Search

We characterized 37 single nucleotide polymorphism (SNP) makers for eelgrass Zostera marina. SNP markers were developed using existing EST (expressed sequence tag)-libraries to locate polymorphic loci and develop primers from the functional expressed genes that are deposited in The ZOSTERA database (V1.2.1). SNP loci were genotyped using a single-base-extension approach which facilitated high-throughput genotyping with minimal optimization time. These markers show a wide range of variability among 25 eelgrass populations and will be useful for population genetic studies including evaluation of population structure, historical demography, and phylogeography. Potential applications include haplotype inference of physically linked SNPs and identification of genes under selection for temperature and desiccation stress.

Rays and skates are an unavoidable part of the by-catch in demersal fisheries. Over the past 40 years, the thornback ray (Raja clavata) has decreased in numbers and even disappeared in some areas, leading to concerns about genetic risk. For this reason, the effective population size (N-e), the migration rate (m) and temporal changes in the genetic diversity were estimated for the population of thornback rays in the Irish Sea and Bristol Channel. Using genotyped, archived and contemporary samples (1965 and 2003-2004), N-e was estimated at 283 individuals (95% CI = 145-857), m at 0.1 (95% CI 0.03-0.25) and the N-e/N ratio between 9 x 10(-5) and 6 x 10(-4). Although these results must be treated with caution, due to the small sample sizes, this is the first attempt to estimate N-e in an elasmobranch species. The low N-e/N ratio suggests that relatively few individuals contribute to the next generation. The combined effect of sex bias, inbreeding, fluctuations in population size and, perhaps most important, the variance in reproductive success may explain the low N-e/N ratio. In addition, the relatively high gene flow between Irish Sea population and other source populations is likely to have had an impact on our estimate, which may be more relevant at the metapopulation scale. No significant loss of genetic diversity was found over the 40-year timeframe and long-term maintenance of the genetic diversity could be due to gene flow.

The last glacial maximum (20 000-18 000 years ago) dramatically affected extant distributions of virtually all northern European biota. Locations of refugia and postglacial recolonization pathways were examined in Fucus serratus (Heterokontophyta; Fucaceae) using a highly variable intergenic spacer developed from the complete mitochondrial genome of Fucus vesiculosus. Over 1500 samples from the entire range of F. serratus were analysed using fluorescent single strand conformation polymorphism. A total of 28 mtDNA haplotypes was identified and sequenced. Three refugia were recognized based on high haplotype diversities and the presence of endemic haplotypes: southwest Ireland, the northern Brittany-Hurd Deep area of the English Channel, and the northwest Iberian Peninsula. The Irish refugium was the source for a recolonization sweep involving a single haplotype via northern Scotland and throughout Scandinavia, whereas recolonization from the Brittany-Hurd Deep refugium was more limited, probably because of unsuitable soft-bottom habitat in the Bay of Biscay and along the Belgian and Dutch coasts. The Iberian populations reflect a remnant refugium at the present-day southern boundary of the species range. A generalized skyline plot suggested exponential population expansion beginning in the mid-Pleistocene with maximal growth during the Eems interglacial 128 000 - 67 000 years ago, implying that the last glacial maximum mainly shaped population distributions rather than demography.

Sargassum polyceratium is widely distributed around the island of Curacao ( Netherlands Antilles) where it inhabits strongly contrasting habitats. Changes in population structure have been followed in three bays with increasing levels of wave exposure at two depths: shallow ( 0 m) and deep ( 18 m). The effects of increasing wave exposure were investigated by studying three deep-water populations; and the effects of depth by studying shallow- and deep-water populations in the two calmer bays. Total density and stage density ( reproductive thalli, juvenile thalli) were determined and total and individual thallus biomass was estimated non-destructively. In the most wave-exposed deep-water population thalli were twice as long with more than twice the biomass than in the calmest deep-water population. Total density and juvenile density were highest in the bay with intermediate wave exposure. Depth was an important factor at both the individual and population level. Shallow-water thalli had basal holdfast areas that were four times larger than those from deep water, and thallus biomass was positively correlated with holdfast area. Shallow-water juveniles invested more in the development of a holdfast and lateral growth than deep-water juveniles. Total biomass per quadrat was up to 10-fold higher in shallow- than in deep-water populations. In shallow- water populations reproductive thalli were present throughout the year whereas in deep-water populations they were present only during autumn and winter. We conclude that both wave exposure and depth affect population structure. Thalli were generally bigger and total biomass higher in the more exposed bay(s) and in shallower water, contradicting the general pattern in macroalgae of reduced size and biomass with increasing wave exposure.

The effects of oceanographic patterns on marine genetic biodiversity along the SW Iberian Peninsula are poorly understood. We addressed the question of whether gene flow in this region depends solely on geographic distance between isolated patches of suitable habitat or if there are superimposed effects correlated with other factors such as current patterns. Zostera noltii, the dwarf eelgrass, is the keystone habitat-structuring seagrass species on intertidal mudflats along the Iberian west coast. We used 9 microsatellite loci to analyze population genetic diversity and differentiation for all existing 8 populations from NW Spain (Ria de Vigo) to SW Spain (Puerto Real, Cadiz). Populations are highly genetically differentiated as shown by high significant F-ST,Wright's fixation index, (0.08 to 0.26) values. A neighbor-joining tree based on Reynold's distances computed from allele frequencies revealed a split between northern and southern populations (bootstrap support of 84 %). This pattern of differentiation can be explained by (1) ocean surface current patterns present during Z. noltii's reproductive season which cause a dispersal barrier between the northern and southern populations of this region, (2) habitat isolation, due to large geographic distances between suitable habitats, preventing frequent gene flow, and (3) small effective population sizes, causing high drift and thus faster differentiation rates.

The seaweed Fucus serratus is hypothesized to have evolved in the North Atlantic and present populations are thought to reflect recolonization from a southern refugium since the last glacial maximum 18 000-20 000 years bp. We examined genetic structure across several spatial scales by analysing seven microsatellite loci in populations collected from 21 localities throughout the species' range. Spatial auto-correlation analysis of seven microsatellite loci revealed no evidence for spatial clustering of alleles on a scale of 100 m despite limited gamete dispersal in F. serratus of approximate to 2 m from parental individuals. Pairwise theta analysis suggested that the minimal panmictic unit for F. serratus was between 0.5 and 2 km. Isolation by distance was significant along some contiguous coastlines. Population differentiation was strong within the Skagerrak-Kattegat-Baltic Seas (SKB) (global theta = 0.17) despite a short history of approximate to 7500 years. A neighbour-joining tree based on Reynold's distances computed from the microsatellite data revealed a central assemblage of populations on the Brittany Peninsula surrounded by four well-supported clusters consisting of the SKB, the North Sea (Ireland, Helgoland), and two populations from the northern Spanish coast. Samples from Iceland and Nova Scotia were most closely aligned with northwest Sweden and Brittany, respectively. When sample sizes were standardized (N = 41), allelic diversity was twofold higher for Brittany populations than for populations to the north and threefold higher than southern populations. The Brittany region may be a refugium or a recolonized area, whereas the Spanish populations most likely reflect present-day edge populations that have undergone repeated bottlenecks as a consequence of thermally induced cycles of recolonization and extinction.

In plant populations alleles often deviate from a random distribution and reveal positive autocorrelation at short distances. In species with both clonal and sexual reproduction, such clustering may be because ramets of the same genet were sampled at nearby locations. Alternatively, clustering may be the result of limited gene flow through pollen or seeds (isolation-by-distance). Were, we modify a conventional spatial autocorrelation analysis using the join-count statistic in order to differentiate between these two causes of genetic structure. We examined the distribution of seven microsatellite loci representing 37 alleles in a 20 x 80 m plot of a perennial population of eelgrass Zostera marina L. In analysing join-counts between all like genotypes we found significant genetic autocorrelation among ramets at distances between 1 and 7 m (P < 0.001). We then excluded joins between clonemates which were identified from the expected likelihood of their seven-locus genotypes. Without joins within genets, no autocorrelation was evident, indicating that most of the significant genetic clustering was caused by clonal spread. At distances up to 27 m, alleles were distributed at random, indicating a panmictic population at this spatial scale. These results illustrate the need for an a priori estimation of genet-ramet structure in clonally reproducing plants in order to avoid erroneous inferences about putative gene flow at various spatial scales

We report that an eelgrass bed Zostera marina L. at the Aland Islands, northern Baltic Sea, is dominated by a. single genotype which extends over an area of approximately 160 x 40 m. In total, 47 individuals were sampled and genotyped for 6 highly polymorphic microsatellite loci. Forty-four of the ramets revealed the same 6-locus genotype, including 3 identical heterozygous loci. They were thus assigned to the same genet (= clone). To our knowledge, it represents the largest marine plant identified thus far. Based on estimates of horizontal rhizome growth rates, this clone may be more than 1000 yr old. The remarkable phenotypic plasiticity of a single genotype which dominates this site illustrates that there is no simple one-to-one relationship between genetic diversity and population persistence in changing and stressful environments.

We examined the spatial distribution of genotypes in a perennial population of eelgrass, Zostera marina L., at two spatial scales. We mapped and sampled 80 ramets in a subtidal area of 20 x 80 m, and an additional 15 ramets in two 1-m(2) sub-quadrats. Ramets were genotyped for seven polymorphic microsatellite loci. Among a total number of 54 genotypes detected, 12 occurred more than once. The ramets of ten of these genotypes occurred in clusters and represented genets based on their expected multi-locus genotype frequencies. The size distribution of genets was uneven with estimated ramet numbers ranging from 2 to 5000. Whereas some areas displayed a high genet diversity, which may indicate past disturbances, large genets (greater than or equal to 10 m(2)) predominated in other locations of the sampled plot. Spatial heterogeneity in clone distribution was also obvious at the smaller sampling scale (15 ramets sam.. pled within m(2)) were the clonal diversity (genets identified/ramets sampled) was 0.24 in one quadrat, and 0.077 in the ether. Ramets belonging to the largest clone were maximally 17 In apart, which corresponds to a genet age of 67 yr based on annual rhizome growth rates. We conclude that the spatial arrangement of clones in seagrasses allows inferences about the past demography and the disturbance regime at a given site which may prove useful for coastal zone management of ecologically valuable seagrass meadows.

Here we describe a rapid method for extracting DNA from dried brown algae material using a microtitre plate system in conjunction with a milling instrument. The method allows the preparation of nuclear and organelle DNA of quality suitable for polymerase chain reaction amplification. It combines high throughput with low cost per sample: DNA from 192 samples can be extracted in c. 3 h for

Postelsia palmaeformis Ruprecht is an annual species, occuring from southern California to Vancouver Island Canada, in upper intertidal sites exposed to extreme wave shock. Because of its limited spore dispersal, discrete and inbred populations are likely on the local scale, yet dispersal of drifting and fertile thalli raises the possibility of outbred populations on a regional scale. M13 minisatellite DNA fingerprinting and random amplified polymorphic DNA (RAPD) markers were used in a complementary fashion to investigate genetic variability among 24 individuals on scales of clusters (= coalesced holdfasts).

Populations along the northern boundary of a marine species' distributional range in the NE Atlantic are expected to harbor lower standing genetic variation as a consequence of post-glacial expansion following the last glacial maximum. Founder events and marginal habitat availability may render the edge populations more vulnerable to anthropogenic stress and less cap able of rapid adaptation to global climate change, a concern for conservation and management. We analyzed meadow architecture, persistence and connectivity within and among 15 locations (600 samples genotyped with 8 microsatellite loci) in 3 fjords in Troms County, Norway (69 degrees N). Whereas global mean allelic diversity (standardized for sample size) was in accordance with previous studies using the same markers, more extensive sampling revealed a broader range of allelic richness (mean = 2.85; range = 1.84 to 4.21) in the regional pool. Genotypic diversity was typically high, whereas large genets were rare (2 out of 15 locations). Population differentiation (F-ST) was 2 to 6 times higher between fjords than within fjords. A Bayesian (STRUCTURE) analysis also strongly supported the genetic distinctness of each fjord. Although 9 locations within the 60 km long Balsfjord were connected by gene flow, demographic connectivity may actually be low, as fixed differences were observed at 6 of the 9 locations, along with significantly positive inbreeding coefficients and strong substructure. Overall, our results suggest that these northern, leading-edge meadows are healthy, but vigilance is required to avoid further losses. Fjord-level management, especially of the larger fjords, will be sufficient to capture the range of variation.

Phylogenetic relationships in the Arctic-Antarctic disjunct seaweed species Acrosiphonia arcta (Dillwyn) J. G. Agardh (Acrosiphoniales, Chlorophyta) were examined using restriction fragment-length polymorphism analysis of the fast-evolving nuclear ribosomal intergenic spacer (IGS) region and random amplified polymorphic DNA (RAPD) markers. Twenty-two isolates collected from 10 different locations in both hemispheres were compared. Five IGS length classes were identified among the 10 locations. Throughout the North Atlantic, IGS regions were found to be extremely homogeneous whereas RAPD patterns revealed subdivided populations that suggest founder effects. Acrosiphonia arcta populations found in the Arctic and North Atlantic oceans are hypothesized to be of Pacific origin. Extensive differences found between Arctic Greenland populations and those in the North Atlantic suggest that colonization of Arctic Greenland occurred as an independent event. Recolonization of the Antarctic peninsula from Southern Chile is favored, whereas the directionality of transequatorial passage along the western coast of the Americas could be in either direction.

Nucleotide sequences of the nuclear ribosomal DNA internal transcribed spacers (ITS1 and ITS2), the 5.8S, and short stretches of the adjacent 18S and 26S coding regions were determined in isolates from four disjunct Cladophora albida (Huds.) Kutz. populations (NE-America, W-Europe, Japan, and W-Australia). The two Pacific isolates share nearly identical ITS sequences as do the two Atlantic isolates. In contrast, interoceanic comparisons exhibit a 21% sequence difference. Variation within ITS regions is useful for identification of population groups on a regional or oceanic scale. However, both spacers are characterized by numerous repeat motifs as well as point mutations, which result in alignment problems at the interspecific level within Cladophora.

Beginning in the 1930s, eelgrass meadows declined throughout the Wadden Sea, leav- ing populations susceptible to extinction through patchiness, low density and isolation. Additional anthropogenic impacts have altered current regimes, nutrients and turbidity — al l of which affect eelgrass. Recent abiotic modeling studies suggest that poor recovery is the result of a regime shift caused by the loss of positive feedbacks between seagrass meadows and their capacity to mediate turbidity. Additionally, it is hypothesized that genetic and demographic factors — in particular, the loss of genetic diversity and patch connectivity — have contributed to lower fitness of eelgrass, thereby further diminishing recovery potential. We assessed genetic diversity and connectivity of Zostera marina among 19 locations, covering some 950 km of coastline between Zeeland, Nether- lands and Langholmen, Sweden. Both allelic and genotypic diversity were high. A Bayesian analysis of population structure revealed 6 significant clusters of subpopulations that are connected by vary- ing degrees of dispersal. Although population divergence was significant at as little as 5 km, isolation by distance was very weak, indicating high connectivity at scales of 150 km. A demographic interpre- tation of these data suggests that realized gene flow is strong and predominantly northward, leaving the western Wadden Sea relatively isolated. The failure of eelgrass to recover in the western Wadden Sea is, therefore, due to both unsuitable physical conditions and low incoming gene flow. Nonethe- less, the greater Wadden Sea can be considered a seed transfer zone providing source material for restoration efforts in any areas where abiotic conditions are more favorable.

A goal of phylogeography is to relate patterns of genetic differentiation to potential historical geographic isolating events. Quaternary glaciations, particularly the one culminating in the Last Glacial Maximum similar to 21 ka (thousands of years ago), greatly affected the distributions and population sizes of temperate marine species as their ranges retreated southward to escape ice sheets. Traditional genetic models of glacial refugia and routes of recolonization include these predictions: low genetic diversity in formerly glaciated areas, with a small number of alleles/haplotypes dominating disproportionately large areas, and high diversity including "private'' alleles in glacial refugia. In the Northern Hemisphere, low diversity in the north and high diversity in the south are expected. This simple model does not account for the possibility of populations surviving in relatively small northern periglacial refugia. If these periglacial populations experienced extreme bottlenecks, they could have the low genetic diversity expected in recolonized areas with no refugia, but should have more endemic diversity (private alleles) than recently recolonized areas. This review examines evidence of putative glacial refugia for eight benthic marine taxa in the temperate North Atlantic. All data sets were reanalyzed to allow direct comparisons between geographic patterns of genetic diversity and distribution of particular clades and haplotypes including private alleles. We contend that for marine organisms the genetic signatures of northern periglacial and southern refugia can be distinguished from one another. There is evidence for several periglacial refugia in northern latitudes, giving credence to recent climatic reconstructions with less extensive glaciation. NSF; Marbef [GOCE-CT-2003-505446]; Intra-European Marie Curie Fellowship [EIF-024781]; 6th European Community Framework Program info:eu-repo/semantics/publishedVersion

Background and Aims Seagrasses are important facilitator species in shallow, soft-bottom marine environments worldwide and, in many places, are threatened by coastal development and eutrophication. One narrow-leaved species (Zostera marina) and one wide-leaved species, variously designated as Z. marina, Z. pacifica or Z. asiatica, are found off the California Channel Islands and adjacent California-Mexico coast. The aim of the present study was to confirm species identification genetically and to link patterns of genetic diversity, connectivity and hybridization among and within the populations with historical sea levels (Ice Age) or the contemporary environment.Methods Samples (n = 11-100) were collected from 28 sites off five California Channel Islands and six sites off the adjacent coast of southern California and Baja California, Mexico. DNA polymorphisms of the rDNA-ITS (internal transcribed spacer) cistron (nuclear), the matK intron (chloroplast) and nine microsatellite loci (nuclear) were examined in a population genetic and phylogeographic context.Key Results All wide-leaved individuals were Z. pacifica, whereas narrow-leaved forms were Z. marina. Microsatellite genotypes were consistent with hybridization between the two species in three populations. The present distribution of Z. pacifica follows a glacial age land mass rather than present oceanographic regimes, but no link was observed between the present distribution of Z. marina and past or present environments. Island populations of Z. marina often were clonal and characterized by low genotypic diversity compared with populations along the Baja California coast. The high level of clonal connectivity around Santa Catalina Island indicated the importance of dispersal and subsequent re-establishment of vegetative fragments.Conclusions The pristine environmental conditions of offshore islands do not guarantee maximum genetic diversity. Future restoration and transplantation efforts of seagrasses must recognize cryptic species and consider the degree of both genetic and genotypic variation in candidate donor populations.

The introduced Fucus evanescens (hermaphroditic) and the native F. serratus (dioecious) have been in secondary contact along the Danish coast of the Kattegat Sea for 60-100 years and dioecious hybrids have been observed at Blushoj for several years. Hybridization in Fucus is unusual because it appears to always involve a hermaphroditic and a dioecious parental pair. We determined the degree and spatial patterns of introgression for 286 individuals using 10 microsatellite loci and cpDNA. Hybrids accounted for nearly 13% of the population, yet parental species were well differentiated (F(ST) = 0.633). The presence of F. evanescens chloroplasts in 100% of F(1) hybrids revealed asymmetrical hybridization. Fucus evanescens cpDNA was observed in 50% of introgressed and 5.4% of pure F. serratus, but no F. serratus cpDNA was found in F. evanescens. In contrast, nuclear DNA introgression was symmetrical with an equal amount (approximate to 1.5%) of genes introgressed into each parental species. Survivorship and viability data suggest selection against hybrids in the hybrid zone.

The common intertidal seaweed Fucus serratus was almost certainly introduced to Iceland and the Faroes by humans from Europe, as previous genetic studies have confirmed that life-history constraints preclude long-distance dispersal. Introduction must have occurred sometime in the 1,000 years between arrival of the first Icelandic settlers c. 900 AD and when the species was first noted in a phycological survey in 1900. We genotyped 19 populations from throughout northern Europe, Iceland, and the Faroes with seven microsatellite loci in order to identify the source or sources of the Icelandic/Faroese populations. Assignment tests indicated that the Smaskjaer area of the Oslofjorden in Norway was the source for the Icelandic populations and the Hafnarfjordur area of Iceland was the likely source for the single Faroese population. The time of introduction to Iceland was probably during the 19th century, whereas introduction to the Faroes occurred during the late 20th century. Additionally, molecular data verified hybridization between the introduced F. serratus and the native F. evanescens.

The marine angiosperm Zostera noltii (dwarf eelgrass), an important facilitator species and food source for invertebrates and waterfowl, predominantly inhabits intertidal habitats along eastern Atlantic shores from Mauritania to southern Norway/Kattegat Sea and throughout the Mediterranean, Black and Azov seas. We used 9 microsatellite loci to characterize population structure at a variety of spatial scales among 33 populations from 11 localities throughout the entire biogeographic range. Isolation by distance analysis suggested a panmictic genetic neighborhood of 100 to 150 km. At the global scale, a neighbor-joining tree based on Reynolds distances revealed strongly-supported groups corresponding to northern Europe, Mauritania and the Black/Azov Sea; separate Mediterranean and Atlantic-Iberian groups were poorly supported. Clones (genets with multiple ramets) were present in most populations but were generally small (ca. < 3 m(2)). Exceptions were found in Mauritania (ca. 29 m in length), the Azov Sea (ca. 40 m in length) and the Black Sea (ca. 50 m in length). Although genetic diversity and allelic richness generally decreased from Mauritania to Denmark, the putative post-glacial recolonization route, both were unexpectedly high among populations from the German Wadden Sea. EVK3-CT-2000-00044

Philopatry to spawning grounds combined with well-known migratory patterns in the flatfish Pleuronectes platessa (plaice) has led to the hypothesis that regional populations may reflect relatively discrete, genetic stocks. Using six microsatellite loci we genotyped 240 adult individuals collected from locations in Norway, the Faeroe plateau, the Irish Sea, the Femer Baelt, Denmark, and the southern North Sea, and 240 0-class juveniles collected from five nursery-ground locations in Iceland, northwest Scotland, two sites in the Wadden Sea, and the Bay of Vilaine in Southern Brittany. The mean number of alloles/locus ranged from 5.3 to 20.4, with a mean of 13.9. Expected heterozygosity was uniformly high across all locations (multilocus H-exp = 0.744+/-0.02). Pairwise comparisons of theta among all 11 locations revealed significant differentiation between Iceland and all other locations (theta = 0.0290*** to 0.0456***), which is consistent with the deep-water barrier to dispersal in plaice. In contrast, no significant differentiation was found among any of the remaining continental-shelf sampling locations. This suggests that regional stocks are themselves composed of several genetic stocks under a model of panmixia which persists even to the spawning grounds. The presence of significant heterozygote deficiencies at all locations (not due to null alleles) suggests a temporal Wahlund effect yet the absence of significant population differentiation among continental shelf localities makes this explanation alone difficult to. reconcile. Sampling of eggs at the spawning grounds will be required to resolve this issue. Causes of the mismatch between genetic and geographical stocks is discussed in the context of high gene flow.

The seaweed Sargassum polyceratium Montagne inhabits a broad spectrum of subtidal and intertidal habitats. Genetic diversity and spatial genetic structure were examined within and among 12 stands using random amplified polymorphic DNA (RAPD) phenotypes. Data were analyzed using analysis of molecular variance (AMOVA) and Shannon's information measure. In both analyses, 60–75% of the variation occurred within stands and 25–40% between stands. These values are consistent with out-crossing, high-dispersal species. Significant differentiation was found among bays ca. 25 km apart (Shannon's G′st averaged 0.37 and pairwise AMOVA Φst values averaged 0.272) and among stands 150–200 m apart within bays (AMOVA Φst values averaged 0.149). Effects of shore (windward vs. leeward), depth, and bay on population structure were tested. These analyses revealed that the factor depth is confounded with shore, and that bays show significant differentiation from each other but are not completely isolated from one another. Mantel tests for differentiation-by-distance were significant along both sides of the island but stronger along the windward side. A neighbor-joining analysis of genetic distances among stands showed that the effects of currents around both tips of the island were especially important for shallow populations. For S. polyceratium, depth and bay promote population differentiation along shores, yet dispersal around the tips of the island simultaneously connects these populations to varying degrees. This study highlights the importance of investigating the relative contribution of habitat factorsin relation to island-scale population structure.

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 N-e of 2440-5000. The overall genetic differentiation among eelgrass populations, assessed as rho (a standardized estimate of Slatkin's R-ST) was 0.384 (95% CI 0.34-0.44, P

Using an enriched genomic library, we developed seven (CT)(n)/(GA)(n) microsatellite loci for eelgrass Zostera marina L. Enrichment is described and highly recommended for genomes in which microsatellites are rare, such as in many plants. A test for polymorphism was performed on individuals from three geographically separated populations (N = 15/population) and revealed considerable genetic variation. The number of alleles per locus varied between five and 11 and the observed heterozygosities for single loci ranged from 0.16 to 0.81 within populations. Mean allele lengths were markedly different among populations, indicating that the identified loci will be useful in studying population structure in Z. marina. As the frequency of the most abundant multilocus genotype within populations was always

Ascophyllum nodosum is dominant seaweed along many rocky intertidal shores throughout the North Atlantic. Next to the kelps, fucalean taxa such as Ascophyllum are the largest macrophytes and provide important habitat for invertebrates. Understanding the underlying genetic structure of natural populations over a range of spatial scales can reveal how the causes of structure may change with scales. Separating population history from population structure may also be elucidated. The analysis is based on six polymorphic microsatellite loci and> 1000 individuals. Strong genetic structure at small spatial scale was found and is consistent with demographic models based on long-lived individuals, low recruitment and many sib matings. At large spatial scales only weak population differentiation was found. This is consistent with recent recolonization of the North Atlantic following the last glacial maximum.