Bottlenecks, drift and differentiation: the fragmented population structureof the saltmarsh beetle Pogonus chalceus

We investigated the distribution of genetic variation within and between 10 fragmented populations of the saltmarsh beetle Pogonus chalceus in the region of Flanders (Belgium) representing all extant populations of the species in that region by using allozyme and microsatellite markers. Beetle population size and habitat area failed to explain a significant part of the genetic variability. Microsatellite allelic diversity was sensitive to population size differences but not to saltmarsh area estimates. Heterozygosities of both marker types and allozyme allelic diversity on the other hand showed no significant correlation to population size and saltmarsh area. There was also no correlation between geographical and genetic distances among populations. Evidence was found for past bottlenecks in some of the smallest populations. Maximum likelihood methods using the coalescent approach revealed that the proportion of common ancestors was also high in those small populations. 35% of our studied individuals, especially in the largest populations showed a relative wing size smaller than 70%. Moreover, only six out of the 10 studied populations showed a few individuals with functional flight musculature. In conclusion, the overall pattern of distribution of genetic variation and the low flight capacity did not support an equilibrium model of population structure in P. chalceus, but mainly suggested a lack of regional equilibrium with both drift and gene flow influences.     

Limited dispersal and its effect on population structure in the milkweed beetle Tetraopes tetraophthalmus

Summary The movement patterns of adult milkweed beetles, Tetraopes tetraphthalmus, were monitored via a mark-recapture technique. Movement or dispersal patterns were studied in two natural populations, one in which the host plant, Asclepias syriaca, was nearly continuously distributed over a 250×90 m area and another where Asclepias was distributed in 17 small discrete patches. In both populations dispersal distances resulting from the flight patterns of the adult beetles were quite short, averaging less than 40 m from the point of first encounter 10 days after marking. Males were shown to be more vagile than females. The distribution of dispersal distances collected from one of the populations was fit to three statistical distributions cited in the literature as expected from dispersal by many small-scale movements or observed in other species. It was found that an equation describing an exponential decay gave the best statistical fit to the data collected here for milkweed beetles. The data is discussed in the context of the effects of the limited dispersal power of the beetles and the distribution of suitable habitat on the population structure of Tetraopes.     

A seascape genetic analysis reveals strong biogeographical structuring driven by contrasting processes in the polyploid saltmarsh species Puccinellia maritima and Triglochin maritima

Little is known about the processes shaping population structure in saltmarshes. It is expected that the sea should act as a powerful agent of dispersal. Yet, in contrast, import of external propagules into a saltmarsh is thought to be small. To determine the level of connectivity between saltmarsh ecosystems at a macro‐geographical scale, we characterized and compared the population structure of two polyploid saltmarsh species, Puccinellia maritima and Triglochin maritima based on a seascape genetics approach. A discriminant analysis of principal components highlighted a genetic structure for both species arranged according to a regional pattern. Subsequent analysis based on isolation‐by‐distance and isolation‐by‐resistance frameworks indicated a strong role of coastal sediment transport processes in delimiting regional structure in P. maritima, while additional overland propagule dispersal was indicated for T. maritima. The identification and comparison of regional genetic structure and likely determining factors presented here allows us to understand the biogeographical units along the UK coast, between which barriers to connectivity occur not only at the species level but at the ecosystem scale. This information is valuable in plant conservation and community ecology and in the management and restoration of saltmarsh ecosystems.     

Spatial extent of analysis influences observed patterns of population genetic structure in a widespread darter species (Percidae)

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Population size,center–periphery,and seed dispersers’ effects on the genetic diversity and population structure of the Mediterranean relict shrub Cneorum tricoccon

The effect of population size on population genetic diversity and structure has rarely been studied jointly with other factors such as the position of a population within the species’ distribution range or the presence of mutualistic partners influencing dispersal. Understanding these determining factors for genetic variation is critical for conservation of relict plants that are generally suffering from genetic deterioration. Working with 16 populations of the vulnerable relict shrub Cneorum tricoccon throughout the majority of its western Mediterranean distribution range, and using nine polymorphic microsatellite markers, we examined the effects of periphery (peripheral vs. central), population size (large vs. small), and seed disperser (introduced carnivores vs. endemic lizards) on the genetic diversity and population structure of the species. Contrasting genetic variation (H_E: 0.04–0.476) was found across populations. Peripheral populations showed lower genetic diversity, but this was dependent on population size. Large peripheral populations showed high levels of genetic diversity, whereas small central populations were less diverse. Significant isolation by distance was detected, indicating that the effect of long‐distance gene flow is limited relative to that of genetic drift, probably due to high selfing rates (F_IS = 0.155–0.887), restricted pollen flow, and ineffective seed dispersal. Bayesian clustering also supported the strong population differentiation and highly fragmented structure. Contrary to expectations, the type of disperser showed no significant effect on either population genetic diversity or structure. Our results challenge the idea of an effect of periphery per se that can be mainly explained by population size, drawing attention to the need of integrative approaches considering different determinants of genetic variation. Furthermore, the very low genetic diversity observed in several small populations and the strong among‐population differentiation highlight the conservation value of large populations throughout the species’ range, particularly in light of climate change and direct human threats.     

Heritability of wing development and body size in a carabid beetle,Pogonus chalceus MARSHAM,and its evolutionary significance

Summary The wing-polymorphic ground beetle Pogonus chalceus MARSHAM was subjected to crossbreeding experiments under different laboratory conditions in order to estimate the genetic and environmental contributions to the total phenotypic variance in different morphological traits related to relative wing development and body size. Heritability of relative wing development appears to be strong. Beetle size also seems genetically determined in some cases, but separation of male and female parent contribution invariably shows a maternal effect. These results are tested in a breeding experiment with a high number of progeny from one parental pair, reared at two temperatures and at two levels of food supply. Relative wing development is not influenced by these environmental conditions, as expected, but different temperatures add significant variance to the body size values. The experimental results are used to explain interdemic variation in these morphological traits, as observed in three isolated field populations. The reproductive effort under optimum breeding conditions is higher in macropterous beetles than in beetles with reduced wings, but this could result from their larger body size. Migtion seems to be the most plausible underlying evolutionary mechanism for the observed wing reduction in older populations.     

Asymmetric gene flow and the evolutionary maintenance of genetic diversity in small,peripheral Atlantic salmon populations

Small populations may be expected to harbour less genetic variation than large populations, but the relation between census size (N), effective population size (N _e), and genetic diversity is not well understood. We compared microsatellite variation in four small peripheral Atlantic salmon populations from the Iberian peninsula and three larger populations from Scotland to test whether genetic diversity was related to population size. We also examined the historical decline of one Iberian population over a 50-year period using archival scales in order to test whether a marked reduction in abundance was accompanied by a decrease in genetic diversity. Estimates of effective population size (N _e) calculated by three temporal methods were consistently low in Iberian populations, ranging from 12 to 31 individuals per generation considering migration, and from 38 to 175 individuals per generation if they were regarded as closed populations. Corresponding N _e/N ratios varied from 0.02 to 0.04 assuming migration (mean=0.03) and from 0.04 to 0.18 (mean=0.10) assuming closed populations. Population bottlenecks, inferred from the excess of heterozygosity in relation to allelic diversity, were detected in all four Iberian populations, particularly in those year classes derived from a smaller number of returning adults. However, despite their small size and declining status, Iberian populations continue to display relatively high levels of heterozygosity and allelic richness, similar to those found in larger Scottish populations. Furthermore, in the R. Asón no evidence was found for a historical loss of genetic diversity despite a marked decline in abundance during the last five decades. Thus, our results point to two familiar paradigms in salmonid conservation: (1)␣endangered populations can maintain relatively high levels of genetic variation despite their small size, and (2) marked population declines may not necessarily result in a significant loss of genetic diversity. Although there are several explanations for such results, microsatellite data and physical tagging suggest that high levels of dispersal and asymmetric gene flow have probably helped to maintain genetic diversity in these peripheral populations, and thus to avoid the negative consequences of inbreeding.     

Fine‐scale Spatial Genetic Structure of Ten Dipterocarp Tree Species in a Bornean Rain Forest

Fine‐scale spatial genetic structure is increasingly recognized as an important factor in the studies of tropical forest trees as it influences genetic diversity of local populations. The biologic mechanisms that generate fine‐scale spatial genetic structure are not fully understood. We studied fine‐scale spatial genetic structure in ten coexisting dipterocarp tree species in a Bornean rain forest using microsatellite markers. Six of the ten species showed statistically significant fine‐scale spatial genetic structure. Fine‐scale spatial genetic structure was stronger at smaller spatial scales (≤ 100 m) than at larger spatial scales (> 100 m) for each species. Multiple regression analysis suggested that seed dispersal distance was important at the smaller spatial scale. At the larger scale (> 100 m) and over the entire sample range (0–1000 m), pollinators and spatial distribution of adult trees were more important determinants of fine‐scale spatial genetic structure. Fine‐scale spatial genetic structure was stronger in species pollinated by less mobile small beetles than in species pollinated by the more mobile giant honeybee (Apis dorsata). It was also stronger in species where adult tree distributions were more clumped. The hypothesized mechanisms underlying the negative correlation between clump size and fine‐scale spatial genetic structure were a large overlap among seed shadows and genetic drift within clumped species.     

Evolutionary history of a dispersal‐associated locus across sympatric and allopatric divergent populations of a wing‐polymorphic beetle across Atlantic Europe

Studying the evolutionary history of trait divergence, in particular those related to dispersal capacity, is of major interest for the process of local adaptation and metapopulation dynamics. Here, we reconstruct the evolution of different alleles at the nuclear‐encoded mitochondrial NADP⁺‐dependent isocitrate dehydrogenase (mtIdh) locus of the ground beetle Pogonus chalceus that are differentially and repeatedly selected in short‐ and long‐winged populations in response to different hydrological regimes at both allopatric and sympatric scales along the Atlantic European coasts. We sequenced 2788 bp of the mtIdh locus spanning a ~7‐kb genome region and compared its variation with that of two supposedly neutral genes. mtIdh sequences show (i) monophyletic clustering of the short‐winged associated mtIDH‐DE haplotypes within the long‐winged associated mtIDH‐AB haplotypes, (ii) a more than tenfold lower haplotype diversity associated with the mtIDH‐DE alleles compared to the mtIDH‐AB alleles and (iii) a high number of fixed nucleotide differences between both mtIDH haplotype clusters. Coalescent simulations suggest that this observed sequence variation in the mtIdh locus is most consistent with a singular origin in a partially isolated subpopulation, followed by a relatively recent spread of the mtIDH‐DE allele in short‐winged populations along the Atlantic coast. These results demonstrate that even traits associated with decreased dispersal capacity can rapidly spread and that reuse of adaptive alleles plays an important role in the adaptive potential within this sympatric mosaic of P. chalceus populations.     

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.     

Impact of river dynamics on the genetic variation of Gypsophila repens (Caryophyllaceae): a comparison of heath forest and more dynamic gravel bank populations along an alpine river

• Alpine rivers are, despite anthropogenic water flow regulation, still often highly dynamic ecosystems. Plant species occurring along these rivers are subject to ecological disturbance, mainly caused by seasonal flooding. Gypsophila repens typically grows at higher altitudes in the Alps, but also occurs at lower altitudes on gravel banks directly along the river and in heath forests at larger distances from the river. Populations on gravel banks are considered non‐permanent and it is assumed that new individuals originate from seed periodically washed down from higher altitudes. Populations in heath forests are, in contrast, permanent and not regularly provided with seeds from higher altitudes through flooding. If the genetic structure of this plant species is strongly affected by gene flow via seed dispersal, then higher levels of genetic diversity in populations but less differentiation among populations on gravel banks than in heath forests can be expected.
• In this study, we analysed genetic diversity within and differentiation among 15 populations of G. repens from gravel banks and heath forests along the alpine River Isar using amplified fragment length polymorphisms (AFLP).
• Genetic diversity was, as assumed, slightly higher in gravel bank than in heath forest populations, but genetic differentiation was, in contrast to our expectations, comparable among populations in both habitat types.
• Our study provides evidence for increased genetic diversity under conditions of higher ecological disturbance and increased seed dispersal on gravel banks. Similar levels of genetic differentiation among populations in both habitat types can be attributed to the species' long lifetime, a permanent soil seed bank and gene flow by pollinators among different habitats/locations.

     

Rural–urban gradients and the population genetic structure of woodland ground beetles

Genetic diversity and differentiation of two carabid beetle species were examined in woodlands along rural–urban gradients in two cities (Brussels, Belgium, and Birmingham, UK), based on allozymes, studied in more than 1000 beetles. Compared to Abax ater, Pterostichus madidus showed higher levels of genetic diversity but lower genetic differentiation, probably because of its ability to survive in non-forest habitats. Higher genetic diversity in both species was observed in Brussels as compared to Birmingham. However, genetic differentiation among sites was higher in Birmingham corresponding to the more extreme degree of fragmentation and isolation between the Birmingham woodlands. The isolation-by-distance model did not explain genetic differentiation among sites within the two regions. Gene diversity in P. madidus Birmingham populations was higher in smaller urban forests with a small perimeter. A similar absence of genetic erosion in smaller and more highly modified (urban) populations was also obtained for A. ater populations from Brussels, with a higher genetic diversity in sites closer to woodland edges. This unexpected result is hypothesised to be the result of an adaptive increase of genetic diversity in more heterogeneous landscapes, closer to woodland edges and in smaller and more perturbed forests.     

Population structure and genetic variation in the endangered Giant Kangaroo Rat (<Emphasis Type="Italic">Dipodomys ingens</Emphasis>)

Populations of the endangered giant kangaroo rat, Dipodomys ingens (Heteromyidae), have suffered increasing fragmentation and isolation over the recent past, and the distribution of this unique rodent has become restricted to 3% of its historical range. Such changes in population structure can significantly affect effective population size and dispersal, and ultimately increase the risk of extinction for endangered species. To assess the fine-scale population structure, gene flow, and genetic diversity of remnant populations of Dipodomys ingens, we examined variation at six microsatellite DNA loci in 95 animals from six populations. Genetic subdivision was significant for both the northern and southern part of the kangaroo rat’s range although there was considerable gene flow among southern populations. While regional gene diversity was relatively high for this endangered species, hierarchical F-statistics of northern populations in Fresno and San Benito counties suggested non-random mating and genetic drift within subpopulations. We conclude that effective dispersal, and therefore genetic distances between populations, is better predicted by ecological conditions and topography of the environment than linear geographic distance between populations. Our results are consistent with and complimentary to previous findings based on mtDNA variation of giant kangaroo rats. We suggest that management plans for this endangered rodent focus on protection of suitable habitat, maintenance of connectivity, and enhancement of effective dispersal between populations either through suitable dispersal corridors or translocations.     

Isolation and characterization of microsatellite loci in the saltmarsh beetle Pogonus chalceus (Coleoptera,Carabidae)

Five microsatellite loci were isolated from the saltmarsh beetle Pogonus chalceus. Polymorphism ranges from six to 16 alleles, and observed and expected heterozygosities range from 0.437–0.764 and 0.588–0.786, respectively. Most loci cross‐amplified well in four other Pogonus species.     

Genetic Population Structure of Local Populations of the Endangered Saltmarsh Sesarmid Crab Clistocoeloma sinense in Japan

During recent decades, over 40% of Japanese estuarine tidal flats have been lost due to coastal developments. Local populations of the saltmarsh sesarmid crab Clistocoeloma sinense, designated as an endangered species due to the limited suitable saltmarsh habitat available, have decreased accordingly, being now represented as small remnant populations. Several such populations in Tokyo Bay, have been recognised as representing distributional limits of the species. To clarify the genetic diversity and connectivity among local coastal populations of Japanese Clistocoeloma sinense, including those in Tokyo Bay, mitochondrial DNA analyses were conducted in the hope of providing fundamental information for future conservation studies and an understanding of metapopulation dynamics through larval dispersal among local populations. All of the populations sampled indicated low levels of genetic diversity, which may have resulted from recent population bottlenecks or founder events. However, the results also revealed clear genetic differentiation between two enclosed-water populations in Tokyo Bay and Ise-Mikawa Bay, suggesting the existence of a barrier to larval transport between these two water bodies. Since the maintenance of genetic connectivity is a requirement of local population stability, the preservation of extant habitats and restoration of saltmarshes along the coast of Japan may be the most effective measures for conservation of this endangered species.     

How river structure and biological traits influence gene flow: a population genetic study of two stream invertebrates with differing dispersal abilities

1. Determined by landscape structure as well as dispersal‐related traits of species, connectivity influences various key aspects of population biology, ranging from population persistence to genetic structure and diversity. Here, we investigated differences in small‐scale connectivity in terms of gene flow between populations of two ecologically important invertebrates with contrasting dispersal‐related traits: an amphipod (Gammarus fossarum) with a purely aquatic life cycle and a mayfly (Baetis rhodani) with a terrestrial adult stage. 2. We used highly polymorphic markers to estimate genetic differentiation between populations of both species within a Swiss pre‐alpine catchment and compared these results to the broader‐scale genetic structure within the Rhine drainage. Landscape genetic approaches were used to test for correlations of genetic and geographical structures and in‐stream barrier effects. 3. We found overall very weak genetic structure in populations of B. rhodani. In contrast, G. fossarum showed strong genetic differentiation, even at spatial scales of a few kilometres, and a clear pattern of isolation by distance. Genetic diversity decreased from downstream towards upstream populations of G. fossarum, suggesting asymmetric gene flow. Correlation of genetic structure with landscape topography was more pronounced in the amphipod. Our study also indicates that G. fossarum might be capable of dispersing overland in headwater regions and of crossing small in‐stream barriers. 4. We speculate that differences in dispersal capacity but also habitat specialisation and potentially the extent of local adaptation could be responsible for the differences in genetic differentiation found between the two species. These results highlight the importance of taking into account dispersal‐related traits when planning management and conservation strategies.     

Vagility and the Monsoon Rain Forest Archipelago of Northern Australia: Patterns of Genetic Diversity in Syzygium nervosum (Myrtaceae)1

Syzygium nervosum is a common monsoon rain forest tree. Its habitat in Australia consists of small rain forest patches that are scattered through a savanna matrix. It is a mast flowering canopy species that produces large quantities of fruits fed on by mobile frugivores such as birds and fruit bats. The genetic diversity of this species was investigated, especially in relation to rain forest patch size, geographic isolation, and geographic distribution. Syzygium nervosum was found to have high levels of genetic diversity within populations (H_e= 0.307). Diversity among populations, however, was relatively low (Fsr = 0.118), and was not spatially structured across its geographic range in Australia. This is thought to have been caused by relatively frequent gene flow among populations (N_m= 1.67), mediated primarily by mobile frugivores. Genetic diversity was not correlated with patch size or isolation. It is thought that seed dispersal by frugivores has acted to expand the effective population size of this species beyond the individual rain forest patch, and thus has prevented the substantial loss of genetic diversity that otherwise would have been observed. Thus this species is dependent upon these frugivores for the maintenance of its genetic diversity and hence its long-term viability. These results lend support to theories of post-Holocene expansion of rain forest by vagile species in northern Australia.     

The influence of the ability to disperse on generation length and population size in the flour beetle, Tribolium castaneum

Abstract.

• 1 In Tribolium castaneum Herbst, in which dispersal is genetically determined, it is possible to select for strains that are characterized by high and low dispersal, High dispersal (HD) beetles are better adapted than low dispersal (LD) beetles for colonization, as can be seen from differences that exist between the two groups with regard to several life-history parameters.
• 2 Comparisons were made between the dynamics of seventy-two HD and seventy-two LD populations. Generation length in HD populations was significantly shorter than in LD populations. After 9¹/₂ weeks, in open treatments (from which dispersal away from the set was allowed), HD populations had more beetles than LD populations, whereas in closed treatments (from which dispersal away from the set was not allowed) the opposite was true.
• 3 These findings may explain the maintenance of the genetic variability of dispersal behaviour in natural populations of T.castmeurn.

     

High biogeographical and evolutionary value of Canary Island pine populations out of the elevational pine belt: the case of a relict coastal population

Aim Marginal populations are frequently neglected in static views of vegetation types, particularly when defining conservation reserves. The biogeographical and evolutionary importance of a marginal and endangered population of Pinus canariensis is addressed in this study to ascertain the need for conservation action. Diversity loss between adults and offspring and patterns of seed dispersal and recruitment were examined to provide evidence of recent degradation of marginal P. canariensis pinewoods. The scientific basis for the provision of sound conservation policies was investigated by elucidating the factors responsible for significant population structure. Location An isolated low‐density pinewood community confined to the Arguineguin ravine, in south Gran Canaria, Canary Islands. Methods Two cohorts, of centenary trees (those older than 100 years) and young recruits, respectively, were found in a detailed inventory of the pine population in the Arguienguin ravine. Chloroplast and nuclear microsatellites were compared to assess the levels of genetic diversity between adults and recruits. Spatial genetic structure and parentage analysis based on highly polymorphic nuclear and chloroplast microsatellites were examined to test limitations in seed dispersal. The underlying environmental factors that led to a clustering effect in the population were tested using point pattern methodologies. Results Centenary trees retain high levels of genetic diversity and effective population size, suggesting a wider extension of the pinewood forests in the past. A significant loss of genetic diversity was detected between adults and recruits. Pinus canariensis dispersal distances were among the longest ever reported for anemochorous species, suggesting that environmental factors account for recruit clustering. Cluster models showed that recruits tend to aggregate in dry streambeds, where soil and water accumulation favours establishment. Main conclusions Boundary populations of P. canariensis are subjected to fragmentation and reduction in effective population size as a result of human impact. Marginal populations were denser in the past and currently require specific conservation efforts. A severe reduction in genetic diversity compromises the future of present populations. Streambeds appear to play a major role in recruit establishment, but data suggest the absence of limitations to seed dispersal.