ASYMMETRIC INTROGRESSION OF MITOCHONDRIAL DNA AMONG EUROPEAN POPULATIONS OF BLUE MUSSELS (MYTILUS SPP.)

Abstract.—Mytilus edulis and M. galloprovincialis are two blue mussel species that coexist in western Europe. Previously, we reported that M. galloprovincialis populations contain female and male haplotypes that are fixed in M. edulis populations as well as unique haplotypes. This study assesses whether paraphyly for these species is due to introgression or incomplete lineage extinction. The lineage extinction hypothesis predicts that the shared mtDNA haplotypes in M. galloprovincialis will be significantly diverged from those in M. edulis and form distinct sequence clades. In contrast, the introgression hypothesis proposes that M. edulis haplotypes have only recently been introduced into M. galloprovincialis through hybridization with relatively little divergence accumulating between the shared RFLP haplotypes. We examined 80 mtl6S gene sequences for both the maternal and paternal mtDNA lineages from mussels sampled from various European populations and found strong support for the introgression hypothesis. In addition, we found that M. edulis mtDNA haplotypes appear to be introgressing into mussel populations in the Baltic Sea, which have predominantly M. trossulus nuclear genotypes, indicating that introgressive hybridization is prevalent among European mussel populations.     

RECRUITMENT AND LIFE SPAN OF TWO NATURAL MUSSEL POPULATIONS PERNA PERNA(LINNAEUS) AND MYTILUS GALLOPROVINCIALIS (LAMARCK) FROM THE ALGERIAN COAST

Coexisting populations of the mussels, Perna perna and Mytilusgalloprovincialis, were monitored at two sites on the NorthAfrican coast, east of Algiers, over a five year period (1985–1989).While spatfalls were observed throughout the year, only themajor spring-summer recruitment, which occurred during favourableweather conditions, contributed to the renewal of both musselspecies at both sites. Very high densities (>10, 000 ind.m^–2) were observed at both sites, but the mussel bedswere composed principally of young and small specimens due toharvesting of the largest animals for use as bait and for humanconsumption. The maximal length observed was 75 mm in P. pernaand 49 mm in M. galloprovincialis. The life span of the specieswas low, 12–24 months in P. perna and 11–28 monthsin M. galloprovincialis. This survey showed that M. galloprovincialisbecame dominant in both mussel beds due to its resistance todisturbance by human activities. (Received 5 January 1995; accepted 18 April 1995)     

Development of a real-time PCR assay for detection of Mytilus species specific alleles: Application to a sampling survey in Scotland

Shellfish aquaculture is a growing industry in Scotland, dominated by the production of the mussel Mytilus edulis, the native species. Recently the discovery of Mytilus galloprovincialis and Mytilus trossulus together with M. edulis and all 3 hybrids in cultivation in some Scottish sea lochs led to questions regarding the distribution of mussel species in Scotland. The establishment of an extensive sampling survey, involving the collection of mussels at 34 intertidal sites and 10 marinas around Scotland, motivated the development of a high-throughput method for identification of Mytilus alleles from samples. Three Taqman®-MGB probes and one set of primers were designed, based on the previously described Me 15/16 primers targeting the adhesive protein gene sequence, and samples were screened for the presence of M. edulis, M. galloprovincialis and M. trossulus alleles using real-time PCR. Mytilus edulis alleles were identified in samples from all 44 sites. Mytilus galloprovincialis alleles were found together with M. edulis alleles extensively in northern parts of the west and east coasts. Mytilus trossulus alleles were identified in samples from 6 sites in the west and south-west of Scotland. Because M. trossulus is generally undesirable in cultivation and therefore preventing the geographical spread of this species across Scotland is considered beneficial by the shellfish aquaculture industry, these 6 samples were further analysed for genotype frequencies using conventional PCR. Although distribution of the non-native species M. galloprovincialis and M. trossulus have proven to be more widespread than previously thought, there is no evidence from our study of either M. trossulus or M. galloprovincialis acting as an invasive species in Scotland. The real-time PCR method developed in this study has proven to be a rapid and effective tool for the identification of M. edulis, M. galloprovincialis and M. trossulus alleles from samples and should prove useful in future surveys, ecological or aquaculture management related studies in both unispecific and mixed species areas of these species.     

Three species of Mytilus and their hybrids identified in a Scottish Loch: natives, relicts and invaders?

Three species of the mussel, Mytilus, occur in the North Atlantic region, M. edulis, M. galloprovincialis and M. trossulus, and hybrid zones are present where their distributions overlap. M. edulis is a native species in the UK. M. galloprovincialis originated in the Mediterranean and its distribution extends northwards along the Atlantic seaboard to Scotland. Baltic Sea mussels have a M. trossulus ancestry but are highly introgressed by M. edulis. In recent decades, farming of mussels on long-line rope culture systems has been introduced into Scotland. On farms in Loch Etive, a form of mussel with a fragile shell and a different shape to either M. edulis or M. galloprovincialis has been increasing in frequency over recent years. Samples of fragile shelled, normal strong shelled and intermediate mussel types were sampled from two farms in 2006 and compared with samples of M. edulis, M. galloprovincialis and M. trossulus from other sources where their species identity is well established. Abundance relative to depth, shell strength, condition index and shell morphology were analysed together with 5 allozyme loci and one nuclear DNA genetic marker (Me 15/16). The fragile shelled mussels, and many of those classed as intermediate, were identified as a mixture of M. trossulus and M. trossulus x M. edulis hybrids. This identification was strongly supported by both morphological and genetic data and is the first record of the presence of M. trossulus in UK waters. M. trossulus in Loch Etive are most likely to be a post-glacial relict population restricted to the low salinity area of the Loch that has recently increased in abundance due to commercial mussel growing activity. In addition, individual mussels of all three species and their hybrids were detected amongst Loch Etive mussels. This is the first genetic demonstration of all three species and their hybrids occurring together in one location in the Atlantic region and provides a unique opportunity to study the processes of speciation, divergence, and introgression in the genus Mytilus.     

Influence of genotype and geography on shell shape and morphometric trait variation among North Atlantic blue mussel (Mytilus spp.) populations

The influence of geography and genotype on shell shape (outline) and trait (morphometric) variation among North Atlantic blue mussels and their hybrids has been examined. Shape differences among reference taxa (Mytilus trossulus, Mytilus edulis and Mytilus galloprovincialis) were consistent with an association between taxon‐specific genes and shape genes. Newfoundland M. edulis × M. trossulus populations and northern Quebec M. trossulus populations exhibited an uncoupling of taxon‐specific genes from shape genes, whereas Nova Scotia M. trossulus populations and SW England M. edulis × M. galloprovincialis populations exhibited an association between taxon‐specific genes and shape genes. We found no evidence of a geographic effect (NE versus NW Atlantic) for shape variation, indicating that the genotype effect is stronger than any geographic effect at macrogeographic scales. Pronounced differences were observed in trait variability consistent with an association between taxon‐specific genes and trait genes in European populations, and trait divergence of New York M. edulis from all European mussels. Trait variability in mussels from Newfoundland, Nova Scotia and northern Quebec indicated an uncoupling of taxon genes from trait genes, whereas trait variability in SW England M. edulis × M. galloprovincialis populations was consistent with background genotype, indicating a strong association between taxon genes and trait genes. A pronounced macrogeographic split (NE versus NW Atlantic) regardless of taxonomy was observed, indicating that geography exerts a greater influence than genotype on trait variation at the macrogeographic scale. This is consistent with pronounced within‐taxon genetic divergence, indicative of different selection regimes or more likely of different evolutionary histories of mussels on either side of the North Atlantic. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96 , 875–897.     

A first report on coexistence and hybridization of <Emphasis Type="Italic">Mytilus trossulus</Emphasis> and <Emphasis Type="Italic">M. edulis</Emphasis> mussels in Greenland

Semi-sessile Mytilus mussels are used as indicators of climate changes, but their geographic distribution is not sufficiently known in the Arctic. The aim of this study was to investigate the taxonomic status and genetic differentiation of Mytilus populations in a Northwest Greenlandic fjord at Maarmorilik, impacted by contaminations from a former mine. In this study, mussels were collected at three sites differing in exposure to environmental factors. A total of 54 polymorphic SNPs found in the Mytilus EST and DNA sequences analyzed were successfully applied to 256 individuals. The results provided the first evidence for the existence of M. trossulus in Greenland. The mussel from M. trossulus and M. edulis taxa are shown to coexist and hybridize in the fjord. The three studied sites were found to differ significantly in the distribution of taxa with a higher prevalence of M. trossulus in the inner fjord. The identified M. edulis × M. trossulus hybrids mostly had a hybrid index score of about 0.5, indicating a similar number of alleles characteristic for M. trossulus and M. edulis. There was a low number of backcrosses between ‘pure’ taxa and hybrids. This newly discovered hybrid zone between the two taxa is unique in comparison with the Canadian populations. As Mytilus mussels in Greenland hitherto have been regarded as the one taxon M. edulis, the results have importance for biogeography and future monitoring and environmental studies.     

GENETICS OF PHYSIOLOGICAL DIFFERENTIATION WITHIN THE MARINE MUSSEL GENUS MYTILUS

Two divergent taxa in the marine mussel genus Mytilus are largely isolated geographically and are routinely exposed to distinctly different thermal environments. We tested the hypothesis that the two taxa are physiologically differentiated with respect to temperature and examined the evolved adaptations allowing one of the taxa to exploit habitats where warm-temperate conditions prevail for prolonged periods. We first analyzed the physiological response to high temperature of mussels collected from a hybrid population containing members of both pure taxa, F, hybrids, and a variety of introgressed genotypes. The experimental temperature of 23°C was chosen to be permissive to the taxon that occurs in warm-temperate regions (Mytilus galloprovincialis) and restrictive to the cold-water taxon (Mytilus edulis). The results show that the two taxa are physiologically differentiated. Under the experimental conditions, M. galloprovincialis exhibited a threefold higher feeding rate and a slightly elevated metabolic rate compared with M. edulis. These differences did not result in a significant difference in net energy balance between the two taxa, probably because of an interaction between physiological response and food availability. However, M. galloprovincialis grew significantly faster in the field, indicating that the physiological differences observed in the laboratory also occur in nature. Numerous introgressed genotypes provided the opportunity to test for cosegregation between the physiological differences and four highly differentiated genetic markers. Two of the markers (esterase and octopine dehydrogenase) cosegregate with variation in feeding rate and shell growth and explained most of the physiological differences observed between taxa. A strong concordance existed between these two loci, suggesting that they may be linked and may mark segregation of the same linkage group. The results suggest that the physiological differentiation between these taxa may be controlled by a few genes (perhaps only one) each with large effect.     

SPATIAL VARIATION IN THE FAUNA ASSOCIATED WITH MYTILUS EDULIS ON A WAVE-EXPOSED ROCKY SHORE

The fauna associated with Mytilus edulis at a rocky wave-exposedlocality in North Wales is reported. A total of fifty-nine taxawith representatives from most of the main invertebrate phylawas recorded. In terms of species richness the community wasdominated by crustaceans (25 taxa), molluscs (10 taxa) and cheliceratearthropods (9 taxa), mainly mites. Fora-miniferans and crustaceanswere the most abundant taxa, representing 25% and 23% of thetotal associated fauna respectively. Nemerteans and nema-todeswere also well represented. The mussel bed was broadly stratifiedinto three distinct layers each dominated by a single species.Thus, the foramini-feran Ammonia batavus was restricted to thesediment in the bottom layer, the barnacle Semibalanus balanoidesoccurred only on the outer exposed surfaces of the mussel shells,whilst the tiny brooding bivalve, Lasaea rubra, nestled mainlyamongst the complex matrix of byssal threads and shell fragmentsin the middle layer of the bed. Species richness and diversitydecreased systematically with tidal elevation and both theseindices were also signifocantly depressed amongst mussel communitiesfrom high-shore tide pools. Inclination of the rock surfacehad little or no effect on the mussel populations or their associatedfauna. Small mussel patches generally supported fewer taxa perunit area and had a lower diversity than larger, more extensivepatches. These marked, small-scale spatial variations exhibitedby the fauna associated with M. edulis appear to be relatedlargely to the degree of aerial and tidal exposure, mussel density,the amount of accumulated sediment and mussel patch size. (Received 2 August 1993; accepted 8 November 1993)     

A population genetic study of the hybrid zone of Mytilus trossulus Gould, 1850 and an introduced species,M. galloprovincialis Lamarck, 1819, (Bivalvia: Mytilidae) in peter the great bay in the Sea of Japan

This paper examines the genetic variability of the Pacific mussel Mytilus trossulus and an introduced Atlantic species, M. galloprovincialis, in the northwestern Sea of Japan (Peter the Great Bay and Kievka Bay). The genotyping of individuals from eight populations was carried out using eight polymorphic enzyme loci and two nuclear DNA markers (Me-5 and ITS-1,2); the occurrence frequency of parent species and their hybrids was determined. The enzyme and nuclear markers demonstrated concordant genetic variation. The genotypes of the native species M. trossulus were predominant in the samples studied. The frequency of the introduced species M. galloprovincialis in the total material was relatively low; however, it reached 42 ± 2% in samples that were collected in Possjet Bay near the town of Zarubino in a zone of active international navigation. In this area the greatest number of hybrids was found as well. It is concluded that the invasion of M. galloprovincialis in the northwestern Sea of Japan is continuing; permanent populations of this mussel appeared in Possjet Bay that were not recorded here previously.     

Bioinvasion threatens the genetic integrity of native diversity and a natural hybrid zone: smooth‐shelled blue mussels (Mytilus spp.) in the Strait of Magellan

Smooth‐shelled blue mussels of the Mytilus edulis species complex are widely distributed bivalve molluscs whose introductions threaten native marine biodiversity (non‐indigenous species – NIS). The aim of the present study was to identify the species and hybrids of Mytilus present in the Magellan Region (southern Chile). Results indicate that three mussel species of the Mytilus edulis complex are found in the region – M. edulis, M. chilensis (or the Southern Hemisphere lineage of Mytilus galloprovincialis), and M. galloprovincialis of Northern Hemisphere origin. For the first time, alleles of the introduced M. trossulus are reported from the Southern Hemisphere. In the Strait of Magellan the native Pacific blue mussel, Mytilus chilensis and the native Atlantic blue mussel, Mytilus edulis, meet and mix at a natural hybrid zone (about 125 km in length). This is the first record of a natural Mytilus hybrid zone in the Southern Hemisphere and is also the first record of the co‐occurrence of genes from all four Mytilus species in any one region. These results contribute to the knowledge of the biodiversity and delimitation of mussel species in southern South America, and highlight how introduced species may threaten the genetic integrity of native species through hybridization and introgression.     

Physiological energetics and biogeographic range limits of three congeneric mussel species

Closely related species with different physiological tolerances and distributions make ideal systems for documenting range shifts in response to a changing climate. Mytilus edulis, M. trossulus, and M. galloprovincialis are sibling species of marine mussels with distinct biogeographical ranges that are correlated with sea surface temperatures. We determined the scope for growth of these three species at a range of temperatures to determine if energetics could predict their distributions. Scope for growth (SFG) represents energy available for growth and/or reproduction above that necessary for maintenance requirements. The SFG of M. galloprovincialis, the species known to inhabit the warmest habitats, was shifted towards warmer temperatures compared to the other two species, remaining positive until nearly 30 °C. M. edulis, a cold-temperate species, maintained a positive SFG up to 23 °C. M. trossulus, a boreal species, generally was not able to maintain a positive SFG above 17 °C. The warm end of each species’ range correlated strongly with the point at which that species’ SFG became negative in summer and fall. Energetics at cold temperatures did not predict the cold end of the species’ ranges, as there was no clear SFG advantage to explain the dominance of M. trossulus in cold habitats. As sea surface temperatures continue to warm with climate change, the energetics of these three species provide a basis for developing mechanistic models predicting future distribution and productivity changes in mussel populations.     

Characterization of six microsatellite loci in the Baltic blue mussel <Emphasis Type="Italic">Mytilus trossulus</Emphasis> and cross-species amplification in North Sea <Emphasis Type="Italic">Mytilus edulis</Emphasis>

The blue mussel Mytilus trossulus occurs in the Pacific and in the North Atlantic. We developed and characterized six microsatellite loci for Baltic M. trossulus. Seventeen microsatellite loci were screened, of which six were polymorphic. The number of alleles among 50 individuals ranged from 3 to 13 and the observed and expected heterozygosity were 0.09–0.46 and 0.34–0.86, respectively. The loci were also tested for cross amplification in M. edulis, in which four of the six microsatellite loci were successfully amplified.     

MORPHOLOGICAL AND ALLOZYMIC VARIATION IN POPULATIONS OF MYTILUS GALLOPROVINCIALISFROM THE AEGEAN SEA

Eight populations of Mytilus galloprovincialis from differentcoasts of the Northern and Central Aegean Sea have been investigatedat the morphological and allozymic level. A significant degreeof variation was found within and among the populations. Theaverage degree of heterozygosity ranges from 0.085 to 0.14 andit is lower than those reported for other populations of thesame species. No correlation was found between the degree ofheterozygosity and the morphological variability among the populationsexamined. (Received 14 May 1992; accepted 1 October 1992)     

KARYOTYPES OF FIVE SUBANTARCTIC BIVALVE SPECIES

Chromosomes of five subantarctic species were studied from mitoticmetaphases using cell suspension techniques. Among the Protobranchia,Malletiagigan-lea has a diploid chromosome number of 2n = 38with five metacentric, seven submetacentric, five sub-telocentricand one telocentric pairs, and Yoldia (Aequiyoldia) woodwardialso shows 2n = 38 but with ten metacentric, six submetacentricand three sub-telocentric pairs. Among the Lamellibranchia,the pterioidan Limatula pygmaea has 2n = 38 with six metacentric,eleven submetacentric and two sub-telocentric pairs, the veneroidanCyclocardia astar-toides has 2n = 30 with five metacentric andten telocentric pairs and the anomalodesmatan Laternula ellipticahas 2n = 40 with two metacentric, one submetacentric, two subtelocentricand fifteen telocentric pairs. Our results indicate that in all the Nuculoidea studied so far,a diploid number of 2n = 38 has been found. In addition, thekaryotypes show a close overall appearance in relative lengthswith a majority of meta-centric-submetacentric chromosomes.The species differ in the proportion of the different morphologicaltypes of chromosomes. In the Lamellibranchia, each species correspondsto a particular case within their respective orders (Pterioida,Veneroida, Anomalodesmata). (Received 22 January 1990; accepted 28 March 1990)     

Study of Two Species of Mussels, Mytilus trossulusand Mytilus galloprovincialis(Bivalvia, Mytilidae), and Their Hybrids in Peter the Great Bay of the Sea of Japan with the Use of PCR Markers

Bivalve mollusks of the genus Mytilus(M. trossulusand M. galloprovincialis) occurring in Peter the Great Bay of the Sea of Japan were first studied in Russia. A region of nonrepetitive sequences of the gene encoding the polyphenolic adhesive protein bissus was used as a species-specific genetic marker. After amplification using specific primers, a 126-bp fragment was found to amplify in all representatives ofM. galloprovincialiscollected from driftwood in the gulf Posset (the southwestern part of Peter the Great Bay). M. trossulusspecimens from the same region were shown to have a 168-bp fragment. In Vostok Gulf (the eastern part of Peter the Great Bay), both artificially grown mussels and those from natural habitats contained a 168-bp fragment or two fragments (126- and 168-bp) that corresponded to a hybrid form between the above species. The possibility of using this genetic marker to identify closely related Mytilusstrains and their hybrids in similar habitats, near the Primorye coast in particular, was demonstrated. The presence of approximately 9% of hybrid specimens confirms that a zone of hybridization between M. trossulusand M. galloprovincialismay exist in this region.     

Effect of Sodium on Phosphate Uptake in Unicellular and Filamentous Cyanobacteria

The effect of Na⁺ on phosphate uptake was studied in four strainsof cyanobacteria: Synechococcus PCC 7942, Gloeothece PCC 6501,Phormidium sp. and Chlorogloeopsis PCC 6912. Phosphate uptakewas stimulated by Na⁺ in all cases. Li⁺ and K⁺ acted as partialanalogues for Na⁺. Half-saturation [K_1/2(Na⁺)] of phosphateuptake was reached with Na⁺ concentrations ranging from 317µM in Chlorogloeopsis to 659 µM in Phormidium. Theconcentration of phosphate required to reach half-saturationof phosphate uptake [K_1/2(P_i)]was not changed by the presenceof Na⁺. (Received April 11, 1994; Accepted July 5, 1994)     

DISTRIBUTION AND PRODUCTION OF THREE HYDROBIA SPECIES (GASTROPODA: HYDROBIIDAE) IN A SHALLOW COASTAL LAGOON IN THE BAY OF CADIZ, SPAIN

The abundance, life span, growth and production of the mud snailsHydrobia minoricensis, H. ulvae and H. ventrosa in a semi-naturallagoon system were studied by taking monthly samples at threesites during 1991 and 1992. The most abundant species, H. minoricensisoccurred at mean densities of 12834 to 26264 snails m^–2(10.7 to 25.8g dry weigh m^–2), depending on the site.The least abundant species, H. ulvae, occurred at mean densitiesof 185 to 353 snails m^–2 (3.2 to 2.2g dry weight m^–2).The numerical abundance and biomass of the three Hydrobia specieswere positively related to the biomass of benthic macroalgae(P<0.01). Although H. ulvae egg capsules were recorded throughoutthe year, newly hatched snailsof this species were not observed,in contrast to the other two species. The early spring and summercohorts of H. minoricensis and H. ventrosa seemed to be themost numerous. The average life spans of these two species wereestimated to be about 18 and 13 months respectively. Annualproduction estimates for the whole lagoon system were 29.0 (6.3),5.5 (0.8) and 5.2 (1.0)g dry weight (ash-free dry weight) m^–2yr^–1 for H. minoricensis, H. ulvae and H. ventrosa respectively.The annual P/B ratio was about 2 for H. minoricensis and H.ventrosa. (Received 5 July 1994; accepted 5 October 1994)     

The Control Region of Maternally and Paternally Inherited Mitochondrial Genomes of Three Species of the Sea Mussel Genus Mytilus

Species of the mussel genus Mytilus possess maternally and paternally transmitted mitochondrial genomes. In the interbreeding taxa Mytilus edulis and M. galloprovincialis, several genomes of both types have been fully sequenced. The genome consists of the coding part (which, in addition to protein and RNA genes, contains several small noncoding sequences) and the main control region (CR), which in turn consists of three distinct parts: the first variable (VD1), the conserved (CD), and the second variable (VD2) domain. The maternal and paternal genomes are very similar in gene content and organization, even though they differ by >20% in primary sequence. They differ even more at VD1 and VD2, yet they are remarkably similar at CD. The complete sequence of a genome from the closely related species M. trossulus was previously reported and found to consist of a maternal-like coding part and a paternal-like and a maternal-like CR. From this and from the fact that it was extracted from a male individual, it was inferred that this is a genome that switched from maternal to paternal transmission. Here we provide clear evidence that this genome is the maternal genome of M. trossulus. We have found that in this genome the tRNA^Gln in the coding region is apparently defective and that an intact copy of this tRNA occurs in the CR, that one of the two conserved domains is missing essential motifs, and that one of the two first variable domains has a high rate of divergence. These features may explain the large size and mosaic structure of the CR of the maternal genome of M. trossulus. We have also obtained CR sequences of the maternal and paternal genomes of M. californianus, a more distantly related species. We compare the control regions from all three species, focusing on the divergence among genomes of different species origin and among genomes of different transmission routes.     

Nonhomologous Recombination Between the Large Unassigned Region of the Male and Female Mitochondrial Genomes in the Mussel, Mytilus trossulus

Doubly uniparental inheritance of mtDNA (DUI) is commonly observed in several genera of bivalves. Under DUI, female offspring inherit mtDNA from their mothers, while male offspring inherit mtDNA from both parents but preferentially transmit the paternally inherited mtDNA to their sons. Several studies have shown that the female- and male-specific mtDNA lineages in blue mussels, Mytilus spp., vary by upward of 20% at the nucleotide level. In addition to high levels of nucleotide substitution, the present study observed substantial gender-based length polymorphism in the presumptive mitochondrial control region (=large unassigned region; LUR) of North American M. trossulus. In this species, female lineage LUR haplotypes are over 2 kb larger than male lineage LUR haplotypes. Analysis of sequence data for these length variants indicates that the F LUR haplotypes of North American M. trossulus contain sequences similar to the F lineage control region in the congeners M. edulis and M. galloprovincialis. Relative to the F LUR in the latter two species, however, the F lineage LUR haplotypes in M. trossulus contain two large sequence insertions, each nearly 1 kb in size. One of these insertions has high sequence similarity to the male lineage LUR of M. trossulus. The tandem arrangement of F and M control region sequences in the F lineage LUR of M. trossulus is most likely the result of nonhomologous recombination between the male and the female mitochondrial genomes in M. trossulus, a finding that has important implications regarding the transmission and evolution of blue mussel mitochondrial genomes. [Reviewing Editor: Dr. Martin Kreitman]     

Survey and management of mussel Mytilus species in Scotland

The important ecological role of Mytilus mussels in marine ecosystems, their high abundance in coastal waters and the demand for human consumption has made them a target species for aquaculture. Mussel cultivation is the most important and rapidly growing sector of the Scottish shellfish aquaculture industry and until recently production was considered to be based exclusively on the native species Mytilus edulis. However, the sympatric occurrence of M. edulis, M. trossulus, M. galloprovincialis and their hybrids in cultivation has recently been reported and significant production losses (over 50% at some sites) have been attributed to the presence of fragile-shelled M. trossulus. Given the ecological and economical importance of these species, an urgent need arose for a wider understanding of Mytilus species distribution on Scottish coasts and its implication for the sustainability of the Scottish shellfish industry. Here we present a summary of a 3-year project established within the “ECOsystem approach to SUstainable Management of the Marine Environment and its living Resources” (ECOSUMMER) Marie Curie network to address this need. We developed DNA-based molecular assays for the detection and surveillance of the different Mytilus species in Scotland. Several potential management strategies have been explored, aimed at favouring M. edulis production at mixed-species sites, but these have so far not been found to provide the reliable efficacy necessary for adoption by the industry. Complete eradication of M. trossulus from economically affected areas in Scotland may be unrealistic, especially considering that its introduction and distribution mechanisms in the environment remain uncertain. Area-specific solutions to managing the problem may thus be required, which may or may not involve eradication and fallowing (clearance of mussels from production sites for a given period of time). Nevertheless, the current distribution of M. trossulus is limited and its spread outside its existing range is clearly undesirable. Any management solutions must also be accompanied by an industry wide strategy and awareness, for example, through the development of an industry supported code of good practice.