Local adaptation and oceanographic connectivity patterns explain genetic differentiation of a marine diatom across the North Sea–Baltic Sea salinity gradient

Drivers of population genetic structure are still poorly understood in marine micro‐organisms. We exploited the North Sea–Baltic Sea transition for investigating the seascape genetics of a marine diatom, Skeletonema marinoi. Eight polymorphic microsatellite loci were analysed in 354 individuals from ten locations to analyse population structure of the species along a 1500‐km‐long salinity gradient ranging from 3 to 30 psu. To test for salinity adaptation, salinity reaction norms were determined for sets of strains originating from three different salinity regimes of the gradient. Modelled oceanographic connectivity was compared to directional relative migration by correlation analyses to examine oceanographic drivers. Population genetic analyses showed distinct genetic divergence of a low‐salinity Baltic Sea population and a high‐salinity North Sea population, coinciding with the most evident physical dispersal barrier in the area, the Danish Straits. Baltic Sea populations displayed reduced genetic diversity compared to North Sea populations. Growth optima of low salinity isolates were significantly lower than those of strains from higher native salinities, indicating local salinity adaptation. Although the North Sea–Baltic Sea transition was identified as a barrier to gene flow, migration between Baltic Sea and North Sea populations occurred. However, the presence of differentiated neutral markers on each side of the transition zone suggests that migrants are maladapted. It is concluded that local salinity adaptation, supported by oceanographic connectivity patterns creating an asymmetric migration pattern between the Baltic Sea and the North Sea, determines genetic differentiation patterns in the transition zone.     

Genetic population structure of turbot (Scophthalmus maximus L.) supports the presence of multiple hybrid zones for marine fishes in the transition zone between the Baltic Sea and the North Sea

Genetic population structure of turbot (Scophthalmus maximus L.) in the Northeast Atlantic was investigated using eight highly variable microsatellite loci. In total 706 individuals from eight locations with temporal replicates were assayed, covering an area from the French Bay of Biscay to the Aaland archipelago in the Baltic Sea. In contrast to previous genetic studies of turbot, we found significant genetic differentiation among samples with a maximum pairwise FST of 0.032. Limited or no genetic differentiation was found among samples within the Atlantic/North Sea area and within the Baltic Sea, suggesting high gene flow among populations in these areas. In contrast, there was a sharp cline in genetic differentiation going from the low saline Baltic Sea to the high saline North Sea. The data were explained best by two divergent populations connected by a hybrid zone; however, a mechanical mixing model could not be ruled out. A significant part of the genetic variance could be ascribed to variation among years within locality. Nevertheless, the population structure was relatively stable over time, suggesting that the observed pattern of genetic differentiation is biologically significant. This study suggests that hybrid zones are a common phenomenon for marine fishes in the transition area between the North Sea and the Baltic Sea and highlights the importance of using interspecific comparisons for inferring population structure in high gene flow species such as most marine fishes.     

A microsatellite-based estimation of clonal diversity and population subdivision in Zostera marina, a marine flowering plant

We examined the genetic population structure in eelgrass (Zostera marina L.), the dominant seagrass species of the northern hemisphere, over spatial scales from 12 km to 10 000 km using the polymorphism of DNA microsatellites. Twelve populations were genotyped for six loci representing a total of 67 alleles. Populations sampled included the North Sea (four), the Baltic Sea (three), the western Atlantic (two), the eastern Atlantic (one), the Mediterranean Sea (one) and the eastern Pacific (one). Microsatellites revealed substantial genetic variation in a plant group with low allozyme diversity. Average expected heterozygosities per population (monoclonal populations excluded) ranged from 0.32 to 0.61 (mean = 0. 48) and allele numbers varied between 3.3 and 6.7 (mean = 4.7). Using the expected frequency of multilocus genotypes within populations, we distinguished ramets from genetic individuals (i.e. equivalent to clones). Differences in clonal diversity among populations varied widely and ranged from maximal diversity (i.e. all ramets with different genotype) to near or total monoclonality (two populations). All multiple sampled ramets were excluded from further analysis of genetic differentiation within and between populations. All but one population were in Hardy-Weinberg equilibrium, indicating that Zostera marina is predominantly outcrossing. From a regression of the pairwise population differentiation with distance, we obtained an effective population size Ne of 2440-5000. The overall genetic differentiation among eelgrass populations, assessed as rho (a standardized estimate of Slatkin's RST) was 0.384 (95% CI 0.34-0.44, P < 0.001). Genetic differentiation was weak among three North Sea populations situated 12-42 km distant from one another, suggesting that tidal currents result in an efficient exchange of propagules. In the Baltic and in Nova Scotia, a small but statistically significant fraction of the genetic variance was distributed between populations (rho = 0.029-0. 053) at scales of 15-35 km. Pairwise genetic differentiation between European populations were correlated with distance between populations up to a distance of 4500 km (linear differentiation-by-distance model, R2 = 0.67). In contrast, both Nova Scotian populations were genetically much closer to North Sea and Baltic populations than expected from their geographical distance (pairwise rho = 0.03-0.08, P < 0.01). A biogeographical cluster of Canadian with Baltic/North Sea populations was also supported using a neighbour-joining tree based on Cavalli-Sforza's chord distance. Relatedness between populations may be very different from predictions based on geographical vicinity.     

Population genetic history of the dreissenid mussel invasions: expansion patterns across North America

This study tests population genetic patterns across the Eurasian dreissenid mussel invasions of North America—encompassing the zebra mussel Dreissena polymorpha (1986 detection) and the quagga mussel D. rostriformis bugensis (detected in 1990, which now has largely displaced the former in the Great Lakes). We evaluate their source-spread relationships and invasion genetics using 9–11 nuclear microsatellite loci for 583 zebra mussels (21 sites) and 269 quagga mussels (12 sites) from Eurasian and North American range locations, with the latter including the Great Lakes, Mississippi River basin, Atlantic coastal waterways, Colorado River system, and California reservoirs. Additionally, mtDNA cytochrome b gene sequences are used to verify species identity. Our results indicate that North American zebra mussels originate from multiple non-native northern European populations, whereas North American quagga mussels trace to native estuaries in the Southern Bug and Dnieper Rivers. Invasive populations of both species show considerable genetic diversity and structure (zebra F _ST = 0.006–0.263, quagga F _ST = 0.008–0.267), without founder effects. Most newer zebra mussel populations have appreciable genetic diversity, whereas quagga mussel populations from the Colorado River and California show some founder effects. The population genetic composition of both species changed over time at given sites; with some adding alleles from adjacent populations, some losing them, and all retaining closest similarity to their original composition. Zebra mussels from Kansas and California appear genetically similar and assign to a possible origin from the St. Lawrence River, whereas quagga mussels from Nevada and California assign to a possible origin from Lake Ontario. These assignments suggest that overland colonization pathways via recreational boats do not necessarily reflect the most proximate connections. In conclusion, our microsatellite results comprise a valuable baseline for resolving present and future dreissenid mussel invasion pathways.     

Phylogeographic evidence for the postglacial colonization of the North and Baltic Sea coasts from inland glacial refugia by Triglochin maritima L.

We investigated the geographical distribution of genetic variation in 67 individuals of Triglochin maritima from 38 localities across Europe using AFLP markers. Analysis of genetic variation resulted in the recognition of two major genetic groups. Apart from few geographical outliers, these are distributed (1) along the Atlantic coasts of Portugal, Spain and France and (2) in the North Sea area, the Baltic Sea area, at central European inland localities, the northern Adriatic Sea coast and the Mediterranean coast of southwest France. Considering possible range shifts of T. maritima in reaction to Quaternary climatic changes as deduced from the present-day northern temperature limit of the species, Quaternary changes of coastline in the North Sea area and the very recent origin of the Baltic Sea, we conclude that the coastal populations of T. maritima in the North Sea and Baltic Sea areas originated from inland populations.     

Genetic traits in the bivalve Mytilus from Europe, with an emphasis on Arctic populations

Genetic and some ecophysiological traits of mussels collected in the European Arctic, up to their northeastern distribution limit in the Barents Sea, were studied and compared with traits of mussels from the Mediterranean, Atlantic and Baltic. The genetic traits of these populations were analysed by isoenzyme electrophoresis on seven loci in order to assess the Mytilus complex to which populations in the Arctic region belong. Ecophysiological variables, the weight-index and glycogen were analysed to assess the physiological fitness of the populations. Three distinct groups were recognised: (1) Mytilus (edulis) galloprovincialis in the Mediterranean and Spain, (2) M. (edulis) edulis along the Atlantic coast from the Netherlands northwards into Russia, and (3) the Baltic Mytilus (edulis) trossulus. The mussels from populations in the Russian Arctic all belong to the Atlantic Mytilus (edulis) edulis group. The genetic variability and ecophysiological measures indicated that the sub-Arctic White Sea mussel populations have a relatively lower performance capacity, whereas those in the Arctic at the edge of their northern distribution showed a surprisingly strong performance. Accepted: 14 June 2000     

Regional environmental pressure influences population differentiation in turbot (Scophthalmus maximus)

Unravelling the factors shaping the genetic structure of mobile marine species is challenging due to the high potential for gene flow. However, genetic inference can be greatly enhanced by increasing the genomic, geographical or environmental resolution of population genetic studies. Here, we investigated the population structure of turbot (Scophthalmus maximus) by screening 17 random and gene‐linked markers in 999 individuals at 290 geographical locations throughout the northeast Atlantic Ocean. A seascape genetics approach with the inclusion of high‐resolution oceanographical data was used to quantify the association of genetic variation with spatial, temporal and environmental parameters. Neutral loci identified three subgroups: an Atlantic group, a Baltic Sea group and one on the Irish Shelf. The inclusion of loci putatively under selection suggested an additional break in the North Sea, subdividing southern from northern Atlantic individuals. Environmental and spatial seascape variables correlated marginally with neutral genetic variation, but explained significant proportions (respectively, 8.7% and 10.3%) of adaptive genetic variation. Environmental variables associated with outlier allele frequencies included salinity, temperature, bottom shear stress, dissolved oxygen concentration and depth of the pycnocline. Furthermore, levels of explained adaptive genetic variation differed markedly between basins (3% vs. 12% in the North and Baltic Sea, respectively). We suggest that stable environmental selection pressure contributes to relatively strong local adaptation in the Baltic Sea. Our seascape genetic approach using a large number of sampling locations and associated oceanographical data proved useful for the identification of population units as the basis of management decisions.     

Population genetic structure of a rare unionid (<Emphasis Type="Italic">Lampsilis cariosa</Emphasis>) in a recently glaciated landscape

The yellow lampmussel (Lampsilis cariosa) is a rare unionid species in need of conservation, as it is declining throughout most of its Atlantic slope range in North America. Because freshwater mussels rely on a fish host for dispersal of their larvae, barriers to the movement of hosts, such as habitat fragmentation by dams, may indirectly affect population genetic structure. We used microsatellite loci to assess genetic variation for L. cariosa within and among three river drainages in the northern part of its range, which emerged from glaciation only ∼ ∼8–10 kya. Despite this relatively recent emergence, significant differences were observed among populations both within and among drainages, possibly because low effective population sizes meant that populations of these mussels achieved drift-migration equilibrium rapidly following glaciation. L. cariosa individuals could be assigned to their own drainages with 89.3% accuracy. Among-population differences were modest, however, in comparison to differences observed in another study of rare mussels south of the recently glaciated region. L. cariosa populations exhibited significant isolation by distance, but there was no additional variation explained by the number, size, or age of intervening dams. An understanding of mussel population genetic structure provides information, useful for conservation planning, on patterns of isolation and connectivity among populations.     

An environmental gradient dominates ecological and genetic differentiation of marine invertebrates between the North and Baltic Sea

Environmental gradients have emerged as important barriers to structuring populations and species distributions. We set out to test whether the strong salinity gradient from the marine North Sea to the brackish Baltic Sea in northern Europe represents an ecological and genetic break, and to identify life history traits that correlate with the strength of this break. We accumulated mitochondrial cytochrome oxidase subunit 1 sequence data, and data on the distribution, salinity tolerance, and life history for 28 species belonging to the Cnidaria, Crustacea, Echinodermata, Mollusca, Polychaeta, and Gastrotricha. We included seven non‐native species covering a broad range of times since introduction, in order to gain insight into the pace of adaptation and differentiation. We calculated measures of genetic diversity and differentiation across the environmental gradient, coalescent times, and migration rates between North and Baltic Sea populations, and analyzed correlations between genetic and life history data. The majority of investigated species is either genetically differentiated and/or adapted to the lower salinity conditions of the Baltic Sea. Species exhibiting population structure have a range of patterns of genetic diversity in comparison with the North Sea, from lower in the Baltic Sea to higher in the Baltic Sea, or equally diverse in North and Baltic Sea. Two of the non‐native species showed signs of genetic differentiation, their times since introduction to the Baltic Sea being about 80 and >700 years, respectively. Our results indicate that the transition from North Sea to Baltic Sea represents a genetic and ecological break: The diversity of genetic patterns points toward independent trajectories in the Baltic compared with the North Sea, and ecological differences with regard to salinity tolerance are common. The North Sea–Baltic Sea region provides a unique setting to study evolutionary adaptation during colonization processes at different stages by jointly considering native and non‐native species.     

Genetic variability and phylogeographical patterns of a nonindigenous species invasion: a comparison of exotic vs. native zebra and quagga mussel populations

There have been few investigations of the number of founding sources and amount of genetic variability that lead to a successful nonindigenous species invasion, although genetic diversity is believed to play a central role. In the present study, population genetic structure, diversity and divergence patterns were analysed for the zebra mussel Dreissena polymorpha [n=280 samples and 63 putative randomly amplified polymorphic DNA (RAPDs) gene loci] and the quagga mussel D. bugensis (n=136 and 52 loci) from 10 nonindigenous North American and six Eurasian sampling sites, representing their present‐day ranges. Results showed that exotic populations of zebra and quagga mussels had surprisingly high genetic variability, similar to those in the Eurasian populations, suggesting large numbers of founding individuals and consistent with the hypothesis of multiple colonizations. Patterns of genetic relationships indicate that the North American populations of D. polymorpha likely were founded by multiple source populations from north‐western and northcentral Europe, but not from southcentral or eastern Europe. Sampling areas within North America also were significantly divergent, having levels of gene flow and migration about twice those separating long‐established Eurasian populations. Samples of D. bugensis in Lakes Erie and Ontario were significantly different, with the former being more closely related to a native population from the Dnieper River, Ukraine. No evidence for a founder effect was discerned for either species.     

Adaptive and Neutral Genetic Variation and Colonization History of Atlantic Salmon, Salmo salar

Synopsis I combined neutral microsatellite markers with the major histocompatibility complex (MHC) class IIB to study genetic differentiation and colonization history in Atlantic salmon, Salmo salar, in the Baltic Sea and in the north-eastern Atlantic. Baltic salmon populations have lower levels of microsatellite genetic variation, in terms of heterozygosity and allelic richness than Atlantic populations, confirming earlier findings with other genetic markers, suggesting that the Baltic Sea populations have been exposed to genetic bottlenecks, most likely at a founding event. On the other hand, the level of MHC variation was similar in the Baltic and in the north-eastern Atlantic, indicating that positive balancing selection has increased the level of MHC-variation. Both microsatellite and MHC class IIB genetic variation give strong support to the hypothesis that the Baltic salmon are of a biphyletic origin, the southern population in this study is strongly differentiated from both the northern Baltic salmon populations and from the north-eastern Atlantic populations. Salmon may have colonized the northern Baltic Sea either from the south, via the so called “N?rke strait” or from the north, via a proposed historical connection between the White Sea and the northern Baltic. At microsatellites, no significant isolation-by distance was found at either colonization route. At the MHC, populations were significantly isolated by distance when assuming that colonization occurred via the “N?rke strait”.     

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.     

Microsatellite markers reveal clear geographic structuring among threatened noble crayfish (Astacus astacus) populations in Northern and Central Europe

Noble crayfish (Astacus astacus L.), the most highly valued freshwater crayfish in Europe, is threatened due to a long-term population decline caused mainly by the spread of crayfish plague. Reintroduction of the noble crayfish into restored waters is a common practice but the geographic and genetic origin of stocking material has rarely been considered, partially because previous genetic studies have been hampered by lack of nuclear gene markers with known inheritance. This study represents the first large scale population genetic survey of the noble crayfish (633 adults from 18 locations) based on 10 newly developed microsatellite markers. We focused primarily on the Baltic Sea area (Estonia, Finland and Sweden) where the largest proportion of the remaining populations exists. To allow comparisons, samples from the Black Sea catchment (the Danube drainage) were also included. Two highly differentiated population groups were identified corresponding to the Baltic Sea and the Black Sea catchments, respectively. The Baltic Sea catchment populations had significantly lower genetic variation and private allele numbers than the Black Sea catchment populations. Within the Baltic Sea area, a clear genetic structure was revealed with population samples corresponding well to their geographic origin, suggesting little impact of long-distance translocations. The clear genetic structure strongly suggests that the choice of stocking material for re-introductions and supplemental releases needs to be based on empirical genetic knowledge.     

Adaptive major histocompatibility complex (MHC) and neutral genetic variation in two native Baltic Sea fishes (perch Perca fluviatilis and zander Sander lucioperca) with comparisons to an introduced and disease susceptible population in Australia (P. fluviatilis): assessing the risk of disease epidemics

This study assessed the major histocompatibility complex (MHC) and neutral genetic variation and structure in two percid species, perch Perca fluviatilis and zander Sander lucioperca, in a unique brackish ecosystem, the Baltic Sea. In addition, to assess the importance of MHC diversity to disease susceptibility in these populations, comparisons were made to an introduced, disease susceptible, P. fluviatilis population in Australia. Eighty‐three MHC class II B exon 2 variants were amplified: 71 variants from 92 P. fluviatilis samples, and 12 variants from 82 S. lucioperca samples. Microsatellite and MHC data revealed strong spatial genetic structure in S. lucioperca, but not P. fluviatilis, across the Baltic Sea. Both microsatellite and MHC data showed higher levels of genetic diversity in P. fluviatilis from the Baltic Sea compared to Australia, which may have facilitated the spread of an endemic virus, EHNV in the Australian population. The relatively high levels of genetic variation in the Baltic Sea populations, together with spatial genetic structure, however, suggest that there currently seems to be little risk of disease epidemics in this system. To ensure this remains the case in the face of ongoing environmental changes, fisheries and habitat disturbance, the conservation of local‐scale genetic variation is recommended.     

Genetic differentiation of southeast Baltic populations of sea trout inferred from single nucleotide polymorphisms

Sea trout (Salmo trutta m. trutta) is a migratory form of brown trout common in the Baltic Sea. Nine populations from the southeast Baltic (Poland; Lithuania; Denmark, Bornholm; Estonia and Russia) were genotyped using iPLEX Gold technology (Sequenom) with 62 informative SNPs. A diagnostic panel of 23 SNPs was applied to estimate genetic differentiation and assess the population structure of Baltic sea trout. The highest level of pairwise F_ST differences was observed between the Russian (East Gulf of Finland) and Polish (Baltic main basin) populations. The lowest differences were between the two Polish and the Polish and Lithuanian populations. A genetic similarity was noted between the Estonian Riguldi River and Danish Bornholm populations, and this finding was supported by a Bayesian and factorial correspondence analysis. Diversity within populations was highest for populations from Estonia and lowest for the Lithuanian population. Genetic structure analysis indicated that individuals from the nine populations were clustered into four groups.     

Evidence of a hybrid-zone in Atlantic cod (Gadus morhua) in the Baltic and the Danish Belt Sea revealed by individual admixture analysis

The study of hybrid zones is central to our understanding of the genetic basis of reproductive isolation and speciation, yet very little is known about the extent and significance of hybrid zones in marine fishes. We examined the population structure of cod in the transition area between the North Sea and the Baltic Sea employing nine microsatellite loci. Genetic differentiation between the North Sea sample and the rest increased along a transect to the Baltic proper, with a large increase in level of differentiation occurring in the Western Baltic area. Our objective was to determine whether this pattern was caused purely by varying degrees of mechanical mixing of North Sea and Baltic Sea cod or by interbreeding and formation of a hybrid swarm. Simulation studies revealed that traditional Hardy-Weinberg analysis did not have sufficient power for detection of a Wahlund effect. However, using a model-based clustering method for individual admixture analysis, we were able to demonstrate the existence of intermediate genotypes in all samples from the transition area. Accordingly, our data were explained best by a model of a hybrid swarm flanked by pure nonadmixed populations in the North Sea and the Baltic Sea proper. Significant correlation of gene identities across loci (gametic phase disequilibrium) was found only in a sample from the Western Baltic, suggesting this area as the centre of the apparent hybrid zone. A hybrid zone for cod in the ecotone between the high-saline North Sea and the low-saline Baltic Sea is discussed in relation to its possible origin and maintenance, and in relation to a classical study of haemoglobin variation in cod from the Baltic Sea/Danish Belt Sea, suggesting mixing of two divergent populations without interbreeding.     

GENETIC DIVERSITY WITHIN BALTIC SEA POPULATIONS OF NODULARIA (CYANOBACTERIA)1

The determination of the genetic structure of microbial populations has, until recently, required the establishment of many independent clonal cultures for genotypic analysis. In such studies it has been necessary to assume that isolates able to grow in laboratory culture are representative of the full range of diversity within the natural population. In order to test this assumption we used the polymerase chain reaction (PCR) to amplify the intergenic spacer region of the Phycocyanin operon (PC-IGS) from filaments of Nodularia taken both from clonal cultures and from natural populations in the Baltic Sea. Analysis of the nucleotide sequences revealed more variation among 16 cultured isolates than within 23 single filaments sampled from a natural population. As a means of rapidly determining population genetic structure we designed and used mixtures of allele-specific amplification primers in diagnostic PCRs to identify which PC-IGS allele was present in single filaments from natural cyanobacterial assemblages. Using this method, we determined the PC-IGS genotype of 156 filaments from 9 sampling stations throughout the central basin of the Baltic Sea in July 1996. Our results show that two distinct genotypes of Nodularia are present in the population at all stations. Although the two types were present in approximately equal numbers, they were not distributed uniformly.     

Population genetic structure of northern Dolly Varden char <Emphasis Type="Italic">Salvelinus malma malma</Emphasis> in Asia and North America

The level of genetic differentiation of northern Dolly Varden char Salvelinus malma malma from Asia and North America was evaluated using the data on mtDNA variation (regions ND1/ND2, ND5/ND6, and Cytb/D-loop) obtained by means of PCR-RFLP analysis. For S. m. malma, the mean values of haplo-type and nucleotide diversity were 0.5261 ± 0.00388 and 0.001558, respectively. The mean estimate of the population nucleotide divergence constituted 0.055%. It was demonstrated that S. m. malma on the most part of the species range examined (drainages of the Beaufort Sea, Chukotka Sea, Bering Sea, and the Sea of Okhotsk) was characterized by the population genetic structure with the low level of genetic differentiation and divergence. At the same time, populations from the Pacific Ocean Gulf of Alaska demonstrated marked genetic differentiation, supported by the high pairwise G4ST values (from 0.4198 to 0.5211) and nucleotide divergence estimates (mean divergence, 0.129%), from Asian and North American populations. Analysis of molecular variance (AMOVA) showed that most of the mtDNA variation in S. m. malma fell in the intrapopulation component (72.5%). At the same time, the differences between the populations (21.1%) and between the regions (6.4%) made lower contribution to the total variation.     

Population genetic structure of three freshwater mussel (Unionidae) species within a small stream system: significant variation at local spatial scales

1. Unionid mussels are highly threatened, but little is known about genetic structure in populations of these organisms. We used allozyme electrophoresis to examine partitioning of genetic variation in three locally abundant and widely distributed species of mussels from a catchment in Ohio. 2. Within‐population variation was similar to that previously reported for freshwater mussels, but genotype frequencies exhibited heterozygote deficiencies in many instances. All three species exhibited significant among‐population variation. Evidence of isolation‐by‐distance was found in Elliptio dilatata and Ptychobranchus fasciolaris, while Lampsilis siliquoidea showed no geographical pattern of among‐population variation. 3. Our results suggest that the isolating effects of genetic drift were greater in L. siliquoidea than in the other species. Differentiation of populations occurred at a much smaller spatial scale than has previously been found in freshwater mussels. Differences among species may reflect differences in the dispersal abilities of fishes that serve as hosts for the glochidia larvae of mussels. 4. Based on our results, we hypothesise that species of mussels that are common to large rivers exhibit relatively large amounts of within‐population genetic variation and little differentiation over large geographical distances. Conversely, species typical of small streams show lower within‐population genetic variation and populations will be more isolated. If this hypothesis can be supported, it may prove useful in the design of conservation strategies that maintain the genetic structure of target species.