochondrial genome sequences provokes high confidence dating approaches. However, even if the congruence between mitochondrial and nuclear markers is reasonable, the resulting topologies are frequently questionable. The unique opportunity to study the evolutionary history of two independent mitochondrial genomes in one phylogenetic context exists in the freshwater mussels family Unionidae. The two lineages function under doubly uniparental inheritance since well before the emergence of the family. Despite the relatively high number of available complete sequences of maternally inherited genomes, comparative analyses are limited by the small number of sequences of counterpart paternally inherited genomes. We have sequenced for the first time the representative set of five sequences (two maternal and three paternal) from the species Unio crassus. Comparative analysis of the phylogenies reconstructed using relevant mitogenomic data available in GenBank (13 species in total) reveal that single - genome inferences are congruent only if the relaxed clock is assumed.

Invasive blue mussels threaten regional scale genetic diversity in mainland and remote offshore locations: the need for baseline data and enhanced protection in the Southern Ocean

Human-mediated biological transfers of species have substantially modified many ecosystems with profound environmental and economic consequences. However, in many cases, invasion events are very hard to identify because of the absence of an appropriate baseline of information for receiving sites/regions. In this study, use of high-resolution genetic markers (single nucleotide polymorphisms - SNPs) highlights the threat of introduced Northern Hemisphere blue mussels (Mytilus galloprovincialis) at a regional scale to Southern Hemisphere lineages of blue mussels via hybridization and introgression. Analysis of a multispecies SNP dataset reveals hotspots of invasive Northern Hemisphere blue mussels in some mainland New Zealand locations, as well as the existence of unique native lineages of blue mussels on remote oceanic islands in the Southern Ocean that are now threatened by invasive mussels. Samples collected from an oil rig that has moved between South Africa, Australia, and New Zealand were identified as invasive Northern Hemisphere mussels, revealing the relative ease with which such non-native species may be moved from region to region. In combination, our results highlight the existence of unique lineages of mussels (and by extension, presumably of other taxa) on remote offshore islands in the Southern Ocean, the need for more baseline data to help identify bioinvasion events, the ongoing threat of hybridization and introgression posed by invasive species, and the need for greater protection of some of the world's last great remote areas.

Genetic, Ecological and Morphological Distinctness of the Blue Mussels Mytilus trossulus Gould and M. edulis L. in the White Sea

Two blue mussel lineages of Pliocene origin, Mytilus edulis (ME) and M. trossulus (MT), co-occur and hybridize in several regions on the shores of the North Atlantic. The two species were distinguished from each other by molecular methods in the 1980s, and a large amount of comparative data on them has been accumulated since that time. However, while ME and MT are now routinely distinguished by various genetic markers, they tend to be overlooked in ecological studies since morphological characters for taxonomic identification have been lacking, and no consistent habitat differences between lineages have been reported. Surveying a recently discovered area of ME and MT co-occurrence in the White Sea and employing a set of allozyme markers for identification, we address the issue whether ME and MT are true biological species with distinct ecological characteristics or just virtual genetic entities with no matching morphological and ecological identities. We find that: (1) in the White Sea, the occurrence of MT is largely concentrated in harbors, in line with observations from other subarctic regions of Europe; (2) mixed populations of ME and MT are always dominated by purebred individuals, animals classified as hybrids constituting only ca. 18%; (3) in terms of shell morphology, 80% of MT bear a distinct uninterrupted dark prismatic strip under the ligament while 97% of ME lack this character; (4) at sites of sympatry MT is more common on algal substrates while ME mostly lives directly on the bottom. This segregation by the substrate may contribute to maintaining reproductive isolation and decreasing competition between taxa. We conclude that while ME and MT are not fully reproductively isolated, they do represent clearly distinguishable biological, ecological and morphological entities in the White Sea. It remains to be documented whether the observed morphological and ecological differences are of a local character, or whether they have simply been overlooked in other contact zones.

Mitogenomics of recombinant mitochondrial genomes of Baltic Sea Mytilus mussels

Recombination in the control region (CR) of Mytilus mitochondrial DNA (mtDNA) was originally reported based on the relatively short, sequenced fragments of mitochondrial genomes. Recombination outside the CR has been reported recently with the suggestion that such processes are common in Mytilus. We have fully sequenced a set of 11 different mitochondrial haplotypes representing the high diversity of paternally inherited mitochondrial genomes of Baltic Sea Mytilus mussels, including the haplotype close to the native Mytilus trossulus mitochondrial genome, which was thought to have been entirely eliminated from this population. Phylogenetic and comparative analysis showed that the recombination is limited to the vicinity of the CR in all sequenced genomes. Coding sequence comparison indicated that all paternally inherited genomes showed increased accumulation of nonsynonymous substitutions, including the genomes which switched their transmission route very recently. The acquisition of certain CR sequences through recombination with highly divergent paternally inherited genomes seems to precede and favor the switch, but it is not a prerequisite for this process. Interspecies hybridization in the Baltic Sea during the recent 10,000 years created conditions for both structural and evolutionary mitochondrial instability which resulted in the observed variation and dynamics of mtDNA in Baltic Sea Mytilus mussels. In conclusion, the data shows that the effects of mitochondrial recombination are limited to the CR of few phylogenetic lineages.

Glacial history of the European marine mussels Mytilus, inferred from distribution of mitochondrial DNA lineages

Mussels of the genus Mytilus have been used to assess the circumglacial phylogeography of the intertidal zone. These mussels are representative components of the intertidal zone and have rapidly evolving mitochondrial DNA, suitable for high resolution phylogeographic analyses. In Europe, the three Mytilus species currently share mitochondrial haplotypes, owing to the cases of extensive genetic introgression. Genetic diversity of Mytilus edulis, Mytilus trossulus and Mytilus galloprovincialis was studied using a 900-bp long part of the most variable fragment of the control region from one of their two mitochondrial genomes. To this end, 985 specimens were sampled along the European coasts, at sites ranging from the Black Sea to the White Sea. The relevant DNA fragments were amplified, sequenced and analyzed. Contrary to the earlier findings, our coalescence and nested cladistics results show that only a single M. edulis glacial refugium existed in the Atlantic. Despite that, the species survived the glaciation retaining much of its diversity. Unsurprisingly, M. galloprovincialis survived in the Mediterranean Sea. In a relatively short time period, around the climatic optimum at 10 ky ago, the species underwent rapid expansion coupled with population differentiation. Following the expansion, further contemporary gene flow between populations was limited.

Mitochondrial control region variability in Mytilus galloprovincialis populations from the central-Eastern Mediterranean Sea

The variable domain 1 (VD1) domain of the control region and a small segment of the rrnaL gene of the F mtDNA type were sequenced and analyzed in 174 specimens of Mytilus galloprovincialis. Samples were collected from eight locations in four Central-Eastern (CE) Mediterranean countries (Italy, Croatia, Greece and Turkey). A new primer, specific for the F mtDNA type, was designed for the sequencing procedure. In total 40 different haplotypes were recorded, 24 of which were unique. Aside from the two populations situated in Thermaikos gulf (Northern Aegean, Greece), relatively high levels of haplotype and nucleotide diversity were estimated for both Central and Eastern Mediterranean populations. Eight out of the 40 haplotypes were shared by at least three populations while two of them were found in all populations. ΦST and cluster analysis revealed lack of structuring among CE Mediterranean populations with the exception of those located at the Sea of Marmara and Croatian coast which were highly differentiated. Apart from the species' inherit dispersal ability, anthropogenic activities, such as the repeated translocations of mussel spat, seem to have played an important role in shaping the current genetic population structure of CE M. galloprovincialis mussels.

Co-expressed mitochondrial genomes: recently masculinized, recombinant mitochondrial genome is co-expressed with the female-transmitted mtDNA genome in a male Mytilus trossulus mussel from the Baltic Sea

BACKGROUND

Few exceptions have been described from strict maternal inheritance of mitochondrial DNA in animals, including sea mussels (Mytilidae), clams (Donacidae, Veneridae and Solenidae) and freshwater mussels (Unionoidae) order. In these bivalves mitochondria and their DNA are transferred through two separate routes. The females inherit only the maternal mitochondrial DNA whereas the males inherit maternal as well as paternal mitochondrial DNA, which is usually present only in gonads and sperm. The mechanism controlling this phenomenon is unclear but leads to the existence of two separate mitochondrial DNA lineages in a single species. The lineages are usually well differentiated: up to 20-50% divergence in nucleotide sequence. Occasionally, a maternal mitochondrial DNA can invade the paternal transmission route, eventually replacing the diverged M-type and lowering the divergence. Such role reversal (masculinization) event has happened recently in the Mytilus population of the Baltic Sea which consists of M. edulis × M. trossulus hybrids, but the functional status of the resulting mitochondrial genome was unknown.

RESULTS

In this paper we sequenced transcripts from one specimen that was identified as male carrying both the female mitochondrial genome and a recently masculinized mitochondrial genome. Additionally, the analysis of the control region has showed that the recently masculinized, recombinant genome, not only has an M-type control region and all coding regions derived from the F-type, but also is transcriptionally active along side the maternally inherited F-type genome. In the comparative analysis, the two genomes exhibit different substitution patterns, typical for the M vs. F genome comparisons. The genetic distances and ratios of non-synonymous substitutions also suggest that one of the genomes is transitioning from the maternal to the paternal inheritance mode, consistent with its recent masculinization.

CONCLUSION

We have shown, for the first time, that the recently masculinized mitochondrial genome is active and that it accumulates excess of non-synonymous substitutions across its coding sequence. This suggests, that, under certain cytonuclear incompatibility conditions, masculinization may serve to restore the endangered functionality of the paternally inherited genome. This is also another example of a mitochondrial genome in which the recombination in the control region predated its transition from paternal to maternal transmission route.