Postglacial Colonization Routes Research Articles

Phylogeography, hybridization, and species discovery in the Etheostoma nigrum complex (Percidae: Etheostoma: Boleosoma)

The history of riverine fish diversification is largely a product of geographic isolation. Physical barriers that reduce or eliminate gene flow between populations facilitate divergence via genetic drift and natural selection, eventually leading to speciation. For freshwater organisms, diversification is often the product of drainage basin rearrangements. In young clades where the history of isolation is the most recent, evolutionary relationships can resemble a tangled web. One especially recalcitrant group of freshwater fishes is the Johnny Darter (Etheostoma nigrum) species complex, where traditional taxonomy and molecular phylogenetics indicate a history of gene flow and conflicting inferences of species diversity. Here we assemble a genomic dataset using double digest restriction site associated DNA (ddRAD) sequencing and use phylogenomic and population genetic approaches to investigate the evolutionary history of the complex of species that includes E. nigrum, E. olmstedi, E. perlongum, and E. susanae. We reveal and validate several evolutionary lineages that we delimit as species, highlighting the need for additional work to formally describe the diversity of the Etheostoma nigrum complex. Our analyses also identify gene flow among recently diverged lineages, including one instance involving E. susanae, a localized and endangered species. Phylogeographic structure within the Etheostoma nigrum species complex coincides with major geologic events, such as parallel divergence in river basins during Pliocene inundation of the Atlantic coastal plain and multiple northward post-glacial colonization routes tracking river basin rearrangements. Our study serves as a nuanced example of how low dispersal rates coupled with geographic isolation among disconnected river systems in eastern North America has produced one of the world’s freshwater biodiversity hotspots.

Phylogeographic analysis and species distribution modelling of the wood frog Batrachyla leptopus (Batrachylidae) reveal interglacial diversification in south western Patagonia.

BackgroundThe evolutionary history of southern South American organisms has been strongly influenced by Pleistocene climate oscillations. Amphibians are good models to evaluate hypotheses about the influence of these climate cycles on population structure and diversification of the biota, because they are sensitive to environmental changes and have restricted dispersal capabilities. We test hypotheses regarding putative forest refugia and expansion events associated with past climatic changes in the wood frog Batrachyla leptopus distributed along ∼1,000 km of length including glaciated and non-glaciated areas in southwestern Patagonia.MethodsUsing three mitochondrial regions (D-loop, cyt b, and coI) and two nuclear loci (pomc and crybA1), we conducted multilocus phylogeographic analyses and species distribution modelling to gain insights of the evolutionary history of this species. Intraspecific genealogy was explored with maximum likelihood, Bayesian, and phylogenetic network approaches. Diversification time was assessed using molecular clock models in a Bayesian framework, and demographic scenarios were evaluated using approximate Bayesian computation (ABC) and extended Bayesian skyline plot (EBSP). Species distribution models (SDM) were reconstructed using climatic and geographic data.ResultsPopulation structure and genealogical analyses support the existence of four lineages distributed north to south, with moderate to high phylogenetic support (Bootstrap > 70%; BPP > 0.92). The diversification time of B. leptopus’ populations began at ∼0.107 mya. The divergence between A and B lineages would have occurred by the late Pleistocene, approximately 0.068 mya, and divergence between C and D lineages was approximately 0.065 mya. The ABC simulations indicate that lineages coalesced at two different time periods, suggesting the presence of at least two glacial refugia and a postglacial colonization route that may have generated two southern lineages (p = 0.93, type I error: <0.094, type II error: 0.134). EBSP, mismatch distribution and neutrality indexes suggest sudden population expansion at ∼0.02 mya for all lineages. SDM infers fragmented distributions of B. leptopus associated with Pleistocene glaciations. Although the present populations of B. leptopus are found in zones affected by the last glacial maximum (∼0.023 mya), our analyses recover an older history of interglacial diversification (0.107–0.019 mya). In addition, we hypothesize two glacial refugia and three interglacial colonization routes, one of which gave rise to two expanding lineages in the south.

Altitudinal, latitudinal and longitudinal responses of cloud forest species to Quaternary glaciations in the northern Neotropics

AbstractThe glaciations of the Quaternary caused changes in the geographical distributions of species associated with tropical montane cloud forests. The most obvious effect of the glacial conditions was the downward displacement of cloud forest species, thus giving opportunities for population connectivity in the lowlands. Considerable attention has been paid to these altitudinal changes, but latitudinal and longitudinal movements remain poorly understood in the northern Neotropics. Here, I use ecological niche modelling to generate palaeodistributions of small-eared shrews (Mammalia: Soricidae) closely associated with cloud forests in the mountain systems of Mexico and then retrodict their range shifts during the Last Glacial Maximum (LGM), one of the coldest periods of the Quaternary. The results suggest that cloud forest species not only migrated downwards in response to global cooling and dryness but also migrated latitudinally and longitudinally onto those slopes that maintained moist conditions (other slopes remained unsuitable during the LGM), thus revealing a hitherto unknown route for postglacial colonization of cloud forest species. This scenario of past distributional change probably had genetic and demographic implications and has repercussions for the identification of areas of refugia and postglacial colonization routes of cloud forest species.

Latitudinal divergence in a widespread amphibian: Contrasting patterns of neutral and adaptive genomic variation.

Stochastic effects from demographic processes and selection are expected to shape the distribution of genetic variation in spatially heterogeneous environments. As the amount of genetic variation is central for long-term persistence of populations, understanding how these processes affect variation over large-scale geographical gradients is pivotal. We investigated the distribution of neutral and putatively adaptive genetic variation, and reconstructed demographic history in the moor frog (Rana arvalis) using 136 individuals from 15 populations along a 1,700-km latitudinal gradient from northern Germany to northern Sweden. Using double digest restriction-site associated DNA sequencing we obtained 27,590 single nucleotide polymorphisms (SNPs), and identified differentiation outliers and SNPs associated with growing season length. The populations grouped into a southern and a northern cluster, representing two phylogeographical lineages from different post-glacial colonization routes. Hybrid index estimation and demographic model selection showed strong support for a southern and northern lineage and evidence of gene flow between regions located on each side of a contact zone. However, patterns of past gene flow over the contact zone differed between neutral and putatively adaptive SNPs. While neutral nucleotide diversity was higher along the southern than the northern part of the gradient, nucleotide diversity in differentiation outliers showed the opposite pattern, suggesting differences in the relative strength of selection and drift along the gradient. Variation associated with growing season length decreased with latitude along the southern part of the gradient, but not along the northern part where variation was lower, suggesting stronger climate-mediated selection in the north. Outlier SNPs included loci involved in immunity and developmental processes.

Genetic basis of amphibian larval development along a latitudinal gradient: Gene diversity, selection and links with phenotypic variation in transcription factor C/EBP-1.

Ectotherm development rates often show adaptive divergence along climatic gradients, but the genetic basis for this variation is rarely studied. Here, we investigated the genetic basis for phenotypic variation in larval development in the moor frog Rana arvalis from five regions along a latitudinal gradient from Germany to northern Sweden. We focused on the C/EBP-1 gene, a transcription factor associated with larval development time. Allele frequencies at C/EBP-1 varied strongly among geographical regions. Overall, the distribution of alleles along the gradient was in concordance with the dual post-glacial colonization routes into Scandinavia, with a large number of alleles exclusively present along the southern colonization route. Only three of 38 alleles were shared between the routes. Analysis of contemporary selection on C/EBP-1 showed divergent selection among the regions, probably reflecting adaptation to the local environmental conditions, although this was especially strong between southern and northern regions coinciding also with lineages from different colonization routes. Overall, the C/EBP-1 gene has historically been under purifying selection, but two specific amino acid positions showed significant signals of positive selection. These positions showed divergence between southern and northern regions, and we suggest that they are functionally involved in the climatic adaptation of larval development. Using phenotypic data from a common garden experiment, we found evidence for specific C/EBP-1 alleles being correlated with larval development time, suggesting a functional role in adaptation of larval development to large-scale climatic variation.

Post-glacial colonization routes coincide with a life-history breakpoint along a latitudinal gradient.

Although adaptive divergence along environmental gradients has repeatedly been demonstrated, the role of post-glacial colonization routes in determining phenotypic variation along gradients has received little attention. Here, we used a hierarchical QST -FST approach to separate the roles of adaptive and neutral processes in shaping phenotypic variation in moor frog (Rana arvalis) larval life histories along a 1,700km latitudinal gradient across northern Europe. This species has colonized Scandinavia via two routes with a contact zone in northern Sweden. By using neutral SNP and common garden phenotypic data from 13 populations at two temperatures, we showed that most of the variation along the gradient occurred between the two colonizing lineages. We found little phenotypic divergence within the lineages; however, all phenotypic traits were strongly diverged between the southern and northern colonization routes, with higher growth and development rates and larger body size in the north. The QST estimates between the colonization routes were four times higher than FST , indicating a prominent role for natural selection. QST within the colonization routes did not generally differ from FST , but we found temperature-dependent adaptive divergence close to the contact zone. These results indicate that lineage-specific variation can account for much of the adaptive divergence along a latitudinal gradient.

Phylogeography of a widely distributed species reveals a cryptic assemblage of distinct genetic lineages needing separate conservation strategies

Genetic structure in broadly distributed herbs may be shaped by past climate events, leading to distinct genetic lineages, and possibly to different Evolutionary Significant Units (ESUs). We identified the potential glacial refugia, postglacial (re)colonization routes, contact zones and ESUs in the widespread but locally rare Silene nutans to propose appropriate conservation strategies related to migration history-related genetic structure. We sequenced four plastid DNA markers from 136 populations covering S. nutans European distribution range. We built a network on concatenated sequences using statistical parsimony and performed Bayesian and Maximum Likelihood analyses to build a phylogenetic tree. Haplotype genetic diversity and between-population differentiation (GST, NST) were calculated. Based on 23 plastid haplotypes, populations were ascribed to seven strongly differentiated genetic lineages. Potential refugia were identified: in the Iberian and Italian Peninsulas, expanding to south-western France and the Alps, respectively; in south-central France, reaching England and Belgium; an eastern refugium, following a north-westward expansion; in the Balkans, expanding to north-eastern Italy, and in the Carpathians. Several contact zones were identified. Silene nutans consists of a cryptic assemblage of genetically distinct taxa, with at least seven ESUs. Evaluating the distribution range, the abundance, and the conservation status of each ESU is necessary for the preservation of the S. nutans species complex at European and regional levels. Ex situ conservation and genetic restoration involving plant translocations must consider the phylogeographic history and the distinct genetic lineages to avoid the introduction of poorly adapted plants and outbreeding depression leading to non-viable hybrid progeny, especially in contact zones.

Drift, selection, or migration? Processes affecting genetic differentiation and variation along a latitudinal gradient in an amphibian

BackgroundPast events like fluctuations in population size and post-glacial colonization processes may influence the relative importance of genetic drift, migration and selection when determining the present day patterns of genetic variation. We disentangle how drift, selection and migration shape neutral and adaptive genetic variation in 12 moor frog populations along a 1700 km latitudinal gradient. We studied genetic differentiation and variation at a MHC exon II locus and a set of 18 microsatellites.ResultsUsing outlier analyses, we identified the MHC II exon 2 (corresponding to the β-2 domain) locus and one microsatellite locus (RCO8640) to be subject to diversifying selection, while five microsatellite loci showed signals of stabilizing selection among populations. STRUCTURE and DAPC analyses on the neutral microsatellites assigned populations to a northern and a southern cluster, reflecting two different post-glacial colonization routes found in previous studies. Genetic variation overall was lower in the northern cluster. The signature of selection on MHC exon II was weaker in the northern cluster, possibly as a consequence of smaller and more fragmented populations.ConclusionOur results show that historical demographic processes combined with selection and drift have led to a complex pattern of differentiation along the gradient where some loci are more divergent among populations than predicted from drift expectations due to diversifying selection, while other loci are more uniform among populations due to stabilizing selection. Importantly, both overall and MHC genetic variation are lower at northern latitudes. Due to lower evolutionary potential, the low genetic variation in northern populations may increase the risk of extinction when confronted with emerging pathogens and climate change.

Unique postglacial evolution of the hornbeam (Carpinus betulus L.) in the Carpathians and the Balkan Peninsula revealed by chloroplast DNA

The Balkan Peninsula is one of the largest and most important European glacial refugia. However, the evolutionary history and phylogeographic pattern of temperate tree species that survived in the Balkans glacial refugia and their contribution to the genetic structure of the current population in the Carpathian Mountains remains poorly understood.Using polymerase chain reaction - restriction fragment length polymorphism (PCR-RFLP), and extensive population sampling, we explored the phylogeographic pattern of Carpinus betulus in both the Balkan Peninsula and the Carpathian region. We aimed to determine the locations of potential glacial refugia, in order to delineate post-glacial colonization routes, and also to test if northern cryptic refugia had persisted during the Last Glacial Maximum (LGM).Our results provide strong support for the existence of multiple refugia similar to the ‘refugia-within-refugia’ scenario, which would suggest that Carpinus betulus has experienced a complex evolutionary history.In agreement with pollen data, our findings corroborate with previous hypotheses suggesting that hornbeam has a distinct postglacial evolution in the southeast of Europe (Carpathians and Balkan Peninsula) compared with that in Western Europe.Three postglacial re-colonization routes, from three distinct effective glacial refugia: (1) the Dinaric Alps; (2) the Pirin and Rhodope Mountains and (3) the Strandzha Mountains, were detected within the Balkan Peninsula. The pattern of cpDNA haplotypes distribution across the Ukrainian Carpathians revealed a “suture zone”, which is a consequence of contact due to postglacial re-colonization between hornbeam populations originating from all three effective glacial refugia.The peculiar haplotype geographical structure found in C. betulus and the identified multiple glacial refugia in the Balkans may have direct consequences in the management and conservation of hornbeam forest genetic resources.

Subterranean termite phylogeography reveals multiple postglacial colonization events in southwestern Europe.

A long‐standing goal of evolutionary biology is to understand how paleoclimatic and geological events shape the geographical distribution and genetic structure within and among species. Using a diverse set of markers (cuticular hydrocarbons, mitochondrial and nuclear gene sequences, microsatellite loci), we studied Reticulitermes grassei and R. banyulensis, two closely related termite species in southwestern Europe. We sought to clarify the current genetic structure of populations that formed following postglacial dispersal from refugia in southern Spain and characterize the gene flow between the two lineages over the last several million years. Each marker type separately provided a fragmented picture of the evolutionary history at different timescales. Chemical analyses of cuticular hydrocarbons and phylogenetic analyses of mitochondrial and nuclear genes showed clear separation between the species, suggesting they diverged following vicariance events in the Late Miocene. However, the presence of intermediate chemical profiles and mtDNA introgression in some Spanish colonies suggests ongoing gene flow. The current genetic structure of Iberian populations is consistent with alternating isolation and dispersal events during Quaternary glacial periods. Analyses of population genetic structure revealed postglacial colonization routes from southern Spain to France, where populations underwent strong genetic bottlenecks after traversing the Pyrenees resulting in parapatric speciation.

Mating system patterns of natural populations of Pinus koraiensis along its post-glacial colonization route in northeastern China.

To understand the genetic mechanisms underlying the endangerment of Pinus koraiensis, we studied the mating system of 49 families of this species in 3 natural populations along its post-glacial colonization route across ~1500 km in northeastern China using the chloroplast simple sequence repeat technique. We analyzed 11 polymorphic loci with clear and repeating bands, and we calculated the multi-locus outcrossing rate (tm), single-locus outcrossing rate, inbreeding index, and fixation index (F). Intra-population variation was not observed, but a large inter-population variation was observed in the outcrossing rate, and the tm increased from 0.767 (the south population) to 0.962 (the north population) along the post-glacial colonization route. The tm values within a population did not change with time over 2 consecutive years. The F values for the 3 populations were <0, which indicates an excess of heterozygotes. The mean effective number of alleles, Shannon diversity index, and Nei's genetic diversity index did not show a south-north pattern. The north population had the highest outcrossing rate but the lowest genetic diversity. The average genetic differentiation of P. koraiensis populations was 0.1251, which was within the average range of woody plants with outcrossing and wind pollination. This study suggests that the current endangerment of P. koraiensis is not related to its genetic structure; perhaps it is mainly caused by man-made and natural disturbances such as deforestation and fire. Therefore, reducing disturbances and enhancing habitats, rather than the genetic aspects, play more important roles in the long-term protection of P. koraiensis.

Glacial refugia and post-glacial colonization patterns in European bryophytes

Most species are assumed to have survived south or east of the ice sheet covering northern Europe during the last glacial maximum. Molecular and macrofossil evidence suggests, however, that some species may have survived in ice-free areas in Scandinavia. In plants, inbreeding and vegetative growth are associated with low genetic load and enhanced survival in small, isolated populations. These characteristics are often found in bryophytes, possibly allowing them to survive extreme conditions in isolated refugia and also within ice sheets. Here, we review the Holocene bryophyte history in Europe highlighting main glacial refugia and post-glacial colonization routes. Also, meta-analyses are performed to investigate if distribution ranges and genetic structure are associated with life-history traits. Bryophytes survived the last glaciation in several refugia, but there is no unequivocal evidence of survival within the Scandinavian ice sheet. Northern Europe was colonized from southern, eastern and western Europe, as well as North America. Species with small spores have broader distribution ranges than species with large spores, and high frequency of sporophyte production is associated with limited genetic differentiation between populations.

Use of multiple markers demonstrates a cryptic western refugium and postglacial colonisation routes of Atlantic salmon (Salmo salar L.) in northwest Europe

Glacial and postglacial processes are known to be important determinants of contemporary population structuring for many species. In Europe, refugia in the Italian, Balkan and Iberian peninsulas are believed to be the main sources of species colonising northern Europe after the glacial retreat; however, there is increasing evidence of small, cryptic refugia existing north of these for many cold-tolerant species. This study examined the glacial history of Atlantic salmon in western Europe using two independent classes of molecular markers, microsatellites (nuclear) and mitochondrial DNA variation. Alongside the well-documented refuge in the Iberian Peninsula, evidence for a cryptic refuge in northwest France is also presented. Critically, methods utilised to estimate divergence times between the refugia indicated that salmon in these two regions had diverged a long time before the last glacial maximum; coalescence analysis (as implemented in the program IMa2) estimated divergence times at around 60 000 years before present. Through the examination of haplotype frequencies, previously glaciated areas of northwest Europe, that is, Britain and Ireland, appear to have been colonised from salmon expanding out of both refugia, with the southwest of England being the primary contact zone and exhibiting the highest genetic diversity.

Longitudinal gradients in the phylogenetic community structure of European Tenebrionidae (Coleoptera) do not coincide with the major routes of postglacial colonization

Species specific colonization abilities and biotic and abiotic filters influence the local and regional faunal composition along colonization trajectories. Using a recent compilation of the occurrences of 1373 darkling beetle (Tenebrionidae) species and subspecies in 49 European countries and major islands, we reconstructed the tenebrionid postglacial colonization of middle and northern Europe from southern European glacial refuges and linked species composition to latitudinal and longitudinal gradients in phylogenetic relatedness across Europe. The majority of European islands and mainland countries appeared to be phylogenetically clustered. We did not find significant latitudinal trends in average phylogenetic relatedness of regional faunas along the supposed postglacial colonization routes but detected a strong positive correlation between mean relatedness and longitude of mainland faunas and an opposite negative correlation for island faunas. The strength of phylogenetic relatedness in the regional tenebrionid faunas decreased significantly with latitude and to a lesser degree with longitude. These findings are in accordance with two trajectories of postglacial colonization from centres in Spain and middle Asia that caused a strong longitudinal trend in the phylogenetic relatedness. Subsequent pair‐wise analyses of species co‐occurrences showed that species of similar distributional ranges tend to be phylogenetically clustered and species of different spatial distribution to be phylogenetically segregated. Both findings are in accordance with the concept of ‘range size heritability’. Our study demonstrates that taxonomic data are sufficiently powerful to infer continental wide patterns in phylogenetic relatedness that can be linked to colonization history and geographic information.

Gene exchange across a postglacial contact zone in Fraxinus excelsior L.

AbstractHybridization between divergent lineages of common ash Fraxinus excelsior L. was studied in Slovakia and adjacent regions of Hungary, the Czech Republic and Poland in the contact zone between two postglacial colonization routes originating from different glacial refugia. Thirty-eight common ash populations were studied using a combination of 6 maternally inherited chloroplast microsatellite markers and 7 biparentally inherited nuclear microsatellites (nSSR). Two predominant chloroplast lineages were identified, between which the boundary was very sharp. Populations containing a mixture of different haplotypes were found only in the immediate proximity of the boundary. Bayesian analysis of population structure based on nSSR loci revealed the existence of two clusters, which largely coincided with chloroplast lineages. Both haplotype frequencies and proportions of clusters identified by the Bayesian analysis exhibited a clinal transition over the hybrid zone, with cline widths of 36 km for chloroplast haplotype frequencies (reflecting gene flow by seeds) and 275 km for Bayesian clusters based on nSSR (reflecting gene flow by pollen and seeds). Chloroplast haplotype diversity increased along the boundary between lineages not only because of admixture, but also due to the presence of rare haplotypes. In contrast, diversity at nuclear loci did not exhibit any geographical trend.

Postglacial colonization of Europe by the barbastelle bat: agreement between molecular data and past predictive modelling

The barbastelle (Barbastella barbastellus) is a rare forest bat with a wide distribution in Europe. Here, we combine results from the analysis of two mtDNA fragments with species distribution modelling to determine glacial refugia and postglacial colonization routes. We also investigated whether niche conservatism occurs in this species. Glacial refugia were identified in the three southern European peninsulas: Iberia, Italy and the Balkans. These latter two refugia played a major role in the postglacial colonization process, with their populations expanding to England and central Europe, respectively. Palaeo-distribution models predicted that suitable climatic conditions existed in the inferred refugia during the last glacial maximum (LGM). Nevertheless, the overlap between the current and the LGM distributions was almost inexistent in Italy and in the Balkans, meaning that B. barbastellus populations were forced to shift range between glacial and interglacial periods, a process that probably caused some local extinctions. In contrast, Iberian populations showed a 'refugia within refugium' pattern, with two unconnected areas containing stable populations (populations that subsisted during both glacial and interglacial phases). Moreover, the match between LGM models and the refugial areas determined by molecular analysis supported the hypothesis of niche conservatism in B. barbastellus. We argue that geographic patterns of genetic structuring, altogether with the modelling results, indicate the existence of four management units for conservation: Morocco, Iberia, Italy and UK, and Balkans and central Europe. In addition, all countries sampled possessed unique gene pools, thus stressing the need for the conservation of local populations.

Multiple glacial refugia and postglacial colonization routes inferred for a woodland geophyte, Cyclamen purpurascens: patterns concordant with the Pleistocene history of broadleaved and coniferous tree species

The phylogeographical history of mid-altitude woodland herbs that depend on moist and shaded forest habitats is poorly understood. Here, we analysed the genetic structure of Cyclamen purpurascens, a mountainous calcicolous perennial, to test hypotheses regarding its glacial survival in single or multiple refugia and postglacial colonization routes, and to explore how they are congruent with the histories inferred for temperate trees and other mountainous herbs. We gathered AFLP data and chloroplast DNA sequences (trnD-trnT region) from 68 populations spanning the entire distribution range (the Jura Mountains, Alps, western Carpathians, Dinarides). Both genetic markers revealed two main phylogeographical groups (phylogroups) in C. purpurascens. Additionally, AFLP data detected a more detailed structure of five phylogroups: two widespread, showing east−west geographical separation, and three local ones, restricted to somewhat disjunct, marginal regions of the species range. We suggest that C. purpurascens survived the last glaciation in two main regions, the foothills of the Southern Limestone Alps and the Karst area of the north-western Dinarides, and possibly also in microrefugia in the Western Carpathians. The glacial persistence and colonization routes of this woodland herb are highly concordant with those inferred for several temperate trees, especially the European beech. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105, 741–760.

High latitudes and high genetic diversity: Phylogeography of a widespread boreal bird, the gray jay (Perisoreus canadensis)

We describe range-wide phylogeographic variation in gray jays (Perisoreus canadensis), a boreal Nearctic corvid that occurs today primarily in recently glaciated regions. Phylogenetic analysis of mitochondrial DNA (1041 base pairs ND2 gene; N=205, 50 localities) revealed four reciprocally monophyletic groups. One widespread clade occurs across the North American boreal zone, from Newfoundland to Alaska and southwest into Utah. Three other clades occur at lower latitudes in the montane West in Colorado, the northern Rocky Mountains, and the Pacific Northwest respectively. The geographic distribution of clades in gray jays corresponds with a general pattern that is emerging for boreal taxa, having one widespread northern clade and one or more geographically restricted southwestern clades. Population genetic analyses indicate that the larger boreal clade is genetically structured and harbors significantly more genetic diversity than those clades occurring at lower latitudes. Species distribution modeling (SDM) revealed multiple putative Pleistocene refugia including several occurring at higher latitudes. We suggest that multiple post-glacial colonization routes, some of which originate from these northern refugia, are responsible for the relatively high genetic diversity at high latitudes. Conversely, lower latitude clades show little variation, probably as a result of historical restriction to smaller geographical areas with smaller long-term population sizes. This ‘upside-down’ pattern of genetic diversity contrasts with the conventional view that populations of north-temperate species occupying previously glaciated habitats should possess lower levels of diversity than their southern counterparts.

Phylogeography, historical demography and postglacial colonization routes of two amphi-Atlantic distributed amphipods

To evaluate the influence of the ice ages on patterns of genetic diversity and to test generally accepted perspectives related to glacial refugia and proposed post-glacial colonization pathways in the North Atlantic area, we sequenced a portion of the mitochondrial cytochrome c oxidase subunit I of two amphi-Atlantic distributed amphipods, Gammarus duebeni (n = 418) and Gammarus oceanicus (n = 242). Both species showed several populations with a combination of highly reduced levels of genetic diversity and significant demographic expansion in previously glaciated sites, consistent with an expected extinction and recolonization scenario. Within G. oceanicus, two reciprocally monophyletic clades were found. One clade consisted of the specimens from populations of the American St. Lawrence River and two populations south of it. The other clade included specimens from populations of Europe, Greenland, Iceland and Svalbard, as well as from the Hudson Bay. For G. duebeni, we found no private American haplotypes, and post-glacial colonization from two founder populations is likely. A fixed genetic discontinuity was observed in both species between Iceland and Greenland on the one hand and Europe on the other hand. Our data hence indicate that the proposed postglacial colonization of America from Europe via the Faroe Islands, Iceland and Greenland could be more complex than previously thought and that parts of the North Atlantic region were not colonized from the Eastern Atlantic coasts. Unlike other studies, we speculate that there must be a Nordic glacial refugium for both species, which could be located in Greenland. The Greenlandic populations of both species exhibited private haplotypes, but signals of demographic expansion, conflicting with our theory. It is possible that the refugial character of the Greenlandic populations was erased by the LGM. Furthermore, we found evidence for glacial refugia along the Norwegian coast, the English Channel (G. duebeni) and on the Faroe Islands (G. oceanicus).

From the Western Alps across Central Europe: Postglacial recolonisation of the tufa stream specialist Rhyacophila pubescens (Insecta, Trichoptera)

BackgroundDispersal rates, i.e. the effective number of dispersing individuals per unit time, are the product of dispersal capacity, i.e. a species physiological potential for dispersal, dispersal behaviour, i.e. the decision to leave a habitat patch in favour of another, and connectivity of occupied habitat. Thus, dispersal of species that are highly specialised to a certain habitat is limited by habitat availability. Species inhabiting very stable environments may also adopt a sedentary life-style. Both factors should lead to strong genetic differentiation in highly specialised species inhabiting stable environments. These two factors apply to our model species Rhyacophila pubescens a highly specialised freshwater insect that occurs in tufa springs, a very stable habitat.ResultsWe examined the genetic population structure and phylogeography using range-wide mtCOI sequence and AFLP data from 333 individuals of R. pubescens. We inferred the location of Pleistocene refugia and postglacial colonisation routes of R. pubescens, and examined ongoing local differentiation. Our results indicate intraregional differentiation with a high number of locally endemic haplotypes, that we attributed to habitat specificity and low dispersal rates of R. pubescens. We observed high levels of genetic diversity south of the Alps and genetic impoverishment north of the Alps. Estimates of migrants placed the refugium and the source of the colonisation in the Dauphiné Alps (SW Alps).ConclusionsThis is the first example of an aquatic insect with a colonisation route along the western margin of the Alps to the Central European highlands. The study also shows that specialisation to a stable environment may have promoted a behavioural shift to decreased dispersal rates, leading to stronger local population differentiation than in less specialised aquatic insects. Alternatively, the occurrence of highly specialised tufa spring habitats may have been more widespread in the past, leading to range regression and fragmentation among present day R. pubescens populations.