Fine-scale genetic breaks driven by historical range dynamics and ongoing density-barrier effects in the estuarine seaweed Fucus ceranoides L

ABSTRACT: BACKGROUND: Factors promoting the emergence of sharp phylogeographic breaks include restricted dispersal, habitat discontinuity, physical barriers, disruptive selection, mating incompatibility, genetic surfing and secondary contact. Disentangling the role of each in any particular system can be difficult, especially when species are evenly distributed across transition zones and dispersal barriers are not evident. The estuarine seaweed Fucus ceranoides provides a good example of highly differentiated populations along its most persistent distributional range at the present rear edge of the species distribution, in NW Iberia. Intrinsic dispersal restrictions are obvious in this species, but have not prevented F. ceranoides from vastly expanding its range northwards following the last glaciation, implying that additional factors are responsible for the lack of connectivity between neighbouring southern populations. In this study we analyze 22 consecutive populations of F. ceranoides along NW Iberia to investigate the processes generating and maintaining the observed high levels of regional genetic divergence. RESULTS: Variation at seven microsatellite loci and at mtDNA spacer sequences was concordant in revealing that Iberian F. ceranoides is composed of three divergent genetic clusters displaying nearly disjunct geographical distributions. Structure and AFC analyses detected two populations with an admixed nuclear background. Haplotypic diversity was high in the W sector and very low in the N sector. Within each genetic cluster, population structure was also pervasive, although shallower. CONCLUSIONS: The deep divergence between sectors coupled with the lack of support for a role of oceanographic barriers in defining the location of breaks suggested 1) that the parapatric genetic sectors result from the regional reassembly of formerly vicariant sub-populations, and 2) that the genetic discontinuities at secondary contact zones (and elsewhere) are maintained despite normal migration rates. We conclude that colonization and immigration, as sources of gene-flow, have very different genetic effects. Migration between established populations is effectively too low to prevent their differentiation by drift or to smooth historical differences inherited from the colonization process. F. ceranoides, but possibly low-dispersal species in general, appear to be unified to a large extent by historical, non-equilibrium processes of extinction and colonization, rather than by contemporary patterns of gene flow.     

Contemporary genetic structure of Xantus's Hummingbird (Basilinna xantusii) in the Baja California peninsula

Genetic structure and phylogeographic patterns of natural populations are of great importance in assessing the conservation status of species. These population properties can be estimated using molecular markers of either mitochondrial DNA (mtDNA) or nuclear DNA (nDNA) to understand the historical, ecological and dispersal patterns that influence genetic exchange within and between populations. Basilinna xantusii is a sexually dimorphic hummingbird endemic to the Baja California Peninsula (BCP). It comprises three ancestral mitochondrial lineages linked to vicariant events, late Pleistocene climate changes and the geographical distribution of oases. This study aimed to determine and understand the current population genetic structure of this hummingbird. The genotypes of 16 microsatellite loci from 100 individuals collected across the geographical range of this species were compared with mtDNA sequences previously published. Cluster analyses identified five populations, two with almost no genetic admixture in the northern part of the BCP and three others with varying levels of admixed ancestry across the BCP. In San José de Magdalena, at the northernmost end of the range of Xantus's Hummingbird, 40% of individuals collected belong to one genetic cluster and the remaining 60% to another, both genetic clusters showing very little admixed ancestry. We hypothesize that, despite being in sympatry, these individuals do not interbreed, unlike the other populations where individuals showed ancestry coefficients of the other genetic groups. The philopatric behaviour of males and the long-range dispersal capacity of females probably determine the observed genetic differentiation pattern. The mito-nuclear discordance detected could be due to the molecular markers used and to female-biased dispersal. Gene flow is asymmetric in this species, being greater from north to south than vice versa, which is probably related to differences in the seasonality of precipitation across the BCP and to urbanization of the oases.     

The divergence of two independent lineages of an endemic Chinese gecko,Gekko swinhonis,launched by the Qinling orogenic belt

The genetic structure and demographic history of an endemic Chinese gecko, Gekko swinhonis, were investigated by analysing the mitochondrial cytochrome b gene and 10 microsatellite loci for samples collected from 27 localities. Mitochondrial DNA data provided a detailed distribution of two highly divergent evolutionary lineages, between which the average pairwise distance achieved was 0.14. The geographic division of the two lineages coincided with a plate boundary consisting of the Qinling and Taihang Mts, suggesting a historical vicariant pattern. The orogeny of the Qinling Mts, a dispersal and major climatic barrier of the region, may have launched the independent lineage divergence. Both lineages have experienced recent expansion, and the current sympatric localities comprised the region of contact between the lineages. Individual‐based phylogenetic analyses of nucDNA and Bayesian‐clustering approaches revealed a deep genetic structure analogous to mtDNA. Incongruence between nucDNA and mtDNA at the individual level at localities outside of the contact region can be explained by the different inheritance patterns and male‐biased dispersal in this species. High genetic divergence, long‐term isolation and ecological adaptation, as well as the morphological differences, suggest the presence of a cryptic species.     

Fine-scale genetic structure, estuarine colonization and incipient speciation in the marine silverside fish Odontesthes argentinensis

The identification of incipient ecological species represents an opportunity to investigate current evolutionary process where adaptive divergence and reproductive isolation are associated. In this study we analysed the genetic structure of marine and estuarine populations of the silverside fish Odontesthes argentinensis using nine microsatellite loci and 396 bp of the mitochondrial DNA (mtDNA) control region. Our main objective was to investigate the relationship among estuarine colonization, divergent selection and speciation in silversides. Significant genetic structure was detected among all marine and estuarine populations. Despite the low phylogeographic structure in mtDNA haplotypes, there was clear signal of local radiations of haplotypes in more ancient populations. Divergence among marine populations was interpreted as a combined result of homing behaviour, isolation by distance and drift. On the other hand, ecological shifts due to the colonization of estuarine habitats seem to have promoted rapid adaptive divergence and reproductive isolation in estuarine populations, which were considered as incipient ecological species. This conclusion is supported by the existence of a set of environmental factors required for successful reproduction of estuarine ecotypes. The pattern of genetic structure indicates that phenotypic and reproductive divergence evolved in the face of potential gene flow between populations. We suggest that the 'divergence-with-gene-flow' model of speciation may account for the diversification of estuarine populations. The approach used can potentially identify 'incipient estuarine species', being relevant to the investigation of the evolutionary relationships of silversides in several coastal regions of the world.     

Dispersal in a plain landscape: short-distance genetic differentiation in southwestern Manitoba wolves, Canada

The effects of human-caused fragmentation require further study in landscapes where physical dispersal barriers and natural ecological transitions can be discounted as causes for population genetic structure. We predict that fragmentation can reduce dispersal across such barrier-free landscapes because dispersal also is limited by a perception of risk. Considerable fragmentation has occurred in the Riding Mountain National Park (RMNP) region in Manitoba, Canada, during the past 60 years. We examine data from 13 autosomal microsatellites to determine whether fragmentation is correlated with genetic population structure in wolves (Canis lupus). Moderate and significant differentiation between RMNP and a genetic cluster identified 30 km farther north (F _ST = 0.053, 95% CI [0.031–0.073]) is consistent with predicted effects of fragmentation. The RMNP population cluster represents at least seven wolf packs followed weekly by radio tracking during 2003–2006. Distinct mtDNA haplotypes have been identified in the Park and no successful wolf dispersal from RMNP has been documented in several multi-year tracking studies since 1974. Tracking data also indicate that some wolves might be reluctant to leave RMNP. Although the influence of behaviour and local adaptation require investigation, human-caused fragmentation appears to have caused cryptic genetic structure on fine spatiotemporal scales in a vagile species that is: (1) not influenced by physical movement barriers or historical ecological discontinuities in our study area, and; (2) able to live relatively close to humans. The Great Plains is now an intensely human-managed landscape. Detection of cryptic genetic structure could therefore function as an important indicator in conservation management.     

East-West cranial differentiation in pre-Columbian populations from Central and North America

In a recent study we found that crania from South Amerindian populations on each side of the Andes differ significantly in terms of craniofacial shape. Western populations formed one morphological group, distributed continuously over 14,000km from the Fuegian archipelago (southern Chile) to the Zulia region (northwestern Venezuela). Easterners formed another group, distributed from the Atlantic Coast up to the eastern foothills of the Andes. This differentiation is further supported by several genetic studies, and indirectly by ecological and archaeological studies. Some authors suggest that this dual biological pattern is consistent with differential rates of gene flow and genetic drift operating on both sides of the Cordillera due to historical reasons. Here we show that such East-West patterning is also observable in North America. We suggest that the "ecological zones model" proposed by Dixon, explaining the spread of the early Americans along a Pacific dispersal corridor, combined with the evolution of different population dynamics in both regions, is the most parsimonious mechanism to explain the observed patterns of within- and between-group craniofacial variability.     

Phylogeographic and population genetic analyses reveal Pleistocene isolation followed by high gene flow in a wide ranging,but endangered,freshwater mussel

Freshwater organisms of North America have had their contemporary genetic structure shaped by vicariant events, especially Pleistocene glaciations. Life history traits promoting dispersal and gene flow continue to shape population genetic structure. Cumberlandia monodonta, a widespread but imperiled (IUCN listed as endangered) freshwater mussel, was examined to determine genetic diversity and population genetic structure throughout its range. Mitochondrial DNA sequences and microsatellite loci were used to measure genetic diversity and simulate demographic events during the Pleistocene using approximate Bayesian computation (ABC) to test explicit hypotheses explaining the evolutionary history of current populations. A phylogeny and molecular clock suggested past isolation created two mtDNA lineages during the Pleistocene that are now widespread. Two distinct groups were also detected with microsatellites. ABC simulations indicated the presence of two glacial refugia and post-glacial admixture of them followed by simultaneous dispersal throughout the current range of the species. The Ouachita population is distinct from others and has the lowest genetic diversity, indicating that this is a peripheral population of the species. Gene flow within this species has maintained high levels of genetic diversity in most populations; however, all populations have experienced fragmentation. Extirpation from the center of its range likely has isolated remaining populations due to the geographic distances among them.     

Multilocus genetic analyses and spatial modeling reveal complex population structure and history in a widespread resident North American passerine (Perisoreus canadensis)

An increasing body of studies of widely distributed, high latitude species shows a variety of refugial locations and population genetic patterns. We examined the effects of glaciations and dispersal barriers on the population genetic patterns of a widely distributed, high latitude, resident corvid, the gray jay (Perisoreus canadensis), using the highly variable mitochondrial DNA (mtDNA) control region and microsatellite markers combined with species distribution modeling. We sequenced 914 bp of mtDNA control region for 375 individuals from 37 populations and screened seven loci for 402 individuals from 27 populations across the gray jay range. We used species distribution modeling and a range of phylogeographic analyses (haplotype diversity, Φ_ST, SAMOVA, F_ST, Bayesian clustering analyses) to examine evolutionary history and population genetic structure. MtDNA and microsatellite markers revealed significant genetic differentiation among populations with high concordance between markers. Paleodistribution models supported at least five potential areas of suitable gray jay habitat during the last glacial maximum and revealed distributions similar to the gray jay's contemporary during the last interglacial. Colonization from and prolonged isolation in multiple refugia is evident. Historical climatic fluctuations, the presence of multiple dispersal barriers, and highly restricted gene flow appear to be responsible for strong genetic diversification and differentiation in gray jays.     

Inferring contemporary levels of gene flow and demographic history in a local population of the leaf beetle Gonioctena olivacea from mitochondrial DNA sequence variation

We have studied mitochondrial DNA variation in a local population of the leaf beetle species Gonioctena olivacea, to check whether its apparent low dispersal behaviour affects its pattern of genetic variation at a small geographical scale. We have sampled 10 populations of G. olivacea within a rectangle of 5 x 2 km in the Belgian Ardennes, as well as five populations located approximately along a straight line of 30 km and separated by distances of 3-12 km. For each sampled individual (8-19 per population), a fragment of the mtDNA control region was polymerase chain reaction-amplified and sequenced. Sequence data were analysed to test whether significant genetic differentiation could be detected among populations separated by such relatively short distances. The reconstructed genealogy of the mitochondrial haplotypes was also used to investigate the demographic history of these populations. Computer simulations of the evolution of populations were conducted to assess the minimum amount of gene flow that is necessary to explain the observed pattern of variation in the samples. Results show that migration among populations included in the rectangle of 5 x 2 km is substantial, and probably involves the occurrence of dispersal flights. This appears difficult to reconcile with the results of a previous ecological field study that concluded that most of this species dispersal occurs by walking. While sufficient migration to homogenize genetic diversity occurs among populations separated by distances of a few hundred metres to a few kilometres, distances greater than 5 km results in contrast in strong differentiation among populations, suggesting that migration is drastically reduced on such distances. Finally, the results of coalescent simulations suggest that the star-like genealogy inferred from the mtDNA sequence data is fully compatible with a past demographic expansion. However, a metapopulation structure alone (without the need to invoke a population expansion event) cannot be dismissed as the cause of this star shape.     

Genetic Variation and Geographic Differentiation in Mitochondrial DNA of the Horseshoe Crab, LIMULUS POLYPHEMUS

Restriction site variation in mitochondrial DNA (mtDNA) of the horseshoe crab (Limulus polyphemus) was surveyed in populations ranging from New Hampshire to the Gulf Coast of Florida. MtDNA clonal diversity was moderately high, particularly in southern samples, and a major genetic "break" (nucleotide sequence divergence approximately 2%) distinguished all sampled individuals which were north vs. south of a region in northeastern Florida. The area of genotypic divergence in Limulus corresponds to a long-recognized zoogeographic boundary between warm-temperate and tropical marine faunas, and it suggests that selection pressures and/or gene flow barriers associated with water mass differences may also influence the evolution of species widely distributed across such transition zones. On the other hand, a comparison of the mtDNA divergence patterns in Limulus with computer models involving stochastic lineage extinction in species with limited gene flow demonstrates that deterministic explanations need not necessarily be invoked to account for the observations. Experiments to distinguish stochastic from deterministic possibilities are suggested. Overall, the pattern and magnitude of mtDNA differentiation in horseshoe crabs is very similar to that typically reported for freshwater and terrestrial species assayed over a comparable geographic range. Results demonstrate for the first time that, geographically, at least some continuously distributed marine organisms can show considerable mtDNA genetic differentiation.     

Lack of concordance between mtDNA gene flow and population density fluctuations in the bank vole

The genetic structure of bank voles Clethrionomys glareolus was determined from analyses of mitochondrial DNA (mtDNA) sequences, and compared with previous data on geographical synchrony in population density fluctuations. From 31 sample sites evenly spaced out along a 256-km transect in SE Norway a total of 39 distinct mtDNA haplotypes were found. The geographical distribution of the haplotypes was significantly nonrandom, and a cladistic analysis of the evolutionary relationship among haplotypes shows that descendant types were typically limited to a single site, whereas the ancestral types were more widely distributed geographically. This geographical distribution pattern of mtDNA haplotypes strongly indicates that the range and amount of female dispersal is severely restricted and insufficient to account for the previously observed synchrony in population density fluctuations. We conclude that geographical synchrony in this species must be caused by factors that are external to the local population, such as e.g. mobile predators.     

Population genetic structure of two ecologically distinct multimammate rats: the commensal Mastomys natalensis and the wild Mastomys erythroleucus in southeastern Senegal

Using the same set of microsatellite markers, we compared the population genetic structure of two Mastomys species, one being exclusively commensal in southeastern Senegal, and the other being continuously distributed outside villages in this region. Both species were sampled in the same landscape context and at the same spatial scale. According to the expectations based on the degree of habitat patchiness (which is higher for commensal populations in this rural area), genetic diversity was lower and genetic differentiation was higher in commensal populations of Mastomys natalensis than in wild populations of Mastomys erythroleucus. Contrasting estimates of effective dispersal and current migration rates corroborates previous data on differences in social structure between the two species. Isolation-by-distance analyses showed that human-mediated dispersal is not a major factor explaining the pattern of genetic differentiation for M. natalensis, and that gene flow is high and random between M. erythroleucus populations at the spatial scale considered.     

Invasion genetics of nutria (<Emphasis Type="Italic">Myocastor coypus</Emphasis>) in Okayama,Japan, inferred from mitochondrial and microsatellite markers

Nutria (Myocastor coypus) is a large semi-aquatic rodent native to South America, introduced worldwide for fur farming in the early twentieth century. In Japan, 150 individuals were introduced from the USA in 1939, and their feral populations are currently causing serious problems to aquatic ecosystem and agriculture. Okayama Plain is the largest habitat of nutria in Japan, established by the escapees from breeding farms around the middle of the 1940s. Here, we examined genetic structure of Okayama population and inferred gene flow among populations, using mtDNA and ten microsatellite markers (MS), to estimate eradication units for the effectiveness of population control. For mtDNA, two haplotypes (A and B) were detected in cytochrome b region. Haplotype A was widely distributed in Okayama Plain, while haplotype B was mainly observed around Yoshii River. For MS, Okayama population showed high genetic diversity, comparable to USA and Argentine populations. Genetic differentiation was recognized among drainages with a significant isolation-by-distance pattern. Multivariate analyses and Bayesian clustering method suggested two genetic clusters and radial dispersal around the coast of the Kojima Bay, while these clusters did not necessarily concord with mtDNA haplotypes in distribution. Genetic heterogeneity tended to be higher in males than in females, and females exhibited a higher relatedness than males in Asahi River. These results suggest that nutria in Okayama Plain originated from farming sites downstream in Yoshii and Takahashi Rivers and have expanded its distribution along rivers via tributaries. Mitochondrial-nuclear discordance seems to be due to male-biased dispersal in nutria.     

Restricted dispersal in a long-distance migrant bird with patchy distribution,the great reed warbler

In patchily distributed species dispersal connects local populations into metapopulations. Reliable quantifications of dispersal are therefore crucial to understanding the population dynamics and genetic structure of such metapopulation systems. The great reed warbler (Acrocephalus arundinaceus) inhabits eutrophic lakes and has a patchy breeding distribution. In this study we investigated the dispersal pattern of the great reed warbler based on an extensive capture-recapture effort covering a large census area (22,500 km²). At two adjacent breeding sites (10 km apart) in southern Central Sweden, the "main study area", we ringed the majority of adult and nestling great reed warblers between 1992 and 1999. In 1998 and 1999, we opportunistically searched for territorial males at the majority of the Swedish breeding sites, and were able to examine about 56% of all males in the region. Analyses of recaptured males demonstrated that philopatry predominated. Sixty-nine percent of the recruiting nestlings returned to breed in the main study area (their natal area), and 92% of the resighted adults were found at the same breeding locality in both study years. Breeding dispersal was significantly more restricted than natal dispersal. Additional data on natal and breeding dispersal within the main study area in 1992-1999 suggested that females were as philopatric as males. The overall high level of philopatry, with only occasional longer dispersal distances documented, yielded a root-mean-square dispersal distance of 33 km per generation. High philopatry, short dispersal distances and similar dispersal patterns of male and female great reed warblers contrast the findings among birds in general, but conform to data of species having patchy breeding habitat and isolated populations. Restricted dispersal suggests limited gene flow even among several Swedish populations, which is in line with some previous findings of the population ecology of the great reed warbler: (1) structured mtDNA lineages among European populations; (2) small-scale population differences in song patterns; and (3) low genetic variation and occurrence of inbreeding depression in our main study population.     

Population genetic structure and male-biased dispersal in the queenless ant Diacamma cyaneiventre

In this study we investigated the population genetic structure of the queenless ant Diacamma cyaneiventre. This species, lacking winged queens, is likely to have a restricted female dispersal. We used both mitochondrial and microsatellite markers to assess the consequence of such restricted female dispersal at three geographical scales: within a given locality (< 1 km), between localities within a given region (< 10 km) and between regions (> 36 km). Within a locality, a strong population structure was observed for mitochondrial DNA (mtDNA) whereas weak or nonexistent population genetic structure was observed for the microsatellites (around 5% of the value for mtDNA). Male gene flow was estimated to be about 20-30 times higher than female gene flow at this scale. At a larger spatial scale, very strong genetic differentiation for both markers was observed between localities - even within a single region. Female dispersal is nonexistent at these scales and male dispersal is very restricted, especially between regions. The phylogeographical structure of the mtDNA haplotypes as well as the very low genetic diversity of mtDNA within localities indicate that new sites are colonized by a single migration event from adjacent localities, followed by successive colony fissions. These patterns of genetic variability and differentiation agree with what is theoretically expected when colonization events are kin-structured and when, following colonization, dispersion is mainly performed by males.     

Spatial structure of lemming populations (Dicrostonyx groenlandicus) fluctuating in density

The pattern and scale of the genetic structure of populations provides valuable information for the understanding of the spatial ecology of populations, including the spatial aspects of density fluctuations. In the present paper, the genetic structure of periodically fluctuating lemmings (Dicrostonyx groenlandicus) in the Canadian Arctic was analysed using mitochondrial DNA (mtDNA) control region sequences and four nuclear microsatellite loci. Low genetic variability was found in mtDNA, while microsatellite loci were highly variable in all localities, including localities on isolated small islands. For both genetic markers the genetic differentiation was clear among geographical regions but weaker among localities within regions. Such a pattern implies gene flow within regions. Based on theoretical calculations and population census data from a snap-trapping survey, we argue that the observed genetic variability on small islands and the low level of differentiation among these islands cannot be explained without invoking long distance dispersal of lemmings over the sea ice. Such dispersal is unlikely to occur only during population density peaks.     

Novel patterns of historical isolation, dispersal, and secondary contact across Baja California in the Rosy Boa (Lichanura trivirgata)

Mitochondrial DNA (mtDNA) sequence variation was examined in 131 individuals of the Rosy Boa (Lichanura trivirgata) from across the species range in southwestern North America. Bayesian inference and nested clade phylogeographic analyses (NCPA) were used to estimate relationships and infer evolutionary processes. These patterns were evaluated as they relate to previously hypothesized vicariant events and new insights are provided into the biogeographic and evolutionary processes important in Baja California and surrounding North American deserts. Three major lineages (Lineages A, B, and C) are revealed with very little overlap. Lineage A and B are predominately separated along the Colorado River and are found primarily within California and Arizona (respectively), while Lineage C consists of disjunct groups distributed along the Baja California peninsula as well as south-central Arizona, southward along the coastal regions of Sonora, Mexico. Estimated divergence time points (using a Bayesian relaxed molecular clock) and geographic congruence with postulated vicariant events suggest early extensions of the Gulf of California and subsequent development of the Colorado River during the Late Miocene-Pliocene led to the formation of these mtDNA lineages. Our results also suggest that vicariance hypotheses alone do not fully explain patterns of genetic variation. Therefore, we highlight the importance of dispersal to explain these patterns and current distribution of populations. We also compare the mtDNA lineages with those based on morphological variation and evaluate their implications for taxonomy.     

Comparative phylogeography and the history of endemic vertebrates in the Wet Tropics rainforests of Australia

We examine the effects of historical climate change on vertebrate differentiation in tropical rainforest by comparing phylogeographic patterns in six species of widespread rainforest-restricted herpetofauna from throughout the Wet Tropics of Australia. Qualitative and quantitative comparisons of phylogeographic structure reveal strikingly similar patterns of pre-Pleistocene vicariant population differentiation on either side of a previously identified biogeographic break (variously referred to as the Black Mountain Barrier or Corridor; BMC). While divergence across the BMC antedates the Pleistocene, the impact of Quaternary climate change is apparent in populations on either side of the BMC. Consistent with palaeoclimatological reconstructions for the region, the distribution and degree of mtDNA diversity suggests that populations were fragmented and reduced to multiple refugia during Pleistocene glacial periods, with expansion following Holocene rainforest recovery. This pattern is repeated on both sides of the BMC, but substantial differences in the amount and distribution of mtDNA diversity within species indicate the importance of species-specific ecological characteristics. The historical processes of extinction and (re)colonization revealed by the comparative phylogeographic analysis of mtDNA sequences substantiate earlier suggestions that current regional patterns of species distribution and diversity in the Wet Tropics are largely determined by local extinctions and subsequent recolonization driven by Quaternary climate changes.     

Effects of hydrographic barriers on population genetic structure of the sea star Coscinasterias muricata (Echinodermata, Asteroidea) in the New Zealand fiords

New Zealand's 14 deep-water fiords possess persistent salinity stratification and mean estuarine circulation that may serve to isolate populations of marine organisms that have a dispersal larval phase. In order to investigate this idea, we analysed the population structure of the sea star Coscinasterias muricata using a mitochondrial DNA marker. Genetic differentiation among populations of C. muricata was analysed using 366 base pairs of mtDNA D-loop. We compared populations from the fiords with several others sampled from around New Zealand. At a macro-geographical scale (> 1000 km), restricted gene flow between the North and South Islands was observed. At a meso-geographical scale (10-200 km), significant population structure was found among fiords and between fiords and open coast. The pattern of population genetic structure among the fiords suggests a secondary contact between a northern population and a southern one, separated by a contact or mixing zone. These populations may have diverged by the effects of random genetic drift and population isolation as a consequence of the influence of estuarine circulation on dispersal. In northern Fiordland, genetic structure approximated an isolation by distance model. However, the pattern in genetic differences suggests that distance alone cannot explain the most divergent populations and that fiord hydrography may increase the effect of genetic drift within populations in the fiords. Finally, our study indicates that populations within the fiords underwent recent rapid expansion, followed most probably by genetic drift due to a lack of gene flow among the fiords.     

Drifting fronds and drifting alleles: range dynamics,local dispersal and habitat isolation shape the population structure of the estuarine seaweed Fucus ceranoides

Aim The seaweed Fucus ceranoides is restricted to spatially discrete estuarine habitats and lacks planktonic dispersal phases; it is therefore expected to exhibit strong population differentiation. Its cold‐temperate affinities and mtDNA variation imply that the northern part of the species’ range, where F. ceranoides is now ubiquitous, was recently colonized after the onset of the last deglaciation, potentially resulting in areas with greater genetic homogeneity. Here we examine the population structure of F. ceranoides to test these predictions, emphasizing the contrasting genetic signatures of limited dispersal in refugial versus recently colonized regions. Location North‐eastern Atlantic estuaries from Portugal to Norway. Methods A total of 504 individuals from 21 estuarine sites spanning the entire range of F. ceranoides were sampled and genotyped for nine microsatellite loci. Population structure was inferred from several genotypic and allele‐frequency analyses. Geographical patterns of genetic diversity were used to reconstruct the historical biogeography of the species. Results Genetic diversity and regional population differentiation showed a consistent decline with increasing latitude. Southernmost populations harboured most of the endemic variation, whereas the northern populations (> 55° N) were almost fixed for the same alleles across loci. In southern and central regions of its distribution, F. ceranoides showed striking population subdivision, with many of the sampled estuaries corresponding to coherent genetic units that were easily discriminated from one another with standard clustering methods. Main conclusions The geographical pattern of genetic diversity supports the long‐term refugial status of Iberia and a post‐glacial range expansion of F. ceranoides into previously glaciated latitudes. Despite the species’ capacity to colonize newly available habitats, the genetic structure of F. ceranoides outside the recently (re)colonized range reveals that gene flow between populations is extremely low. This study provides a remarkable example of how infrequent and spatially limited dispersal can have contrasting effects at the scales of meta‐population (connectivity) versus range dynamics (habitat tracking), and of how dispersal restrictions can result in either genetic divergence or homogeneity depending on the maturity and demographic conditions of the populations.