Northern glacial refugia for the pygmy shrew Sorex minutus in Europe revealed by phylogeographic analyses and species distribution modelling

The southern European peninsulas (Iberian, Italian and Balkan) are traditionally recognized as glacial refugia from where many species colonized central and northern Europe after the Last Glacial Maximum (LGM). However, evidence that some species had more northerly refugia is accumulating from phylogeographic, palaeontological and palynological studies, and more recently from species distribution modelling (SDM), but further studies are needed to test the idea of northern refugia in Europe. Here, we take a rarely implemented multidisciplinary approach to assess if the pygmy shrew Sorex minutus, a widespread Eurasian mammal species, had northern refugia during the LGM, and if these influenced its postglacial geographic distribution. First, we evaluated the phylogeographic and population expansion patterns using mtDNA sequence data from 123 pygmy shrews. Then, we used SDM to predict present and past (LGM) potential distributions using two different training data sets, two different algorithms (Maxent and GARP) and climate reconstructions for the LGM with two different general circulation models. An LGM distribution in the southern peninsulas was predicted by the SDM approaches, in line with the occurrence of lineages of S. minutus in these areas. The phylogeographic analyses also indicated a widespread and strictly northern‐central European lineage, not derived from southern peninsulas, and with a postglacial population expansion signature. This was consistent with the SDM predictions of suitable LGM conditions for S. minutus occurring across central and eastern Europe, from unglaciated parts of the British Isles to much of the eastern European Plain. Hence, S. minutus likely persisted in parts of central and eastern Europe during the LGM, from where it colonized other northern areas during the late‐glacial and postglacial periods. Our results provide new insights into the glacial and postglacial colonization history of the European mammal fauna, notably supporting glacial refugia further north than traditionally recognized.     

A new scenario for the quaternary history of European beech populations: palaeobotanical evidence and genetic consequences

Here, palaeobotanical and genetic data for common beech (Fagus sylvatica) in Europe are used to evaluate the genetic consequences of long-term survival in refuge areas and postglacial spread. Four large datasets are presented, including over 400 fossil-pollen sites, 80 plant-macrofossil sites, and 450 and 600 modern beech populations for chloroplast and nuclear markers, respectively. The largely complementary palaeobotanical and genetic data indicate that: (i) beech survived the last glacial period in multiple refuge areas; (ii) the central European refugia were separated from the Mediterranean refugia; (iii) the Mediterranean refuges did not contribute to the colonization of central and northern Europe; (iv) some populations expanded considerably during the postglacial period, while others experienced only a limited expansion; (v) the mountain chains were not geographical barriers for beech but rather facilitated its diffusion; and (vi) the modern genetic diversity was shaped over multiple glacial-interglacial cycles. This scenario differs from many recent treatments of tree phylogeography in Europe that largely focus on the last ice age and the postglacial period to interpret genetic structure and argue that the southern peninsulas (Iberian, Italian and Balkan) were the main source areas for trees in central and northern Europe.     

A new cytochrome b phylogroup of the common vole (Microtus arvalis) endemic to the Balkans and its implications for the evolutionary history of the species

The phylogeographic architecture of the common vole, Microtus arvalis, has been well‐studied using mitochondrial DNA and used to test hypotheses relating to glacial refugia. The distribution of the five described cytochrome b (cyt b) lineages in Europe west of Russia has been interpreted as a consequence of postglacial expansion from both southern and central European refugia. A recently proposed competing model suggests that the ‘cradle’ of the M. arvalis lineages is in western central Europe from where they dispersed in different directions after the Last Glacial Maximum. In the present study, we report a new cyt b lineage of the common vole from the Balkans that is not closely related to any other lineage and whose presence might help resolve these issues of glacial refugia. The Balkan phylogroup occurs along the southern distributional border of M. arvalis in central and eastern Bosnia and Herzegovina, Montenegro, and eastern Serbia. Further north and west in Slovenia, Bosnia and Herzegovina, and Serbia, common voles belong to the previously‐described Eastern lineage, whereas both lineages are sympatric in one site in Bosnia and Herzegovina. The Balkan phylogroup most reasonably occupied a glacial refugium already known for various Balkan endemic species, in contrast to the recently proposed model. South‐east Europe is an absolutely crucial area for understanding the postglacial colonization history of small mammals in Europe and the present study adds to the very few previous detailed phylogeographic studies of this region. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 788–796.     

Unravelling the evolutionary history and future prospects of endemic species restricted to former glacial refugia

The contemporary distribution and genetic composition of biodiversity bear a signature of species’ evolutionary histories and the effects of past climatic oscillations. For many European species, the Mediterranean peninsulas of Iberia, Italy and the Balkans acted as glacial refugia and the source of range recolonization, and as a result, they contain disproportionately high levels of diversity. As these areas are particularly threatened by future climate change, it is important to understand how past climatic changes affected their biodiversity. We use an integrated approach, combining markers with different evolutionary rates and combining phylogenetic analysis with approximate Bayesian computation and species distribution modelling across temporal scales. We relate phylogeographic processes to patterns of genetic variation in Myotis escalerai, a bat species endemic to the Iberian Peninsula. We found a distinct population structure at the mitochondrial level with a strong geographic signature, indicating lineage divergence into separate glacial refugia within the Iberian refugium. However, microsatellite markers suggest higher levels of gene flow resulting in more limited structure at recent time frames. The evolutionary history of M. escalerai was shaped by the effects of climatic oscillations and changes in forest cover and composition, while its future is threatened by climatically induced range contractions and the role of ecological barriers due to competition interactions in restricting its distribution. This study warns that Mediterranean peninsulas, which provided refuge for European biodiversity during past glaciation events, may become a trap for limited dispersal and ecologically limited endemic species under future climate change, resulting in loss of entire lineages.     

Post-glacial colonization of Western Europe brown bears from a cryptic Atlantic refugium out of the Iberian Peninsula

The European brown bear (Ursus arctos) shows a particular phylogeography that has been used to illustrate the model for contraction-expansion dynamics related to glacial refugia in Southern European peninsulas. Recent studies, however, have nuanced the once generally accepted paradigm, indicating the existence of cryptic refugia for some species further north. In this paper we collected available data on chronology and mitochondrial haplotypes from Western European brown bears, adding new sequences from present day individuals from the Cantabrian (North Iberia) area, in order to reconstruct the dynamics of the species in the region. Both genetics and chronology show that the Iberian Pleistocene lineages were not the direct ancestors of the Holocene ones, the latter entering the Peninsula belatedly (around 10,000 years BP) with respect to other areas such as the British Isles. We therefore propose the existence of a cryptic refugium in continental Atlantic Europe, from where the bears would expand as the ice receded. The delay in the recolonization of the Iberian Peninsula could be due to the orographic characteristics of the Pyrenean-Cantabrian region and to the abundant presence of humans in the natural entrance to the Peninsula.     

Maternal genomic variability of the wild boar (Sus scrofa) reveals the uniqueness of East‐Caucasian and Central Italian populations

The phylogeography of the European wild boar was mainly determined by postglacial recolonization patterns from Mediterranean refugia after the last ice age. Here we present the first analysis of SNP polymorphism within the complete mtDNA genome of West Russian (n = 8), European (n = 64), and North African (n = 5) wild boar. Our analyses provided evidence of unique lineages in the East‐Caucasian (Dagestan) region and in Central Italy. A phylogenetic analysis revealed that these lineages are basal to the other European mtDNA sequences. We also show close connection between the Western Siberian and Eastern European populations. Also, the North African samples were clustered with the Iberian population. Phylogenetic trees and migration modeling revealed a high proximity of Dagestan sequences to those of Central Italy and suggested possible gene flow between Western Asia and Southern Europe which was not directly related to Northern and Central European lineages. Our results support the presence of old maternal lineages in two Southern glacial refugia (i.e., Caucasus and the Italian peninsula), as a legacy of an ancient wave of colonization of Southern Europe from an Eastern origin.     

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.     

Mitochondrial phylogeography of the Woodmouse (Apodemus sylvaticus) in the Western Palearctic region

We sequenced 965 base pairs of the mitochondrial DNA cytochrome b from 102 woodmice (Apodemus sylvaticus) collected from 40 European localities. The aims of the study were to answer the following questions. (i) Did the Mediterranean peninsulas play a role as refuge for woodmice? (ii) Is genetic variability of A. sylvaticus higher in the Mediterranean region compared with northern Europe? (iii) Are the patterns of the postglacial colonization of Europe by woodmice similar to those presently recognized for other European species? The results provide a clear picture of the impact of the Quaternary glaciations on the genetic and geographical structure of the woodmouse. Our analyses indicate a higher genetic variability of woodmice in the Mediterranean peninsulas compared to northern Europe, suggesting a role of the former as refuge regions for this small mammal. An original pattern of postglacial colonization is proposed where the Iberian and southern France refuge populations colonized almost all European regions. The Sicilian population appears to be very differentiated and highly variable. This emphasizes the importance of this island as a ‘hot spot’ for the intraspecific genetic diversity of the woodmouse. Finally, woodmice in North Africa originated from southwestern Europe, most probably as a result of a recent anthropogenic introduction.     

A dynamic history of admixture from Mediterranean and Carpathian glacial refugia drives genomic diversity in the bank vole

Understanding the historical contributions of differing glacial refugia is key to evaluating the roles of microevolutionary forces, such as isolation, introgression, and selection in shaping genomic diversity in present‐day populations. In Europe, where both Mediterranean and extra‐Mediterranean (e.g., Carpathian) refugia of the bank vole (Clethrionomys glareolus) have been identified, mtDNA indicates that extra‐Mediterranean refugia were the main source of colonization across the species range, while Mediterranean peninsulas harbor isolated, endemic lineages. Here, we critically evaluate this hypothesis using previously generated genomic data (>6,000 SNPs) for over 800 voles, focusing on genomic contributions to bank voles in central Europe, a key geographic area in considering range‐wide colonization. The results provide clear evidence that both extra‐Mediterranean (Carpathian) and Mediterranean (Spanish, Calabrian, and Balkan) refugia contributed to the ancestry and genomic diversity of bank vole populations across Europe. Few strong barriers to dispersal and frequent admixture events in central Europe have led to a prominent mid‐latitude peak in genomic diversity. Although the genomic contribution of the centrally located Carpathian refugium predominates, populations in different parts of Europe have admixed origins from Mediterranean (28%–47%) and the Carpathian (53%–72%) sources. We suggest that the admixture from Mediterranean refugia may have provisioned adaptive southern alleles to more northern populations, facilitating the end‐glacial spread of the admixed populations and contributing to increased bank vole diversity in central Europe. This study adds critical details to the complex end‐glacial colonization history of this well‐studied organism and underscores the importance of genomic data in phylogeographic interpretation.     

Multilocus phylogeography of the European ground squirrel: cryptic interglacial refugia of continental climate in Europe

The theory of classical and cryptic Pleistocene refugia is based mainly on historical changes in temperature, and the refugia are usually defined within a latitudinal gradient. However, the gradient of oceanic–continental climate (i.e. longitudinal) was also significantly variable during glacial cycles with important biotic consequences. Range‐wide phylogeography of the European ground squirrel (EGS) was used to interpret the evolutionary and palaeogeographical history of the species in Europe and to shed light on its glacial–interglacial dynamic. The EGS is a steppe‐inhabiting species and the westernmost member of the genus in the Palaearctic region. We have analysed 915 specimens throughout the present natural range by employing mitochondrial DNA sequences (cytochrome b gene) and 12 nuclear microsatellite markers. The reconstructed phylogeography divides the species into two main geographical groups, with deep substructuring within both groups. Bulgaria is the centre of the ancestral area, and it also has the highest genetic diversity within the species. The northernmost group of the EGS survived in the southern part of Pannonia throughout several glacial–interglacial cycles. Animals from this population probably repeatedly colonized areas further to the north and west during the glacial periods, while in the interglacial periods, the EGS distribution contracted back to this Pannonian refugium. The EGS thus represents a species with a glacial expansion/interglacial contraction palaeogeographical dynamics, and the Pannonian and southeastern Balkanian steppes are supported as cryptic refugia of continental climate during Pleistocene interglacials.     

Mitochondrial phylogeography,contact zones and taxonomy of grass snakes (Natrix natrix,N. megalocephala)

Grass snakes (Natrix natrix) represent one of the most widely distributed snake species of the Palaearctic region, ranging from the North African Maghreb region and the Iberian Peninsula through most of Europe and western Asia eastward to the region of Lake Baikal in Central Asia. Within N. natrix, up to 14 distinct subspecies are regarded as valid. In addition, some authors recognize big‐headed grass snakes from western Transcaucasia as a distinct species, N. megalocephala. Based on phylogenetic analyses of a 1984‐bp‐long alignment of mtDNA sequences (ND4+tRNAs, cyt b) of 410 grass snakes, a nearly range‐wide phylogeography is presented for both species. Within N. natrix, 16 terminal mitochondrial clades were identified, most of which conflict with morphologically defined subspecies. These 16 clades correspond to three more inclusive clades from (i) the Iberian Peninsula plus North Africa, (ii) East Europe and Asia and (iii) West Europe including Corso‐Sardinia, the Apennine Peninsula and Sicily. Hypotheses regarding glacial refugia and postglacial range expansions are presented. Refugia were most likely located in each of the southern European peninsulas, Corso‐Sardinia, North Africa, Anatolia and the neighbouring Near and Middle East, where the greatest extant genetic diversity occurs. Multiple distinct microrefugia are inferred for continental Italy plus Sicily, the Balkan Peninsula, Anatolia and the Near and Middle East. Holocene range expansions led to the colonization of more northerly regions and the formation of secondary contact zones. Western Europe was invaded from a refuge within southern France, while Central Europe was reached by two distinct range expansions from the Balkan Peninsula. In Central Europe, there are two contact zones of three distinct mitochondrial clades, and one of these contact zones was theretofore completely unknown. Another contact zone is hypothesized for Eastern Europe, which was colonized, like north‐western Asia, from the Caucasus region. Further contact zones were identified for southern Italy, the Balkans and Transcaucasia. In agreement with previous studies using morphological characters and allozymes, there is no evidence for the distinctiveness of N. megalocephala. Therefore, N. megalocephala is synonymized with N. natrix.     

A highly divergent mitochondrial DNA lineage of Microtus agrestis in southern Europe

The Mediterranean peninsulas constitute important areas for endemism and intraspecific variation, and are likely places for cryptic biodiversity. We assessed the phylogeographic pattern of field voles (Microtus agrestis) in southern and central Europe by sequence analysis of a 385-bp fragment of the mitochondrial cytochrome b gene in 74 specimens from 44 localities. The majority of samples consisted of skulls collected from owl pellets. The data revealed a highly distinct cytochrome b lineage in an area ranging from Portugal to Hungary. This southern field vole phylogroup differed by a sequence divergence of 5.6-7.1% from the remaining haplotypes, a level of divergence comparable to that found between known Microtus sibling species. However, this ancient phylogeographic break that dates back many glacial cycles has not been recognised previously by either morphology or karyotype. The southern cytochrome b lineage was further divided into two well-defined sublineages that appear to have derived from different glacial refugia in the Iberian Peninsula.     

Colonization of Northern Europe by Zygaena filipendulae (Lepidoptera)

Northern and mountainous ice sheets have expanded and contracted many times due to ice ages. Consequently, temperate species have been confined to refugia during the glacial periods wherefrom they have recolonized warming northern habitats between ice ages. In this study, we compare the gene CYP405A2 between different populations of the common burnet moth Zygaena filipendulae from across the Western Palearctic region to illuminate the colonization history of this species. These data show two major clusters of Z. filipendulae populations possibly reflecting two different refugial populations during the last ice age. The two types of Z. filipendulae only co‐occur in Denmark, Sweden, and Scotland indicating that Northern Europe comprise the hybridization zone where individuals from two different refugia met after the last ice age. Bayesian phylogeographic and ecological clustering analyses show that one cluster probably derives from an Alpe Maritime refugium in Southern France with ancestral expansive tendencies to the British Isles in the west, touching Northern Europe up to Denmark and Sweden, and extending throughout Central Europe into the Balkans, the Peleponnes, and South East Europe. The second cluster encompasses East Anatolia as the source area, from where multiple independent dispersal events to Armenia, to the Alborz mountains in north‐western Iran, and to the Zagros mountains in western Iran are suggested. Consequently, the classical theory of refugia for European temperate species in the Iberian, Italian, and Balkan peninsulas does not fit with the data from Z. filipendulae populations, which instead support more Northerly, mountainous refugia.     

Species persistence in northerly glacial refugia of Europe: a matter of chance or biogeographical traits?

Aim The southern European peninsulas (Iberian, Italian and Balkan) are considered to have been refugia for many European species of plants and animals during the climatic extremes of the Pleistocene ice ages. A number of recent studies (fossil and genetic), however, have provided evidence for full‐glacial survival of some species beyond these peninsulas. Here we explore the biogeographical traits of these species, and ask whether they possessed certain characteristics that enabled them to persist in more northerly refugia. Location Europe. Methods Fossil and genetic evidence for refugial localities of species that survived in Europe during the last full‐glacial was obtained from the literature (totalling 90 species: 34 woody plants and 56 vertebrates). Forty‐seven of these species (23 woody plants and 24 vertebrates) had fossil evidence, whereas the remaining 43 species (11 woody plants and 32 vertebrates) had only genetic evidence. All species were scored according to their present geographical distribution, habitat preference and life‐history traits. The species were classified on the basis of these traits using hierarchical cluster analysis. Analysis of similarities was used to examine differences in vertebrate and woody plant species groups that survived only in southerly refugia and those that also persisted in more northerly locations. Non‐metric multi‐dimensional scaling was used to examine patterns observed between and within groups. Results Results from our analysis of species with fossil and genetic evidence for survival in refugia reveal that species that survived only in southerly refugia were large‐seeded trees or thermophilous vertebrates. In contrast, species that had a full‐glacial distribution, including more northerly locations, were wind‐dispersed, habitat‐generalist trees with the ability to reproduce vegetatively, and habitat‐generalist mammals with present‐day northerly distributions. Main conclusions Analysis of the geographical distribution, habitat preference and life‐history traits of the species studied suggests that underlying biogeographical traits may have determined their response to Pleistocene glaciation. The traits most commonly found in present populations with a northerly distribution in Europe enabled the same species to exist much farther north than the southern European peninsulas during the full‐glacial. It is possible that many of these species are now in restricted populations, within the ‘warm‐stage’ refugia of the current interglacial. The northerly full‐glacial survival of a number of woody plants and vertebrate species has significant implications for understanding migration rates of these species in response to climate change. It also has important implications for understanding current patterns of genetic diversity of European species. We suggest that both fossil and genetic evidence should be used to identify and prioritize for conservation of refugial localities in southern and northern Europe.     

Colonization routes of Pinus sylvestris inferred from distribution of mitochondrial DNA variation

Understanding the present-day distribution of molecular variation requires knowledge about the history of the species. Past colonization routes and locations of refugia of Scots pine (Pinus sylvestris) were inferred from variation in mitochondrial DNA in material collected from 37 populations located in countries within, and immediately adjacent to the continent of Europe. Two mitochondrial regions, nad1 intron (exon B/C) and nad7 intron 1, were included in the study. Differentiation in maternally inherited mitochondria was high (G _ST′ = 0.824). Two new haplotypes were found at the nad7 intron 1. The occurrence of a 5-bp indel variant was restricted to the Turkish Kalabak population and a 32 bp only found in Central, Eastern, and Northern Europe. The complete absence of the 32-bp indel from the Mediterranean peninsulas supports the view that coniferous forests existed outside these areas during the last glacial maximum, and these populations contributed to the subsequent colonization of the northern parts of Europe. P. sylvestris shares features of its glacial and postglacial history with two other northern, cold-tolerant tree species, Picea abies and Betula sp. These three species differ from many other European trees for which pollen core and molecular evidence indicate colonization from southern refugia after the last glacial period.     

Species distribution modeling and molecular markers suggest longitudinal range shifts and cryptic northern refugia of the typical calcareous grassland species Hippocrepis comosa (horseshoe vetch)

Calcareous grasslands belong to the most diverse, endangered habitats in Europe, but there is still insufficient information about the origin of the plant species related to these grasslands. In order to illuminate this question, we chose for our study the representative grassland species Hippocrepis comosa (Horseshoe vetch). Based on species distribution modeling and molecular markers, we identified the glacial refugia and the postglacial migration routes of the species to Central Europe. We clearly demonstrate that H. comosa followed a latitudinal and due to its oceanity also a longitudinal gradient during the last glacial maximum (LGM), restricting the species to southern refugia situated on the Peninsulas of Iberia, the Balkans, and Italy during the last glaciation. However, we also found evidence for cryptic northern refugia in the UK, the Alps, and Central Germany. Both species distribution modeling and molecular markers underline that refugia of temperate, oceanic species such as H. comosa must not be exclusively located in southern but also in western of parts of Europe. The analysis showed a distinct separation of the southern refugia into a western cluster embracing Iberia and an eastern group including the Balkans and Italy, which determined the postglacial recolonization of Central Europe. At the end of the LGM, H. comosa seems to have expanded from the Iberian refugium, to Central and Northern Europe, including the UK, Belgium, and Germany.     

Extra-Mediterranean refugia: The rule and not the exception?

ABSTRACT: Some decades ago, biogeographers distinguished three major faunal types of high importance for Europe: (i) Mediterranean elements with exclusive glacial survival in the Mediterranean refugia, (ii) Siberian elements with glacial refugia in the eastern Palearctic and only postglacial expansion to Europe and (iii) arctic and/or alpine elements with large zonal distributions in the periglacial areas and postglacial retreat to the North and/or into the high mountain systems. Genetic analyses have unravelled numerous additional refugia both of continental and Mediterranean species, thus strongly modifying the biogeographical view of Europe. This modified notion is particularly true for the so-called Siberian species, which in many cases have not immigrated into Europe during the postglacial period, but most likely have survived the last, or even several glacial phases, in extra-Mediterranean refugia in some climatically favourable but geographically limited areas of southern Central and Eastern Europe. Recently, genetic analyses revealed that typical Mediterranean species have also survived the Last Glacial Maximum in cryptic northern refugia (e.g. in the Carpathians or even north of the Alps) in addition to their Mediterranean refuge areas.     

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.     

Modified dispersal‐related traits in disjunct populations of bird‐dispersed Frangula alnus (Rhamnaceae): a result of its Quaternary distribution shifts?

Many European tree species survived Pleistocene glaciations in Mediterranean refugia and rapidly recolonized temperate Europe afterwards. Inter‐ and postglacial migration processes are assumed to have catalized evolutionary optimizations of dispersal‐related traits, but up to now empirical evidence is lacking in vertebrate‐dispersed plants. We investigated if south Iberian glacial relict and central European “colonizer” populations of the bird‐dispersed tree Frangula alnus have experienced differentiations of dispersal‐related traits which increase the mobility of northern populations. A comparison of lifetime reproductive strategy, disperser guilds, ripening phenology, and fruit design revealed considerable differences. Compared to south Iberian conspecifics, central European plants were considerably smaller and experienced a highly accelerated generation turnover. In south Iberian populations seed dispersal was carried out almost completely by resident birds which occurred in constant abundances throughout the ripening season. In contrast, central European seeds were dispersed by migrants whose abundances changed considerably during the ripening season. Several bird species were involved in both study areas but rendered different importance for seed dispersal. The fruit ripening pattern was highly asynchronous throughout the ripening season in south Iberia, while central European trees showed a complex ripening sequence which resulted in a significant correlation between fruit abundance and changing disperser availability. Central European fruits were smaller and showed a considerably smaller seed load than south Iberian fruits, thus presumably being more attractive for their small‐sized main dispersers (Sylvia warblers). Chemical analyses revealed significant differences in contents of water, glucose, fructose, proteins, ash. and phenolic compounds. The extensive differentiation of dispersal‐related traits in F. alnus suggests that even weak selective pressures by frugivores may induce evolutionary adjustments of dispersal traits over large time scales. We suggest that the differences we observe today evolved during the species' distribution shifts in the Quaternary.     

Rangewide phylogeography of a bird-dispersed Eurasian shrub: contrasting Mediterranean and temperate glacial refugia

We studied the phylogeography of alder buckthorn (Frangula alnus), a bird-dispersed shrub or small tree distributed over most of Europe and West Asia and present in three of the four main refugia of West Palaearctic temperate woody plants: the Iberian Peninsula, the Balkans and Anatolia. A total of 78 populations from 21 countries were analysed for chloroplast DNA variation using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP), and 21 different haplotypes were distinguished. We found a very strong overall population differentiation (GST = 0.81) and phylogeographical structure, and a sharp contrast between the haplotype-rich refugia and the almost completely uniform area of postglacial colonization. The haplotype network comprises three lineages made up of haplotypes from the Iberian Peninsula, Anatolia with the Caucasus, and temperate Europe. The Iberian and the Anatolian branches represent parts of a major lineage that spans over the whole northern Mediterranean Basin and some neighbouring areas and probably dates back to the Tertiary. Many haplotypes of this lineage are distributed locally and most populations are fixed for a single haplotype; these populations have apparently been very stable since their establishment, experiencing negligible gene flow and few mutations. The temperate European lineage consists of one very widespread and abundant plus six locally distributed haplotypes. Four of them are located in Southeast Europe, the putative refugium of all extant temperate European populations. Contrary to populations from Iberia and Anatolia, F. alnus populations from the southeastern European refugium have most genetic variation within populations. Bird-mediated seed dispersal has apparently allowed not only a very rapid postglacial expansion of F. alnus but also subsequent regular seed exchanges between populations of the largely continuous species range in temperate Europe. In contrast, the disjunct F. alnus populations persisting in Mediterranean mountain ranges seem to have experienced little gene flow and have therefore accumulated a high degree of differentiation, even at short distances. Populations from the southern parts of the glacial refugia have contributed little to the postglacial recolonization of Europe, but their long-term historical continuity has allowed them to maintain a unique store of genetic variation.