Genetic diversity and fitness in peripheral and central populations of the European tree frog Hyla arborea

Genetic diversity is expected to decrease in small and isolated populations as a consequence of founder effects, bottlenecks, inbreeding and genetic drift. In this study we analyse temporal and spatial effects on genetic variation and progeny viability of the European tree frog (Hyla arborea) at two scales. First, the Swedish distribution has been isolated from the continental distribution for more than 8000 thousand years, and secondly, within Sweden, recent habitat alterations that have taken place during this century have increased isolation between local populations. Genetic variation and progeny survival in relation to isolation was studied within the entire Swedish distribution of the tree frog. Allozyme electrophoresis analysis of froglets, sampled across the Swedish distribution, revealed a low overall genetic variation (1.06 alleles/locus) at the protein level in comparison with continental populations (1.54-1.68 alleles/locus). However, egg hatchability (97%) and early larval survival (95%) were not lower than in other parts of the tree frog distribution or in other anuran species. Within the Swedish distribution, early larval survival was lower in isolated breeding ponds than in more central ones. However, no differences in genetic variation were found in relation to isolation. Polymorphism was detected only at a single locus, and was restricted geographically to the eastern part of the Swedish distribution. Bottlenecks due to climatic changes and fragmentation of suitable habitat (primarily natural pastures with ponds) are suggested as possible causes of the low genetic diversity of the Swedish tree frog population.     

Genetic differentiation in Fomitopsis pinicola (Schwarts: Fr.) Karst studied by means of arbitrary primed-PCR

Generic variation within and among one Finnish and three Swedish populations of Fomitopsis pinicola (Schwarts: Fr.) Karst. were studied by amplifying DNA from hap-loid isolates originating from single spore cultures using two arbitrary primers. Analysis offspring from single fruit bodies revealed only three pairs of codominant alleles among 42 variable genetic markers, the remaining 38 segregated independently. Genetic similarity was measured in terms of Euclidean distance. Individuals in the Finnish population tended to form a distinct cluster in the principal component analysis. Variation within and among populations/regions was partitioned by Analysis of Molecular Variance — AMOVA. Within population variation accounted for 91.6% of the total genetic variation. The remaining 7.68% was accounted for by variation between the Finnish population and each of the three Swedish ones. Variation among the Swedish populations accounted for only 0.72% of the total variation. Wright's F^st was 0.17 for all four populations and 0.13 for the three Swedish populations. These relatively low values indicate that there is gene flow among all populations or that they are derived from a common ancestral population. The observed pattern of genetic variation is probably the result of effective spore dispersal and the continuous distribution of this common early successional species.     

IS PINUS SYLVESTRIS RESISTANCE TO PINE TWIST RUST ASSOCIATED WITH FITNESS COSTS OR BENEFITS?

Seven-year-old Pinus sylvestris were studied in two field trials with respect to height growth and injury caused by the fungal pathogen Melampsora pinitorqua. Each trial comprised single-tree progenies from two to 11 parent trees from each of 45 pine populations in northern Sweden (open pollinated) and Finland (control pollinated with a pollen mixture from 22 north Swedish trees). Heritability estimates were in the range of 6-18%. Most of genetic variation in height growth and rust resistance in Swedish populations and in height growth in Finnish populations occurred within populations (86-100%). In populations of Finnish origin variation in rust resistance consisted of more equal among- and within-population components (68% and 32%, respectively). Family genetic correlations between pathogen resistance and tree height the previous year (disease-free environment) were positive among Finnish populations and Swedish coast populations, suggesting that pathogen resistance confers fitness benefits even in the absence of disease, whereas Swedish inland populations showed a negative correlation indicative of fitness costs. Patterns of genetic correlations at the population level tended to be just the reverse compared with those found at the family level. No genotype × trial interactions were detected for any of the examined characters. Prerequisites for establishment of a genecological balance between host and parasite are discussed.     

Microevolutionary Patterns and Molecular Markers: The Genetics of Geographic Variation in Ascaris suum

Molecular markers have been used only rarely to characterize the population genetic structure of nematodes. Published studies have suggested that different taxa may show distinct genetic architectures. Isoenzyme and RAPD markers have been used to investigate geographic variation of Ascaris suum at the level of infrapopulations (nematodes within individual hosts), within localities, and among geographic regions. Independent estimates of genetic differentiation among population samples based on isoenzyme and RAPD data showed similar patterns and substantial correlation. Heterozygote deficiencies within infrapopulations and large values for inbreeding coefficients among infrapopulations suggested that the composition of these populations was not consistent with a model of random recruitment from a large panmictic pool of life-cycle stages. Both isoenzyme and RAPD markers revealed moderate levels of genetic differentiation among samples representing infrapopulations and localities. Of total gene diversity, 9.4% (isoenzyme) and 9.2% (RAPD) was partitioned among infrapopulations. Geographic localities accounted for 7.8% (isoenzyme) and 6.2% (RAPD) of total diversity. Only infrapopulations from the same farm had low levels of differentiation.     

RAPD and morphological analysis of the rare plant species Vicia pisiformis (Fabaceae)

The amount and pattern of genetic variation was surveyed in two Swedish and three Czech populations of the rare perennial forest plant Vicia pisiformis. This species has a mainly easterly-continental European distribution and has few and small populations in Sweden. It is classified as 'vulnerable' on the Swedish Red Data list. Seeds from natural populations were collected and grown under controlled conditions in growth chambers. The variation was estimated in growth and fecundity traits and with Random Amplified Polymorphic DNA, RAPD. Low inter- and intra-population variation in RAPD-markers was found using 11 primers, with a similarity index (Wetton) for the families of 0.98. In contrast, multivariate analysis of variance showed significant morphological differences within and between populations. Also in the univariate ANOVAs, a number of the traits showed significant between-and/or within population differentiation. Cluster analysis for the morphological traits and RAPDs (UPGMA) did not structure the variation of families in accordance with their geographical distance. A Mantel test based on comparisons between Mahalonobfs and Jaccard distance for morphological and RAPD data, respectively, did not reveal any significant correlation between the two matrices. It is concluded that if a genetic conservation program is to be applied on Vicia pisiformis , different sampling strategies are needed to capture morphological vs RAPD variation. This is, to our knowledge, the first investigation that compares RAPD and morphological variation in a threatened plant species.     

Patterns of genetic variation in Gymnadenia conopsea, the fragrant orchid

Gymnadenia conopsea (L.) R. Br., or the fragrant orchid, is one of many plant species negatively influenced by new practices in agriculture and forestry during the last decades. This study describes the level of microsatellite variation within and among 10 Swedish populations of this species. It was not possible to detect strong effects of small population size or fragmentation. In general, the species had high genetic variation within and low genetic divergence among populations, although the correlation between population size and number of alleles was close to significance at the 95% level. Also, a significant isolation by distance effect was observed, indicating the presence of modest restrictions in gene dispersal between the investigated populations.     

Intraspecific molecular variation in the seaweed fly Coelopa frigida consistent with behavioural distinctness of British and Swedish populations

The major aim of this study was to compare the intraspecific variation and genetic structure of the behaviourally distinct British and Swedish populations of the seaweed fly Coelopa frigida. C. frigida has been the subject of intense study into the basis of female choice. The behaviour of British females is consistent with a 'good genes' model, whereas that of the Swedish flies suggests a Fisher process, in which the difference between the former and the latter is defined by female choice increasing offspring viability in 'good genes' models. Through a study of variability in the mitochondrial cytochrome oxidase II gene from more than 600 flies, we show that there is clear differentiation at the molecular level between the two countries' populations, with an FST of > 75% and no shared haplotypes. Tajima's test reveals an excess of rare variants relative to expectation, which, if not the result of selective sweep, indicates either a population expansion or purifying selection against weakly deleterious variants. Within the two populations, substantial subpopulation differentiation is observed in the UK, where there is also evidence of isolation by distance. Swedish populations exhibit lower variability, and no evidence of isolation by distance, with the latter result possibly being related to the continuous distribution of suitable habitat. The pattern of intraspecific variation is explainable by a combination of contemporary and also historical factors. British and Swedish populations may have been descended from at least two separate founding populations during the recolonization of these areas following Pleistocene glaciations.     

Evolution in populations of Swedish sand lizards: genetic differentiation and loss of variability revealed by multilocus DNA fingerprinting

The Swedish sand lizard (Lacerta agilis) is a relict species from the period of warmth following the last glacial episode and has a fragmented distribution in central Sweden and a more continuous distribution in the southern part of the country. We used this model system of colonization–extinction for a study of genetic variability within and among Swedish populations from different parts of the distribution range using multilocus DNA fingerprinting. The results from the Swedish populations are then contrasted with those from a large Hungarian population in the centre of the species geographical distribution range, which is likely to closely resemble the ancestral founding population of Sweden. Swedish populations have a low level of genetic variability compared with the Hungarian reference population, which showed a genetic variability within the range described for outbred populations. Within the Swedish populations, the average bandsharing was 0.61, the mean heterozygosity 0.45 and the estimated number of alleles 2.7. The figures for the Hungarian population were a bandsharing of 0.19, a heterozygosity of 0.89 and an estimated number of alleles of 9.8. A population bottleneck, common to all Swedish sand lizards, is indicated by less than 20% of the alleles in the Hungarian population being retained in the Swedish populations, and higher bandsharing similarity between different Swedish populations (0.33) as opposed to the Hungarian population (0.19). The limited variability found in Swedish sand lizards is strongly subdivided between populations, with an average F_ST of 0.32, indicating a very limited gene flow between the isolated populations, as well as between populations in the region where the sand lizard has a more or less continuous distribution (F_ST = 0.41).     

Colonization, genetic diversity, and evolution in the Swedish sand lizard, Lacerta agilis (Reptilia, Squamata)

The Swedish sand lizard ( Lacerta agilis ) is a relict species from the post-glacial warmth period. From the geological history of this region, and more recent data on population fragmentation due to disturbance by man, it can be surmised that the Swedish sand lizards passed through at least one population bottleneck in relatively recent times. We tested this hypothesis by investigating the amount and structuring of genetic variability in six microsatellite loci in ten lizard populations from different parts of Sweden. We contrasted these data against those from a Hungarian population which we have reason to assume strongly resembles the founder population for Swedish sand lizards. The average number of alleles per locus in Sweden was 3.3, and these alleles were common in almost all populations, whereas the average number of alleles in the Hungarian population was 8.0. Likewise, the level of expected heterozygosity was lower in the Swedish populations (0.45) compared to the Hungarian population (0.70). The lower variability in Swedish populations is probably a consequence of a common population bottleneck during the immigration subsequent to the latest glacial period. The remaining variability is strongly subdivided between populations (F_ST=0.30) with the main genetic differences being between rather than within populations. Despite the marked isolation of the populations and the present small population sizes (N= 10–300 adults), the Swedish relict populations show a surprisingly high level of observed heterozygosity, indicating that small population size is probably a recent phenomenon.     

Higher genetic variation estimated by microsatellites compared to isoenzyme markers in Aedes aegypti from Rio de Janeiro

Aedes aegypti populations from five districts in Rio de Janeiro were analyzed using five microsatellites and six isoenzyme markers, to assess the amount of variation and patterns of gene flow at local levels. Microsatellite loci were polymorphic enough to detect genetic differentiation of populations collected at small geographic scales (e.g. within a city). Ae. aegypti populations were highly differentiated as well in the city center as in the outskirt. Thus, dengue virus propagation by mosquitoes could be as efficient in the urban area as in the outskirt of Rio de Janeiro, the main entry point of dengue in Brazil.     

Molecular phylogeography and cryptic speciation in the mosses, Mielichhoferia elongata and M. mielichhoferiana (Bryaceae)

Nucleotide sequence variation in the ITS1-5.8S-ITS2 region of nuclear ribosomal DNA (nrDNA) from 70 populations of Mielichhoferia elongata and M. mielichhoferiana, plus two outgroup species, was analysed using maximum parsimony and maximum likelihood methods. High levels of nucleotide substitution and numerous insertion-deletion events were detected within and between the two species. M. elongata is monophyletic with regard to nrDNA variation, but M. mielichhoferiana is paraphyletic. (M. elongata is nested within it.) A clade within M. mielichhoferiana provides evidence of vicariance, with North American and Scandinavian sister groups of populations. Two major clades are resolved in M. elongata by sequence data that are completely congruent with previous isozyme work. One clade includes populations from both North America and Europe whereas the other is strictly North American. These two clades, resolved by multiple independent loci, clearly represent cryptic species within the morphologically uniform M. elongata. Certain geographical areas, most notably southwestern Colorado in Ouray and San Juan Counties, harbour diverse populations of M. elongata with distinct phylogenetic and phylogeographical histories. Morphologically indistinguishable but phylogenetically distant populations were detected a few metres apart at one site. In contrast, all populations collected over hundreds of kilometres in California belong to a single clade. Arctic North American populations belong to a clade that includes disjunct populations in Alaska, northern Ellesmere Island, and the northeastern USA, but not subarctic Swedish populations, which are more closely related to plants from the Rocky Mountains. Morphological uniformity belies complex infraspecific phylogenetic patterns within M. elongata and M. mielichhoferiana.     

Genetic variation in Arabidopsis suecica and its parental species A. arenosa and A. thaliana

Random amplified polymorphic DNA (RAPD) markers were used to estimate the level of genetic variation in Swedish accessions of the allopolyploid Arabidopsis suecica and its parental species A. thaliana and A. arenosa. The results showed clear differences among the three species with respect to the level of variation. A. arenosa was highly variable, A. thaliana showed a moderate level of variation whereas A. suecica was much less variable than the two other species. An extended analysis covering 19 Swedish populations of A. suecica corroborated the low level of variation in this species, yet 16 unique phenotypes were observed. No isolation by distance was observed. When the genetic variation was partitioned among and within populations of A. suecica, the results showed that the majority of the variation (81%) occurred among populations. This result is interpreted as a strong indication that A. suecica is autogamous in nature.     

Low genetic variation support bottlenecks in Scandinavian red deer

Loss of genetic variation from genetic drift during population bottlenecks has been shown for many species. Red deer (Cervus elaphus) may have been exposed to bottlenecks due to founder events during postglacial colonisation in the early Holocene and during known population reductions in the eighteenth and nineteenth centuries. In this study, we assess loss of genetic variation in Scandinavian red deer due to potential bottlenecks by comparing microsatellite (n = 14) and mitochondrial DNA variation in the Norwegian and Swedish populations with the Scottish, Lithuanian and Hungarian populations. Bottlenecks are also assessed from the M ratio of populations, heterozygosity excess and from hierarchical Bayesian analyses of their demographic history. Strong genetic drift and differentiation was identified in both Scandinavian populations. Microsatellite variation was lower in both Scandinavian populations compared with the other European populations and mitochondrial DNA variation was especially low in the Swedish population where only one unique haplotype was observed. Loss of microsatellite alleles was demonstrated by low M ratios in all populations except the Hungarian. M ratios’ were especially low in the Scandinavian populations, indicating additional or more severe bottlenecks. Heterozygosity excess compared with the expectation from the number of observed microsatellite alleles suggested a recent bottleneck of low severity in the Norwegian population. Hierarchical Bayesian coalescent analyses consistently yielded estimates of a large ancestral and a small current population size in all investigated European populations and suggested the onset of population decline to be between 5,000 and 10,000 years ago, which coincide well with postglacial colonisation.     

The structure of allozyme and leaf shape variation in isolated, range-margin populations of the shrub Hippocrepis emerus (Leguminosae)

We investigated allozyme variation and phenotypic variation in leaf shape in 15 populations of the shrub Hippocrepis emerus (Leguminosae) from the three isolated, regional populations in Scandinavia and analysed patterns of differentiation and the hierarchical structuring of diversity on different geographic scales There are pronounced geographic differences between the Norwegian and Swedish isolates of the species and most of the polymorphic allozyme loci show reciprocal fixation in the two isolates The Scandinavian populations of H emerus are not only disjunct on a macrogeographic scale but also show considerable disjunction within regions Within the Oland regional population, a central group of populations shows low levels of Inter-population differentiation the Gotland group of populations is related to this core group of Oland populations The geographically marginal populations on Oland are spatially isolated and show a higher degree of divergence between populations than does the central group of Oland populations We interpret genetic divergence between the marginal populations in terms of genetic drift - as a result of historical fluctuations in habitat availability and population size     

A biochemical genetic analysis of populations of the northern pike, Esox lucius L., from Europe and North America

Genetic variation as identified by starch gel electrophoresis and enzyme staining was studied in seven populations of the northern pike, Esox lucius L. Samples derived from populations in northern U.S., Canada, Swedish fresh water, Swedish Baltic, the Netherlands, England and Ireland were analysed for variation at between 10 and 26 enzyme-coding loci. Polymorphism was found to be restricted to three loci: Sod-1 (mitochondrial superoxide dismutase), Ldh-2 (heart-type lactate dehydrogenase) and Got-1 (cytoplasmic glutamateoxalate transaminase). Mean heterozygosity ( H ) over all seven population samples was estimated to be 0·019. This extremely low variability, relative to reported data for other fish species, is discussed in terms of the structure and possible origins of pike populations.     

Population genetic structure of the noxious weed Amaranthus retroflexus in Central Europe

Genetic variation was assessed in a range of populations of Amaranthus retroflexus using isoenzyme analysis. Population genetic diversity was measured by evaluating patterns of variation at six putatively neutral isoenzyme loci (comprising 24 putative alleles) within and among 20 populations of A. retroflexus collected in different habitats: ruderal habitats, cereal fields and hop gardens. Amaranthus retroflexus is a noxious weed of North American origin that infests various crops. Overall, A. retroflexus displayed moderate levels of genetic diversity in comparison with other herbaceous plants. The percentage of polymorphic loci was 50.0%, with mean values of 2.01, 0.142 and 0.227 for the average number of alleles per polymorphic locus (A), observed heterozygosity (H_o) and expected heterozygosity (H_e), respectively. A discrepancy between observed and expected heterozygosity and significant differences from H-W expectation indicate that there is an excess of homozygotes in many populations. As a result, there is strong evidence of inbreeding within populations (F_IS = 0.382) and significant population differentiation (F_ST = 0.270). Even though the species is partly autogamous, inbreeding does not lead to strong inbreeding depression resulting from self-pollination, as inbreeding has no effect on the success of the species in today's countryside. Moreover, allele frequencies detected in agricultural habitats (i.e., cereal fields and hop gardens) differed from those detected in populations collected from ruderal habitats, which is probably caused by systematic application of herbicides in agricultural ecosystems.     

Population differentiation of Phlebotomus perniciosus in Spain following postglacial dispersal

Comparative sequencing of mitochondrial cytochrome b (Cyt b) and isoenzyme analyses have not resolved the population structure of the Iberian lineage of the sandfly Phlebotomus perniciosus, the most widespread vector of Leishmania infantum (Protozoa, Trypanosomatidae) to humans and dogs in the western Mediterranean subregion. Allelic variation at trinucleotide microsatellite loci was investigated in 13 Spanish populations of P. perniciosus. Four out of five loci showed significant differentiation between (pairwise F(ST)>0.23), but not within (pairwise F(ST)&<0.05), two regional groups of populations (southern and northeastern). All Cyt b sequences belonged to the Iberian lineage, which differs by six fixed nucleotide differences from the typical lineage found in northwest Africa, Malta and Italy. The northeastern group of Spanish populations had a reduced number of microsatellite alleles (16 out of the 29 present in the southern populations), indicating its derivation as a peripheral isolate following the species' dispersal from a southern Ice Age refuge 8000-12 000 years ago. Pairwise F(ST) values did not increase with geographical distance between populations, over distances of 246-850 km (between regions) and 16-491 km (within regions). This suggests that the two regional groups of populations remain isolated, but that within each region there are no significant permanent barriers to gene flow between contiguous populations. These findings will help to predict the capacity of this sandfly to disperse, and originate new foci of leishmaniasis, in response to climate warming.     

Climatic adaptation in an isolated and genetically impoverished amphibian population

The capacity of populations to respond adaptively to environmental change is essential for their persistence. Isolated populations often harbour reduced genetic variation, which is predicted to decrease adaptive potential, and can be detrimental under the current scenarios of global change. In this study, we examined climatic adaptation in larval life history traits in the pool frog Rana lessonae along a latitudinal gradient across the northern distribution area of the species, paying special attention to the isolated and genetically impoverished fringe populations in central Sweden. Larvae from eight populations within three geographic areas (Poland, Latvia and Sweden) were reared under three temperatures (19, 22 and 26°C) in a common garden laboratory experiment. We found clear evidence for latitudinal adaptation in R. lessonae populations, larvae from higher latitudes growing and developing faster than low‐latitude ones. Larvae from the Swedish populations were able to compensate for the effects of cooler temperatures and a shorter growth season with genetically higher growth and development rates (i.e. countergradient variation) in the two higher temperature treatments, but there was no difference among the populations at the lowest temperature treatment, which is likely to be close to the temperature limiting growth in R. lessonae. Our results demonstrate that isolated and genetically impoverished populations can be locally adapted, and identify the Swedish fringe populations as a significant conservation unit adapted to the northern environmental conditions.     

Genetic diversity and phenotypic variation in marginal populations of the locally endangered speciesHordeum secalinum (Poaceae)

Electrophoresis was used to compare variation in hordein polypeptide patterns and isozymes in five marginal populations ofHordeum secalinum (four Swedish and one Danish population) and four populations along the Atlantic coast from Denmark to Spain. Hordein patterns were uniform within and among Swedish and Danish populations whereas the materials from France and Spain were divergent. The banding patterns for nine isozymes (AAT, IDH, MDH, PGD, GPI, ME, ACP, EST, and PRX) agreed with those reported for otherHordeum species. All putative loci were monomorphic with the exception of PRX, which showed three different phenotypes. The five marginal populations varied in seed set and cultivated plants differed in phenological traits.Dedicated to emer. Univ.-Prof. DrFriedrich Ehrendorfer on the occasion of his 70th birthday     

Landscape structure, clonal propagation, and genetic diversity in Scandinavian populations of Arabidopsis lyrata (Brassicaceae)

Colonization history, landscape structure, and environmental conditions may influence patterns of neutral genetic variation because of their effects on gene flow and reproductive mode. We compared variation at microsatellite loci within and among 26 Arabidopsis lyrata populations in two disjunct areas of its distribution in northern Europe (Norway and Sweden). The two areas probably share a common colonization history but differ in size (Norwegian range markedly larger than Swedish range), landscape structure (mountains vs. coast), and habitat conditions likely to affect patterns of gene flow and opportunities for sexual reproduction. Within-population genetic diversity was not related to latitude but was higher in Sweden than in Norway. Population differentiation was stronger among Norwegian than among Swedish populations (F(ST) = 0.23 vs. F(ST) = 0.18). The frequency of clonal propagation (proportion of identical multilocus genotypes) increased with decreasing population size, was higher in Norwegian than in Swedish populations, but was not related to altitude or substrate. Differences in genetic structure are discussed in relation to population characteristics and range size in the two areas. The results demonstrate that the possibility of clonal propagation should be considered when developing strategies for sampling and analyzing data in ecological and genetic studies of this emerging model species.