Comparing latitudinal and upstream–downstream gradients: life history traits of invasive mosquitofish

Aim Variation of life history traits along spatial gradients is poorly understood in invasive species and particularly in freshwater fish. We aimed to examine life history variation in a highly invasive fish (Gambusia holbrooki) along latitudinal and upstream–downstream river gradients and to assess the effects of age on this variation. We hypothesized similar responses in populations inhabiting environments more favourable to this species (lower latitudes and lower reaches of rivers). Location European rivers from southern Spain to southern France. Methods We sampled mosquitofish from the lowest reaches of ten river basins along 6° of latitude in the Mediterranean region and seven sites along the upstream–downstream gradient in three of the rivers. We examined abundance, population structure, size‐at‐age and other life history traits along these gradients. Results As hypothesized, lower reaches and lower latitudes both resulted in higher reproductive effort and lower body condition of mosquitofish. However, these patterns explained low per cent variation, were nonlinear and strongly depended on fish age. Independently of fish size, age groups differed in reproductive effort, in the gonadal weight–size relationship and its variation along spatial gradients. Mean size‐at‐age (or overall body size) did not vary with latitude (so the intra‐specific version of Bergmann’s rule or its converse does not apply) and in contrast increased upstream in rivers. Main conclusions Our findings suggest that for life history traits of freshwater organisms, river longitudinal variation plays a role as important as climate, with often differential effects. Our results also illustrate the poor knowledge of spatial variation of many life history traits, which precludes the understanding and prediction of biological invasions in a rapidly changing world.     

Asymmetric dispersal structures a riverine metapopulation of the freshwater pearl mussel Margaritifera laevis

Unidirectional water flow results in the downstream‐biased, asymmetric dispersal of many riverine organisms. However, little is known of how asymmetric dispersal influences riverine population structure and dynamics, limiting our ability to properly manage riverine organisms. A metapopulation of the freshwater pearl mussel Margaritifera laevis may be sensitive to river currents because mussels are repeatedly exposed to downstream drift during floods—a parasitic life stage is the only, limited period (~40 days) during which larvae (glochidia) can move upstream with the aid of host fish. We hypothesized that water‐mediated dispersal would overwhelm upstream dispersal via host fish, and therefore, that upstream subpopulations play a critical role as immigrant sources. To test this hypothesis, we examined the effects of both up‐ and downstream immigrant sources on the size of target subpopulations in the Shubuto River system, Hokkaido, Japan. We found that target subpopulation size was dependent on the upstream distribution range of reproductive subpopulations and the number of upstream tributaries, which are proxies for the number of potential immigrants moving downstream. In contrast, little influence was observed of downstream immigrant sources (proximity to downstream reproductive subpopulations). These results were consistent even after accounting for local environments and stream size. Our finding suggests that upstream subpopulations can be disproportionately important as immigrant sources when dispersal is strongly asymmetric.     

Dispersal potential impacts size clines of grasshoppers across an elevation gradient

Body size is a life history trait that determines the reproductive success of a variety of organisms. Changes in body size may have a genetic component when persistent conditions such as season length and climate select for individuals of an optimal body size and an environmental component when it is influenced on an ecological scale by factors such as weather, food availability, or maternal effects. Along elevational gradients that experience seasonality, insects commonly become smaller with increases in elevation. In this study we test the hypothesis that dispersal potential, an indicator of gene flow, impacts the type of size clines exhibited by insects along elevational gradients and that these differences in local adaptation should lead to predictable changes in their reproductive potential and output. Using two short winged grasshopper species, Aeropedellus clavatus and Melanoplus boulderensis, and two long winged species, Camnula pellucida and Melanoplus sanguinipes, we showed that species with low dispersal potential are associated with significant declines in body size with increases in elevation while species with high dispersal potential displayed no size clines. Consistent with short winged species being more locally adapted, we show that reproductive potential, as measured by the proportion of ovarioles that become functional, do not differ among populations of short winged species, but decline with elevation in the long winged species. While our study failed to show that dispersal potential impacts reproductive output in a consistent and predictable manner (as measured by clutch and egg sizes), we address the possibility that clutch size may not reflect changes in total reproductive output and that changes in egg size may be a plastic trait. We concluded that studies exploring the evolution of body size, the reproductive capacity and species level responses to environmental change should note the importance of dispersal potential in influencing these patterns.     

In-stream and overland dispersal across a river network influences gene flow in a freshwater insect, Calopteryx splendens

Gene flow in riverine species is constrained by the dendritic (branching) structure of the river network. Spatial genetic structure (SGS) of freshwater insects is particularly influenced by catchment characteristics and land use in the surroundings of the river. Gene flow also depends on the life cycle of organisms. Aquatic larvae mainly drift downstream whereas flying adults can disperse actively overland and along watercourses. In-stream movements can generate isolation by distance (IBD) at a local scale and differentiation between subcatchments. However, these patterns can be disrupted by overland dispersal. We studied SGS across the Loire River in the damselfly Calopteryx splendens which is able to disperse along and between watercourses. Our sampling design allowed us to test for overland dispersal effects on genetic differentiation between watercourses. Amplified fragment length polymorphism markers revealed high genetic differentiation at the catchment scale but the genetic structure did not reflect the geographical structure of sampling sites. We observed IBD patterns when considering the distance following the watercourse but also the Euclidean distance, i.e. the shortest distance, between pairs of sites. Altogether, our results support the hypothesis of overland dispersal between watercourses. From a conservation perspective, attention should be paid to the actual pathways of gene flow across complex landscapes such as river networks.     

Latitudinal insect body size clines revisited: a critical evaluation of the saw-tooth model

1.?Insect body size is predicted to increase with decreasing latitude because time available for growth increases. In insects with changing voltinism (i.e. number of generations per season), sharp decreases in development time and body size are expected at season lengths where new generations are added to the phenology of a species, giving rise to saw-tooth clines in these traits across latitudes. Growth rate variation may affect the magnitude of variation in body size or even reverse the saw-tooth cline. 2.?In this study, we analyse latitudinal body size clines in four geometrid moths with changing voltinism in a common laboratory environment. In addition to body size, we measured larval development time and growth rate and genetic correlations among the three traits. 3.?The patterns of clinal variation in body size were diverse, and the theory was not supported even when saw-tooth body size clines were found. Larval development time increased and growth rate decreased consistently with increasing season length, the clines in these traits being uniform. 4.?The consistencies of development time and growth rate clines suggest a common mechanism underlying the observations. Such a mechanism is discussed in relation to the complex interdependencies among the traits.     

Antagonistic selection between adult thorax and wing size in field released Drosophila melanogaster independent of thermal conditions

Attempts to explain size variation in Drosophila and other small insects often focus on the larval stage and association between development time and size, but patterns are also influenced by direct selection on size-related traits in the adults. Here we use multiple field releases of Drosophila melanogaster to test the association between size and one component of field fitness, the ability of Drosophila to locate resources for feeding and breeding. We find antagonistic selection between wing length and thorax length in both males and females, such that capture at baits is higher for flies with relatively larger thorax lengths and smaller wings. However flies with large wings relative to thoraces disperse further as reflected in the longer distances moved to baits. These patterns did not depend strongly on weather conditions, suggesting that selection on adult size is at least partly independent of temperature. Antagonistic selection between size traits can generate changes in size along gradients if the distribution of resources in the environment varies and selects for different dispersal patterns, particularly as dispersal is relatively higher under warmer conditions.     

Quantifying phenotypic gradients in freshwater snails: a case study in Lithasia (Gastropoda: Pleuroceridae)

Many authors have described a pattern of morphological variation in freshwater bivalves where shells taken from lentic and lotic environments, or headwaters and main stem reaches, appear to exhibit phenotypic gradients in size and shape. For example, mussels taken from headwater reaches tend to possess smooth, less inflated shells compared to the more obese, sculptured individuals downstream. Others observed similar relationships in certain freshwater gastropods, but this variation has not been quantified nor its existence explained in an ecological or evolutionary context. Geometric morphometrics indicated freshwater snails shells from the pleurocerid genus Lithasia from the Duck River, Tennessee, USA, show phenotypic gradients similar to those in freshwater mussels. Shells from upstream areas were narrow and less sculptured on the posterior portions of their body whorls, while downstream shells were more inflated and possessed significantly more sculpture. This phenotypic variation may reduce predation or damage due to dislodging. The nature of the observed plasticity suggests an unidirectional environment similar to that proposed by the river continuum concept. Handling editor: K. Martens     

A trait-based approach to assess the vulnerability of European aquatic insects to climate change

Aquatic insects are the dominant taxon group in most freshwater ecosystems. As temperature is the main driver of their life cycle development, metabolic activity, and geographic distribution, these macroinvertebrates are particularly suitable for large scale and comparative studies of freshwater community responses to climate change. A dataset of bio-ecological traits of 1,942 Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa was used to analyze (1) the relationships among traits, (2) the potential vulnerability of EPT species to climate change, and (3) the geographical occurrence patterns of these potentially endangered species at the scale of European ecoregions. By means of a fuzzy correspondence analysis (FCA), two gradients emerged: (1) a longitudinal gradient, describing successive upstream–downstream features, and (2) a biogeographical gradient, separating endemic and micro-endemic species from widely distributed taxa. Moreover, aquatic insects of southern European ecoregions emerged as those most endangered in terms of potential vulnerability to climate change. Comparative multi-taxon studies provide important new insights into freshwater ecosystem functioning and responses to climate change, and could be the first step toward developing integrative monitoring or assessment tools (e.g., trait-based indicators at the species level) by means of non-arbitrary statistical methods.     

Integrating large‐scale geographic patterns in flight morphology,flight characteristics and sexual selection in a range‐expanding damselfly

While geographic trait variation along environmental clines is widespread, associated patterns in sexual selection remain largely unexplored. Geographic patterns in sexual selection may be expected if 1) phenotypes vary geographically and sexual selection is dependent on the local phenotypes in the population, and if 2) sexual selection is influenced by geographically structured environmental conditions. We quantified geographic variation in flight‐related traits and flight performance in mated and unmated males and tested for geographic variation in sexual selection on these traits in the poleward range‐expanding damselfly Coenagrion scitulum across a set of eleven core and edge populations ordered along thermal gradients in the larval and in the adult stage. We found little support for trait differentiation between core and edge populations, instead we found considerable geographic trait variation along the larval and adult thermal gradients. As expected under time constraints, body mass decreased with shorter larval growth seasons. Lower temperatures during the adult flight period were associated with a higher body mass, a higher flight speed and a higher fat content; these traits likely evolved to buffer flight ability at suboptimal temperatures and to optimize starvation resistance. Across the large geographic scale, we found a consistent higher flight duration in mated males. Instead, sexual selection for higher fat content was stronger in populations with lower adult flight temperatures and sexual selection for lower body mass acted only in edge populations. Our results indicate sexual selection on flight performance to be consistent over a large geographic scale and this despite the clear geographic patterns in sexual selection on the underlying morphological traits. Our results highlight that to fully understand the fitness implications of geographically changing trait patterns, researchers should consider the entire phenotype–performance–fitness axis and incorporate effects of geographically structured life‐stage specific environmental conditions on this axis.     

Limited gene flow may enhance adaptation to local optima in isolated populations of the Roesel's bush cricket (Metrioptera roeselii)

Variation in morphological traits along latitudinal gradients often manifests as size clines. In insects, both positive and negative correlations are seen, and the mechanism behind the response is unclear. We studied variation in seven morphological traits of Roesel's bush cricket, Metrioptera roeselii, sampled from seven latitude-matched-pair populations that were either geographically isolated from or connected to the species continuous distribution range. The aim was to examine whether morphological traits differed between isolated and continuous populations, and whether latitudinal variation was apparent. The data were used to indicate whether variation in trait means originates from plastic responses to the environment or genetic adaptation to local conditions. To evaluate the influence of gene flow on trait means, we analysed the genetic variation in seven microsatellites. Data showed that individuals from isolated populations display a positive relationship between latitude and body size, whereas individuals from continuous populations show little or no such relationship. The combined morphological and genetic data suggest that the isolated populations have adapted to local optima, while gene flow between continuous populations appears to counteract this process.     

Hierarchical large-scale to local-scale influence of abiotic factors in summer-fragmented Mediterranean rivers: structuring effects on fish distributions,assemblage composition and species richness

Summer rainfall shortage and low or no flows in Mediterranean rivers make it difficult for fish to attain their final preferenda and hinder the assembling of fish communities. We investigated in this study the degree to which large-scale features of the watersheds and reach-scale habitats correlated with a non-random structure of fish species' richness, composition and distribution under these conditions by intensively sampling a river system during summer and using Geographical Information Systems data. The results of multivariate ordination and association analyses showed that multi-scale factors were hierarchically related and that they correlated with non-random fish distributions and assemblage structure. They also highlighted large-scale factors shaping reach-scale water availability for fish during summer. Fish species' richness, composition and distribution varied along a gradient of water shortage and connectivity loss which were more severe with the smaller drainage area into the reaches upstream. Fish species' distributions along these gradients were strongly correlated with both life-history traits (adult size) and biogeographical origin (native vs. exotic). The results for the summer environmental segregation of exotic versus small endemic richness characteristics indicate strong species' interactions forcing the latter into the upstream, harsher seasonal habitats. Special fitness costs are to be expected in drought years, in regulated rivers, and in general, in a climate change scenario in Mediterranean river networks. The study emphasizes that the conservation of the threatened Mediterranean freshwater fish faunas requires operating at the proper spatial scale. In particular, reach-scale habitat improvements are adequate but not enough during the summer in fragmenting river networks, unless there is proper watershed-scale management of the flow regimes.     

The structure of clines with fitness-dependent dispersal

Spatial models commonly assume that dispersal does not depend on environmental conditions or phenotype. For example, these assumptions underpin explanations for clines on the basis of a trade-off between dispersal and local adaptation. We reexamine clines when an individual's decisions over whether and where to disperse depend on its fitness. We compare fitness-dependent dispersal with cases where dispersal responds to juvenile survivorship only. Clines are steeper the more responsive dispersal is to environmental conditions for all dispersal behaviors that we consider. Clines eventually become stepped as the responsiveness of dispersal to environmental conditions is increased for half of the dispersal behaviors we consider, but smooth clines are maintained for the remaining cases. Smooth clines are maintained by the biased movement of individuals out of the hybrid zone when individuals move directionally in response to gradients in juvenile survivorship, which is a different mechanism to that maintaining smooth clines in classic cline theory.     

Dispersal of yellow phase Japanese eels <Emphasis Type="Italic">Anguilla japonica</Emphasis> after recruitment in the Kojima Bay-Asahi River system,Japan

The density, size and age distribution were investigated for 233 eels, Anguilla japonica, sampled in fresh and brackish water areas of the Kojima Bay-Asahi River system, Okayama, Japan, to evaluate the possible patterns of dispersal of eels that recruit to this area. Migratory histories of 183 eels were categorized into 5 types depending on the Sr and Ca concentrations in their otoliths: (1) brackish water residents (74 fish, 40.4%), which settled in saline water and remained until capture; (2) freshwater residents (46 fish, 25.1%), which settled in freshwater and remained until capture; (3) upstream shifters (3 fish, 1.6%), which settled in saline water and moved upstream into freshwater; (4) downstream shifters (53 fish, 29.0%), which settled in freshwater and moved downstream into saline water; (5) multiple habitat shifters (7 fish, 3.8%), which shifted their habitats between freshwater and saline water more than twice. For eels captured in the brackish water area, fish density decreased with distance in the downstream direction, while the size and age of eels increased. For eels captured in the freshwater area, size and age were greater than those in the upper-most brackish site. These observations suggest that eels in this system initially accumulate in the lower reaches of the river and then disperse in both upstream and downstream directions following their growth.     

Tributary and mainstem benthic macroinvertebrate communities linked by direct dispersal and indirect habitat alteration

Benthic communities in tributary–mainstem networks might interact via downstream drift of invertebrates or material from tributaries and adult dispersal from the mainstem. Depending on the strength of these interactions, mainstem downstream communities are expected to be more similar to tributary communities due to drift or habitat alteration. Communities not connected by flow are expected to be similar due to adult dispersal but decreasing in similarity with distance from the mainstem. We investigated interactions between invertebrate communities of a 7th order river and 5th order tributary by comparing benthic community structure in the river upstream and downstream of the tributary confluence and upstream in the tributary. Non-metric multidimensional scaling showed invertebrate communities and habitat traits from river locations directly downstream of the tributary clustered tightly, intermediate between tributary and mid-channel river locations. In addition, Bray–Curtis dissimilarity increased between the mainstem and tributary with distance upstream in the tributary. Our results indicate that similarities between mainstem and tributary communities are potentially caused by direct mass effects from tributary to downstream mainstem communities by invertebrate drift and indirect mass effects by habitat restructuring via material delivery from the tributary, as well as potential effects of adult dispersal from the river on proximal tributary communities.     

The effect of genetic and environmental variation on genital size in male Drosophila: canalized but developmentally unstable

The genitalia of most male arthropods scale hypoallometrically with body size, that is they are more or less the same size across large and small individuals in a population. Such scaling is expected to arise when genital traits show less variation than somatic traits in response to factors that generate size variation among individuals in a population. Nevertheless, there have been few studies directly examining the relative sensitivity of genital and somatic traits to factors that affect their size. Such studies are key to understanding genital evolution and the evolution of morphological scaling relationships more generally. Previous studies indicate that the size of genital traits in male Drosophila melanogaster show a relatively low response to variation in environmental factors that affect trait size. Here we show that the size of genital traits in male fruit flies also exhibit a relatively low response to variation in genetic factors that affect trait size. Importantly, however, this low response is only to genetic factors that affect body and organ size systemically, not those that affect organ size autonomously. Further, we show that the genital traits do not show low levels of developmental instability, which is the response to stochastic developmental errors that also influence organ size autonomously. We discuss these results in the context of current hypotheses on the proximate and ultimate mechanisms that generate genital hypoallometry.     

Multiple mechanisms underlie rapid expansion of an invasive alien plant

? With growing concerns over serious ecological problems, a particular challenge is to reveal the complex mechanisms underlying rapid expansion of invasive species. Ageratina adenophora is of particular interest in addressing this question. ? We used geographic information systems and logistic regression to identify the geographic and environmental factors contributing to the presence of A. adenophora. Join-count spatial statistics with reproduction mode examination were employed to elucidate the spatiotemporal dispersal mechanisms. ? Multiple factors have significantly contributed to the rapid expansion of A. adenophora. Its biological traits, favoring dispersal by water and wind coupled with local spatiotemporally heterogeneous geography and ecology, promote invasion downstream and upstream along river valleys, while other factors associated with human activities facilitate its invasion over high mountains and across river valleys, providing new scope for progressive invasions. We further identified an unusual invasion event of A. adenophora subsequent to a great flood that amplified its dispersal ability from vegetative propagules and seeds. ? These findings suggest that dynamic interactions of multiple factors in heterogeneous ecogeographical environments - a 'combinatorial' invasion mechanism - would generate an unexpected invasion rate of an alien species or a seemingly stochastic invasion event.     

Genetic differentiation of submerged plant populations and taxa between habitats

Ceratophyllum spp., Callitriche spp., Zannichellia spp. and Potamogeton pectinatus L. are widespread submerged macrophyte species, often occurring at high abundance and forming an integral part of the vegetation of many types of shallow aquatic systems. Several species occur in both freshwater and brackish water habitats. Most have a mixed reproduction system and can reproduce sexually by seeds and propagate asexually by rhizomes, turions, root tubers or axillary tubers. It is hypothesized that sexual propagules are more important than vegetative fragments to ensure long-distance-dispersal, which in case of frequent bird or water flow-mediated dispersal should lead to lowered genetic differentiation. At a regional level, we used dominant ISSR markers in a multi-species approach and observed the largest clonal differentiation between brackish water and freshwater populations of the western European lowland (Belgium). Differentiation was pronounced at taxon level (e.g. Zannichellia), as a salinity gradient (P. pectinatus) or as a coastal-to-inland conductivity gradient (Callitriche obtusangula). These differences and trends suggested a very limited dispersal at regional level across both habitats and regions. To test the hypothesis whether vegetative reproduction and dispersal may have an important function in maintenance of the species at local scale, we investigated the microsatellite diversity and clonal distribution within and between populations of P. pectinatus from a single catchment, representing upstream forest ponds and downstream river sites along the Woluwe (Brussels, Belgium). Clonal diversity was low on average, however, with a higher number of multilocus genotypes in upstream forest ponds than in downstream river sites. A few but abundant clones were present along various stretches of the river indicating clonal spread and establishment over larger distances within the river. Clonal dispersal at a local scale was more pronounced in river than in pond habitats, indicating a higher relative importance of water flow than bird-mediated dispersal for establishment of P. pectinatus in river sites. Dispersal of seeds and establishment of seedlings were assumed more effective within ponds than in river habitats. Upstream forest ponds can be regarded as source populations and refuges of clonal diversity for recolonization of the more stressful downstream river habitat.     

Molecular and morphological analysis of secondary contact zones of Cottus gobio in Fennoscandia: geographical discordance of character transitions

Northern Europe was postglacially colonized from different directions by distinct phylogeographical lineages of the bullhead Cottus gobio L. (Pisces: Scorpaeniformes). These lineages have then come into contact in coastal habitats of the currently brackish Baltic Sea and in the freshwaters north of it. We studied the patterns of intergradation in the contact zones in four morphometric and six molecular characters. In the north, intergradation between the western (W) and eastern (E) bullhead lineages is found both among rivers (west‐to‐east) and along individual rivers (south‐to‐north). The locations of the transition zones probably relate to the timing of the initial contact, subsequent Baltic shoreline displacement (i.e. emergence of the lower river reaches), and dispersal barriers caused by variations of coastal salinity. The transitions (clines) in different characters are, however, not geographically coincident. Mitochondrial DNA clines are generally found upstream and to the east of the other transitions, and GPI‐1 allozyme clines are mostly shifted downstream in the rivers, and west of the other transitions on the broader scale of the Baltic Sea. The location of the mtDNA clines may best reflect the initial contact between lineages, and the displacement of the other clines could result from dispersal being overall asymmetric (predominantly downstream) and sex‐biased (stronger in males). Alternatively, the non‐coincidence might reflect selection against deleterious cytonuclear character combinations. No clear evidence of reproductive incompatibility between the lineages was seen in local population structures; no remaining genetic correlations were observed locally among traits. In another transition area, a coastal transect in southern Finland, clinal patterns similar to those in the northern contact zone were recorded, but the population compositions could not be explained by simple in situ mixing of any of the putatively pure, invading refugial lineages. Probably, the bullhead stocks that initially came into contact in this southern study area already represented mixtures of the invading lineages. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 81 , 535–552.     

Environmental and spatial effects on coastal stream fishes in the Atlantic rain forest

Contemporary and historical factors influence assemblage structure. The environmental and spatial influences acting on fish organization of rain forest coastal streams in the Atlantic rain forest of Brazil were examined. Fish (and functional traits such as morphology, diet, velocity preference, body size), environmental variables (pH, water conductivity, dissolved oxygen, temperature, stream width, flow, depth, substrate), and altitude were measured from 59 stream reaches. Asymmetric eigenvector maps were used to model the spatial structure considering direction of fish movements. Elevation played an important role—fish abundance, biomass, and richness all decrease with increasing elevation. Fish communities are influenced by both environmental and spatial factors, but downstream movements were shown to be more important in explaining the observed spatial variation than were bidirectional and upstream movements. Spatial factors, as well as environmental variables influenced by the spatial structure, explained most of the variation in fish assemblages. The strong spatial structuring is probably attributable to asymmetric dispersal limitation along the altitudinal profile: Dispersal is likely to be more limiting moving upstream than downstream. These fish assemblages reflect scale-dependent processes: At the stream-reach scale, fish respond to local environmental filters (habitat structure, water chemistry, and food supply), which are in turn influenced by a larger scale, namely the altitudinal gradient expected in steep coastal mountains. Thus, environmental drivers are not independent of spatial factors, and the effects of local factors can be confounded across the altitudinal gradient. These results may have implications for conservation, because downstream reaches are often neglected in management and conservation plans.     

Ancient drainage networks mediated a large‐scale genetic introgression in the East Asian freshwater snails

Biogeography and genetic variation of freshwater organisms are influenced not only by current freshwater connections but also by past drainage networks. The Seto Inland Sea is a shallow enclosed sea in Japan, but geological evidence showed that a large freshwater drainage had intermittently appeared in this area between the late Pliocene and Pleistocene. Here, we demonstrated that this paleodrainage greatly affected the genetic variation of the East Asian freshwater snails, Semisulcospira spp. We found that the mtDNA haplotypes originated in the Lake Biwa endemic Semisulcospira species at the upstream side of the paleodrainage were frequently observed in the riverine Semisulcospira species at its downstream side. The genome‐wide DNA and morphological analyses consistently showed that there was no clear evidence of nuclear introgression between the Lake Biwa endemics and riverine species. These results suggest that the large paleodrainage had facilitated mitochondrial introgression and had broadly spread the introgressed mtDNA haplotypes to its downstream region around the Seto Inland Sea. Our study highlights the role of paleodrainages in shaping the genetic variation of freshwater organisms.