Asymmetric Gene Flow Research Articles

Population differentiation of zander (Sander lucioperca) across native and newly colonized ranges suggests increasing admixture in the course of an invasion.

In addition to ecological factors, evolutionary processes can determine the invasion success of a species. In particular, genetic admixture has the potential to induce rapid evolutionary change, which can result from natural or human-assisted secondary contact between differentiated populations. We studied the recent range expansion of zander in Germany focusing on the interplay between invasion and genetic admixture. Historically, the rivers Elbe and Danube harboured the most north-western source populations from which a north-westward range expansion occurred. This was initiated by introducing zander outside its native range into rivers and lakes, and was fostered by migration through artificial canals and stocking from various sources. We analysed zander populations of the native and invaded ranges using nuclear and mitochondrial genetic markers. Three genetic lineages were identified, which were traced to ancestral ranges. Increased genetic diversity and admixture in the invaded region highlighted asymmetric gene flow towards this area. We suppose that the adaptive potential of the invading populations was promoted by genetic admixture, whereas competitive exclusion in the native areas provided a buffer against introgression by novel genotypes. These explanations would be in line with evidence that hybridization can drive evolutionary change under conditions when new niches can be exploited.

One species or two? Multilocus analysis of nucleotide variation of Melastoma penicillatum and Melastoma sanguineum (Melastomataceae) in Hainan, China

Melastoma comprises more than 20 shrub species and is distributed in tropical Asia and Oceania. Melastoma penicillatum, the most narrowly distributed taxon of this genus in China, was recently incorporated into another morphologically similar species, M. sanguineum. Based on its distinct morphological traits, unique habitat and flowering time, we propose that M. penicillatum should be a distinct species. In this study, we sequenced three nuclear genes and four chloroplast intergenic spacers of M. sanguineum and M. penicillatum at two locations in Hainan to test this hypothesis. There was no sequence variation at all four chloroplast intergenic spacers within and between M. sanguineum and M. penicillatum, suggesting that they should have diverged recently. However, they showed strong divergence at all three nuclear genes and no shared haplotypes were observed between them. Bayesian clustering-based STRUCTURE analysis revealed that individuals of M. sanguineum and M. penicillatum were clustered into their own clades. Fst calculations showed that both taxa had marked population genetic differentiation, but population differentiation in M. penicillatum was stronger than that in M. sanguineum. Stronger population differentiation in M. penicillatum may be caused by its discontinuous high-elevation and understory habitats, which is disadvantageous for bird-mediated dispersal. Our analysis based on the isolation-with-migration (IM) model revealed bidirectional, asymmetrical gene flow in M. sanguineum and little gene flow in M. penicillatum. Taken together, our results indicate that M. penicillatum is a distinct species and should not be incorporated into M. sanguineum. The two species might have diverged recently, and lineage sorting has been incomplete. Differential adaptation to different elevations might have led to speciation in this case.

Spatial and temporal genetic structure of a river-resident Atlantic salmon (Salmo salar) after millennia of isolation.

The river-resident Salmo salar (“småblank”) has been isolated from other Atlantic salmon populations for 9,500 years in upper River Namsen, Norway. This is the only European Atlantic salmon population accomplishing its entire life cycle in a river. Hydropower development during the last six decades has introduced movement barriers and changed more than 50% of the river habitat to lentic conditions. Based on microsatellites and SNPs, genetic variation within småblank was only about 50% of that in the anadromous Atlantic salmon within the same river. The genetic differentiation (FST) between småblank and the anadromous population was 0.24. This is similar to the differentiation between anadromous Atlantic salmon in Europe and North America. Microsatellite analyses identified three genetic subpopulations within småblank, each with an effective population size Ne of a few hundred individuals. There was no evidence of reduced heterozygosity and allelic richness in contemporary samples (2005–2008) compared with historical samples (1955–56 and 1978–79). However, there was a reduction in genetic differentiation between sampling localities over time. SNP data supported the differentiation of småblank into subpopulations and revealed downstream asymmetric gene flow between subpopulations. In spite of this, genetic variation was not higher in the lower than in the upper areas. The meta-population structure of småblank probably maintains genetic variation better than one panmictic population would do, as long as gene flow among subpopulations is maintained. Småblank is a unique endemic island population of Atlantic salmon. It is in a precarious situation due to a variety of anthropogenic impacts on its restricted habitat area. Thus, maintaining population size and avoiding further habitat fragmentation are important.

Remarkably low genetic diversity and strong population structure in common bottlenose dolphins (Tursiops truncatus) from coastal waters of the Southwestern Atlantic Ocean

Knowledge about the ecology of bottlenose dolphins in the Southwestern Atlantic Ocean is scarce. Increased by-catch rates over the last decade in coastal waters of southern Brazil have raised concerns about the decline in abundance of local dolphin communities. Lack of relevant data, including information on population structure and connectivity, have hampered an assessment of the conservation status of bottlenose dolphin communities in this region. Here we combined analyses of 16 microsatellite loci and mitochondrial DNA (mtDNA) control region sequences to investigate genetic diversity, structure and connectivity in 124 biopsy samples collected over six communities of photographically identified coastal bottlenose dolphins in southern Brazil, Uruguay and central Argentina. Levels of nuclear genetic diversity were remarkably low (mean values of allelic diversity and heterozygosity across all loci were 3.6 and 0.21, respectively), a result that possibly reflects the small size of local dolphin communities. On a broad geographical scale, strong and significant genetic differentiation was found between bottlenose dolphins from southern Brazil–Uruguay (SB–U) and Bahia San Antonio (BSA), Argentina (AMOVA mtDNA ΦST = 0.43; nuclear FST = 0.46), with negligible contemporary gene flow detected based on Bayesian estimates. On a finer scale, moderate but significant differentiation (AMOVA mtDNA ΦST = 0.29; nuclear FST = 0.13) and asymmetric gene flow was detected between five neighbouring communities in SB–U. Based on the results we propose that BSA and SB–U represent two distinct evolutionarily significant units, and that communities from SB–U comprise five distinct Management Units (MUs). Under this scenario, conservation efforts should prioritize the areas in southern Brazil where dolphins from three MUs overlap in their home ranges and where by-catch rates are reportedly higher.

Genetic structure of Bemisia tabaci Med populations from home‐range countries, inferred by nuclear and cytoplasmic markers: impact on the distribution of the insecticide resistance genes

Insecticide resistance management in Bemisia tabaci is one of the main issues facing agricultural production today. An extensive survey was undertaken in five Mediterranean countries to examine the resistance status of Med B. tabaci species in its range of geographic origin and the relationship between population genetic structure and the distribution of resistance genes. The investigation combined molecular diagnostic tests, sequence and microsatellite polymorphism studies and monitoring of endosymbionts. High frequencies of pyrethroid (L925I and T929V, VGSC gene) and organophosphate (F331W, ace1 gene) resistance mutations were found in France, Spain and Greece, but not in Morocco or Tunisia. Sequence analyses of the COI gene delineated two closely related mitochondrial groups (Q1 and Q2), which were found either sympatrically (Spain) or separately (France). Only Q1 was observed in Greece, Morocco and Tunisia. Bayesian analyses based on microsatellite loci revealed three geographically delineated genetic groups (France, Spain, Morocco/Greece/Tunisia) and high levels of genetic differentiation even between neighbouring samples. Evidence was also found for hybridisation and asymmetrical gene flow between Q1 and Q2. Med B. tabaci is more diverse and structured than reported so far. On a large geographic scale, resistance is affected by population genetic structure, whereas on a local scale, agricultural practices appear to play a major role.

Phylogeography of Liquidambar styraciflua (Altingiaceae) in Mesoamerica: survivors of a Neogene widespread temperate forest (or cloud forest) in North America?

We investigate the genetic variation between populations of the American sweetgum (Liquidambar styraciflua), a tree species with a disjunct distribution between northeastern Texas and Mexico, by analyzing sequences of two chloroplast DNA plastid regions in Mesoamerica. Our results revealed phylogeographical structure, with private haplotypes distributed in unique environmental space at either side of the Trans-Mexican Volcanic Belt, and a split in the absence of gene flow dating back ca. 4.2–1.4 million years ago (MYA). Species distribution modeling results fit a model of refugia along the Gulf and Atlantic coasts but the present ranges of US and Mesoamerican populations persisted disjunct during glacial/interglacial cycles. Divergence between the US and Mesoamerican (ca. 8.4–2.8 MYA) populations of L. styraciflua and asymmetrical gene flow patterns support the hypothesis of a long-distance dispersal during the Pliocene, with fragmentation since the most recent glacial advance (120,000 years BP) according to coalescent simulations and high effective migration rates from Mesoamerica to the USA and close to zero in the opposite direction. Our findings implicate the Trans-Mexican Volcanic Belt as a porous barrier driving genetic divergence of L. styraciflua, corresponding with environmental niche differences, during the Pliocene to Quaternary volcanic arc episode 3.6 MYA, and a Mesoamerican origin of populations in the USA.

‘Becoming a species by becoming a pest’ or how two maize pests of the genus Ostrinia possibly evolved through parallel ecological speciation events

New agricultural pest species attacking introduced crops may evolve from pre-existing local herbivores by ecological speciation, thereby becoming a species by becoming a pest. We compare the evolutionary pathways by which two maize pests (the Asian and the European corn borers, ACB and ECB) in the genus Ostrinia (Lepidoptera, Crambidae) probably diverged from an ancestral species close to the current Adzuki bean borer (ABB). We typed larval Ostrinia populations collected on maize and dicotyledons across China and eastern Siberia, at microsatellite and mitochondrial loci. We found only two clusters: one on maize (as expected) and a single one on dicotyledons despite differences in male mid-tibia morphology, suggesting that all individuals from dicotyledons belonged to the ABB. We found evidence for migrants and hybrids on both host plant types. Hybrids suggest that field reproductive isolation is incomplete between ACB and ABB. Interestingly, a few individuals with an 'ABB-like' microsatellite profile collected on dicotyledons had 'ACB' mtDNA rather than 'ABB-like' mtDNA, whereas the reverse was never found on maize. This suggests asymmetrical gene flow directed from the ACB towards the ABB. Hybrids and backcrosses in all directions were obtained in no-choice tests. In laboratory conditions, they survived as well as parental strain individuals. In Xinjiang, we found ACB and ECB in sympatry, but no hybrids. Altogether, our results suggest that reproductive isolation between ACB and ABB is incomplete and mostly prezygotic. This points to ecological speciation as a possible evolutionary scenario, as previously found for ECB and ABB in Europe.

New insights into domestication of carrot from root transcriptome analyses.

BackgroundUnderstanding the molecular basis of domestication can provide insights into the processes of rapid evolution and crop improvement. Here we demonstrated the processes of carrot domestication and identified genes under selection based on transcriptome analyses.ResultsThe root transcriptomes of widely differing cultivated and wild carrots were sequenced. A method accounting for sequencing errors was introduced to optimize SNP (single nucleotide polymorphism) discovery. 11,369 SNPs were identified. Of these, 622 (out of 1000 tested SNPs) were validated and used to genotype a large set of cultivated carrot, wild carrot and other wild Daucus carota subspecies, primarily of European origin. Phylogenetic analysis indicated that eastern carrot may originate from Western Asia and western carrot may be selected from eastern carrot. Different wild D. carota subspecies may have contributed to the domestication of cultivated carrot. Genetic diversity was significantly reduced in western cultivars, probably through bottlenecks and selection. However, a high proportion of genetic diversity (more than 85% of the genetic diversity in wild populations) is currently retained in western cultivars. Model simulation indicated high and asymmetric gene flow from wild to cultivated carrots, spontaneously and/or by introgression breeding. Nevertheless, high genetic differentiation exists between cultivated and wild carrots (Fst = 0.295) showing the strong effects of selection. Expression patterns differed radically for some genes between cultivated and wild carrot roots which may be related to changes in root traits. The up-regulation of water-channel-protein gene expression in cultivars might be involved in changing water content and transport in roots. The activated expression of carotenoid-binding-protein genes in cultivars could be related to the high carotenoid accumulation in roots. The silencing of allergen-protein-like genes in cultivated carrot roots suggested strong human selection to reduce allergy. These results suggest that regulatory changes of gene expressions may have played a predominant role in domestication.ConclusionsWestern carrots may originate from eastern carrots. The reduction in genetic diversity in western cultivars due to domestication bottleneck/selection may have been offset by introgression from wild carrot. Differential gene expression patterns between cultivated and wild carrot roots may be a signature of strong selection for favorable cultivation traits.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-895) contains supplementary material, which is available to authorized users.

Range dynamics, rather than convergent selection, explain the mosaic distribution of red-winged blackbird phenotypes.

Geographic distributions of genetic and phenotypic characters can illuminate historical evolutionary processes. In particular, mosaic distributions of phenotypically similar populations can arise from parallel evolution or from irregular patterns of dispersal and colonization by divergent forms. Two phenotypically divergent forms of the red-winged blackbird (Agelaius phoeniceus) show a mosaic phenotypic distribution, with a “bicolored” form occurring disjunctly in California and Mexico. We analyzed the relationships among these bicolored populations and neighboring typical populations, using ∼600 bp of mitochondrial DNA sequence data and 10 nuclear short tandem repeat loci. We find that bicolored populations, although separated by ∼3000 km, are genetically more similar to one other than they are to typical populations separated by ∼400 km. We also find evidence of ongoing gene flow among populations, including some evidence of asymmetric gene flow. We conclude that the current distribution of bicolored forms represents incomplete speciation, where recent asymmetric hybridization with typical A. phoeniceus is dividing the range of a formerly widespread bicolored form. This hypothesis predicts that bicolored forms may suffer extinction by hybridization. Future work will use fine-scaled geographical sampling and nuclear sequence data to test for hybrid origins of currently typical populations and to more precisely quantify the directionality of gene flow.

Investigation of Genetic Structure between Deep and Shallow Populations of the Southern Rock Lobster, Jasus edwardsii in Tasmania, Australia

The southern rock lobster, Jasus edwardsii, shows clear phenotypic differences between shallow water (red coloured) and deeper water (pale coloured) individuals. Translocations of individuals from deeper water to shallower waters are currently being trialled as a management strategy to facilitate a phenotypic change from lower value pale colouration, common in deeper waters, to the higher value red colouration found in shallow waters. Although panmixia across the J. edwardsii range has been long assumed, it is critical to assess the genetic variability of the species to ensure that the level of population connectivity is appropriately understood and translocations do not have unintended consequences. Eight microsatellite loci were used to investigate genetic differentiation between six sites (three shallow, three deep) across southern Tasmania, Australia, and one from New Zealand. Based on analyses the assumption of panmixia was rejected, revealing small levels of genetic differentiation across southern Tasmania, significant levels of differentiation between Tasmania and New Zealand, and high levels of asymmetric gene flow in an easterly direction from Tasmania into New Zealand. These results suggest that translocation among Tasmanian populations are not likely to be problematic, however, a re-consideration of panmictic stock structure for this species is necessary.

Reevaluating a model of gender-biased gene flow among Sub-Saharan Hunter-gatherers and farmers.

In a previous study, we proposed a model for genetic admixture between African hunter-gatherers and food producers, in which we integrated demographic and genetic aspects together with ethnographic knowledge (Destro-Bisol et al. 2004b). In that study it was possible to test the model only using genetic information from widely dispersed and genetically heterogeneous populations. Here we reevaluate the congruence between the model and patterns of genetic variation using an anthropologically and geographically more homogeneous data set that includes Pygmies and farmers from Cameroon, Congo, and the Central African Republic. As implied by the model, the ratios of mtDNA to Y chromosome Nm estimates (effective population size, N, times the migration rate, m; 0.154 in Pygmies and 6.759 in farmers), support an asymmetric gene flow, with a higher Bantu-to-Pygmy gene flow for paternal than for maternal lineages, and vice versa for farmers. Analyses of intra- and interpopulation genetic variation further support the above observation, showing a prevailing effect of genetic drift on maternal lineages and gene flow on paternal lineages among Pygmies, and an opposite pattern among farmers. We also detected differences between patterns for classical and molecular measures of Y chromosome intrapopulation variation, which likely represent signatures of the introgression of Bantu lineages into the gene pool of Pygmy populations. On the whole, our results seem to reflect differences in the demographic history and the degree of patrilocality and polygyny between the two population groups, thus providing further support to our microevolutionary model in an anthropologically coherent framework.

Reevaluating a Model of Gender-Biased Gene Flow among Sub-Saharan Hunter-Gatherers and Farmers

In a previous study, we proposed a model for genetic admixture between African hunter-gatherers and food producers, in which we integrated demographic and genetic aspects together with ethnographic knowledge (). In that study it was possible to test the model only using genetic information from widely dispersed and genetically heterogeneous populations. Here we reevaluate the congruence between the model and patterns of genetic variation using an anthropologically and geographically more homogeneous data set that includes Pygmies and farmers from Cameroon, Congo, and the Central African Republic. As implied by the model, the ratios of mtDNA to Y chromosome Nm estimates (effective population size, N, times the migration rate, m; 0.154 in Pygmies and 6.759 in farmers), support an asymmetric gene flow, with a higher Bantu-to-Pygmy gene flow for paternal than for maternal lineages, and vice versa for farmers. Analyses of intra- and interpopulation genetic variation further support the above observation, showing a prevailing effect of genetic drift on maternal lineages and gene flow on paternal lineages among Pygmies, and an opposite pattern among farmers. We also detected differences between patterns for classical and molecular measures of Y chromosome intrapopulation variation, which likely represent signatures of the introgression of Bantu lineages into the gene pool of Pygmy populations. On the whole, our results seem to reflect differences in the demographic history and the degree of patrilocality and polygyny between the two population groups, thus providing further support to our microevolutionary model in an anthropologically coherent framework.

Living on the edge: how philopatry maintains adaptive potential

Without genetic variation, species cannot cope with changing environments, and evolution does not proceed. In endangered species, adaptive potential may be eroded by decreased population sizes and processes that further reduce gene flow such as philopatry and local adaptations. Here, we focused on the philopatric and endangered loggerhead sea turtle (Caretta caretta) nesting in Cape Verde as a model system to investigate the link between adaptive potential and philopatry. We produced a dataset of three complementary genomic regions to investigate female philopatric behaviour (mitochondrial DNA), male-mediated gene flow (microsatellites) and adaptive potential (major histocompatibility complex, MHC). Results revealed genetically distinct nesting colonies, indicating remarkably small-scale philopatric behaviour of females. Furthermore, these colonies also harboured local pools of MHC alleles, especially at the margins of the population's distribution, which are therefore important reserves of additional diversity for the population. Meanwhile, directional male-mediated gene flow from the margins of distribution sustains the adaptive potential for the entire rookery. We therefore present the first evidence for a positive association between philopatry and locally adapted genomic regions. Contrary to expectation, we propose that philopatry conserves a high adaptive potential at the margins of a distribution, while asymmetric gene flow maintains genetic connectivity with the rest of the population.

Interspecific differentiation and gene flow between two desert poplars inferred from six vacuolar Na+/H+exchanger loci

Populus euphratica Olivier and P. pruinosa Schrenk are known for their tolerance to highly saline and arid habitats, and overlapping distribution. We examined interspecific differentiation and gene flow between these two species at six loci that encode vacuolar Na+/H+ exchanger genes. Interspecific divergence varied greatly between sampled loci and could collectively delimit the two species well. Simulations based on the isolation–migration model suggested gene flow primarily from P. euphratica into P. pruinosa. This asymmetrical gene flow may be related to the adaptive survival of the introgressed individuals. Our findings suggest that these species may have diverged in the presence of gene flow and that local adaptation may have played an important role in maintaining the distinct species lineages by restricting gene flow between them. Our results together indicate that interspecific divergence and gene flow differ greatly between members of the same gene family, possibly due to differential subfunctionalization and/or neofunctionalization during ongoing speciation of these two poplar species.

Population genetics, gene flow, and biogeographical boundaries ofCarcinus aestuarii(Crustacea: Brachyura: Carcinidae) along the European Mediterranean coast

Carcinus aestuarii Nardo, 1847 is a widespread coastal crab species throughout the Mediterranean Sea with a pelagic larval phase. This species tolerates a wide range of environmental conditions and typically inhabits fragmented habitats, such as embayments, lagoons and estuaries. It is therefore a good candidate species for studying and testing different phylogeographical hypotheses in the Mediterranean Sea. By contrast to its Atlantic sister species, Carcinus maenas, studies on the population genetic structure of C. aestuarii in its native range are still scarce. In the present study, specimens from along the European Mediterranean Sea were collected and DNA-sequenced and analyses were applied to discriminate between present day and historical factors influencing the population genetic structure of this species. The results obtained demonstrate the existence of two genetically distinct geographical groups, corresponding to the eastern and western Mediterranean, with further subdivision within the East Mediterranean Basin. A strong asymmetric gene flow was recorded toward the Eastern Basin, which may play a crucial role in shaping the present day biogeographical patterns of this species and potentially other sympatric ones with pelagic larvae. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 771‐790. ADDITIONAL KEYWORDS: biodiversity ‐ cox1 ‐ demographic history ‐ larval dispersal ‐ Mediterranean Sea ‐ phylogeography.

The demographic history of populations experiencing asymmetric gene flow: combining simulated and empirical data

Population structure can significantly affect genetic-based demographic inferences, generating spurious bottleneck-like signals. Previous studies have typically assumed island or stepping-stone models, which are characterized by symmetric gene flow. However, many organisms are characterized by asymmetric gene flow. Here, we combined simulated and empirical data to test whether asymmetric gene flow affects the inference of past demographic changes. Through the analysis of simulated genetic data with three methods (i.e. bottleneck, M-ratio and msvar), we demonstrated that asymmetric gene flow biases past demographic changes. Most biases were towards spurious signals of expansion, albeit their strength depended on values of effective population size and migration rate. It is noteworthy that the spurious signals of demographic changes also depended on the statistical approach underlying each of the three methods. For one of the three methods, biases induced by asymmetric gene flow were confirmed in an empirical multispecific data set involving four freshwater fish species (Squalius cephalus, Leuciscus burdigalensis, Gobio gobio and Phoxinus phoxinus). However, for the two other methods, strong signals of bottlenecks were detected for all species and across two rivers. This suggests that, although potentially biased by asymmetric gene flow, some of these methods were able to bypass this bias when a bottleneck actually occurred. Our results show that population structure and dispersal patterns have to be considered for proper inference of demographic changes from genetic data.

Interspecific hybridization between Scirpus mariqueter Ts. Tang & F.T. Wang and S. planiculmis F. Schmidt and their selfing

Self-pollination and bidirectional interspecific crosses were carried out in Scirpus mariqueter Ts. Tang & F.T. Wang and S. planiculmis F. Schmidt. Self-pollination experiments suggest that S. mariqueter is a completely self-compatible species despite that its flowers are protogynous, whereas S. planiculmis is a predominantly outcrossing species, possessing a low level of self-compatibility. For the interspecific crossing experiments, hybrid seeds were obtained from bidirectional pollination. However, only 4% of the hybrid seeds from the cross of S. mariqueter ♀×S. planiculmis ♂ germinated successfully, with hybrid offsprings exhibiting similar morphological characters to the maternal parent S. mariqueter, while hybrid seeds from the cross of S. planiculmis ♀×S. mariqueter ♂ were unviable. These results suggest asymmetrical gene flow from S. planiculmis to S. mariqueter and the existence of some post-pollination reproductive barriers between these two Scirpus species.

Effects of fine-scale environmental heterogeneity on local genetic structure in Macoma balthica from the Gulf of Gdañsk (southern Baltic Sea)

It is increasingly recognized that populations of marine organisms with potential for large-scale dispersal may exhibit fine-scale genetic structure. The Gulf of Gdansk (Poland) is an interesting setting to study fine-grained population structure in marine organisms as it is characterized by chronic anthropogenic pollution and strong salinity gradients. We investigated, at two nested spatial scales (35 and 7 km), genetic structure among populations of the infaunal tellinid bivalve Macoma balthica. The COI mitochondrial marker revealed a likely mix of evolutionary lineages in the Gulf, with no detectable spatial structure. Seven microsatellite markers detected weak population structure, separating deep and shallow populations within short distances (7 km) and assignment tests suggested asymmetric gene flow among these populations, with no shallow recruits being detected in deep waters. Given the specific environmental conditions encountered at deeper depths in the Gulf (increased salinity, lower temperatures, oxygen depletion, hydrogen sulfide pollution), we suggest that deeper populations may be subjected to local adaptation.

Evidence for asymmetrical divergence-gene flow of nuclear loci, but not mitochondrial loci, between seabird sister species: blue-footed (Sula nebouxii) and Peruvian (S. variegata) boobies.

Understanding the process of speciation requires understanding how gene flow influences divergence. Recent analyses indicate that divergence can take place despite gene flow and that the sex chromosomes can exhibit different levels of gene flow than autosomes and mitochondrial DNA. Using an eight marker dataset including autosomal, z-linked, and mitochondrial loci we tested the hypothesis that blue-footed (Sula nebouxii) and Peruvian (S. variegata) boobies diverged from their common ancestor with gene flow, paying specific attention to the differences in gene flow estimates from nuclear and mitochondrial markers. We found no gene flow at mitochondrial markers, but found evidence from the combined autosomal and z-linked dataset that blue-footed and Peruvian boobies experienced asymmetrical gene flow during or after their initial divergence, predominantly from Peruvian boobies into blue-footed boobies. This gene exchange may have occurred either sporadically between periods of allopatry, or regularly throughout the divergence process. Our results add to growing evidence that diverging species can remain distinct but exchange genes.

Assessing inter-drainage connections: patterns of genetic diversity in an Iberian cyprinid fish

The phylogeography of Iberian freshwater fish species strongly reflects the geomorphological history of Iberian water drainages and formation of mountain ranges. In particular, the Iberian Central Massif limits two ichthyogeographical districts based on fish distribution assemblages: the Northwest district, which comprises all river basins north of the Central massif, and the Central-west district, which comprises the Tejo and Sado basins. Despite this scenario, the cyprinid Pseudochondrostoma polylepis occurs in both the Tejo and Mondego rivers (one either side of the Estrela Mountains, in the western end of the Central Massif). By analysing the population structure and demographic history of this straight-mouth nase, based on both mtDNA and microsatellites, we have found evidence of a relatively recent common origin for both Mondego and Tejo populations. Shared haplotypes and lack of divergent lineages for mitochondrial marker, high levels of admixture and common ancestry at the nuclear level, together with signatures of asymmetrical gene flow from Tejo to Mondego suggest a Mondego colonization through a founder event with an origin in Tejo. We suggest that this colonization might have resulted from the occurrence of river connections/captures between Mondego and Tejo headwater tributaries in the recent past. With this study we reinforce that local-scale geomorphological events and the adaptation/dispersal ability of a species to new environments can have a major impact on its demographic history, colonization processes, and ultimately its distribution. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 656–669.