Recombination in Mitochondrial DNA of European Mussels Mytilus

The effect of mitochondrial recombination on fertilization success in blue mussels

The presence of doubly uniparental inheritance (DUI) in bivalves represents a unique mode of mitochondrial transmission, whereby paternal (male-transmitted M-type) and maternal (female-transmitted F-type) haplotypes are transmitted to offspring separately. Male embryos retain both haplotypes, but the M-type is selectively removed from females. Due to the presence of heteroplasmy in males, mtDNA can recombine resulting in a 'masculinized' haplotype referred to as Mf-type. While mtDNA recombination is usually rare, it has been recorded in multiple mussel species across the Northern Hemisphere. Given that mitochondria are the powerhouse of the cell, different mtDNA haplotypes may have different selective advantages under diverse environmental conditions. This may be particularly important for sperm fitness and fertilization success. In this study we aimed to i) determine the presence, prevalence of the Mf-type in Australian blue mussels (Mytilus sp.) and ii) investigate the effect of Mf-mtDNA on sperm performance (a fitness correlate). We found a high prevalence of recombined mtDNA (≈35 %) located within the control region of the mitochondrial genome, which occurred only in specimens that contained Southern Hemisphere mtDNA. The presence of two female mitotypes were identified in the studied mussels, one likely originating from the Northern Hemisphere, and the other either representing the endemic M. planulatus species or introduced genotypes from the Southern Hemisphere. Despite having recombination events present in a third of the studied population, analysis of sperm performance indicated no difference in fertilization success related to mitotype.

A tale of two paths: The evolution of mitochondrial recombination in bivalves with doubly uniparental inheritance

In most animals, mitochondrial DNA is strictly maternally inherited and non-recombining. One exception to this pattern is called doubly uniparental inheritance (DUI), a phenomenon involving the independent transmission of female and male mitochondrial genomes. DUI is known only from the molluskan class Bivalvia. The phylogenetic distribution of male-transmitted mitochondrial DNA (M mtDNA) in bivalves is consistent with several evolutionary scenarios, including multiple independent gains, losses, and varying degrees of recombination with female-transmitted mitochondrial DNA (F mtDNA). In this study, we use phylogenetic methods to test M mtDNA origination hypotheses and infer the prevalence of mitochondrial recombination in bivalves with DUI. Phylogenetic modeling using site concordance factors supported a single origin of M mtDNA in bivalves coupled with recombination acting over long evolutionary timescales. Ongoing mitochondrial recombination is present in Mytilida and Venerida, which results in a pattern of concerted evolution of F mtDNA and M mtDNA. Mitochondrial recombination could be favored to offset the deleterious effects of asexual inheritance and maintain mitonuclear compatibility across tissues. Cardiida and Unionida have gone without recent recombination, possibly due to an extension of the COX2 gene in male mitochondrial DNA. The loss of recombination could be connected to the role of M mtDNA in sex determination or sexual development. Our results support that recombination events may occur throughout the mitochondrial genomes of DUI species. Future investigations may reveal more complex patterns of inheritance of recombinants, which could explain the retention of signal for a single origination of M mtDNA in protein-coding genes.

Microsatellites as Molecular Markers with Applications in Exploitation and Conservation of Aquatic Animal Populations

A large number of species and taxa has been studied for genetic polymorphism. Microsatellites have been known as hypervariable neutral molecular markers with the highest resolution power in comparison with any other markers. However, the discovery of a new type of molecular marker—single nucleotide polymorphism (SNP) has put the existing applications of microsatellites to the test. To ensure good resolution power in studies of populations and individuals, a number of microsatellite loci from 14 to 20 was often used, which corresponds to about 200 independent alleles. Recently, these numbers have tended to be increased by the application of genomic sequencing of expressed sequence tags (ESTs), and the choice of the most informative loci for genotyping depends on the aims of research. Examples of successful applications of microsatellite molecular markers in aquaculture, fisheries, and conservation genetics in comparison with SNPs have been summarized in this review. Microsatellites can be considered superior markers in such topics as kinship and parentage analysis in cultured and natural populations, the assessment of gynogenesis, androgenesis and ploidization. Microsatellites can be coupled with SNPs for mapping QTL. Microsatellites will continue to be used in research on genetic diversity in cultured stocks, and also in natural populations as an economically advantageous genotyping technique.

No evidence of DUI in the Mediterranean alien species Brachidontes pharaonis (P. Fisher, 1870) despite mitochondrial heteroplasmy

Two genetically different mitochondrial haplogroups of Brachidontes pharaonis (p-distance 6.8%) have been identified in the Mediterranean Sea. This hinted at a possible presence of doubly uniparental inheritance in this species. To ascertain this possibility, we sequenced two complete mitogenomes of Brachidontes pharaonis mussels and performed a qPCR analysis to measure the relative mitogenome copy numbers of both mtDNAs. Despite the presence of two very similar regions composed entirely of repetitive sequences in the two haplogroups, no recombination between mitogenomes was detected. In heteroplasmic individuals, both mitogenomes were present in the generative tissues of both sexes, which argues against the presence of doubly uniparental inheritance in this species.

Identification and function prediction of novel microRNAs in adenosine monophosphate activated protein kinase-activated Sertoli cells of immature boar

This study was carried out with the objective to identify function prediction of novel microRNAs (miRNAs) in immature boar Sertoli cells (SCs) treated with 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR), which is an agonist of adenosine monophosphate-activated protein kinase (AMPK) for regulating cellular energy homeostasis. Two small RNA libraries (control and AICAR treatment) prepared from immature boar SCs were constructed and sequenced by the Illumina small RNA deep sequencing. We identified 77 novel miRNAs and predicted 177 potential target genes for 26 differential novel miRNAs (four miRNAs up-regulation and 22 miRNAs down-regulation) in AICAR-treated SCs. Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway suggested that target genes of differential novel miRNAs were implicated in many biological processes and metabolic pathways. Our findings provided useful information for the functional regulation of novel miRNAs and target mRNAs on AMPK-activated immature boar SCs.

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.

Identification of microRNAs for regulating adenosine monophosphate-activated protein kinase expression in immature boar Sertoli cells in vitro

Adenosine monophosphate-activated protein kinase (AMPK) plays a key role in cellular energy homeostasis and cell proliferation. MicroRNAs (miRNAs) function as posttranscriptional regulators of gene expression in biological processes. It is unclear to whether miRNAs are involved in AMPK-regulated Sertoli cell (SC) proliferation. To further understand the regulation of miRNAs in the immature boar SC proliferation, 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) was added to activate AMPK. By an Illumina small RNA deep sequencing, we obtained sequences and relative expression levels of 272 known mature miRNAs, among which 9 miRNAs were significantly upregulated whereas 16 miRNAs were downregulated following the AICAR treatment. The results identified 38 conserved miRNAs, with 8 significantly downregulated miRNAs whereas no upregulated miRNAs. Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses suggested that miR-1285 was involved in many activities and pathways associated with cell proliferation via targeting on AMPKα2. We validated that AICAR significantly downregulated miR-1285 level in SCs. Transfection of miR-1285 mimic increased the SC viability and cell cycle progression but reduced AMPKα2 mRNA and protein levels, indicating that miR-1285 is involved in the immature boar SC proliferation via downregulating AMPKα2 expression.

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.

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.

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.

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.

Recombinant mitochondrial genome with standard transmission route from Mediterranean mussel Mytilus galloprovincialis

Several bivalve species, including marine mussels Mytilus are atypical in having two gender-specific and highly divergent mtDNA genomes. This peculiar genetic system allows not only the recombination to occur but also facilitates its detection. Previous reports associated the existence of mosaic recombinant haplotypes with the switch of their transmission route. Here we report nearly complete sequence of a mitochondrial genome isolated from a homoplasmic female individual of Mediterranean Mytilus galloprovincialis. The genome has clear phylogenetic affinity with and organization identical to the M. galloprovincialis female haplotypes, in the coding part. However, the genome is very large, approximately 20,600 bp long, exclusively due to a long and complex control region. It contains an array of repeats, some of which are degenerated. A large part of the control region is derived from the paternal genome. This finding shows that not all haplotypes with recombinant control regions must be paternally inherited in Mytilus.

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.

Evidence for a fourteenth mtDNA-encoded protein in the female-transmitted mtDNA of marine Mussels (Bivalvia: Mytilidae)

Background

A novel feature for animal mitochondrial genomes has been recently established: i.e., the presence of additional, lineage-specific, mtDNA-encoded proteins with functional significance. This feature has been observed in freshwater mussels with doubly uniparental inheritance of mtDNA (DUI). The latter unique system of mtDNA transmission, which also exists in some marine mussels and marine clams, is characterized by one mt genome inherited from the female parent (F mtDNA) and one mt genome inherited from the male parent (M mtDNA). In freshwater mussels, the novel mtDNA-encoded proteins have been shown to be mt genome-specific (i.e., one novel protein for F genomes and one novel protein for M genomes). It has been hypothesized that these novel, F- and M-specific, mtDNA-encoded proteins (and/or other F- and/or M-specific mtDNA sequences) could be responsible for the different modes of mtDNA transmission in bivalves but this remains to be demonstrated.

Methodology/Principal Findings

We investigated all complete (or nearly complete) female- and male-transmitted marine mussel mtDNAs previously sequenced for the presence of ORFs that could have functional importance in these bivalves. Our results confirm the presence of a novel F genome-specific mt ORF, of significant length (>100aa) and located in the control region, that most likely has functional significance in marine mussels. The identification of this ORF in five Mytilus species suggests that it has been maintained in the mytilid lineage (subfamily Mytilinae) for ∼13 million years. Furthermore, this ORF likely has a homologue in the F mt genome of Musculista senhousia, a DUI-containing mytilid species in the subfamily Crenellinae. We present evidence supporting the functionality of this F-specific ORF at the transcriptional, amino acid and nucleotide levels.

Conclusions/Significance

Our results offer support for the hypothesis that “novel F genome-specific mitochondrial genes” are involved in key biological functions in bivalve species with DUI.

The mitogenome of Paphia euglypta (Bivalvia: Veneridae) and comparative mitogenomic analyses of three venerids

Extraordinary variation has been found in mitochondrial (mt) genome inheritance, gene content and arrangement among bivalves. However, only few bivalve mt genomes have been comparatively analyzed to infer their evolutionary scenarios. In this study, the complete mt genome of the venerid Paphia euglypta (Bivalvia: Veneridae) was firstly studied and, secondly, it was comparatively analyzed with other venerids (e.g., Venerupis philippinarum and Meretrix petechialis) to better understand the mt genome evolution within a family. Though several common features such as the AT content, codon usage of protein-coding genes, and AT/GC skew are shared by the three venerids, a high level of variability is observed in genome size, gene content, gene order, arrangements and primary sequence of nucleotides or amino acids. Most of the gene rearrangement can be explained by the "tandem duplication and random loss" model. From the observed rearrangement patterns, we speculate that block interchange between adjacent genes may be common in the evolution of mt genomes in venerids. Furthermore, this study presents several new findings in mt genome annotation of V. philippinarum and M. petechialis, and hence we have reannotated the genome of these two species as: (1) the ORF of the formerly annotated cox2 gene in V. philippinarum is deduced by using a truncated "T" codon and a second cox2 gene is identified; (2) the trnS-AGN gene is identified and marked in the mt genome of both venerids. Thus, this study demonstrated a high variability of mt genomes in the Veneridae, and showed the importance of comparative mt genome analysis to interpret the evolution of the bivalve mt genome.

Homologous recombination between highly diverged mitochondrial sequences: examples from maternally and paternally transmitted genomes

Homologous recombination is restricted to sequences of low divergence. This is attributed to the mismatch repairing system (MMR), which does not allow recombination between sequences that are highly divergent. This acts as a safeguard against recombination between nonhomologous sequences that could result in genome imbalance. Here, we report recombination between maternal and paternal mitochondrial genomes of the sea mussel, whose sequences differ by >20%. We propose that the strict maternal inheritance of the animal mitochondrial DNA and the ensuing homoplasmy has relieved the MMR system of the animal mitochondrion from the pressure to tolerate recombination only among sequences with a high degree of similarity.

The atypical presence of the paternal mitochondrial DNA in somatic tissues of male and female individuals of the blue mussel species Mytilus galloprovincialis

BACKGROUND

In animals mtDNA inheritance is maternal except in certain molluscan bivalve species which have a paternally inherited mitochondrial genome (genome M) along with the standard maternal one (genome F). Normally, the paternal genome occurs in the male gonad, but it can be often found, as a minority, in somatic tissues of males and females. This may happen in two ways. One is through "sperm mtDNA leakage" into somatic tissues, a deviation from the normal situation in which the sperm mtDNA vanishes in females or ends up exclusively in the germ line of males. The other is through "egg heteroplasmy", when the egg contains, in small quantities, the paternal genome in addition to maternal genome.

FINDINGS

To test the two hypotheses, we compared the sequences of one of the most variable domains of the M molecule in a somatic tissue (foot) and in the sperm of ten male and in the foot of ten female individuals of M. galloprovincialis. Presence of the M genome was rarer in the foot of females than males. The M genome in the sperm and in the foot of males was identical.

CONCLUSIONS

Given that the surveyed region differs from individual to individual, the identity of the M genome in the foot and the sperm of males supports strongly the hypothesis that, at least for the tissue examined, the presence of the M genome is due to sperm mtDNA leakage.

Scottish Mytilus trossulus mussels retain ancestral mitochondrial DNA: complete sequences of male and female mtDNA genomes

Mytilus trossulus mussels occur in North America and in the Baltic Sea. Recently genetic markers for the three Mytilus subspecies M. edulis, M. galloprovincialis, and M. trossulus, have been detected at Loch Etive in Scotland suggesting mixed ancestry for this population. Of particular interest is the evidence that M. trossulus occurs at Loch Etive because it had not previously been reported in the British Isles. In the present study, analysis of subspecies-specific diagnostic nuclear DNA markers confirms the presence of a high frequency of mussels with M. trossulus ancestry at Loch Etive. The genetic structure suggests hybridisation at an intermediate stage compared with North American populations, where there is little hybridisation, and Baltic populations where there is extensive introgression. This points strongly against a Baltic origin for Loch Etive M. trossulus. The F and M mitochondrial DNA (mtDNA) genomes of Baltic M. trossulus are similar in sequence to the corresponding genomes in M. edulis and believed to be derived by introgression from that subspecies. Both F and M mtDNA genomes are observed at Loch Etive consistent with the presence of doubly uniparental inheritance. Here we provide the complete sequences of the three M. trossulus mtDNA genomes (one F and two M) from Loch Etive. These genomes are extremely similar to the corresponding genomes from ancestral M. trossulus in America but divergent from the genomes for Baltic M. trossulus. This is the first report of ancestral M. trossulus mtDNA genomes in Europe. The F and M genomes are diverged by 26% in nucleotide sequence, similar to other Mytilus F and M genomes. The gene arrangement in the sequenced genomes is also similar to that in other sequenced Mytilus mtDNA genomes. However the two sequenced M genomes differ by 960bp which is caused by a duplication in the main noncoding region (CR). This duplication has not so far been observed in North American populations of M. trossulus. The coding regions of the Loch Etive genomes have no features suggesting that they are other than functional genomes and have K(a)/K(s) values in coding regions less than one indicative of purifying selection. Estimates of divergence times were made for both genomes and are consistent with invasion of Loch Etive by M. trossulus towards the end of the last glacial period.