Geographic and genetic isolation in spring-associated Eurycea salamanders endemic to the Edwards Plateau region of Texas

Populations of neotenic, spring-associated salamanders of the genus Eurycea occupy discontinuous sites throughout the Edwards Plateau of central Texas and many warrant conservation attention. Here we used DNA sequence data from a nuclear (rag1) and a mitochondrial (ND4) gene to determine (1) the extent of genetic isolation among seven Edwards Plateau Eurycea populations and (2) the relationship between genetic distance and both geographic distance and hydrogeological features. Coalescent-based methods detected little gene flow among the sampled Eurycea populations, and we were unable to reject a model of complete isolation for any pair of populations. These findings were consistent with the relatively high genetic distances we detected among the sampled Eurycea populations (pairwise ϕ_ST ranged from 0.249 to 0.924). We detected a positive correlation between genetic distance and geographic distance, which is consistent with a pattern of isolation by distance. However, while controlling for geographic distance, we did not detect a positive relationship between genetic distance and aquifer or river distance. Thus, we found no evidence that aquifers and/or rivers serve as dispersal corridors among isolated Eurycea populations. Based on these results, we have no evidence that re-colonization of spring sites by migrant salamanders following local extirpation would be likely. Our findings indicate that spring-associated Eurycea salamander populations occupying the Edwards Plateau region are genetically isolated, and that each of these populations should be considered a distinct management unit.     

草履蚧不同寄主和地理种群遗传分化的RAPD分析

应用随机扩增多态性DNA(RAPD)技术对草履蚧保定、石家庄、邯郸16个不同寄主地理种群遗传多样性和种群分化进行研究,结果显示4个RAPD引物共扩增出41个多态性位点,多态位点比率为100%。遗传距离指数在0.701—0.4360,平均为0.2395。其中以邯郸枫杨和邯郸垂丝海棠为寄主的草履蚧种群遗传距离最小(0.0701);以石家庄紫叶李和邯郸木槿为寄主的种群遗传距离最大(0.4360)。遗传一致度系数在0.6466—0.9290。说明草履蚧不同种群遗传多样性丰富并存在遗传差异。聚类分析结果表明草履蚧种群遗传多样性同时受到寄主和地理因素的双重影响,且不同寄主草履蚧种群已产生明显的遗传分化。     

Environmental gradients shape the genetic structure of two medicinal Salvia species in Jordan

• Environmental gradients, and particularly climatic variables, exert a strong influence on plant distribution and, potentially, population genetic diversity and differentiation. Differences in water availability can cause among‐population variation in ecological processes and can thus interrupt populations’ connectivity and isolate them environmentally. The present study examines the effect of environmental heterogeneity on plant populations due to environmental isolation unrelated to geographic distance.
• Using AFLP markers, we analyzed genetic diversity and differentiation among 12 Salvia spinosa populations and 13 Salvia syriaca populations from three phytogeographical regions (Mediterranean, Irano‐Turanian and Saharo‐Arabian) representing the extent of the species’ geographic range in Jordan. Differences in geographic location and climate were considered in the analyses.
• For both species, flowering phenology varied among populations and regions. Irano‐Turanian and Saharo‐Arabian populations had higher genetic diversity than Mediterranean populations, and genetic diversity increased significantly with increasing temperature. Genetic diversity in Salvia syriaca was affected by population size, while genetic diversity responded to drought in S. spinosa. For both species, high levels of genetic differentiation were found as well as two well‐supported phytogeographical groups of populations, with Mediterranean populations clustering in one group and the Irano‐Turanian and Saharo‐Arabian populations in another. Genetic distance was significantly correlated to environmental distance, but not to geographic distance.
• Our data indicate that populations from moist vs. arid environments are environmentally isolated, where environmental gradients affect their flowering phenology, limit gene flow and shape their genetic structure. We conclude that environmental heterogeneity may act as driver for the observed variation in genetic diversity.

     

Analysis of genetic diversity in Glyptosternum maculatum (Sisoridae, Siluriformes) populations with AFLP markers

We determined the genetic diversity of geographic populations from three spawning grounds (Nyang River, Lhasa River, Shetongmon Reach of Yarlung Zangbo River) of Glyptosternum maculatum with amplified fragment length polymorphism (AFLP) markers. Five primer combinations detected 332 products, 51 of them (15.4%) were polymorphic in at least one population. The Shetongmon population was found to be the richest in genetic diversity as was indicated by the percentage of polymorphic loci and heterozygosity, followed by the Nyang population and the Lhasa population. The pair-wise genetic distance between populations were all very close, ranging from 0.0015 to 0.0042 with an average of 0.0024. The genetic distance was not proportional to the geographic distance. The analysis of molecular variance demonstrated that all variation occurred within populations. The average estimated fixation index (F _st) of three populations across all polymorphic loci was −0.0184, indicating the absence of genetic differences among the three sampled populations. The differentiation among populations was not significant, and population structure was weak. Our observations will help identify the genetic relationship among populations as the first approach to understand the genetic diversity of Glyptosternum maculatum.     

Genetic variability and structure of the water vole Arvicola amphibius across four metapopulations in northern Norway

Water vole Arvicola amphibius populations have recently experienced severe decline in several European countries as a consequence of both reduction in suitable habitat and the establishment of the alien predator American mink Neovison vison. We used DNA microsatellite markers to describe the genetic structure of 14 island populations of water vole off the coast of northern Norway. We looked at intra‐ and inter‐population levels of genetic variation and examined the effect of distance among pairs of populations on genetic differentiation (isolation by distance). We found a high level of genetic differentiation (measured by F_ST) among populations overall as well as between all pairs of populations. The genetic differentiation between populations was positively correlated with geographic distance between them. A clustering analysis grouped individuals into 7 distinct clusters and showed the presence of 3 immigrants among them. Our results suggest a small geographic scale for evolutionary and population dynamic processes in our water vole populations.     

MICROSATELLITE MARKERS REVEAL POPULATION GENETIC STRUCTURE OF THE TOXIC DINOFLAGELLATE ALEXANDRIUM TAMARENSE (DINOPHYCEAE) IN JAPANESE COASTAL WATERS1

This is the first report to explore the fine‐scale diversity, population genetic structure, and biogeography of a typical planktonic microbe in Japanese and Korean coastal waters and also to try to detect the impact of natural and human‐assisted dispersals on the genetic structure and gene flow in a toxic dinoflagellate species. Here we present the genetic analysis of Alexandrium tamarense (Lebour) Balech populations from 10 sites along the Japanese and Korean coasts. We used nine microsatellite loci, which varied widely in number of alleles and gene diversity across populations. The analysis revealed that Nei's genetic distance correlated significantly with geographic distance in pair‐wise comparisons, and that there was genetic differentiation in about half of 45 pair‐wise populations. These results clearly indicate genetic isolation among populations according to geographic distance and restricted gene flow via natural dispersal through tidal currents among the populations. On the other hand, high P‐values in Fisher's combined test were detected in five pair‐wise populations, suggesting similar genetic structure and a close genetic relationship between the populations. These findings suggest that the genetic structure of Japanese A. tamarense populations has been disturbed, possibly by human‐assisted dispersal, which has resulted in gene flow between geographically separated populations.     

Patterns of genetic differentiation in two annual bromegrasses,Bromus lanceolatus andB. hordeaceus (Poaceae)

Infraspecific genetic differentiation was analysed in two tetraploid annual bromegrasses,Bromus lanceolatus (from N Africa) andB. hordeaceus (from N Africa and France). Genetic analysis of populations was based on allozyme polymorphisms at 17 loci. Different fixed heterozygous phenotypes were scored in both species, according to their allopolyploid origin. In N Africa, more variation occurred among populations ofB. lanceolatus than ofB. hordeaceus. The variation was not randomly distributed among populations of both species. InB. lanceolatus, differentiation was correlated with climatic variables rather than with geographic distance between populations. Higher correlation of genetic differentiation with geographic distance occurred inB. hordeaceus, particularly at large geographic scale, between French and N African populations. Within each region, the populations appeared weakly genetically differentiated, even when belonging to different subspecies.     

Genetic differentiation among cottontails from isolated playa basins

Protein variation in 182 Sylvilagus floridanus from 19 playa basins in Castro Co., Texas was examined using starch-gel electrophoresis. Heterozygote deficiencies were noted for all populations. This heterozygote deficiency may be due to differential selection against heterozygous individuals over the winter months. Results of F-statistics indicated a significant degree of population differentiation at six loci. Nei's genetic distance between populations ranged from 0.20 to 0.388 and a significant association between genetic distance and linear geographic distance among playas was found. These results suggest that genetic exchange and long-distance dispersal may be hindered by expanses of unsuitable habitat.     

Congruence between environmental parameters, morphology and genetic structure in Australia’s most widely distributed eucalypt, Eucalyptus camaldulensis

Eucalyptus camaldulensis is one of the most widely utilised eucalypts. It is also the only eucalypt that occurs across the Australian continent, playing a key ecological role as fauna habitat and in riverbank stabilisation. Despite its ecological and economic importance, uncertainty remains regarding the delineation of genetic and morphological variants. Nine hundred and ninety trees from 97 populations, representing the species’ geographic range were genotyped using 15 microsatellite loci and patterns of diversity compared with restriction fragment length polymorphisms in 29 of these populations. Both markers showed that despite having a riverine distribution, downstream seed dispersal has had less influence than geographic distance on dispersal patterns. Spatial patterns in the distribution of microsatellite genotypes were compared with environmental parameters and boundaries defined by river systems, drainage basins and proposed subspecies. Significant genetic differences among populations within river systems indicated that rivers should not be treated as a single genetic entity in conservation or breeding programmes. Strong geographic trends were evident with 40% of variation in genetic diversity explained by latitude and moisture index. Isolation by distance and significant correlations between genetic distance and environmental parameters for most loci suggest historical factors have had more influence than selection on current patterns of distribution of genetic diversity. Geographic structuring of molecular variation, together with congruence between genetic and morphological variation indicate that E. camaldulensis should be treated as a number of subspecies rather than a single variable taxon. High levels of genetic diversity and geographic trends in the distribution of variation provide a firm basis for further exploration of the species’ genetic resources.     

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.     

A cline of allozyme variation inAbies mariesii

Genetic variation at 22 allozyme loci was examined for 1,003 trees from 11 isolated natural populations ofAbies mariesii covering all except the southernmost region of its geographic range. Genetic diversity within species (H _es=0.063) was low compared to many other long-lived woody species. Most of the genetic variation is found within populations (G _ST=0.144) despite their isolated distribution. Genetic distance between populations was positively correlated with geographic distance. Genetic diversity within populations was generally low (meanH _ep=0.054), but varied across populations in a clinal fashion such that genetic variation decreased with increasing latitude. These genetic characteristics may reflect the distribution history of this species.     

Population structure of the Dory snapper, Lutjanus fulviflamma, in the western Indian Ocean revealed by means of AFLP fingerprinting

The genetic structure of spatially separated populations of the Dory snapper, Lutjanus fulviflamma, was investigated in seven areas along the East African coast and one area in the Comoros archipelago in the western Indian Ocean, using amplified fragment length polymorphism (AFLP). Phylogenetic and multidimensional scaling analyses did not show any clear clustering of individuals into the spatially separated populations. The analysis of molecular variance clearly showed that the variation was partitioned within populations and not between populations, leading to low genetic differentiation among populations. No clear relationship between genetic distance and geographic distance between populations was observed. These observations suggest that populations of Lutjanus fulviflamma have an open structure and are possibly genetically connected on a large geographic scale in the western Indian Ocean.     

Character displacement in sailfin mollies, <Emphasis Type="Italic">Poecilia latipinna</Emphasis>: allozymes and behavior

Synopsis We analyzed variation in allozymes and mating preferences in 12 populations across much of the range of the sailfin molly, Poecilia latipinna. Sailfin mollies can be sympatric with its sexual parasite Amazon mollies, P. formosa. Amazon mollies must co-exist and mate with bisexual males of closely related species (including sailfin mollies) to induce embryogenesis but inheritance is strictly maternal. Where sailfin and Amazon mollies are sympatric there is evidence of reproductive character displacement as males show a significantly stronger mating preference for sailfin molly females over Amazon mollies compared to preferences of males from allopatric populations. From the allozyme data we found a moderate amount of genetic variation across all populations but this variation did not reveal significant partitioning between sympatric and allopatric populations. Additionally, we found no evidence for isolation by distance as genetic distance was not significantly correlated with geographic distance. While allozyme variation also did not significantly correlate with male mating preferences, there was a significant correlation between male mating preferences and geographic distance. This correlation between mating preferences and geographic distance may have arisen from coevolution with Amazon mollies resulting in reproductive character displacement. Taken together, the distribution of genetic and behavioral variation among sympatric and allopatric populations suggests that behavioral evolution has outpaced evolution at the allozyme loci we examined in P. latipinna.     

Genetic diversity and geographic differentiation analysis of duckweed using inter-simple sequence repeat markers

Duckweed, with rapid growth rate and high starch content, is a new alternate feedstock for bioethanol production. The genetic diversity among 27 duckweed populations of seven species in genus Lemna and Spirodela from China and Vietnam was analyzed by ISSR-PCR. Eight ISSR primers generating a reproducible amplification banding pattern had been screened. 89 polymorphic bands were scored out of the 92 banding patterns of 16 Lemna populations, accounting for 96.74% of the polymorphism. 98 polymorphic bands of 11 Spirodela populations were scored out of 99 banding patterns, and the polymorphism was 98.43%. The genetic distance of Lemna varied from 0.127 to 0.784, and from 0.138 to 0.902 for Spirodela, which indicated a high level of genetic variation among the populations studied. The unweighted pair group method with arithmetic average (UPGMA) cluster analysis corresponded well with the genetic distance. Populations from Sichuan China grouped together and so did the populations from Vietnam, which illuminated populations collected from the same region clustered into one group. Especially, the only one population from Tibet was included in subgroup A2 alone. Clustering analysis indicated that the geographic differentiation of collected sites correlated closely with the genetic differentiation of duckweeds. The results suggested that geographic differentiation had great influence on genetic diversity of duckweed in China and Vietnam at the regional scale. This study provided primary guidelines for collection, conservation, characterization of duckweed resources for bioethanol production etc.     

EVOLUTIONARY AND HISTORICAL ANALYSIS OF PROTEIN VARIATION IN THE BLOTCHED FORMS OF SALAMANDERS OF THE ENSATINA COMPLEX (AMPHIBIA: PLETHODONTIDAE)

Geographic variation in 23 to 29 protein-encoding genetic loci was examined in 48 populations of the Ensatina complex, a “ring species” distributed around the Central Valley of California. The samples span two critical links in the chain of morphologically distinct units: the transition from the unblotched to blotched color pattern types in the vicinity of Lassen Peak, northeastern California, and a geographic gap in the range of the complex in the San Gabriel Mountains, southern California. A general pattern of isolation by distance with a regular buildup of genetic distance correlated with increases in geographic distance characterizes the populations studied, with the exception of a little-differentiated group of populations in the northern Sierra Nevada; this region is postulated to be a zone of genetic reticulation characterized by relatively high gene flow. An adaptively significant color pattern is thought to have spread into the northern Sierra Nevada from the south, but protein variants have been introduced both from the north and the south. Genetic distances across the San Gabriel Mountain gap match expectations from the pattern of buildup of genetic distance as a function of geographic distance elsewhere in the complex. A phylogenetic analysis of the protein data supports the reticulation hypothesis; whereas the southernmost populations currently do constitute a monophyletic assemblage, an “extinction experiment” demonstrates that the distinction could be the result of the recent extinction of populations in a present gap in our sampling. The Ensatina complex appears to be a dynamic entity representing several stages in the evolution of species. It is a ring species, and whereas various taxonomic arrangements are possible, no taxonomic changes are proposed.     

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.     

Allozymic diversity of Allium tricoccum (Ait.) Solander var. burdickii Hanes in isolated populations of Nova Scotia (Canada)

 Allium tricoccumvar. burdickii is a rare species in Nova Scotia, growing in isolated populations. Genetic variation was investigated in three populations using cellulose acetate gel electrophoresis. The results showed that all 13 enzyme loci were polymorphic. An excess of heterozygotes was found in all populations. A total of 29 genotypes were detected in the populations. Very few genotypes were common to two populations and none to the three populations. Genetic diversity among populations was compared with geographic distance and previous work. The comparison of the genetic distance matrix to the geographic distance matrix indicated no relationship. F-statistics suggested that the populations are predominantly maintained through vegetative propagation. Occasional sexual reproduction and gene flow might occur in order to maintain high level of variation among populations. Environmental conditions could also influence population genetic structure as they occur in highly different habitats. Received November 13, 2000 Accepted February 26, 2001     

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.     

Regional structuring of genetic variation in short-lived rock pool populations of Branchipodopsis wolfi (Crustacea: Anostraca)

The genetic structure of three metapopulations of the southern African anostracan Branchipodopsis wolfi was compared by analysing allozyme variation at four loci (PGM, GPI, APK, AAT). In total, 17 local populations from three sites (metapopulations) were analysed from rock pools in south-eastern Botswana ranging from 0.2 to 21 m² in surface area. In three populations we found significant deviations from Hardy-Weinberg (H-W) equilibrium at one or more loci due to heterozygote deficiencies. Genetic variability at one site was significantly lower than at the other sites, which may be linked to a greater incidence of extinction and recolonisation, as the basins at this site are shallower and have shorter hydrocycles. Across all local populations, a significant level of population differentiation was revealed. More than 90% of this variation was explained by differentiation among sites (metapopulations), although this differentiation did not correlate with geographic distance, or with environmental variables. Genetic differentiation among populations within metapopulations was low, but significant at all sites. At only one of the sites was a significantly positive association measured between genetic and geographic distance among local populations. Our data suggest that persistent stochastic events and limited effective long-range dispersal appear to dominate genetic differentiation among populations of B. wolfi inhabiting desert rock pools. The lack of association between geographic distance and genetic or ecological differences between rock pool sites is indicative of historical stochastic events. Low heterozygosity, the significant deviations from H-W equilibrium, and the large inter- but low intra-site differentiation are suggestive of the importance of short-range dispersal. Gene flow between metapopulations of B. wolfi appears to be seriously constrained by distances of 2 km or even less. Received: 28 June 1999 / Accepted: 10 January 2000