Diverse spore rains and limited local exchange shape fern genetic diversity in a recently created habitat colonized by long-distance dispersal

Background and Aims

Populations established by long-distance colonization are expected to show low levels of genetic variation per population, but strong genetic differentiation among populations. Whether isolated populations indeed show this genetic signature of isolation depends on the amount and diversity of diaspores arriving by long-distance dispersal, and time since colonization. For ferns, however, reliable estimates of long-distance dispersal rates remain largely unknown, and previous studies on fern population genetics often sampled older or non-isolated populations. Young populations in recent, disjunct habitats form a useful study system to improve our understanding of the genetic impact of long-distance dispersal.

Methods

Microsatellite markers were used to analyse the amount and distribution of genetic diversity in young populations of four widespread calcicole ferns (Asplenium scolopendrium, diploid; Asplenium trichomanes subsp. quadrivalens, tetraploid; Polystichum setiferum, diploid; and Polystichum aculeatum, tetraploid), which are rare in The Netherlands but established multiple populations in a forest (the Kuinderbos) on recently reclaimed Dutch polder land following long-distance dispersal. Reference samples from populations throughout Europe were used to assess how much of the existing variation was already present in the Kuinderbos.

Key Results

A large part of the Dutch and European genetic diversity in all four species was already found in the Kuinderbos. This diversity was strongly partitioned among populations. Most populations showed low genetic variation and high inbreeding coefficients, and were assigned to single, unique gene pools in cluster analyses. Evidence for interpopulational gene flow was low, except for the most abundant species.

Conclusions

The results show that all four species, diploids as well as polyploids, were capable of frequent long-distance colonization via single-spore establishment. This indicates that even isolated habitats receive dense and diverse spore rains, including genotypes capable of self-fertilization. Limited gene flow may conserve the genetic signature of multiple long-distance colonization events for several decades.     

Genetic differentiation and phylogeographical structure of the Brachionus calyciflorus complex in eastern China

Spatio-temporal patterns and processes of genetic differentiation in passively dispersing zooplankton are drawing much attention from both ecologists and evolutionary biologists. Two opposite phylogeographical scenarios have already been demonstrated in rotifers, which consist of high levels of genetic differentiation among populations even on small geographical scales on the one hand and the traditionally known cosmopolitanism that is associated with high levels of gene flow and long-distance dispersal via diapausing stages on the other hand. Here, we analysed the population genetic structure and the phylogeography of the Brachionus calyciflorus species complex in eastern China. By screening a total of 318 individuals from ten locations along a 2320-km gradient and analysing samples from two growing seasons, we aimed at focusing on both small- and large-scale patterns. We identified eight cryptic species and verified species status of two of these by sexual reproduction tests. Samples in summer and winter yielded different cryptic species. The distribution patterns of these genetically distinct cryptic species were diverse across eastern China, from full cosmopolitanism to local endemism. The two most abundant cryptic species BcWIII and BcSW showed a pattern of strong genetic differentiation among populations and no significant isolation by distance. Long-distance colonization, secondary contact and recent range expansion are probably responsible for the indistinct pattern of isolation by distance. Our results suggest that geographical distance is more important than temporal segregation across seasons in explaining population differentiation and the occurrence of cryptic species. We explain the current phylogeographical structure in the B. calyciflorus species complex by a combination of recent population expansion, restricted gene flow, priority effects and long-distance colonization.     

River fragmentation increases localized population genetic structure and enhances asymmetry of dispersal in bullhead (Cottus gobio)

Man-made habitat fragmentation is a major concern in river ecology and is expected to have particularly detrimental effects on aquatic species with limited dispersal abilities, like the bullhead (Cottus gobio). We used ten microsatellite markers to investigate small-scale patterns of gene flow, current dispersal and neutral genetic diversity in a morphologically diverse river where fragmented and unfragmented sections could be compared. We found high genetic differentiation between sampling sites with a maximum F _ST of 0.32 between sites separated by only 35 km. A significant increase of genetic differentiation with geographical distance was observed in the continuous river section as well as in the full dataset which included headwater populations isolated by anthropogenic barriers. Several lines of evidence are consistent with the hypothesis that such barriers completely block upstream movement while downstream dispersal may be little affected. In the unfragmented habitat, dispersal rates were also higher in the direction of water flow than against it. The resulting asymmetry in gene flow likely contributes to the decrease of genetic variation observed from the lower reaches towards the headwaters, which is particularly pronounced in physically isolated populations. Our findings suggest that headwater populations, due to their isolation and low genetic variation, may be particularly vulnerable to extinction.     

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.     

A portrait of a sucker using landscape genetics: how colonization and life history undermine the idealized dendritic metapopulation

Dendritic metapopulations have been attributed unique properties by in silico studies, including an elevated genetic diversity relative to a panmictic population of equal total size. These predictions have not been rigorously tested in nature, nor has there been full consideration of the interacting effects among contemporary landscape features, colonization history and life history traits of the target species. We tested for the effects of dendritic structure as well as the relative importance of life history, environmental barriers and historical colonization on the neutral genetic structure of a longnose sucker (Catostomus catostomus) metapopulation in the Kogaluk watershed of northern Labrador, Canada. Samples were collected from eight lakes, genotyped with 17 microsatellites, and aged using opercula. Lakes varied in differentiation, historical and contemporary connectivity, and life history traits. Isolation by distance was detected only by removing two highly genetically differentiated lakes, suggesting a lack of migration–drift equilibrium and the lingering influence of historical factors on genetic structure. Bayesian analyses supported colonization via the Kogaluk's headwaters. The historical concentration of genetic diversity in headwaters inferred by this result was supported by high historical and contemporary effective sizes of the headwater lake, T‐Bone. Alternatively, reduced allelic richness in headwaters confirmed the dendritic structure's influence on gene flow, but this did not translate to an elevated metapopulation effective size. A lack of equilibrium and upstream migration may have dampened the effects of dendritic structure. We suggest that interacting historical and contemporary factors prevent the achievement of the idealized traits of a dendritic metapopulation in nature.     

Genetic signatures of a range expansion in natura: when clones play leapfrog

The genetic consequences of range expansions have generally been investigated at wide geographical and temporal scales, long after the colonization event. A unique ecological system enabled us to both monitor the colonization dynamics and decipher the genetic footprints of expansion over a very short time period. Each year an epidemic of the poplar rust (Melampsora larici‐populina) expands clonally and linearly along the Durance River, in the Alps. The colonization dynamics observed in 2004 showed two phases with different genetic outcomes. Upstream, fast colonization maintained high genetic diversity. Downstream, the colonization wave progressively faltered, diversity eroded, and differentiation increased, as expected under recurrent founder events. In line with the high dispersal abilities of rust pathogens, we provide evidence for leapfrog dispersal of clones. Our results thus emphasize the importance of colonization dynamics in shaping spatial genetic structure in the face of high gene flow.     

Multiple glacial refugia and contemporary dispersal shape the genetic structure of an endemic amphibian from the Pyrenees

Historical factors (colonization scenarios, demographic oscillations) and contemporary processes (population connectivity, current population size) largely contribute to shaping species’ present‐day genetic diversity and structure. In this study, we use a combination of mitochondrial and nuclear DNA markers to understand the role of Quaternary climatic oscillations and present‐day gene flow dynamics in determining the genetic diversity and structure of the newt Calotriton asper (Al. Dugès, 1852), endemic to the Pyrenees. Mitochondrial DNA did not show a clear phylogeographic pattern and presented low levels of variation. In contrast, microsatellites revealed five major genetic lineages with admixture patterns at their boundaries. Approximate Bayesian computation analyses and linear models indicated that the five lineages likely underwent separate evolutionary histories and can be tracked back to distinct glacial refugia. Lineage differentiation started around the Last Glacial Maximum at three focal areas (western, central and eastern Pyrenees) and extended through the end of the Last Glacial Period in the central Pyrenees, where it led to the formation of two more lineages. Our data revealed no evidence of recent dispersal between lineages, whereas borders likely represent zones of secondary contact following expansion from multiple refugia. Finally, we did not find genetic evidence of sex‐biased dispersal. This work highlights the importance of integrating past evolutionary processes and present‐day gene flow and dispersal dynamics, together with multilocus approaches, to gain insights into what shaped the current genetic attributes of amphibians living in montane habitats.     

Biogeography and diversification of hermit spiders on Indian Ocean islands (Nephilidae: Nephilengys)

The origin of the terrestrial biota of Madagascar and, especially, the smaller island chains of the western Indian Ocean is relatively poorly understood. Madagascar represents a mixture of Gondwanan vicariant lineages and more recent colonizers arriving via Cenozoic dispersal, mostly from Africa. Dispersal must explain the biota of the smaller islands such as the Comoros and the chain of Mascarene islands, but relatively few studies have pinpointed the source of colonizers, which may include mainland Africa, Asia, Australasia, and Madagascar. The pantropical hermit spiders (genus Nephilengys) seem to have colonized the Indian Ocean island arc stretching from Comoros through Madagascar and onto Mascarenes, and thus offer one opportunity to reveal biogeographical patterns in the Indian Ocean. We test alternative hypotheses on the colonization route of Nephilengys spiders in the Indian Ocean and simultaneously test the current taxonomical hypothesis using genetic and morphological data. We used mitochondrial (COI) and nuclear (ITS2) markers to examine Nephilengys phylogenetic structure with samples from Africa, southeast Asia, and the Indian Ocean islands of Madagascar, Mayotte, Réunion and Mauritius. We used Bayesian and parsimony methods to reconstruct phylogenies and haplotype networks, and calculated genetic distances and fixation indices. Our results suggest an African origin of Madagascar Nephilengys via Cenozoic dispersal, and subsequent colonization of the Mascarene islands from Madagascar. We find strong evidence of gene flow across Madagascar and through the neighboring islands north of it, while phylogenetic trees, haplotype networks, and fixation indices all reveal genetically isolated and divergent lineages on Mauritius and Réunion, consistent with female color morphs. These results, and the discovery of the first males from Réunion and Mauritius, in turn falsify the existing taxonomic hypothesis of a single widespread species, Nephilengys borbonica, throughout the archipelago. Instead, we diagnose three Nephilengys species: Nephilengys livida (Vinson, 1863) from Madagascar and Comoros, N. borbonica (Vinson, 1863) from Réunion, and Nephilengys dodo new species from Mauritius. Nephilengys followed a colonization route to Madagascar from Africa, and on through to the Mascarenes, where it speciated on isolated islands. The related golden orb-weaving spiders, genus Nephila, have followed the same colonization route, but Nephila shows shallower divergencies, implying recent colonization, or a moderate level of gene flow across the archipelago preventing speciation. Unlike their synanthropic congeners, N. borbonica and N. dodo are confined to pristine island forests and their discovery calls for evaluation of their conservation status.     

Phylogeography of the pine processionary moth Thaumetopoea wilkinsoni in the Near East

Phylogeographic structure of the eastern pine processionary moth Thaumetopoea wilkinsoni was explored in this study by means of nested clade phylogeographic analyses of COI and COII sequences of mitochondrial DNA and Bayesian estimates of divergence times. Intraspecific relationships were inferred and hypotheses tested to understand historical spread patterns and spatial distribution of genetic variation. Analyses revealed that all T. wilkinsoni sequences were structured in three clades, which were associated with two major biogeographic events, the colonization of the island of Cyprus and the separation of southwestern and southeastern Anatolia during the Pleistocene. Genetic variation in populations of T. wilkinsoni was also investigated using amplified fragment length polymorphisms and four microsatellite loci. Contrasting nuclear with mitochondrial data revealed recurrent gene flow between Cyprus and the mainland, related to the long-distance male dispersal. In addition, a reduction in genetic variability was observed at both mitochondrial and nuclear markers at the expanding boundary of the range, consistent with a recent origin of these populations, founded by few individuals expanding from nearby localities. In contrast, several populations fixed for one single mitochondrial haplotype showed no reduction in nuclear variability, a pattern that can be explained by recurrent male gene flow or selective sweeps at the mitochondrial level. The use of both mitochondrial and nuclear markers was essential in understanding the spread patterns and the population genetic structure of T. wilkinsoni, and is recommended to study colonizing species characterized by sex-biased dispersal.     

Mitochondrial DNA and microsatellite variation in the eider duck (Somateria mollissima) indicate stepwise postglacial colonization of Europe and limited current long-distance dispersal

To unravel the postglacial colonization history and the current intercolony dispersal in the common eider, Somateria mollissima, we analysed genetic variation at a part of the mitochondrial control region and five unlinked autosomal microsatellite loci in 175 eiders from 11 breeding colonies, covering the entire European distribution range of this species. As a result of extreme female philopatry, mitochondrial DNA differentiation is substantial both among local colonies and among distant geographical regions. Our study further corroborates the previous hypothesis of a single Pleistocene refugium for European eiders. A nested clade analysis on mitochondrial haplotypes suggests that (i) the Baltic Sea eider population is genetically closest to a presumably ancestral population and that (ii) the postglacial recolonization progressed in a stepwise fashion via the North Sea region and the Faroe Islands to Iceland. Current long-distance dispersal is limited. Differentiation among colonies is much less pronounced at microsatellite loci. The geographical pattern of this nuclear genetic variation is to a large extent explained by isolation by distance. As female dispersal is very limited, the geographical pattern of nuclear variation is probably explained by male-mediated gene flow among breeding colonies. Our study provides genetic evidence for the assumed prominent postglacial colonization route shaping the present terrestrial fauna of the North Atlantic islands Iceland and the Faroes. It suggests that this colonization had been a stepwise process originating in continental Europe. It is the first molecular study on eider duck populations covering their entire European distribution range.     

Population genetic structure of a common host predicts the spread of white‐nose syndrome,an emerging infectious disease in bats

Landscape complexity influences patterns of animal dispersal, which in turn may affect both gene flow and the spread of pathogens. White‐nose syndrome (WNS) is an introduced fungal disease that has spread rapidly throughout eastern North America, causing massive mortality in bat populations. We tested for a relationship between the population genetic structure of the most common host, the little brown myotis (Myotis lucifugus), and the geographic spread of WNS to date by evaluating logistic regression models of WNS risk among hibernating colonies in eastern North America. We hypothesized that risk of WNS to susceptible host colonies should increase with both geographic proximity and genetic similarity, reflecting historical connectivity, to infected colonies. Consistent with this hypothesis, inclusion of genetic distance between infected and susceptible colonies significantly improved models of disease spread, capturing heterogeneity in the spatial expansion of WNS despite low levels of genetic differentiation among eastern populations. Expanding our genetic analysis to the continental range of little brown myotis reveals strongly contrasting patterns of population structure between eastern and western North America. Genetic structure increases markedly moving westward into the northern Great Plains, beyond the current distribution of WNS. In western North America, genetic differentiation of geographically proximate populations often exceeds levels observed across the entire eastern region, suggesting infrequent and/or locally restricted dispersal, and thus relatively limited opportunities for pathogen introduction in western North America. Taken together, our analyses suggest a possibly slower future rate of spread of the WNS pathogen, at least as mediated by little brown myotis.     

Unexpected diversity in the catfish Pseudancistrus brevispinis reveals dispersal routes in a Neotropical center of endemism: the Guyanas Region

Neotropical freshwater fishes have reached an unrivalled diversity, organized into several areas of endemism, yet the underlying processes are still largely unknown. The topographical and ecological characteristics of the Guyanas Region make it an ideal area of endemism in which to investigate the forces that have shaped this great diversity. This region is thought to be inhabited by species descending from Amazonian ancestors, which would have used two documented routes that, however, hardly explain the entrance of species adapted to running waters. Here, we investigate the evolutionary history of Pseudancistrus brevispinis , a catfish endemic to this region and exclusively found in running waters, thus making it an ideal model for investigating colonization routes and dispersal in such habitats. Our analyses, based on mitochondrial and nuclear markers, revealed an unexpected diversity consisting of six monophyletic lineages within P. brevispinis , showing a disjoint distribution pattern. The lineages endemic to Guyanas coastal rivers form a monophyletic group that originated via an ancestral colonization event from the Amazon basin. Evidence given favours a colonization pathway through river capture between an Amazonian tributary and the Upper Maroni River. Population genetic analyses of the most widespread species indicate that subsequent dispersal among Guyanas coastal rivers occurred principally by temporary connections between adjacent rivers during periods of lower sea level, yet instances of dispersal via interbasin river captures are not excluded. During high sea level intervals, the isolated populations would have diverged leading to the observed allopatric species. This evolutionary process is named the sea level fluctuation (SLF) hypothesis of diversification.     

Local and regional founder effects in lake zooplankton persist after thousands of years despite high dispersal potential

We reconstructed the genetic structure of a planktonic crustacean Daphnia longispina living in high mountain lakes and ponds in the Pyrenees to investigate whether it was shaped by persistent founder effects originating shortly after the last glacial maximum or by ongoing dispersal and effective migration (gene flow). We found that the genetic structure can largely be explained by a single colonization event following gradual deglaciation of the Pyrenees ~10 000–15 000 years ago. Nuclear genetic diversity declined steeply from southeast to northwest, suggestive of serial colonization of available habitats with advancing deglaciation. The spatial genetic structure suggests that founder effects were major determinants of the present‐day diversity, both at the catchment level and at the level of individual water bodies, further supporting extremely low effective migration rates. This study reveals a prime example of a founder effect that is both long lasting and maintained at small spatial scales. Our data suggest a process of isolation by colonization as a result of strong priority effects and monopolization. We found evidence for the spread of haplotypes with Pyrenean ancestry across the Palaearctic over distances up to 5500 km, although the local genetic structure after colonization was hardly influenced by contemporary dispersal. Finally, our data also suggest that mitochondrial mutation rates in the studied populations were seven times higher than typically assumed. Overall, we show that founder effects can persist for centuries even at small spatial scales at which the potential for dispersal is high.     

Species cohesion despite extreme inbreeding in a social spider

Colonial social spiders experience extreme inbreeding and highly restricted gene flow between colonies; processes that question the genetic cohesion of geographically separated populations and which could imply multiple origins from predecessors with limited gene flow. We analysed species cohesion and the potential for long-distance dispersal in the social spider Stegodyphus dumicola by studying colony structure in eastern South Africa and the cohesion between this population and Namibian populations previously published. Data from both areas were (re)analysed for historic demographic parameters. Eastern South African S. dumicola were closely related to an east Namibian lineage, showing cohesion of S. dumicola relative to its sister species. Colony structure was similar in both areas with mostly monomorphic colonies, but haplotype diversity was much reduced in eastern South Africa. Here, the population structure indicated recent population expansion. By contrast, Namibia constitutes an old population, possibly the geographic origin of the species. Both the comparison of the eastern South African and Namibian lineages and the distribution within eastern South Africa show the potential for long-distance dispersal in few generations via colony propagation.     

Does fragmentation of wetlands affect gene flow in sympatric Acrocephalus warblers with different migration strategies?

Wetlands are naturally patchy habitats, but patchiness has been accentuated by the extensive wetlands loss due to human activities. In such a fragmented habitat, dispersal ability is especially important to maintain gene flow between populations. Here we studied population structure, genetic diversity and demographic history of Iberian and North African populations of two wetland passerines, the Eurasian reed warbler Acrocephalus scirpaceus and the moustached warbler Acrocephalus melanopogon. These species are closely related and sympatric in our study sites, but the reed warbler is a widespread long‐distance migrant while the moustached warbler's breeding range is patchier and it is resident or migrates over short distances. Using microsatellite and mtDNA data, we found higher differentiation in moustached than in reed warblers, indicating higher dispersal capability of the latter species. Our results also suggest that the sea limits dispersal in the moustached warbler. However, we found evidence of gene flow between the study sites in both species, indicating a capability to compensate for habitat fragmentation. In most cases, the gene flow was restricted, possibly because of the large distances between study sites (from ca 290 to 960 km) or breeding site fidelity. The reed warbler had higher haplotype diversity, likely due to dispersal from different populations, past admixture event and a larger population size. We found also signs of postglacial population growth for both species and evidence of a recent colonization or re‐colonization of the Mallorca Island by the moustached warbler.     

Inferring long-distance dispersal and topographic barriers during post-glacial colonization from the genetic structure of red maple (Acer rubrum L.) in New England

Aim  This study aims to assess the role of long-distance seed dispersal and topographic barriers in the post-glacial colonization of red maple ( Acer rubrum L.) using chloroplast DNA (cpDNA) variation, and to understand whether this explains the relatively higher northern diversity found in eastern North American tree species compared with that in Europe.
Location  North-eastern United States.
Methods  The distribution of intraspecific cpDNA variation in temperate tree populations has been used to identify aspects of post-glacial population spread, including topographic barriers to population expansion and spread by long-distance seed dispersal. We sequenced c.  370 cpDNA base pairs from 221 individuals in 100 populations throughout the north-eastern United States, and analysed spatial patterns of diversity and differentiation.
Results  Red maple has high genetic diversity near its northern range limit, but this diversity is not partitioned by topographic barriers, suggesting that the northern Appalachian Mountains were not a barrier to the colonization of red maple. We also found no evidence of the patchy genetic structure that has been associated with spread by rare long-distance seed dispersal in previous studies.
Main conclusions  Constraints on post-glacial colonization in eastern North America seem to have been less stringent than those in northern Europe, where bottlenecks arising from long-distance colonization and topographic barriers appear to have strongly reduced genetic diversity. In eastern North America, high northern genetic diversity may have been maintained by a combination of frequent long-distance dispersal, minor topographic obstacles and diffuse northern refugia near the ice sheet.     

Sexual systems and population genetic structure in an annual plant: testing the metapopulation model

The need for reproductive assurance during dispersal, along with the pressure of local mate competition, means that the importance of frequent or repeated colonization is implicit in the literature on sexual system evolution. However, there have been few empirical tests of the association between colonization history and sexual system in plants, and none within a single species. Here we use patterns of genetic diversity to provide such a test in the Mercurialis annua species complex, which spans the range of systems from self-compatible monoecy through androdioecy to dioecy. This variation has been hypothesized to result from differing patterns of metapopulation turnover and recolonization. Because monoecy should be favored during colonization, androdioecy and dioecy will be maintained only in regions with low rates of local extinction and recolonization, and these differences should also be reflected in patterns of neutral genetic diversity. We show that monoecious populations of M. annua display lower within-population genetic diversity than androdioecious populations and higher genetic differentiation than dioecious and androdioecious populations, as predicted by metapopulation models. In contrast, regional diversity in M. annua appears to be primarily a product of postglacial range expansion from two refugia in the eastern and western Mediterranean Basin.     

Temporal genetic dynamics among mosquitofish (<Emphasis Type="Italic">Gambusia holbrooki</Emphasis>) populations in invaded watersheds

The temporal components of genetic diversity and geographical structure of invasive mosquitofish populations are poorly known. Through the genetic monitoring of four consecutive cohorts of Gambusia holbrooki from three different river basins we aimed to determine temporal patterns of regional genetic variation and dispersal rates within invasive populations. Despite showing evidence of strong population size fluctuations, genetic diversity levels were maintained among local cohorts. We only detected temporal allele frequency changes associated with seasonal flooding that did not modify major trends on population structure among cohorts. Downstream gene flow coupled with increased connectivity at lowland locations to increase genetic diversity levels in these areas. A large proportion of local fish (up to 50 %) were dispersers, often originated from locations within the same river basin. High dispersal capability, ecological tolerance, and reproductive traits likely promote river colonization. Finally, our results also confirmed that human-assisted translocations promote within and among basin gene flow and maintained levels of genetic diversity, particularly in upstream locations.     

Conservation genetics of the wood ant, Formica lugubris, in a fragmented landscape

Various intrinsic factors connected to the special features of sociality influence the persistence of social insect populations, including low effective population size, reduced amount of genetic variation easily leading to inbreeding depression, and spatially structured populations. In this work, we studied an isolated, small and fragmented population system of the red wood ant Formica lugubris, and evaluated the impact of social and genetic population structure on the persistence and conservation of the populations. The effective population size was large in our study population because all nests were polygynous. As a result, and despite the apparent isolation, the amount of nuclear genetic variability was similar to that in a nonisolated population system. Lack of inbreeding, as well as a high level of variability, indirectly suggests that this population does not suffer from inbreeding depression. The spatial distribution of genetic variation between local populations suggests intensive, but strongly male-biased, nuclear gene flow. Thus, the persistence of this population system does not seem to be threatened by any immediate social or genetic factor, but colonization of new habitat patches may be difficult because of restricted female dispersal.     

Did postglacial sea-level changes initiate the evolutionary divergence of a Tasmanian endemic raptor from its mainland relative?

Populations on continental islands are often distinguishable from mainland conspecifics with respect to body size, appearance, behaviour or life history, and this is often congruent with genetic patterns. It is commonly assumed that such differences developed following the complete isolation of populations by sea-level rise following the Last Glacial Maximum (LGM). However, population divergence may predate the LGM, or marine dispersal and colonization of islands may have occurred more recently; in both cases, populations may have also diverged despite ongoing gene flow. Here, we test these alternative hypotheses for the divergence between wedge-tailed eagles from mainland Australia (Aquila audax audax) and the threatened Tasmanian subspecies (Aquila audax fleayi), based on variation at 20 microsatellite loci and mtDNA. Coalescent analyses indicate that population divergence appreciably postdates the severance of terrestrial habitat continuity and occurred without any subsequent gene flow. We infer a recent colonization of Tasmania by marine dispersal and cannot discount founder effects as the cause of differences in body size and life history. We call into question the general assumption of post-LGM marine transgression as the initiator of divergence of terrestrial lineages on continental islands and adjacent mainland, and highlight the range of alternative scenarios that should be considered.