Introgression and mitochondrial DNA heteroplasmy in the Baltic populations of mussels Mytilus trossulus and M. edulis

A strong clinal change in salinity occurs between the Baltic Sea and the North Sea, Atlantic Ocean, in the Danish Straits, where hybridization zone between mussels Mytilus edulis and M. trossulus has been reported. Eleven samples of mussels were studied from the Danish Straits and the inner Baltic Sea. Extensive introgression of M. edulis alleles from the North Sea into populations throughout the Baltic was ascertained for mitochondrial DNA (mtDNA) and two nuclear markers (ME15–16 and ITS). In the opposite direction, introgression of M. trossulus alleles into the M. edulis background was observed at the EFbis nuclear marker in populations from Kattegat (Danish Straits). While only M. edulis F (female) mtDNA was present in the Baltic, there were still strong differences in frequencies in the control region length variants between the Danish Straits and the inner Baltic samples, and weaker variation in coding region ND2–COIII haplotype frequencies. In the assays of the two mtDNA regions, various patterns of heteroplasmy were detected in 32% of all the studied individual mussels; this includes the presence of distinct, independently inherited M and F mitochondria in males, as well as the presence of two different distinguishable F genomes. The male-inherited M mtDNA genomes are quite common in the mussels from the Danish Straits, but very rare in males from the inner Baltic. Instead, a recombined control region variant (1r), which seems to have taken over the role of the M genome, was present in a number of specimens in the Baltic. Observations of heteroplasmy for two F genomes in some females and males confirm disruptions of the doubly uniparental inheritance mechanism in the hybrid Baltic Mytilus.     

Genetic variation of mitochondrial DNA in mussel (Mytilus edulis and M. galloprovincialis) populations from South West England and South Wales

Mitochondrial DNA (mtDNA) and allozyme variation were analysed in samples of mussels collected in 1984 and 1985 from four localities in South West England and one locality in South Wales, a region of Britain where the common mussel (Mytilus edulis) occurs sympatrically and hybridises with the Mediterranean mussel (M. galloprovincialis). Significant differences in mtDNA genotype frequencies for three restriction enzymes (BstEII, XbaI, and EcoRI) were observed between mussels from M. galloprovincialis populations (Padstow and Bude) and those from an M. edulis population (Swansea). Some mtDNA genotypes at high-frequency in M. galloprovincialis were not observed in M. edulis, although there was no indication that mtDNA variation provides greater overall diagnostic power than allozyme variation in distinguishing between the two forms of mussel. Construction of a phylogenetic tree of multiple mtDNA genotypes revealed small mutational distances between the genotypes characterising M. edulis and M. galloprovincialis. The results were consistent with predominant mtDNA flow from M. edulis to M. galloprovincialis. This can be explained by the dispersal of larvae to South West England from M. edulis regions to the north and east, but little dispersal in the opposite directions. Samples from two hybrid populations (Whitsand and Croyde) were analysed. mtDNA genotype frequencies at Croyde were in line with predictions made on the basis of two partially diagnostic allozyme loci (Est-D and Odh), mtDNA frequencies at Whitsand were not. Frequencies of some mtDNA genotypes at Whitsand were characteristic of M. edulis, others of M. galloprovincialis. Differential selective mortality or flow of different mtDNA genotypes and allozyme variation are proposed as possible causes of these results.     

Genetic relations between the bivalves Cardium (Cerastoderma) edule,Cardium lamarcki and Cardium glaucum,studied by means of crossed immunoelectrophoresis

Rabbit antisera were raised against Cardium edule (L.) from the Waddensea locality, Skallingen, against Cardium lamarcki (Reeve) from the Danish locality, Dybsø Fjord, and against Cardium glaucum (Bruguière) from the French Mediterranean coast at Ile des Embiez. Allopatric and sympatric populations of C. lamarcki and C. edule from Swedish and Danish localities, and allopatric populations of C. glaucum from the Mediterranean Sea, Red Sea and Black Sea were studied by means of the described method and by starch gel electrophoresis of malate dehydrogenase (MDH). By means of crossed immuno-electrophoresis, using intermediary gels containing antibodies against one of the Cardium types and receiver gels containing antibodies against one of the others, type-specific and type-non-specific reactions were recorded. The results are compared with a similar investigation of Mytilus edulis, M. galloprovincialis and M. californianus. Based on immunological properties, it is shown that C. edule is a valid species different from the C. lamarcki/C. glaucum complex. It is further suggested that C. lamarcki and C. glaucum should still be referred to by these two names as significant genetic differences between them is revealed by this immunological study.     

Phylogeographic patterns of the mysid Mesopodopsis slabberi (Crustacea, Mysida) in Western Europe: evidence for high molecular diversity and cryptic speciation

The phylogeographic patterns among populations of Mesopodopsis slabberi (Crustacea, Mysida), an ecological important mysid species of marine and estuarine habitats, were analysed by means of DNA sequencing of a fragment of the mitochondrial cytochrome c oxidase subunit I (COI) and the 16S ribosomal RNA genes. Samples of M. slabberi collected from five Atlantic and two Western Mediterranean populations were investigated. Very high levels of within-population molecular diversity were observed in all samples (mean h=0.807 and π=0.0083), with exception of the Mediterranean Ebro population which contained only one haplotype. Differentiation among populations was high, and a clear phylogeographic break was observed between the Atlantic and Mediterranean populations. Moreover, a strong differentiation was detected between both populations in the Western Mediterranean Sea (Alicante and Ebro delta), while two divergent lineages occurred in sympatry within the Atlantic Mondego estuary. The high congruence between both the COI and 16S rRNA sequence data, the reciprocal monophyly of the different mitochondrial clades and the levels of nucleotide divergence between them suggest the presence of a complex of cryptic species within M. slabberi. Estimations of divergence time between the different mitochondrial lineages indicate that a split occurred during the late Miocene/early Pliocene. Such a divergence could be concordant with vicariant events during sea-level drops within the Mediterranean region at that time. However, within the Mediterranean Sea, the potential of divergence through ecological diversification cannot be ruled out.     

Mitochondrial DNA variation in king mackerel (Scomberomorus cavalla) from the western Atlantic Ocean and Gulf of Mexico

King mackerel (Scomberomorus cavalla Cuvier) collected in 1992 and 1993 from 13 localities along the Atlantic coast of the southeastern USA and in the northern Gulf of Mexico were surveyed for variation in mitochondrial (mt)DNA and a nuclear-encoded dipeptidase locus (PEPA-2). Both polymorphic and fixed mtDNA restriction sites were identified and mapped using conventional and polymerase chain-reaction (PCR)-based methods. Heterogeneity in mtDNA haplotype frequencies was found only in comparisons of pooled haplotypes from Atlantic localities versus pooled haplotypes from Gulf localities. This finding indicates weak genetic divergence between king mackerel from the Atlantic and those from the Gulf. Frequencies of two PEPA-2 alleles essentially paralleled previous findings: one allele (PEPA-2a) was common among samples from western Gulf localities, whereas the other allele (PEPA-2b) was common among samples from Atlantic and eastern Gulf localities. There was considerable variation in PEPA-2 allele frequencies within broadly-defined regions. Variation in mtDNA haplotypes and PEPA-2 genotypes was independent, as was variation in mtDNA haplotypes with sex or age of individuals. Variation in PEPA-2 genotypes was not independent of sex or age of individuals. The latter result suggests that frequencies of PEPA-2 alleles in samples of king mackerel may stem, in part, from sex and age distributions of individuals within samples, and indicates that caution should be exercised in using allelic variation at PEPA-2 as a measure of population (stock) structure in king mackerel. The discordance in spatial patterning of mtDNA haplotypes versus PEPA-2 alleles across the Gulf (i.e. homogeneity in mtDNA haplotype frequencies versus heterogeneity in PEPA-2 allele frequencies) may be due to either female excess at several localities, sex-biased migration, or both. Observed patterns of genetic variation also are consistent with the hypothesis that king mackerel in the western Atlantic may have been subdivided during Pleistocene glaciation, and that the current distribution of PEPA-2 alleles may be a historical artefact. Received: 17 December 1996 / Accepted: 2 April 1997     

Phylogeography of Cerastoderma glaucum (Bivalvia: Cardiidae) across Europe: a major break in the Eastern Mediterranean

Molecular variation of the lagoon cockle Cerastoderma glaucum (Poiret, 1789) was examined across the species range along European coasts, from the northern Baltic to the Black Sea and the Caspian Sea. A major phylogeographic break in mitochondrial COI gene sequences (divergence 6.2%) separated a group of Ponto-Caspian and Aegean Sea haplotypes from those to the west of the Peloponnese peninsula in the Mediterranean and in the Atlantic-Baltic sector. A similar subdivision, yet not entirely geographically coincident, was found at 16 allozyme loci (Nei's distance D=0.15). The results imply a long-term isolation of populations in parts of the Eastern Mediterranean or Black Sea basins through the Pleistocene. The subdivision does not concur with previous views on the systematics of C. glaucum complex, but the pattern is notably similar to that described earlier from some fish species. Marked phylogeographic structuring was also found at lower level within the major Mediterranean-Atlantic phylogroup of C. glaucum, which was divided into six regional or local haplotype subgroups. Divergence of these groups may date back one or several major Pleistocene climatic cycles. Local mtDNA diversity was particularly high in a sample from the Ionian Sea, whose mitochondrial identity was of the Mediterranean-Atlantic type, while its nuclear characters were more strongly associated with the Ponto-Caspian type. Patterns of shallower, star-phylogeny type diversity within the Ponto-Caspian phylogroup and in the Baltic Sea area may represent more recent, post-glacial generation of variation.     

Allozyme variation in European populations of the oyster Ostrea edulis

Variations at 22 enzyme coding loci were surveyed in 11 populations of the oyster Ostrea edulis L., which were sampled between 1988 and 1990 along the Atlantic and Mediterranean coasts of Europe. Atlantic oyster beds suffered a steady decline during the last century, and restocking of beds with oysters of foreign origin has probably resulted in a high degree of interbreeding of natural oyster stocks from all Atlantic Europe. Our study confirms the low levels of genetic variability previously reported for the oyster populations from the Atlantic coasts, and extends it to the Mediterranean coasts. The locus arginine-kinase (ARK ^*) exhibited a high degree of interpopulation differentiation (F _ST=0.289), resulting from extensive variation in gene frequencies along a geographical cline. However, the overall genetic differentiation between populations was slight, and similar to that reported for other local populations of bivalves (mean genetic distance between populations is 0.010, mean F _ST=0.062). A general pattern of increasing differentiation along the coastline in an Atlantic-mediterranean direction emerged; but genetic differentiation among the Atlantic populations was not significantly lower than that observed among the Mediterranean populations. This and other results suggest that the effects of extensive transplantation of oysters among various areas in Europe are detectable only in some particular localities. The geographical distribution of low-frequency alleles suggests a restriction to gene flow outwards from the Mediterranean Sea, across the Straits of Gibraltar.     

Genetic structure of populations of two species of Chthamalus (Crustacea: Cirripedia) in the north-east Atlantic and Mediterranean

Protein electrophoresis on starch gels was used to investigate population genetic structure of the barnacles Chthamalus montagui Southward and C. stellatus (Poli) over their north-east Atlantic and Mediterranean ranges. In each species, a single locus exhibited marked differentiation of allele frequencies between Atlantic and Mediterranean localities; in C. stellatus, genetic differentiation between the two basins had not previously been noted. In both species, mean heterozygosity per locus appeared higher in the Mediterranean samples than in the Atlantic, and Mediterranean populations had more alleles at the loci studied. Possible explanations for the differentiation between the Atlantic and the Mediterranean populations are discussed. Received: 30 May 1996 / Accepted: 17 September 1996     

Genetic structure and allozyme variation of sea bass (Dicentrarchus labrax and D. punctatus) in the Mediterranean Sea

This paper reports data on 28 allozyme loci in wild and artificially reared sea bass (Dicentrarchus labrax) samples, originating from either coastal lagoon or marine sites in the Mediterranean Sea. F _ST analysis (θ estimator) indicated strong genetic structuring among populations; around 34% of the overall genetic variation is due to interpopulation variation. Pairwise θ estimates showed that, on average, the degree of genetic structuring was much higher between marine populations than between samples from lagoons. Six polymorphic loci showed differences in allele frequencies between marine and lagoon samples. Multivariate analyses of individual allozymic profiles and of allele frequencies suggested that different arrays of genotypes prevail in lagoons compared to marine samples, particularly at those loci that, on the basis of previous acclimation experiments, had been implicated in adaptation to freshwater. On the other hand, variation at “neutral” allozyme loci reflects to a greater extent the geographic location of populations. Allozyme differentiation was also studied in a D. labrax population from the Portuguese coast. Average genetic distance between this population and the Mediterranean populations was quite high (Nei's D = 0.236) and calls into question the taxonomic status of the Portuguese population. Finally, genetic relationships between D. labrax and D. punctatus were evaluated. Average Nei's D was 0.648, revealing high genetic differentiation between the two species, even for two sympatric populations of these species in Egypt; thus gene flow was not indicated between species. Received: 24 October 1996 / Accepted: 27 November 1996     

Atlantic-Mediterranean and within-Mediterranean molecular variation in <Emphasis Type="Italic">Coris julis</Emphasis> (L. 1758) (Teleostei,Labridae)

Sequence variation in the mitochondrial control region was studied in the Mediterranean rainbow wrasse (Coris julis), a species with pronounced pelagic larval phase inhabiting the Mediterranean Sea and the adjacent coastal eastern Atlantic Ocean. A total of 309 specimens from 19 sampling sites were analysed with the aim of elucidating patterns of molecular variation between the Atlantic and the Mediterranean as well as within the Mediterranean Sea. Phylogeographic analyses revealed a pronounced structuring into a Mediterranean and an Atlantic group. Samples from a site at the Moroccan Mediterranean coast in the Alboran Sea showed intermediate frequencies of “Mediterranean” and “Atlantic” haplotypes. We recognised a departure from molecular neutrality and a star-like genealogy for samples from the Mediterranean Sea, which we propose to have happened due to a recent demographic expansion. The results are discussed in the light of previous studies on molecular variation in fish species between the Atlantic and the Mediterranean and within the Mediterranean.     

Size-dependent, spatial and temporal genetic variation at a leucine aminopeptidase (LAP) locus among blue mussel (Mytilus galloprovincialis) populations along a salinity gradient

Biochemical genetic variation at a leucine aminopeptidase (LAP) locus is related to salinity variation in several marine bivalve molluscs. This paper details an investigation of the Long Island Sound model of LAP selection (LAP genotype-dependent mortality occurring among newly settled Mytilus edulis mussels) carried out in 1997 among three M. galloprovincialis mussel populations along the salinity gradient of Wellington Harbour, New Zealand. Significant LAP genotypic heterogeneity was observed at the LAP locus between small (<25 mm shell length) and large (>25 mm shell length) M. galloprovincialis from Petone and Eastbourne (the two sites experiencing the greatest salinity variation), whereas genotypic heterogeneity was not significantly different between small and large mussels from Seatoun (the site experiencing the least salinity variation). The Lap ³ allele decreased in frequency and the Lap ⁴ allele increased in frequency at Petone and Eastbourne, whereas the Lap ³ and Lap ⁴ allele frequencies remained effectively constant at Seatoun. Both these findings are consistent with the Long Island Sound model of selection. At all three locations, the Lap ^3,3 genotype decreased in frequency from small to large mussels, whereas the Lap ^3,4 genotype increased in frequency from small to large mussels. All other LAP genotypes occurred at low frequencies (<0.10) at all three locations and showed no evidence of frequency change from small to large-size mussels nor evidence of clinal change among the three locations. These genotype frequency data possibly indicate that the Lap ^3,3 genotype is at a selective disadvantage compared to the Lap ^3,4 genotype at all three locations, and that this selective disadvantage is related to the extent of salinity variation which exists at each location. Further investigation is required before it can be determined if the Long Island Sound model of selection best describes the size-dependent and location-specific changes in LAP allele and genotype frequencies along this salinity gradient. Comparison of the population genetic structure at the LAP locus in 1995 and in 1997 revealed a profound change from heterozygote excesses to heterozygote deficiencies for all three M. galloprovincialis populations. The reason for the change is unknown, but the change indicates that population genetic structure at the LAP locus is highly variable in time, but consistent in space, among these M.␣galloprovincialis populations. Received: 5 February 1998 / Accepted: 27 May 1998     

A biochemical genetic study of population structure in queen scallop (Chlamys opercularis) stocks in the Northern Irish Sea

Electrophoresis was used to survey a number of enzymes for loci of potential value as stock markers in Chlamys opercularis (L.). Two loci, Pgi and Pgd, were then studied in larger samples from each of two localities around the Isle of Man. One Pgi allele was found to be restricted to a particular year-class in both populations. This was confirmed by subsequent studies of three further populations, in each of which the same allele occurred only in the one year-class. Otherwise, no significant differences in allele frequencies were found between year-classes or between populations in any of the samples used. From the results it is concluded that there is temporal variation in the sources of recruitment of C. opercularis stocks around the Isle of Man. Possible origins of larvae contributing to these stocks are discussed in the light of what is known of current circulation patterns in the northern Irish Sea.     

Heterozygosity and growth in transplanted mussels

Growth comparisons were made involving mussels (Mytilus spp.) collected from five different localities in Britain in 1980–1981. Two of the localities, Mumbles, South Wales, and Bude, Southwest England, have pure populations of M. edulis and M. galloprovincialis, respectively. The other three, Whitsand Bay, Southwest England, Croyde Bay, Southwest England and Robin Hood's Bay, Northeast England, have hybrid populations with both M. edulis and M. galloprovincialis ancestry. To make growth comparisons, mussels from different populations were mixed in oyster nets and transplanted to three different localities. After periods of transplantation varying between several months and one year, growth was assessed by measuring increase in shell length or dry body weight. Starch-gel electrophoresis was used to assay variation in the transplanted mussels at three allozyme loci partially diagnostic for M. edulis and M. galloprovincialis. The results provide evidence of small growth differences between populations and between allozyme genotypes within populations. These differences accounted for no more than a few percent of the total variation in growth between mussels. Statistically significant results were obtained, but were frequently found not to be reproducible. There is no clear evidence of a growth difference between M. edulis and M. galloprovincialis. Allozyme heterozygotes appear to have growth rates intermediate between allozyme homozygotes; this study thus fails to provide evidence for overdominance with respect to growth rate.     

Application of random amplified polymorphic DNA (RAPD) markers to evaluate intraspecific genetic variation in red mullet (Mullus barbatus)

The random amplified polymorphic DNA (RAPD) technique was used to evaluate genetic affinities among eight red mullet (Mullus barbatus L., 1758) samples from the Mediterranean Sea. Twenty-nine random primers were used. Despite the variability which was found within samples, no specific RAPD marker for the discrimination of the populations was detected. The data analysis revealed that the genetic diversity among populations is positively related to their geographic distances. The results of this study indicate that the estimated intraspecific variation may be more␣pronounced with RAPD markers than with allozymes when the two approaches are applied on the same populations. Received: 7 November 1997 / Accepted: 30 May 1998     

Genetic variation between geographic populations of the amphipods Gammarus zaddachi and G. salinus

Spatial and temporal patterns of gene-enzyme variation were estimated in the sibling species Gammarus zaddachi Sexton and G. salinus Spooner by starch gel electrophoresis. Twenty-one G. zaddachi and 18 G. salinus populations from coastal and estuarine areas in the Baltic Sea, North Sea and other localities of north-western Europe were surveyed. Both amphipods display similar electrophoresis patterns of the enzyme systems studied. Considerable interspecific and interpopulational differences were detected in allele frequencies at three highly polymorphic loci, phosphoglucose isomerase (PGI), glutamate oxalacetate transaminase (GOT) and arginine phosphokinase (APK). G. zaddachi exhibits a pronounced genetic heterogeneity in most areas of the sampled range. Populations from northern French and western English coasts differ significantly from the other samples in allele frequencies at the PGI or APK locus, respectively. Baltic populations are widely uniform in their genetic composition but can be distinguished from samples taken at North Sea sites in allele frequencies at the APK locus. The latter reveal a clinal variation, ranging from the Danish to the French coast. In contrast to G. zaddachi, a low degree of genetic differentiation was observed among the G. salinus populations examined. This indicates that migration and interregional mixing may be more important in maintaining the genetic structure than in G. zaddachi which, compared to G. salinus, prefers habitats of lower salinity levels. Evidently, less extensive dispersal capabilities owing to the confinement of G. zaddachi to brackish waters of dilute salt concentrations may account for a diminished gene flow and considerable genetic separation of local populations. This assumption is supported by the genetic homogeneity documented in Baltic G. zaddachi populations. In view of the low and constant salinities in wide areas of this brackish-water sea such barriers do not exist. Survey studies performed with selected populations over a 3-yr period demonstrated a general pattern of temporal constancy in the allozyme variation observed.     

Genetic differentiation of the California spiny lobster Panulirus interruptus (Randall, 1840) along the west coast of the Baja California Peninsula, Mexico

Identifying reproductive stocks in commercial species is relevant to fishery management strategies. We obtained muscle samples of California spiny lobster (Panulirus interruptus) from six localities along the west coast of the Baja California Peninsula and analyzed the genetic structure using mtDNA RFLPs. Our results indicated that all localities shared the same major haplotypes and showed a spatial homogeneity in the distribution of haplotype frequencies. An analysis of molecular variance indicated that only 0.84% of the genetic variability was explained by differences among localities and was not significantly different from zero. Weak divergences were found between Bahía Magdalena, the most southerly locality, in relation to other populations. Major oceanographic processes along this coast, combined with a long larval period that supports passive transport among localities, are suggested to explain the results.     

Phylogeography of the sea star Marthasterias glacialis (Asteroidea, Echinodermata): deep genetic divergence between mitochondrial lineages in the north-western mediterranean

We explore the phylogeography of the broadcast spawner Marthasterias glacialis along south Europe and Azores. Sequences of the cytochrome c oxidase gene from 225 specimens, belonging to 10 localities, were analysed. We found 73 haplotypes grouped within two lineages (divergence 2.9%). One lineage was Atlanto–Mediterranean, whereas another one was exclusively Mediterranean. Estimation of lineages split goes back to 830,000–580,000 (±120,000) years ago. This suggests that sea-level oscillations during the Pleistocene glaciations promoted gene flow interruption, lineage divergence between basins and cryptic speciation. Secondary contact between populations allowed a recolonization of the Mediterranean by the Atlantic lineage. When animals of the Atlanto–Mediterranean lineage were considered separately, F _st index and AMOVA did not show significant differences between populations along either the Iberian Peninsula or basins. Isolation by distance between populations was not detected, and only populations of Plymouth and Azores showed significant differences to all the others. The remoteness of Azores islands might explain the structure of this population. Haphazard arrival of larvae and local extinctions rather than contemporary restricted gene flow might be responsible for the distinctive population structure of Plymouth.     

Genetic structure of the flounders Platichthys flesus and P. stellatus at different geographic scales

The genetic structure of the flounders Platichthys flesus L. and P. stellatus Pallas was investigated on different spatial scales through analysis of allozyme variation at 7 to 24 polymorphic loci in samples collected from different regions (Baltic Sea, North Sea, Brittany, Portugal, western Mediterranean, Adriatic Sea, Aegean Sea and Japan) in 1984 to 1987. No geographic variation was evident within a region. Some pattern of differentiation by distance was inferred within the Atlantic, while the Mediterranean comprised three geographically isolated populations and was itself geographically isolated from the Atlantic (fixed allele differences at up to three loci were found among P. flesus populations from the Atlantic, the western Mediterranean, the Adriatic Sea, the Aegean Sea and also P. stellatus from the coast of Japan). Sea temperature during the reproductive period probably acts as a barrier to gene flow between populations. Genetic distances among European flounder populations (P. flesus) were higher than, or of the same magnitude as, the genetic distance between Pacific (P. stellatus) and European flounder populations, suggesting that P. flesus is paraphyletic and/or there is no phylogenetic basis to recognising P. stellatus as a different species. The divergence between P. flesus and P. stellatus was thus inferred to be more recent than the divergence between the present P. flesus populations from the NE Atlantic and eastern Mediterranean. The eastern Mediterranean populations are thought to originate from the colonisation of the Mediterranean by a proto-P. flesus/P. stellatus ancestor, whereas the present western Mediterranean population has undergone a more recent colonisation event by P. flesus. Patterns of mitochondrial DNA variation, established on a smaller array of P. flesus samples, were in accordance with the geographic patterns inferred from the allozyme survey. In addition, they supported the hypothesis of a two-step colonisation of the western Mediterranean. These results contribute to our understanding of the biogeography of the Mediterranean marine fauna, especially the group of boreal remnants to which P. flesus belongs. Received: 7 February 1997 / Accepted: 26 March 1997     

Mitochondrial DNA lineages in the European populations of mussels (<Emphasis Type="Italic">Mytilus</Emphasis> spp.)

Marine mussels (Mytilus spp.) belong to a group of benthic species crucial to coastal ecosystems in Europe and are important for the cultivation industry. In the present study, the nuclear adhesive protein marker (Me15/16) was used for identification of Mytilus species in coastal areas, on a large geographic scale in Europe. Pure M. edulis populations were found in the White Sea and Iceland. M. edulis, M. trossulus and their hybrids were found in the Baltic Sea and the North Sea (Oosterschelde, The Netherlands). M. galloprovincialis, M. edulis and their hybrids occurred in Ireland. M. galloprovincialis populations were observed in the Sea of Azov (Black Sea), the Mediterranean and Portugal. The mitochondrial (mt) DNA coding-region ND2-COIII was studied by PCR (polymerase chain reaction) and RFLP (restriction fragment length polymorphism) assay methods. The mtDNA control region was studied by PCR. Substantial differentiation in the frequency of female haplotypes among the studied populations in Europe was observed. Despite isolation between the Mytilus taxa on a macro-geographic scale, considerable mitochondrial gene flow occurred between populations, with introgression in hybrid zones on a more local geographic scale in Europe. MtDNA of the Atlantic Iberian (Portugal) population of M. galloprovincialis was more similar to mtDNA in populations of M. galloprovincialis and M. edulis from the Atlantic coasts of the Ireland and M. edulis from the North Sea, than to M. galloprovincialis from the Mediterranean. Lower polymorphism of mtDNA in populations of the Baltic and Azov Sea mussels in comparison with other European populations was observed and can be explained by the recent history of both seas after the Pleistocene glaciation. In the M. galloprovincialis population from the Azov Sea, the presence of the male-inherited (M) genome was demonstrated for the first time by sequencing the control region and was observed at high frequency. Possible influence of mussel culture on geographic distribution of the Mytilus taxa in Europe is discussed.     

Mitochondrial DNA differentiation and population structure in red drum (Sciaenops ocellatus) from the Gulf of Mexico and Atlantic Ocean

Variation in mitochondrial (mt)DNA was examined among 473 red drum (Sciaenops ocellatus) sampled in 1988 and 1989 from nearshore localities in the northern Gulf of Mexico (Gulf) and the Atlantic coast of the southeastern United States (Atlantic). Data were combined with those from a previous study to generate a total of 871 individuals sampled from 11 localities in the Gulf and 5 localities in the Atlantic. Individuals assayed were from the 1986 and 1987 year-classes. A total of 118 composite mtDNA genotypes (haplotypes) was found. The percentage nucleotide sequence divergence among the 118 haplotypes ranged from 0.184 to 1.913, with a mean (±SE) of 0.878±0.004. MtDNA nucleon diversities and intrapopulational nucleotide-sequence divergence values were similar over all Gulf and Atlantic localities, and were high relative to most fish species surveyed to date. These data indicate that the perceived decline in red drum abundance appears not to have affected the genetic variability base of the species. Significant heterogeneity in the frequencies of at least four haplotypes was detected between pooled samples from the Gulf vs pooled samples from the Atlantic. No heterogeneity was found among localities from the Gulf or localities from the Atlantic. High levels of gene flow among all localities were inferred from F _ST values (a measure of the variance in mtDNA haplotype frequencies) and from Slatkin's qualitative and quantitative analyses. Parsimony and phenetic analyses revealed no strong evidence for phylogeographic cohesion of localities, although there was weak support for cohesion of four of five localities from the Atlantic. These data indicate that the red drum population is subdivided, with weakly differentiated subpopulations (stocks) occurring in the northern Gulf and along the Atlantic coast of southeastern USA. Spatial autocor-relation analysis and heterogeneity tests of haplotype frequencies among regions within the Gulf supported the hypothesis of increased gene flow among neighboring localities; i.e., migration of individuals within the Gulf may be inversely related to geographic distance from an estuary or bay of natal origin. Estimates of evolutionary effective female-population size indicate that the red drum subpopulations may be large.