Genetic variability in geographic populations of the natterjack toad (Bufo calamita)

Across altitudinal and latitudinal gradients, the proportion of suitable habitats varies, influencing the individual dispersal that ultimately can produce differentiation among populations. The natterjack toad (Bufo calamita) is distributed across a wide geographic range that qualifies the species as interesting for a geographic analysis of its genetic variability. Five populations of B. calamita in the Sierra de Gredos (Spain) were studied in an altitudinal gradient ranging from 750 to 2270 m using microsatellite markers. In addition, we analyzed the latitudinal genetic variation in B. calamita within a global European distribution using genetic diversity parameters (mean number of alleles per locus [M(a)] and expected heterozygosity [H(E)]) obtained from our results and those published in the literature. The low level of genetic differentiation found between populations of B. calamita (F(st) ranging from 0.0115 to 0.1018) and the decreases in genetic diversity with altitude (M(a) from 13.6 to 8.3, H(E) from 0.82 to 0.74) can be interpreted by the combined effects of discontinuous habitat, produced mainly by the high slopes barriers and geographic distance. In the latitudinal gradient, genetic diversity decreases from south to north as a consequence of the colonization of the species from the Pleistocene refugium. We conclude that the genetic variability in B. calamita along its wide altitudinal and latitudinal geographic distribution mainly reflects the colonization history of the species after the last glacial period.     

Gene flow between populations of two invertebrates in springs

1. Using allozymes, we analysed genetic structure of the freshwater gastropod Bythinella dunkeri and the freshwater flatworm Crenobia alpina. The two species are habitat specialists, living almost exclusively in springs. The sampled area in Hesse (Germany) covers a spatial scale of 20 km and includes two river drainages. From the biology of the two species we expected little dispersal along rivers. However, the possibility exists that groundwater provide suitable pathways for dispersal. 2. In B. dunkeri heterozygosity decreased from west to east. For some alleles we found clines in this geographic direction. These clines generated a positive correlation between geographic distance and genetic differentiation. Furthermore patterns of genetic variation within populations suggested that populations may have been faced with bottlenecks and founder effects. If populations are not in population genetic equilibrium, such founder effects would also explain the rather high amount of genetic differentiation between populations (10%). 3. For C. alpina the mean number of alleles decreased with increasing isolation of populations. Genetic differentiation between populations contributed 19% to the total genetic variation. Genetic differentiation was not correlated to geographic distance, but compared with B. dunkeri variability of pairwise differentiation between pairs of populations was higher in C. alpina. 4. Overall B. dunkeri appears to be a fairly good disperser, which may use groundwater as dispersal pathway. Furthermore populations seem to be not in equilibrium. In contrast C. alpina forms rather isolated populations with little dispersal between springs and groundwater seems to play no important role for dispersal.     

Microgeographic variation in Caiman latirostris

In theory, geographic scale is related to genetic variation at the population level, whereas microgeographic scale may reveal intra-population structure such as social groups and families. In the present work, both levels of genetic variation in the broad-snouted caiman (Caiman latirostris) were evaluated in small wetlands associated with the Piracicaba River and some of its tributaries in the state of S?o Paulo, Brazil. Genetic variation was determined using microsatellite DNA markers originally developed for the American alligator (Alligator mississipiensis) and previously tested in pedigreed captive broad-snouted caimans. Using these markers, we were able to detect variability among individuals from different sites, even those within a small geographic distance. Genetic results suggest that the groups sampled at each site are composed predominantly of related individuals. A possible combination of high mortality and low natality rates results in a low number of successfully dispersed individuals per generation. Future studies using a recently constructed Caiman latirostris microsatellite library (Zucoloto et al., 2002) might help us to understand metapopulation processes that may be occurring within this species.     

Does thermal plasticity align with local adaptation? An interspecific comparison of wing morphology in sepsid flies

Although genetic and plastic responses are sometimes considered as unrelated processes, their phenotypic effects may often align because genetic adaptation is expected to mirror phenotypic plasticity if adaptive, but run counter to it when maladaptive. Because the magnitude and direction of this alignment has further consequences for both the tempo and mode of adaptation, they are relevant for predicting an organisms’ reaction to environmental change. To better understand the interplay between phenotypic plasticity and genetic change in mediating adaptive phenotypic variation to climate variability, we here quantified genetic latitudinal variation and thermal plasticity in wing loading and wing shape in two closely related and widespread sepsid flies. Common garden rearing of 16 geographical populations reared across multiple temperatures revealed that wing loading decreases with latitude in both species. This pattern could be driven by selection for increased dispersal capacity in the cold. However, although allometry, sexual dimorphism, thermal plasticity and latitudinal differentiation in wing shape all show similar patterns in the two species, the relationship between the plastic and genetic responses differed between them. Although latitudinal differentiation (south to north) mirrored thermal plasticity (hot to cold) in Sepsis punctum, there was no relationship in Sepsis fulgens. While this suggests that thermal plasticity may have helped to mediate local adaptation in S. punctum, it also demonstrates that genetic wing shape differentiation and its relation to thermal plasticity may be complex and idiosyncratic, even among ecologically similar and closely related species. Hence, genetic responses can, but do not necessarily, align with phenotypic plasticity induced by changing environmental selection pressures.     

Genetic diversity of Stipa grandis P.Smirn populations across the species’ range in the Inner Mongolia Plateau of China

In order to investigate the genetic diversity of dominant species under the background of climate change and grassland utilization in the Inner Mongolia Plateau of China, we sampled seven Stipa grandis populations along an increasing aridity gradient in the present study. The Nei’s gene diversity of populations (He) was estimated to be 0.15 and the percentage of polymorphic loci (PPL) was 49.28%. The genetic differentiation among populations (Φ_ST) was 0.2431. There was a significant relationship between genetic distance and geographic distance among the S. grandis populations by Mantel's test. The genetic diversity was significantly correlated with longitude and annual mean precipitation, which suggested that a combination of climatic factors affected the genetic diversity. The populations in the marginal habitat should be paid more attention because of their low genetic diversity and its significance for conservation of the whole species.     

Population at the edge: increased divergence but not inbreeding towards northern range limit in Acer campestre

Ecological conditions shape natural distribution of plants. Populations are denser in optimal habitats but become more fragmented in the areas of suboptimal environmental conditions. Usually, fragmentation increases towards the limits of species distribution. Fragmented populations are often characterised by decreased genetic variation, and this effect is frequent in peripheral populations, mostly due to the reduced effective population size. Interestingly, the genetic consequences of fragmentation seem to be relatively weak in forest trees. Using microsatellite markers, we assessed the impact of population fragmentation on the genetic structure of a European tree species Acer campestre. Within the study area, this medium-size wind-dispersed and insect-pollinated tree reveals a gradual decrease in population density towards the northern range limit. Over the distance of 150 km, we detected the significant decrease in allelic richness, heterozygosity as well as an increase in the rate of population divergence along with latitude. On the other hand, we failed to show that the observed patterns of genetic structure result from the variation in population densities. Moreover, inbreeding levels revealed no association with both density and geographic location, suggesting that pollen limitation does not occur, even at the range margin. As we showed that there is no difference in a dispersal scale between low- and high-density populations in the study species, we argue that the genetic structure is a result of postglacial recolonization. However, unlike many other forest trees, A. campestre showed the sharp latitudinal genetic pattern at a very restricted spatial scale. Limited dispersal and high fragmentation are likely the reasons.     

DEPENDENCE OF GENE FLOW ON GEOGRAPHIC DISTANCE IN TWO SOLITARY CORALS WITH DIFFERENT LARVAL DISPERSAL CAPABILITIES

When the level of gene flow among populations depends upon the geographic distance separating them, genetic differentiation is relatively enhanced. Although the larval dispersal capabilities of marine organisms generally correlate with inferred levels of average gene flow, the effect of different modes of larval development on the association between gene flow and geographic distance remains unknown. In this paper, I examined the relationship between gene flow and distance in two co-occurring solitary corals. Balanophyllia elegans broods large, nonfeeding planulae that generally crawl only short distances from their place of birth before settling. In contrast, Paracyathus stearnsii free-spawns and produces small planktonic larvae presumably capable of broad dispersal by oceanic currents. I calculated F-statistics using genetic variation at six (P. stearnsii) or seven (B. elegans) polymorphic allozyme loci revealed by starch gel electrophoresis, and used these F-statistics to infer levels of gene flow. Average levels of gene flow among twelve Californian localities agreed with previous studies: the species with planktonic, feeding larvae was less genetically subdivided than the brooding species. In addition, geographic isolation between populations appeared to affect gene flow between populations in very different ways in the two species. In the brooding B. elegans, gene flow declined with increasing separation, and distance explained 31% of the variation in gene flow. In the planktonically dispersed P. stearnsii distance of separation between populations at the scale studied (10–1000 km) explained only 1% of the variation in gene flow between populations. The mechanisms generating geographic genetic differentiation in species with different modes of larval development should vary fundamentally as a result of these qualitative differences in the dependence of gene flow on distance.     

Genetic variation of chloroplast and nuclear markers in natural populations of hazelnut (Corylus avellana L.) in Germany

Corylus avellana L. (hazel) is a long-lived, monoecious and wind-pollinated shrub species, widespread all over Europe. In Germany, hazel is intensively traded and planted, and thus is of central interest from a nature conservancy point of view. To assess the within- and between-population differentiation of hazel, 20 natural populations (18 from Germany, one from Italy and one from Hungary) were investigated genetically. Seven isozyme systems comprising 11 gene loci were analysed in up to 100 samples (average 92.6) per population, amplified fragment length polymorphisms (AFLP) were analysed in up to 50 samples (average 47.4) and nine cpDNA-SSR markers were assessed in 20 samples per population. Results for overall isozyme variability with Na 2.46 alleles per locus, allelic diversity (Ne) 1.39, expected heterozygosity He 21 % and 79 % polymorphic loci were in accordance with the findings of previous studies. The respective values for AFLPs were lower, but both marker systems revealed the same level of about 3.5 % differentiation between populations. For cpSSR only the Italian sample showed within-population variation and the two haplotypes were completely differentiated from all other populations expressing a unique genetic structure with one single haplotype. Among the three marker systems AFLPs showed the best ability to differentiate between populations. While only one isozyme locus revealed significant differentiation, 41 AFLP loci showed highly significant differentiation between all populations, but 26 loci when only German populations were considered. Consequently geographic differentiation analyses focused mainly on molecular markers. Mantel tests showed significant correlations between genetic and geographic distance, but in the unweighted pair-group method with arithmetic mean analyses, adjacent populations did not always form clusters. While chloroplast markers were able to clearly distinguish only the Hungarian population, the nuclear markers revealed clear spatial genetic structures. The correlations between geographic and genetic distance was high for AFLPs. The correlograms illustrate this effect for all populations as well as for the German populations.     

Latitudinal divergence of common frog (Rana temporaria) life history traits by natural selection: evidence from a comparison of molecular and quantitative genetic data

The relative roles of natural selection and direct environmental induction, as well as of natural selection and genetic drift, in creating clinal latitudinal variation in quantitative traits have seldom been assessed in vertebrates. To address these issues, we compared molecular and quantitative genetic differentiation between six common frog (Rana temporaria) populations along an approximately 1600 km long latitudinal gradient across Scandinavia. The degree of population differentiation (QST approximately 0.81) in three heritable quantitative traits (age and size at metamorphosis, growth rate) exceeded that in eight (neutral) microsatellite loci (FST = 0.24). Isolation by distance was clear for both neutral markers and quantitative traits, but considerably stronger for one of the three quantitative traits than for neutral markers. QST estimates obtained using animals subjected to different rearing conditions (temperature and food treatments) revealed some environmental dependency in patterns of population divergence in quantitative traits, but in general, these effects were weak in comparison to overall patterns. Pairwise comparisons of FST and QST estimates across populations and treatments revealed that the degree of quantitative trait differentiation was not generally predictable from knowledge of that in molecular markers. In fact, both positive and negative correlations were observed depending on conditions where the quantitative genetic variability had been measured. All in all, the results suggest a very high degree of genetic subdivision both in neutral marker genes and genes coding quantitative traits across a relatively recently (< 9000 years) colonized environmental gradient. In particular, they give evidence for natural selection being the primary agent behind the observed latitudinal differentiation in quantitative traits.     

Effects of geography and life history traits on genetic differentiation in benthic marine fishes

Dispersal of planktonic larvae can create connections between geographically separated adult populations of benthic marine animals. How geographic context and life history traits affect these connections is largely unresolved. We use data from genetic studies (species level F_ST) of benthic teleost fishes combined with linear models to evaluate the importance of transitions between biogeographic regions, geographic distance, egg type (benthic or pelagic eggs), pelagic larval duration (PLD), and type of genetic marker as factors affecting differentiation within species. We find that transitions between biogeographic regions and egg type are significant and consistent contributors to population genetic structure, whereas PLD does not significantly explain population structure. Total study distance frequently contributes to significant interaction terms, particularly in association with genetic markers, whereby F_ST increases with study distance for studies employing mtDNA sequences, but allozyme and microsatellite studies show no increase in F_ST with study distance. These results highlight the importance of spatial context (biogeography and geographic distance) in affecting genetic differentiation and imply that there are inherent differences in dispersal ability associated with egg type. We also find that the geographic distance over which the maximum pairwise F_ST between populations occurs (relative to total study distance) is highly variable and can be observed at any scale. This result is consistent with stochastic processes inflating genetic differentiation and/or insufficient consideration of geographic and biological factors relevant to connectivity.     

Geographic variation in body size, sexual size dimorphism and fitness components of a seed beetle: local adaptation versus phenotypic plasticity

Variation in body size, growth and life history traits of ectotherms along latitudinal and altitudinal clines is generally assumed to represent adaptation to local environmental conditions, especially adaptation to temperature. However, the degree to which variation along these clines is due to adaptation vs plasticity remains poorly understood. In addition, geographic patterns often differ between females and males – e.g. sexual dimorphism varies along latitudinal clines, but the extent to which these sex differences are due to genetic differences between sexes vs sex differences in plasticity is poorly understood. We use common garden experiments (beetles reared at 24, 30 and 36°C) to quantify the relative contribution of genetically‐based differentiation among populations vs phenotypic plasticity to variation in body size and other traits among six populations of the seed‐feeding beetle Stator limbatus collected from various altitudes in Arizona, USA. We found that temperature induces substantial plasticity in survivorship, body size and female lifetime fecundity, indicating that developmental temperature significantly affects growth and life history traits of S. limbatus. We also detected genetic differences among populations for body size and fecundity, and genetic differences among populations in thermal reaction norms, but the altitude of origin (and hence mean temperature) does not appear to explain these genetic differences. This and other recent studies suggest that temperature is not the major environmental factor that generates geographic variation in traits of this species. In addition, though there was no overall difference in plasticity of body size between males and females (when averaged across populations), we did find that the degree to which dimorphism changed with temperature varied among populations. Consequently, future studies should be extremely cautious when using only a few study populations to examine environmental effects on sexual dimorphism.     

Environmental versus geographical effects on genomic variation in wild soybean (Glycine soja) across its native range in northeast Asia

A fundamental goal in evolutionary biology is to understand how various evolutionary factors interact to affect the population structure of diverse species, especially those of ecological and/or agricultural importance such as wild soybean (Glycine soja). G. soja, from which domesticated soybeans (Glycine max) were derived, is widely distributed throughout diverse habitats in East Asia (Russia, Japan, Korea, and China). Here, we utilize over 39,000 single nucleotide polymorphisms genotyped in 99 ecotypes of wild soybean sampled across their native geographic range in northeast Asia, to understand population structure and the relative contribution of environment versus geography to population differentiation in this species. A STRUCTURE analysis identified four genetic groups that largely corresponded to the geographic regions of central China, northern China, Korea, and Japan, with high levels of admixture between genetic groups. A canonical correlation and redundancy analysis showed that environmental factors contributed 23.6% to population differentiation, much more than that for geographic factors (6.6%). Precipitation variables largely explained divergence of the groups along longitudinal axes, whereas temperature variables contributed more to latitudinal divergence. This study provides a foundation for further understanding of the genetic basis of climatic adaptation in this ecologically and agriculturally important species.     

Exploring wild genetic resources of Musa acuminata Colla distributed in the humid forests of southern Western Ghats of peninsular India using ISSR markers

Musa acuminata ssp. burmannica, one of the wild progenitors contributing 'A genome' to the present-day dessert bananas, has a long evolutionary history intervened by human activities. In this study, ISSR markers were used to analyze the pattern of genetic variation and differentiation in 32 individuals along with two reference samples (viz., Musa acuminata ssp. burmannicoides, var. Calcutta 4 and Musa balbisiana) of wild Musa, which corresponded to three populations across the biodiversity-rich hot spot of southern Western Ghats of India. High levels of genetic diversity were revealed both at the species and population levels, using Nei's diversity indices. The hierarchical analysis of molecular variance showed pronounced genetic differentiation, as 96?% of the total variance was fixed within population and only 4?% among populations. Nei's genetic differentiation coefficient (G (ST)?=?0.1823) and low gene flow (Nm?=?1.18) further confirmed this. The positive correlation (Mantel test) between geographic distance and genetic distance (r?=?0.338 P?相似文献   

Environmental correlates of geographic divergence in a phenotypic trait: A case study using bat echolocation

Divergence in phenotypic traits may arise from the interaction of different evolutionary forces, including different kinds of selection (e.g., ecological), genetic drift, and phenotypic plasticity. Sensory systems play an important role in survival and reproduction, and divergent selection on such systems may result in lineage diversification. Such diversification could be largely influenced by selection in different environments as a result of isolation by environment (IbE). We investigated this process using geographic variation in the resting echolocation frequency of the horseshoe bat species, Rhinolophus damarensis, as a test case. Bats were sampled along a latitudinal gradient ranging from 16°S to 32°S in the arid western half of southern Africa. We measured body size and peak resting frequencies (RF) from handheld individual bats. Three hypotheses for the divergence in RF were tested: (1) James’ Rule, (2) IbE, and (3) genetic drift through isolation by distance (IbD) to isolate the effects of body size, local climatic conditions, and geographic distance, respectively, on the resting frequency of R. damarensis. Our results did not support genetic drift because there was no correlation between RF variation and geographic distance. Our results also did not support James' Rule because there was no significant relationship between (1) geographic distances and RF, (2) body size and RF, or (3) body size and climatic variables. Instead, we found support for IbE in the form of a correlation between RF and both region and annual mean temperature, suggesting that RF variation may be the result of environmental discontinuities. The environmental discontinuities coincided with previously reported genetic divergence. Climatic gradients in conjunction with environmental discontinuities could lead to local adaptation in sensory signals and directed dispersal such that gene flow is restricted, allowing lineages to diverge. However, our study cannot exclude the role of processes like phenotypic plasticity in phenotypic variation.     

RELATIONSHIPS BETWEEN INFERRED LEVELS OF GENE FLOW AND GEOGRAPHIC DISTANCE IN A PHILOPATRIC CORAL,BALANOPHYLLIA ELEGANS

When the dispersal capability of a species is considerably less than its geographic range, genetic differences between populations should increase with the distance separating those populations. This pattern should be most evident in linearly distributed species. The sessile solitary cup coral Balanophyllia elegans lives along nearly the entire Pacific coast of North America, yet its crawling larvae usually settle within 40 cm of their birthplace. In this paper, I document geographic patterns of allozyme differentiation within and among populations of B. elegans and estimate the proportion of observed geographic pattern attributable to gene flow between adjacent populations. Genetic subdivision among localities separated by up to 3000 km was high (F_ST = 0.283, SE = 0.038). Inferred gene flow between pairs of localities (, individuals per generation) correlated inversely with the geographic distance between those localities, consistent with the pattern expected for a species at equilibrium in which gene flow occurred exclusively between adjacent localities. Within localities, patches separated by 4 to 30 m were also significantly subdivided, but genetic differentiation between patches did not vary significantly with the distance separating them. Simulations revealed that the power to detect genetic pattern expected from gene flow between adjacent populations increased with both the number of loci used to infer gene flow and the heterozygosity of those loci. Simulations also verified that when geographic distance poorly approximated the number of steps between populations, reduced major-axis regression more accurately portrayed the structural relationship between gene flow and separation than did ordinary least-squares regression. Attenuation of gene flow with distance explained 15% of the between-locality pattern of genetic differentiation in B. elegans. The remaining variation appeared to be due to neither natural selection nor a recent rangewide recolonization. Loci from the northern sampled localities, however, had fewer alleles than those from the remainder of the range, suggesting these localities had been recolonized recently following Pleistocene cooling.     

Population genetic differences along a latitudinal cline between original and recently colonized habitat in a butterfly

BACKGROUND: Past and current range or spatial expansions have important consequences on population genetic structure. Habitat-use expansion, i.e. changing habitat associations, may also influence genetic population parameters, but has been less studied. Here we examined the genetic population structure of a Palaeartic woodland butterfly Pararge aegeria (Nymphalidae) which has recently colonized agricultural landscapes in NW-Europe. Butterflies from woodland and agricultural landscapes differ in several phenotypic traits (including morphology, behavior and life history). We investigated whether phenotypic divergence is accompanied by genetic divergence between populations of different landscapes along a 700 km latitudinal gradient. METHODOLOGY/PRINCIPAL FINDINGS: Populations (23) along the latitudinal gradient in both landscape types were analyzed using microsatellite and allozyme markers. A general decrease in genetic diversity with latitude was detected, likely due to post-glacial colonization effects. Contrary to expectations, agricultural landscapes were not less diverse and no significant bottlenecks were detected. Nonetheless, a genetic signature of recent colonization is reflected in the absence of clinal genetic differentiation within the agricultural landscape, significantly lower gene flow between agricultural populations (3.494) than between woodland populations (4.183), and significantly higher genetic differentiation between agricultural (0.050) than woodland (0.034) pairwise comparisons, likely due to multiple founder events. Globally, the genetic data suggest multiple long distance dispersal/colonization events and subsequent high intra- and inter-landscape gene flow in this species. Phosphoglucomutase deviated from other enzymes and microsatellite markers, and hence may be under selection along the latitudinal gradient but not between landscape types. Phenotypic divergence was greater than genetic divergence, indicating directional selection on some flight morphology traits. MAIN CONCLUSIONS/SIGNIFICANCE: Clinal differentiation characterizes the population structure within the original woodland habitat. Genetic signatures of recent habitat expansion remain, notwithstanding high gene flow. After differentiation through drift was excluded, both latitude and landscape were significant factors inducing spatially variable phenotypic variation.     

Isolation-by-distance in landscapes: considerations for landscape genetics

In landscape genetics, isolation-by-distance (IBD) is regarded as a baseline pattern that is obtained without additional effects of landscape elements on gene flow. However, the configuration of suitable habitat patches determines deme topology, which in turn should affect rates of gene flow. IBD patterns can be characterized either by monotonically increasing pairwise genetic differentiation (for example, F_ST) with increasing interdeme geographic distance (case-I pattern) or by monotonically increasing pairwise genetic differentiation up to a certain geographical distance beyond which no correlation is detectable anymore (case-IV pattern). We investigated if landscape configuration influenced the rate at which a case-IV pattern changed to a case-I pattern. We also determined at what interdeme distance the highest correlation was measured between genetic differentiation and geographic distance and whether this distance corresponded to the maximum migration distance. We set up a population genetic simulation study and assessed the development of IBD patterns for several habitat configurations and maximum migration distances. We show that the rate and likelihood of the transition of case-IV to case-I F_ST–distance relationships was strongly influenced by habitat configuration and maximum migration distance. We also found that the maximum correlation between genetic differentiation and geographic distance was not related to the maximum migration distance and was measured across all deme pairs in a case-I pattern and, for a case-IV pattern, at the distance where the F_ST–distance curve flattens out. We argue that in landscape genetics, separate analyses should be performed to either assess IBD or the landscape effects on gene flow.     

Genetic diversity and structure of Cordyceps sinensis populations from extensive geographical regions in China as revealed by inter-simple sequence repeat markers

Cordyceps sinensis is one of the most valuable medicinal caterpillar fungi native to China. However, its productivity is extremely limited and the species is becoming endangered. The genetic diversity of eighteen C. sinensis populations across its major distributing regions in China was evaluated by inter-simple sequence repeat (ISSR) markers. A total of 141 markers were produced in 180 individuals from the 18 populations, of which 99.3% were polymorphic. The low average of Shannon (0.104) and Nei index (0.07) of the 18 populations indicates that there are little genetic variations within populations. For all 18 populations, estimates of total gene diversity (H_T), gene diversity within populations (H_S), coefficient of genetic differentiation (G_ST), and gene flow (Nm) were 0.170, 0.071, 0.583, and 0.357, respectively. This pattern suggests that the genetic diversity of C. sinensis is low and most of the ISSR variations are found among populations with little gene exchange. The 18 populations are divided into five groups based on the genetic distance and the grouping pattern matches with the geographic distribution along the latitudinal gradient. The five groups show obvious difference in the G_ST and Nm values. Therefore, the genetic diversification of C. sinensis populations may be determined by geographic isolation and the combined effects of life history characters and the interaction with host insect species. The information illustrated by this study is useful for selecting in situ conservation sites of C. sinensis.     

Genetic diversity of the natural populations of Arabidopsis thaliana in China

Although extensive studies have been conducted on the genetic structure of Arabidopsis thaliana (A. thaliana) populations worldwide, the populations from China have never been studied. In this study, we collected 560 individuals from 19 natural populations of A. thaliana distributed in East China along the lower reaches of the Yangtze River, and two populations from northwest China (Xinjiang Province). We adopted two kinds of molecular marker, inter-simple sequence repeats (ISSRs) and random amplified polymorphic DNA (RAPDs) to investigate the genetic diversity within and among populations, and the correlation between the genetic and geographic distances. Thirteen ISSR primers produced 165 polymorphic bands (PPB) (96%) and 11 RAPD primers produced 162 polymorphic bands (98%) in about 560 individuals. The two marker systems generated similar patterns of genetic diversity in these natural populations. The AMOVA analysis indicated about 42-45% of the total genetic variation existed within populations, and found possible geographic structure. The Mantel test revealed a significant correlation between the geographic distance and the genetic distance of these populations in general. A close genetic relationship was found among four populations in the Jiangxi Province, and these always appeared clustered together as a monophyletic group in unweighted pair-group method with arithmetic averages dendrograms based on both ISSR and RAPD data sets. Based on the observation of recolonization and extinction of naturally distributed populations of A. thaliana, and the pattern of their genetic differentiation, the distribution of this species in China might be a result of natural dispersal under the strong influence of human activity.     

RELATIONSHIP OF GENETIC DIVERSITY AND NICHE CENTRALITY: A SURVEY AND ANALYSIS

The distribution of genetic diversity within and among populations in relation to species’ geographic ranges is important to understanding processes of evolution, speciation, and biogeography. One hypothesis predicts that natural populations at geographic range margins will have lower genetic diversity relative to those located centrally in species’ distributions owing to a link between geographic and environmental marginality; alternatively, genetic variation may be unrelated with geographic marginality via decoupling of geographic and environmental marginality. We investigate the predictivity of geographic patterns of genetic variation based on geographic and environmental marginality using published genetic diversity data for 40 species (insects, plants, birds, mammals, worms). Only about half of species showed positive relationships between geographic and environmental marginality. Three analyses (sign test, multiple linear regression, and meta‐analysis of correlation effect sizes) showed a negative relationship between genetic diversity and distance to environmental niche centroid, but no consistent relationship of genetic diversity with distance to geographic range center.