SELECTION FOR NATIVE CHARACTERS IN HYBRIDS BETWEEN TWO LOCALLY ADAPTED PLANT SUBSPECIES

Gene exchange between locally adapted plant populations can have significant evolutionary consequences, including changes in genetic diversity, introduction of adaptive or maladaptive traits, disruptive of coadaptive gene complexes, and the creation of new ecotypes or even species. The potential for introgression between divergent populations will depend on the strength of selection against nonnative characters. Morphologically variable F₂ hybrids of two Gilia capitata subspecies were used to evaluate the strength of phenotypic selection and the response to selection in the home habitats of each subspecies. At both sites, traits diagnostic of the subspecies were subject to significant phenotypic selection, probably mediated by direct selection on unmeasured correlated characters. Phenotypic selection favored native morphologies in all but a single case; leaf shape of one subspecies was favored in both habitats. The strength of selection varied between sites, with one site selecting more strongly against nonnative characters. Offspring of the F₂ hybrids showed a significant evolutionary response to selection when grown in a common environment. Evolution was in the direction of similarity with the subspecies native to the site where selection was imposed. This result reveals that native character states are adaptive and suggests that selection will maintain native morphologies even after a substantial influx of genes from an ecologically and morphologically distinct, and locally adapted subspecies.     

Contrasting patterns of genome‐wide polymorphism in the native and invasive range of the marine mollusc Crepidula fornicata

Selection processes are believed to be an important evolutionary driver behind the successful establishment of nonindigenous species, for instance through adaptation for invasiveness (e.g. dispersal mechanisms and reproductive allocation). However, evidence supporting this assumption is still scarce. Genome scans have often identified loci with atypical patterns of genetic differentiation (i.e. outliers) indicative of selection processes. Using microsatellite‐ and AFLP‐based genome scans, we looked for evidence of selection following the introduction of the mollusc Crepidula fornicata. Native to the northwestern Atlantic, this gastropod has become an emblematic invader since its introduction during the 19th and 20th centuries in the northeastern Atlantic and northeastern Pacific. We examined 683 individuals from seven native and 15 introduced populations spanning the latitudinal introduction and native ranges of the species. Our results confirmed the previously documented high genetic diversity in native and introduced populations with little genetic structure between the two ranges, a pattern typical of marine invaders. Analysing 344 loci, no outliers were detected between the introduced and native populations or in the introduced range. The genomic sampling may have been insufficient to reveal selection especially if it acts on traits determined by a few genes. Eight outliers were, however, identified within the native range, underlining a genetic singularity congruent with a well‐known biogeographical break along the Florida. Our results call into question the relevance of AFLP genome scans in detecting adaptation on the timescale of biological invasions: genome scans often reveal long‐term adaptation involving numerous genes throughout the genome but seem less effective in detecting recent adaptation from pre‐existing variation on polygenic traits. This study advocates other methods to detect selection effects during biological invasions—for example on phenotypic traits, although genome scans may remain useful for elucidating introduction histories.     

Molecular signatures of lineage‐specific adaptive evolution in a unique sea basin: the example of an anadromous goby Leucopsarion petersii

Climate changes on various time scales often shape genetic novelty and adaptive variation in many biotas. We explored molecular signatures of directional selection in populations of the ice goby Leucopsarion petersii inhabiting a unique sea basin, the Sea of Japan, where a wide variety of environments existed in the Pleistocene in relation to shifts in sea level by repeated glaciations. This species consisted of two historically allopatric lineages, the Japan Sea (JS) and Pacific Ocean (PO) lineages, and these have lived under contrasting marine environments that are expected to have imposed different selection regimes caused by past climatic and current oceanographic factors. We applied a limited genome‐scan approach using seven candidate genes for phenotypic differences between two lineages in combination with 100 anonymous microsatellite loci. Neuropeptide Y (NPY) gene, which is an important regulator of food intake and potent orexigenic agent, and three anonymous microsatellites were identified as robust outliers, that is, candidate loci potentially under directional selection, by multiple divergence‐ and diversity‐based outlier tests in comparisons focused on multiple populations of the JS vs. PO lineages. For these outlier loci, populations of the JS lineage had putative signals of selective sweeps. Additionally, real‐time quantitative PCR analysis using fish reared in a common environment showed a higher expression level for NPY gene in the JS lineage. Thus, this study succeeded in identifying candidate genomic regions under selection across populations of the JS lineage and provided evidence for lineage‐specific adaptive evolution in this unique sea basin.     

Multiple origins and incursions of the Atlantic barnacle Chthamalus proteus in the Pacific

Chthamalus proteus, a barnacle native to the Caribbean and western Atlantic, was introduced to the Pacific within the last few decades. Using direct sequencing of mitochondrial DNA (COI), we characterized genetic variation in native and introduced populations and searched for genetic matches between regions to determine if there were multiple geographical sources and introduction points for this barnacle. In the native range, we found great genetic differences among populations (max. F_ST = 0.613) encompassing four lineages: one endemic to Panama, one endemic to Brazil, and two occurring Caribbean-wide. All four lineages were represented in the Pacific, but not equally; the Brazilian lineage was most prevalent and the Panamanian least common. Twenty-one individuals spread among nearly every island from where the barnacle is known in the Pacific, exactly matched six haplotypes scattered among Curaçao, the Netherlands Antilles; St John, US Virgin Islands; Puerto Rico; and Brazil, confirming a multigeographical origin for the Pacific populations. Significant genetic differences were also found in introduced populations from the Hawaiian Islands (F_CT = 0.043, P < 0.001), indicating introduction events have occurred at more than one locality. However, the sequence, timing and number of arrival events remains unknown. Possible reasons for limited transport of this barnacle through the Panama Canal are discussed. This and a preponderance of Brazilian-type individuals in the Pacific suggest an unexpected route of entry from around Cape Horn, South America. Unification in the Pacific of historically divergent lineages of this barnacle raises the possibility for selection of ‘hybrids’ with novel ecological adaptations in its new environment.     

Insight into the roles of selection in speciation from genomic patterns of divergence and introgression in secondary contact in venomous rattlesnakes

Investigating secondary contact of historically isolated lineages can provide insight into how selection and drift influence genomic divergence and admixture. Here, we studied the genomic landscape of divergence and introgression following secondary contact between lineages of the Western Diamondback Rattlesnake (Crotalus atrox) to determine whether genomic regions under selection in allopatry also contribute to reproductive isolation during introgression. We used thousands of nuclear loci to study genomic differentiation between two lineages that have experienced recent secondary contact following isolation, and incorporated sampling from a zone of secondary contact to identify loci that are resistant to gene flow in hybrids. Comparisons of patterns of divergence and introgression revealed a positive relationship between allelic differentiation and resistance to introgression across the genome, and greater‐than‐expected overlap between genes linked to lineage‐specific divergence and loci that resist introgression. Genes linked to putatively selected markers were related to prominent aspects of rattlesnake biology that differ between populations of Western Diamondback rattlesnakes (i.e., venom and reproductive phenotypes). We also found evidence for selection against introgression of genes that may contribute to cytonuclear incompatibility, consistent with previously observed biased patterns of nuclear and mitochondrial alleles suggestive of partial reproductive isolation due to cytonuclear incompatibilities. Our results provide a genome‐scale perspective on the relationships between divergence and introgression in secondary contact that is relevant for understanding the roles of selection in maintaining partial isolation of lineages, causing admixing lineages to not completely homogenize.     

Genetic resilience of <Emphasis Type="Italic">Daphnia</Emphasis> populations following experimental removal of introduced fish

Dramatic changes in environmental conditions or community composition may impose severe selective pressures on resident populations. These changes in the selective regime can lead to demographic bottlenecks or local extinction. The consequence of demographic contraction is often a reduction of standing genetic variation. Since the level of adaptive genetic variation in populations plays an important role in persistence and adaptive response, understanding genetic resilience and the time course for re-establishment of genetic diversity following demographic perturbations is a critical component of assessing the consequences of changing environments. The introduction of nonnative fish into historically fishless lakes is a particularly dramatic environmental change frequently contributing to demographic bottlenecks and local extinction of native populations. We examine the quantitative- and molecular-genetic recovery of two alpine populations of the zooplankton Daphnia melanica from the Sierra Nevada, California, USA. These populations were extirpated by introduced salmonids and subsequently re-established following the experimental removal of nonnative fish. We obtained data for nuclear and mitochondrial markers and conducted a common-garden experiment to assess the levels of molecular- and quantitative-genetic variation following experimental fish removal. Reestablished D. melanica populations attained levels of nuclear genetic diversity only slightly lower than surrounding fishless populations in the first year following fish removal and substantial mitochondrial and quantitative-genetic diversity within 8 years. This high level of genetic resilience was likely facilitated by multiple sources of genetic variation, including immigration from neighboring populations and hatching from a local reservoir of diapausing eggs. Our results highlight the genetic resilience of taxa with reservoirs of genetic variation in seed or egg banks.     

Patterns of contemporary phenotypic selection and flower integration in the hummingbird‐pollinated Nicotiana glauca between populations with different flower–pollinator combinations

We studied six populations of the hummingbird‐pollinated Nicotiana glauca to determine if the marked differences in the degree of floral‐pollinator mismatch between populations promote divergences in the pattern of pollinator‐mediated phenotypic selection on single traits and on the evolution of complexes of many interacting floral traits. We found evidence that flower phenotype is being shaped by pollinator‐mediated phenotypic selection, since corolla length was consistently under contemporary directional or stabilizing selection. Weak directional selection for longer corollas was found in two populations with low flower–pollinator mismatch; much stronger directional selection was detected for shorter corollas in two populations with high flower–pollinator mismatch; finally, the remaining two populations with intermediate flower–pollinator mismatch showed stabilizing selection for corolla length. N. glauca populations differed in every flower character measured but variations in pollinator‐mediated selection among populations were only observed for corolla length. Multiple covariation among traits was favoured, as suggested by the predominately functional patterns of integration and selection of complexes of many interacting floral traits. This was consistent with the patterns of correlational selection exhibited by four of the six populations, where corolla length was under significant selection in combination with corolla width, style length or stamen length. Overall floral integration was relatively high in all populations but phenotypic integration patterns were not clearly accounted by the degree of flower–pollinator mismatch or type of phenotypic selection, suggesting that trait covariation at the entire flower level is not explained by the current scenario of pollinator‐mediated selection.     

Evaluation of strategies to conserve and restore intraspecific biodiversity of brown trout: outcomes from genetic monitoring in the French Alps

Introduction of nonnative cultured fish is one of the most important threats to native salmonid populations. In brown trout, more than a century of stocking practices has led to a large hybridization between initially geographically isolated lineages, threatening native populations and thereby intraspecific diversity. In the French region of Haute-Savoie, managers and scientists implemented together three management strategies (genetic refuge, direct translocation of wild spawners and stocking with native fry) on 19 test sites for more than 15 years, in the aim to recover pure or nearly pure native populations. Here we propose an assessment of the different management strategies based on a synthetic analysis of the evolution of the introgression rate. While none of the implemented strategies completely achieves the initial objective to restore pure native populations, they differ in their efficiency: introgression rates tend to decrease quickly when direct translocation of native spawners of stocking with native fry strategies are used. The genetic refuge strategy shows slower and more heterogeneous changes of introgression rates. In general, pure nonnative fish are efficiently removed but at the cost of an increased presence of hybrids. Our results imply that intraspecific dynamics react quickly to management practices and that these changes are probably fueled by evolutionary feedbacks that are not yet well understood.     

Genetic relationships of the marine invasive crab parasite <Emphasis Type="Italic">Loxothylacus panopaei</Emphasis>: an analysis of DNA sequence variation,host specificity,and distributional range

Host specificity is a key variable of the niche breath of parasites that can be an important determinant of a parasite’s ability to invade new areas. There is increasing evidence that many parasite species may comprise a variety of genetically variable lineages, which differ in host specificity and geographic range. In this study, we (1) explored the extent of diversity in the invasive parasitic barnacle Loxothylacus panopaei (Rhizocephala) infecting mud crabs (2) examined the geographic origin for the invasive lineage and (3) assessed if further southward spread of the parasite may be impeded. Along the US Atlantic coast, L. panopaei infects different hosts in its invaded range (Chesapeake Bay to north of Cape Canaveral) compared to one portion of the native range in Southeast Florida. This difference was reflected in genetic lineages on two independent loci, mitochondrial cytochrome oxidase I and nuclear cytochrome c. Both loci were concordant in that they showed one lineage infecting crabs of the genus Panopeus in the native range and one lineage infecting Eurypanopeus depressus and Rhithropanopeus harrisii hosts in the invaded range and in the Gulf of Mexico, thus indicating Gulf of Mexico populations as the most likely source of introduction into Chesapeake Bay. Interestingly, the nuclear marker resolved an additional lineage of parasites infecting panopeid hosts in the native range. All three parasite lineages were well supported, but a decision about species status must await further analyses. Since its introduction in the 1960s, the invasive L. panopaei lineage has expanded its range southward along the US Atlantic coast, now almost reaching the northern limit of native Panopeus-infecting lineages at Cape Canaveral, Florida. We hypothesized that parasite-free E. depressus in Southeast Florida, living in sympatry with infected panopeid populations, might be resistant to infection by the invasive lineage. Our infection experiments rejected this hypothesis, suggesting that any impediment to further southward range expansion might be expected from temperature regimes of the subtropical zoogeographic region south of Cape Canaveral.     

Increase in nonnative understorey vegetation cover after nonnative conifer removal and passive restoration

Nonnative conifers are widespread in the southern hemisphere, where their use as plantation species has led to adverse ecosystem impacts sometimes intensified by invasion. Mechanical removal is a common strategy used to reduce or eliminate the negative impacts of nonnative conifers, and encourage native regeneration. However, a variety of factors may preclude active ecological restoration following removal. As a result, passive restoration – unassisted natural vegetation regeneration – is common following conifer removal. We asked, ‘what is the response of understorey cover to removal of nonnative conifer stands followed by passive restoration?' We sampled understorey cover in three site types: two‐ to ten‐year‐old clearcuts, native forest and current plantations. We then grouped understorey species by origin (native/nonnative) and growth form, and compared proportion and per cent cover of these groups as well as of bare ground and litter between the three site types. For clearcuts, we also analysed the effect of time since clearcut on the studied variables. We found that clearcuts had a significantly higher average proportion of nonnative understorey vegetation cover than native forest sites, where nonnative vegetation was nearly absent. The understorey of clearcut sites also averaged more overall vegetation cover and more nonnative vegetation cover (in particular nonnative shrubs and herbaceous species) than either plantation or native forest sites. Notably, 99% of nonnative shrub cover in clearcuts was the invasive nonnative species Scotch broom (Cytisus scoparius). After ten years of passive recovery since clearcutting, the proportion of understorey vegetation cover that is native has not increased and remains far below the proportion observed in native forest sites. Reduced natural regeneration capacity of the native ecosystem, presence of invasive species in the surrounding landscape and land‐use legacies from plantation forestry may inhibit native vegetation recovery and benefit opportunistic invasives, limiting the effectiveness of passive restoration in this context. Abstract in Spanish is available with online material.     

Rapid genome‐wide evolution in Brassica rapa populations following drought revealed by sequencing of ancestral and descendant gene pools

There is increasing evidence that evolution can occur rapidly in response to selection. Recent advances in sequencing suggest the possibility of documenting genetic changes as they occur in populations, thus uncovering the genetic basis of evolution, particularly if samples are available from both before and after selection. Here, we had a unique opportunity to directly assess genetic changes in natural populations following an evolutionary response to a fluctuation in climate. We analysed genome‐wide differences between ancestors and descendants of natural populations of Brassica rapa plants from two locations that rapidly evolved changes in multiple phenotypic traits, including flowering time, following a multiyear late‐season drought in California. These ancestor‐descendant comparisons revealed evolutionary shifts in allele frequencies in many genes. Some genes showing evolutionary shifts have functions related to drought stress and flowering time, consistent with an adaptive response to selection. Loci differentiated between ancestors and descendants (F_ST outliers) were generally different from those showing signatures of selection based on site frequency spectrum analysis (Tajima's D), indicating that the loci that evolved in response to the recent drought and those under historical selection were generally distinct. Very few genes showed similar evolutionary responses between two geographically distinct populations, suggesting independent genetic trajectories of evolution yielding parallel phenotypic changes. The results show that selection can result in rapid genome‐wide evolutionary shifts in allele frequencies in natural populations, and highlight the usefulness of combining resurrection experiments in natural populations with genomics for studying the genetic basis of adaptive evolution.     

Drosophila suzukii wing spot size is robust to developmental temperature

Phenotypic plasticity is an important mechanism allowing adaptation to new environments and as such it has been suggested to facilitate biological invasions. Under this assumption, invasive populations are predicted to exhibit stronger plastic responses than native populations. Drosophila suzukii is an invasive species whose males harbor a spot on the wing tip. In this study, by manipulating developmental temperature, we compare the phenotypic plasticity of wing spot size of two invasive populations with that of a native population. We then compare the results with data obtained from wild‐caught flies from different natural populations. While both wing size and spot size are plastic to temperature, no difference in plasticity was detected between native and invasive populations, rejecting the hypothesis of a role of the wing‐spot plasticity in the invasion success. In contrast, we observed a remarkable stability in the spot‐to‐wing ratio across temperatures, as well as among geographic populations. This stability suggests either that the spot relative size is under stabilizing selection, or that its variation might be constrained by a tight developmental correlation between spot size and wing size. Our data show that this correlation was lost at high temperature, leading to an increased variation in the relative spot size, particularly marked in the two invasive populations. This suggests: (a) that D. suzukii's development is impaired by hot temperatures, in agreement with the cold‐adapted status of this species; (b) that the spot size can be decoupled from wing size, rejecting the hypothesis of an absolute constraint and suggesting that the wing color pattern might be under stabilizing (sexual) selection; and (c) that such sexual selection might be relaxed in the invasive populations. Finally, a subtle but consistent directional asymmetry in spot size was detected in favor of the right side in all populations and temperatures, possibly indicative of a lateralized sexual behavior.     

The effects of nonnative interactions on protein folding rates: theory and simulation

Proteins are minimally frustrated polymers. However, for realistic protein models, nonnative interactions must be taken into account. In this paper, we analyze the effect of nonnative interactions on the folding rate and on the folding free energy barrier. We present an analytic theory to account for the modification on the free energy landscape upon introduction of nonnative contacts, added as a perturbation to the strong native interactions driving folding. Our theory predicts a rate-enhancement regime at fixed temperature, under the introduction of weak, nonnative interactions. We have thoroughly tested this theoretical prediction with simulations of a coarse-grained protein model, by using an off-lattice C(alpha)model of the src-SH3 domain. The strong agreement between results from simulations and theory confirm the nontrivial result that a relatively small amount of nonnative interaction energy can actually assist the folding to the native structure.     

Population differentiation and behavioural association of the two ‘personality’ genes DRD4 and SERT in dunnocks (Prunella modularis)

Quantifying the variation in behaviour‐related genes within and between populations provides insight into how evolutionary processes shape consistent behavioural traits (i.e. personality). Deliberate introductions of non‐native species offer opportunities to investigate how such genes differ between native and introduced populations and how polymorphisms in the genes are related to variation in behaviour. Here, we compared the genetic variation of the two ‘personality’ genes, DRD4 and SERT, between a native (United Kingdom, UK) and an introduced (New Zealand, NZ) population of dunnocks, Prunella modularis. The NZ population showed a significantly lower number of single nucleotide polymorphisms (SNPs) compared to the UK population. Standardized F’_st estimates of the personality genes and neutral microsatellites indicate that selection (anthropogenic and natural) probably occurred during and post the introduction event. Notably, the largest genetic differentiation was found in the intronic regions of the genes. In the NZ population, we also examined the association between polymorphisms in DRD4 and SERT and two highly repeatable behavioural traits: flight‐initiation distance and mating status (promiscuous females and cobreeding males). We found 38 significant associations (for different allele effect models) between the two behavioural traits and the studied genes. Further, 22 of the tested associations showed antagonistic allele effects for males and females. Our findings illustrate how introduction events and accompanying ecological changes could influence the genetic diversity of behaviour‐related genes.     

Multiple electrofishing as a mitigate tool for removing nonnative Atlantic brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis> L.) threatening a native Mediterranean brown trout population

In the native range of the brown trout (Salmo trutta L.) in Europe, the hybridization of native populations by nonnative domesticated strains introduced by stocking is one of the most serious threats to the long-term conservation of diversity within this species. With the objective of conserving and restoring the native gene pool, fishery managers are beginning to implement various management strategies at the local scale. Nevertheless, few case studies have been published that investigate the effectiveness of the various different conservation strategies for native brown trout populations. In the Chevenne Creek, a small French mountain stream, we tested the strategy of removing nonnative individuals by multiple electrofishing carried out by fishery managers in order to evaluate its feasibility and effectiveness for eliminating a nonnative population threatening a native population. Electrofishing produced major reductions in the nonnative population between 2006 and 2009, with 82–100% of nonnative individuals being removed over a period of 4 years. Nevertheless, despite multiple-electrofishing campaigns, this nonnative population was not entirely eradicated, and some natural recruitment persisted. The young of the year and subadults were less effectively removed than the adults. The results suggest that repeated electrofishing campaigns can be used by managers to reduce the nonnative brown trout population with the objective of conserving the native gene pool, but the removal operation must be continued for at least 4 consecutive years. This strategy, which is feasible in small streams, has to be followed by complementary operations to allow the restoration of a new, native, self-sustainable brown trout population.     

Microevolution of symbiotic Bradyrhizobium populations associated with soybeans in east North America

Microevolution and origins of Bradyrhizobium populations associated with soybeans at two field sites (A and B, 280 km apart in Canada) with contrasting histories of inoculation was investigated using probabilistic analyses of six core (housekeeping) gene sequences. These analyses supported division of 220 isolates in five lineages corresponding either to B. japonicum groups 1 and 1a or to one of three novel lineages within the genus Bradyrhizobium. None of the isolates from site A and about 20% from site B (the only site with a recent inoculation history) were attributed to inoculation sources. The data suggest that most isolates were of indigenous origin based on sequence analysis of 148 isolates of soybean‐nodulating bacteria from native legumes (Amphicarpaea bracteata and Desmodium canadense). Isolates from D. canadense clustered with B. japonicum group 1, whereas those from A. bracteata were placed in two novel lineages encountered at soybean field sites. One of these novel lineages predominated at soybean sites and exhibited a significant clonal expansion likely reflecting selection by the plant host. Homologous recombination events detected in the 35 sequence types from soybean sites had an effect on genetic diversification that was approximately equal to mutation. Interlineage transfer of core genes was infrequent and mostly attributable to gyrB that had a history of frequent recombination. Symbiotic gene sequences (nodC and nifH) of isolates from soybean sites and native legumes clustered in two lineages corresponding to B. japonicum and B. elkani with the inheritance of these genes appearing predominantly by vertical transmission. The data suggest that soybean‐nodulating bacteria associated with native legumes represent a novel source of ecologically adapted bacteria for soybean inoculation.     

Population admixture and high larval viability among urban toads

In terms of evolutionary biology, a population admixture of more than two distinct lineages may lead to strengthened genetic variation through hybridization. However, a population admixture arising from artificial secondary contact poses significant problems in conservation biology. In urban Tokyo, a population admixture has emerged from two lineages of Japanese common toad: native Bufo japonicus formosus and nonnative B. japonicus japonicus, of which the latter was introduced in the early 20th century. To evaluate the degree of genetic disturbance in the admixed population of these two subspecies, we analyzed genotypes of toads distributed within and outside Tokyo by assessing mtDNA and seven microsatellite loci. We found that the introduced B. japonicus japonicus genotype dominates six local populations in the Tokyo admixture zone and was clearly derived from past introgressive hybridization between the two subspecies. These observations were supported by morphological assessments. Furthermore, the average larval survival rate in Tokyo was significantly higher than that outside Tokyo, suggesting that the temporary contribution of introduced toads occurred through introgression. The fitness of toads in urban Tokyo may thus be increasing with the assistance of nonnative individuals.     

Genomics of invasion: diversity and selection in introduced populations of monkeyflowers (Mimulus guttatus)

Global trade and travel is irreversibly changing the distribution of species around the world. Because introduced species experience drastic demographic events during colonization and often face novel environmental challenges from their native range, introduced populations may undergo rapid evolutionary change. Genomic studies provide the opportunity to investigate the extent to which demographic, historical and selective processes shape the genomic structure of introduced populations by analysing the signature that these processes leave on genomic variation. Here, we use next‐generation sequencing to compare genome‐wide relationships and patterns of diversity in native and introduced populations of the yellow monkeyflower (Mimulus guttatus). Genome resequencing data from 10 introduced populations from the United Kingdom (UK) and 12 native M. guttatus populations in North America (NA) demonstrated reduced neutral genetic diversity in the introduced range and showed that UK populations are derived from a geographic region around the North Pacific. A selective‐sweep analysis revealed site frequency changes consistent with selection on five of 14 chromosomes, with genes in these regions showing reduced silent site diversity. While the target of selection is unknown, genes associated with flowering time and biotic and abiotic stresses were located within the swept regions. The future identification of the specific source of origin of introduced UK populations will help determining whether the observed selective sweeps can be traced to unsampled native populations or occurred since dispersal across the Atlantic. Our study demonstrates the general potential of genome‐wide analyses to uncover a range of evolutionary processes affecting invasive populations.     

Patterns of subspecific anthropogenic introgression in two salmonid genera

Introgressive hybridizations have often been observed between native and introduced trouts of North America (Oncorhynchus spp.) and Europe (Salmo spp.), including some lineages that have been isolated for more than a million years. These observations have suggested that introgression is the expected result between introduced and indigenous conspecific salmonids. However, an examination of published information reveals a high variability in such anticipated gene flow. Many studies have noted the relative ease of translocating freshwater over anadromous salmonids, and this difference has been related to the more complex adaptations of anadromous populations (e.g., freshwater and marine residence, smoltification, juvenile and adult migration) that obstruct their translocation to conspecifically colonized areas. This contrast extends to introgressive capabilities where examples of introgression among freshwater populations predominate. Despite intensive introductions of non-native salmonids, indigenous anadromous populations commonly resist introgression. However, within major lineages, anadromous populations appear to be more susceptible to introgression. Measuring the extent and dynamics of such introgressions remains challenging because subgroups within major lineages lie on or below the threshold for detection by molecular genetic markers. These substructures appear to reflect the more rapid evolution of directional selection promoting, for instance, temporal or microgeographic divergence within a population unit defined by genetic markers. Consequently, management that assumes panmixia within a particular region based on even intensive molecular genetic analysis will inevitably erode and prevent reformation of this substructure to the detriment of the overall genetic variability and productivity.