Małgorzata Zbawicka

Doubly Uniparental Inheritance Is Associated With High Polymorphism for Rearranged and Recombinant Control Region Haplotypes in Baltic Mytilus trossulus

Many bivalve species, including mussels of the genus Mytilus, are unusual in having two mtDNA genomes, one inherited maternally (the F genome) and the other inherited paternally (the M genome). The sequence differences between the genomes are usually great, indicating ancient divergence predating speciation events. However, in Mytilus trossulus from the Baltic, both genomes are similar to the F genome from the closely related M. edulis. This study analyzed the mtDNA control region structure in male and female Baltic M. trossulus mussels. We show that a great diversity of structural rearrangements is present in both sexes. Sperm samples are dominated by recombinant haplotypes with M. edulis M-like control region segments, some having large duplications. By contrast, the rearranged haplotypes that dominate in eggs lack segments from this M genome. The rearrangements can be explained by a combination of tandem duplication, deletion, and intermolecular recombination. An evolutionary pathway leading to the recombinant haplotypes is suggested. The data are also considered in relation to the hypothesis that the M. edulis M-like control region sequence is necessary to confer the paternal role on genomes that are otherwise F-like.

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.

Genetic biodiversity in the Baltic Sea: species-specific patterns challenge management

Information on spatial and temporal patterns of genetic diversity is a prerequisite to understanding the demography of populations, and is fundamental to successful management and conservation of species. In the sea, it has been observed that oceanographic and other physical forces can constitute barriers to gene flow that may result in similar population genetic structures in different species. Such similarities among species would greatly simplify management of genetic biodiversity. Here, we tested for shared genetic patterns in a complex marine area, the Baltic Sea. We assessed spatial patterns of intraspecific genetic diversity and differentiation in seven ecologically important species of the Baltic ecosystem—Atlantic herring (Clupea harengus), northern pike (Esox lucius), European whitefish (Coregonus lavaretus), three-spined stickleback (Gasterosteus aculeatus), nine-spined stickleback (Pungitius pungitius), blue mussel (Mytilus spp.), and bladderwrack (Fucus vesiculosus). We used nuclear genetic data of putatively neutral microsatellite and SNP loci from samples collected from seven regions throughout the Baltic Sea, and reference samples from North Atlantic areas. Overall, patterns of genetic diversity and differentiation among sampling regions were unique for each species, although all six species with Atlantic samples indicated strong resistence to Atlantic-Baltic gene-flow. Major genetic barriers were not shared among species within the Baltic Sea; most species show genetic heterogeneity, but significant isolation by distance was only detected in pike and whitefish. These species-specific patterns of genetic structure preclude generalizations and emphasize the need to undertake genetic surveys for species separately, and to design management plans taking into consideration the specific structures of each species.

Mitochondrial DNA variation in populations of the mussel Mytilus trossulus from the Southern Baltic

The mussel Mytilus trossulus is an important component of the Baltic brackish water ecosystem. The genetic structure of mussel (M. trossulus) populations was studied in sites along the Polish coast, Southern Baltic for two segments of mitochondrial DNA (mtDNA). The mode of inheritance of Mytilus mtDNA is termed doubly uniparental; two genomes are passed independently down the female (the F genome) and male (the M genome) lines of descent. The M genome has not been detected at high frequency in M. trossulus, thus the present study focuses on the F genome. PCR and RFLP analysis was used to characterise haplotypes in the coding region ND2-COIII; PCR was used to detect length variants in a major noncoding region. Significant differentiation between populations was observed in the frequency of 24 coding region haplotypes and 14 different length variants. For the three most frequent coding region haplotypes, two (I and III) are associated with the length variants, whereas the third (II) is monomorphic for a single variant of short length. It is suggested that variant II is derived by introgression from a related species, M. edulis, and may be resistant to expansion in the noncoding region. In both regions studied, the Ewens–Watterson test reveals significant deviations from neutrality with an excess of rare variants. This might be due to selection against slightly deleterious variants and is consistent with previously published results for Mytilus taxa. The present study also points towards the potential utility of mtDNA length variation in studies of population differentiation of Mytilus.

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 worlds last great remote areas.

Native and invasive taxa on the Pacific coast of South America: Impacts on aquaculture, traceability and biodiversity of blue mussels (Mytilus spp.)

Gaining new knowledge of the native distributions of species (phylogeography) is more and more difficult in a world affected by anthropogenic disturbance, in particular by species translocations. Increasingly, molecular markers are required to support decisions about the taxonomy of native vs. introduced species, and the existence of their hybrids, to answer phylogeographic questions. In many fields, including aquaculture, traceability and food security, taxonomic and phylogeographic knowledge is key to the successful management and conservation of biodiversity. The Pacific coast of Chile is one of the last regions without a clear and agreed understanding of the taxonomy and systematics of smooth‐shelled blue mussels of the genus Mytilus. A panel of 49 bi‐allelic single nucleotide polymorphisms (SNPs) was genotyped in 338 Mytilus individuals collected from nine Chilean and five reference populations. All analyses confirmed the hypothesis that the native Chilean blue mussel is genetically distinct from the reference species M. edulis, M. galloprovincialis and M. trossulus. These results support the hypothesis of a unique evolutionary history of the native Chilean blue mussel on the Pacific coast of South America. It is therefore concluded that the native blue mussel from Chile should be recognized as M. chilensis Hupé 1854. We confirmed a recent Mediterranean origin of introduced M. galloprovincialis on the coast of Chile. This knowledge advances the understanding of global phylogeography of blue mussels and their bioinvasions and harmonizes taxonomy in the context of aquaculture production, seafood traceability, labelling and trade.

Mytilus trossulus in NW Greenland is genetically more similar to North Pacific than NW Atlantic populations of the species

Changes in climate-related factors such as ice coverage, water temperature, and ocean currents have been proposed to facilitate an increased interchange of species in the High Arctic between the Pacific and Atlantic oceans. In Greenland, the colonization of the mussel species Mytilus edulis has been suggested to have occurred recently and exclusively from Northwest Atlantic populations. The source population for its sibling species Mytilus trossulus is however unknown, and therefore we aimed to explore its genetic origin. Using 54 SNP markers, M. trossulus was identified from three Greenland blue mussel populations collected in 2012–2014 and the relative similarities to Northwest Atlantic and North Pacific M. trossulus populations were assessed. Populations were found to fall into two clades. The North Pacific is the most likely source for the northern Greenland M. trossulus probably as a result of occasional postglacial long-distance dispersal through the Bering Strait.Our findings in M. trossulus are in agreements with the predictions that climate change will, in addition to driving a northward expansion of temperate-boreal species into the Arctic Ocean, increase the rate of trans-Arctic interchange between the Atlantic and the Pacific oceans.