Chloroplast DNA phylogeography of the argan tree of Morocco

Genome-wide identification of SSR markers from coding regions for endangered Argania spinosa L. skeels and construction of SSR database: AsSSRdb

Microsatellites [simple sequence repeats (SSRs)] are one of the most widely used sources of genetic markers, particularly prevalent in plants. Despite their importance in various applications, a comprehensive genome-wide identification of coding sequence (CDS)-associated SSR markers in the Argania spinosa L. genome has yet to be conducted. In this study, 66 280 CDSs containing 5351 SSRs within 4535 A. spinosa L. CDSs were identified. Among these, tri-nucleotide motifs (58.96%) were the most common, followed by hexa-nucleotide (15.71%) and di-nucleotide motifs (13.32%). The predominant SSR motif in the tri-nucleotide category was AAG (24.4%), while AG (94.1%) was the most abundant among di-nucleotide repeats. Furthermore, the extracted CDSs containing SSRs were subjected to functional annotation; 3396 CDSs (74.88%) exhibited homology with known proteins, 3341 CDSs (73.7%) were assigned Gene Ontology terms, 1004 CDSs were annotated with Enzyme Commission numbers, and 832 (18.3%) were annotated with KEGG pathways. A total of 3475 primer pairs were designed, out of which 3264 were successfully validated in silico against the A. spinosa L. genome, with 99.6% representing high-resolution markers yielding no more than three products. Additionally, the SSR markers demonstrated a low rate of transferability through in-silico verification in two species within the Sapotaceae family. Furthermore, we developed an online database, the "Argania spinosa L. SSR database: https://as-fmmdb.shinyapps.io/asssrdb/" (AsSSRdb) to provide access to the CDS-associated SSRs identified in this study. Overall, this research provides valuable marker resources for DNA fingerprinting, genetic studies, and molecular breeding in argan and related species. Database URL: https://as-fmmdb.shinyapps.io/asssrdb/.

Molecular and Technological Characterization of Saccharomyces cerevisiae from Sourdough

DNA markers help detect the intraspecific genetic diversity of yeast strains. Eight ISSR (Inter Simple Sequence Repeats) primers were used to assess the intraspecific diversity of Saccharomyces cerevisiae (n = 96) from different populations (n = 3), evaluate the technological characteristics, and investigate trait-loci associations. The primers amplified 154 reproducible and scorable bands, of which 79.87% were polymorphic. The UPGMA (unweighted pair group method with arithmetic mean) dendrogram clustered 96 isolates into two main clusters, supported by STRUCTURE HARVESTER results (ΔK = 2). Analysis of molecular variance (AMOVA) indicated significant genetic differences between (15%) and within the populations (85%) (p < 0.001). Twenty-nine genetically distinct strains were selected for the technological characterization. Principal component analysis (PCA) revealed that five strains with high fermentation capacity, leavening activity, high growth index at 37 °C, and harsh growth conditions were technologically relevant. Trait-loci association analyses indicated that the highest correlation (r = 0.60) was recorded for the fermentation capacity on the 8th and 113th loci, amplified by ISSR-1 and ISSR-6 primers, respectively (p < 0.05). The strains yielding high performances and the associated loci amplified by ISSR markers possess a high potential to generate locus-specific primers to target the strains with high fermentation capacity.

Ethnobotany, phytochemistry and biological properties of Argan tree (Argania spinosa (L.) Skeels) (Sapotaceae) - A review

ETHNOPHARMACOLOGICAL RELEVANCE

The argan [Argania spinosa (L.) Skeels] is one of the most important floristic resource in Morocco, it is the only representative of the Sapotaceae family and Argania genus found in Morocco. This tree is fully exploited by the native populations for nutrition, medication and cosmetics. The argan oil extracted from seed is the main tree product for his large use.

AIM OF THE REVIEW

This review describes the traditional uses, chemical composition and biological activities of different the argan tree parts.

MATERIALS AND METHODS

This review covers the literature available from 1972 to 2021. The informations were collected from electronic databases Scopus, PubMed, Web of Science, SciFinder and Google Scholar.

RESULTS

Argan oil have been used for nutrition, and to treat several diseases, namely rheumatisms, hypercholesterolemia, atherosclerosis, lung infections, newborn gastrointestinal disorders, diabetes, skin and hair hydration. The other parts of Argan tree have been used to treat intestinal disorders, dermatosis, and hair caring, with additional uses such as livestock nutrition, carpentry and heating. The argan oil is primarily composed of unsaturated fatty acids mainly oleic and linoleic acids furthermore the chemical composition, of the others part, are very diversified (flavonoids, terpenoids, triacylglycerols, saponins. …). Diverse biological activities have been reported for argan oil, such as antioxidant, skin water retention, hair protection, cholesterol stabilization, antidiabetic, anticancer and antibacterial. Antimicrobial activities have been reported for argan leaves essential oils, when the fruit pulp organic extract presented very interesting antioxidant activity due to the presence of polyphenols. The argan cake is the seed waste produced during the extraction process, it is traditionally used for skin care and for livestock nutrition. Different biological activities of argan cake have been cited essentially antioxidant, haemoprotective, anti-inflammatory and antimicrobial.

When ecological marginality is not geographically peripheral: exploring genetic predictions of the centre-periphery hypothesis in the endemic plant Lilium pomponium

Background

Quantifying variation of genetic traits over the geographical range of species is crucial for understanding the factors driving their range dynamics. The center-periphery hypothesis postulates, and many studies support, the idea that genetic diversity decreases and genetic differentiation increases toward the geographical periphery due to population isolation. The effects of environmental marginality on genetic variation has however received much less attention.

Methods

We tested the concordance between geographical and environmental gradients and the genetic predictions of center-periphery hypothesis for endemic Lilium pomponium in the southern Alps.

Results

We found little evidence for concordance between genetic variation and both geographical and environmental gradients. Although the prediction of increased differentiation at range limits is met, genetic diversity does not decrease towards the geographical periphery. Increased differentiation among peripheral populations, that are not ecologically marginal, may be explained by a decrease in habitat availability that reduces population connectivity. In contrast, a decrease of genetic diversity along environmental but not geographical gradients may be due to the presence of low quality habitats in the different parts of the range of a species that reduce effective population size or increase environmental constraints. As a result, environmental factors may affect population dynamics irrespective of distance from the geographical center of the range. In such situations of discordance between geographical and environmental gradients, the predictions of decreasing genetic diversity and increasing differentiation toward the geographical periphery may not be respected.

Agro-Fruit-Forest Systems Based on Argan Tree in Morocco: A Review of Recent Results

The argan tree, Argania spinosa (L.) Skeels, is a horticultural forestry species characterized by its endemicity and adaptation to arid and semi-arid zones in the southwest of Morocco. Despite its limited geographical distribution, argan tree presents large genetic diversity, suggesting that improvement of argan is possible. This species plays important ecological, and socioeconomic roles in the sustainable development of the country. The integration of arganiculture into Moroccan agricultural policy has been implemented through a sector strategy, which is fully aligned with the conservation and regeneration of argan forest. A. spinosa is suitable for incorporation into different agroforestry productive systems under agro-fruit-forest model and its domestication will provide a powerful means of socio-economic and environmental management. Here, we provide an overview of the argan tree literature and highlight the specific aspects of argan stands, as agro-forest systems, with the aim of developing an adequate strategy of conservation and domestication of this species. We introduce promising programs and projects for argan plantations and arganiculture, which have been adopted to relieve anthropogenic pressure on the natural argan forest.

Complete Chloroplast Genome of Argania spinosa: Structural Organization and Phylogenetic Relationships in Sapotaceae

Argania spinosa (Sapotaceae), an important endemic Moroccan oil tree, is a primary source of argan oil, which has numerous dietary and medicinal proprieties. The plant species occupies the mid-western part of Morocco and provides great environmental and socioeconomic benefits. The complete chloroplast (cp) genome of A. spinosa was sequenced, assembled, and analyzed in comparison with those of two Sapotaceae members. The A. spinosa cp genome is 158,848 bp long, with an average GC content of 36.8%. The cp genome exhibits a typical quadripartite and circular structure consisting of a pair of inverted regions (IR) of 25,945 bp in length separating small single-copy (SSC) and large single-copy (LSC) regions of 18,591 and 88,367 bp, respectively. The annotation of A. spinosa cp genome predicted 130 genes, including 85 protein-coding genes (CDS), 8 ribosomal RNA (rRNA) genes, and 37 transfer RNA (tRNA) genes. A total of 44 long repeats and 88 simple sequence repeats (SSR) divided into mononucleotides (76), dinucleotides (7), trinucleotides (3), tetranucleotides (1), and hexanucleotides (1) were identified in the A. spinosa cp genome. Phylogenetic analyses using the maximum likelihood (ML) method were performed based on 69 protein-coding genes from 11 species of Ericales. The results confirmed the close position of A. spinosa to the Sideroxylon genus, supporting the revisiting of its taxonomic status. The complete chloroplast genome sequence will be valuable for further studies on the conservation and breeding of this medicinally and culinary important species and also contribute to clarifying the phylogenetic position of the species within Sapotaceae.

Genetic, Morphological, and Biochemical Diversity of Argan Tree (Argania spinosa L.) (Sapotaceae) in Tunisia

Argan trees are normally endemic to Morocco and Algeria, but hundreds of argan trees exist in Tunisia, some introduced from Morocco and some from unknown origins. The aim of the present study was to evaluate the genetic, morphological, and biochemical diversity of the argan trees in Tunisia. In this study, we used morphometric data collected from vegetative tissue, as well as pomological characteristics related to fruits, stones, and kernels. Genetic variation in 60 trees of Tunisian Argania spinosa L. was estimated using inter-simple sequence repeats (ISSRs). Mutation screening and genotyping by high-resolution melting (HRM) was performed to detect delta-6-desaturase (D6D) variants in the tested individuals, and finally fatty acid analysis of argan leaves with gas chromatography (GC) was performed. The plant materials used in this study originated from four different sites in Tunisia. Analysis of morphological characteristics showed large variability both within and between the studied collections. The analysis of ISSR polymorphisms gave information about the diversity within and between populations. HRM analysis showed that all 60 argan individuals were grouped into 10 different categories. The results of the gas chromatography analysis showed that the presence of omega-3 fatty acids EPA and DHA was noticeable in some argan leaves.

Tracking Host Trees: The Phylogeography of Endemic Psittacanthus sonorae (Loranthaceae) Mistletoe in the Sonoran Desert

The host dependence of mistletoes suggests that they track the distributions of their hosts. However, the factors that determine the geographic distribution of mistletoes are not well understood. In this study, the phylogeography of Psittacanthus sonorae was reconstructed by sequencing one nuclear (ITS) and two plastid (trnL-F and atpB-rbcL) regions of 148 plants from populations separated by the Sea of Cortez. Divergence time and gene flow were estimated to gain insight into the historical demography and geographic structuring of genetic variation. We also described and mapped the spatial distribution of suitable habitat occupied by P. sonorae and its most common host Bursera microphylla in the Sonoran Desert, along with their responses to Quaternary climate fluctuations using environmental data and ecological niche modeling (ENM). We detected environmental and genetic differentiation between the peninsular and continental P. sonorae populations. Population divergence occurred during the Pleistocene, around the time of the Last Glacial Maximum. No signals of population growth were detected, with net gene flow moving from the continent to the peninsula. ENM models indicate decoupled responses by the mistletoe and its main host to past climate changes. For the Last Interglacial to the present, most models produce only partial areas of overlap on both the peninsula and the continent. Our results support a scenario of Late-Pleistocene isolation and divergence with asymmetrical gene flow between peninsular and continental P. sonorae populations. Continental populations migrated to the peninsula and the spatial isolation probably produced genetic differentiation under different environmental conditions.

First draft genome assembly of the Argane tree ( Argania spinosa)

Background: The Argane tree ( Argania spinosa L. Skeels) is an endemic tree of mid-western Morocco that plays an important socioeconomic and ecologic role for a dense human population in an arid zone. Several studies confirmed the importance of this species as a food and feed source and as a resource for both pharmaceutical and cosmetic compounds. Unfortunately, the argane tree ecosystem is facing significant threats from environmental changes (global warming, over-population) and over-exploitation. Limited research has been conducted, however, on argane tree genetics and genomics, which hinders its conservation and genetic improvement. Methods: Here, we present a draft genome assembly of A. spinosa. A reliable reference genome of  A. spinosa was created using a hybrid  de novo assembly approach combining short and long sequencing reads. Results: In total, 144 Gb Illumina HiSeq reads and 7.6 Gb PacBio reads were produced and assembled. The final draft genome comprises 75 327 scaffolds totaling 671 Mb with an N50 of 49 916 kb. The draft assembly is close to the genome size estimated by k-mers distribution and covers 89% of complete and 4.3 % of partial Arabidopsis orthologous groups in BUSCO. Conclusion: The A. spinosa genome will be useful for assessing biodiversity leading to efficient conservation of this endangered endemic tree. Furthermore, the genome may enable genome-assisted cultivar breeding, and provide a better understanding of important metabolic pathways and their underlying genes for both cosmetic and pharmacological.

First draft genome assembly of the Argane tree ( Argania spinosa)

Background: The Argane tree ( Argania spinosa L. Skeels) is an endemic tree of mid-western Morocco that plays an important socioeconomic and ecologic role for a dense human population in an arid zone. Several studies confirmed the importance of this species as a food and feed source and as a resource for both pharmaceutical and cosmetic compounds. Unfortunately, the argane tree ecosystem is facing significant threats from environmental changes (global warming, over-population) and over-exploitation. Limited research has been conducted, however, on argane tree genetics and genomics, which hinders its conservation and genetic improvement. Methods: Here, we present a draft genome assembly of A. spinosa. A reliable reference genome of  A. spinosa was created using a hybrid  de novo assembly approach combining short and long sequencing reads. Results: In total, 144 Gb Illumina HiSeq reads and 7.6 Gb PacBio reads were produced and assembled. The final draft genome comprises 75 327 scaffolds totaling 671 Mb with an N50 of 49 916 kb. The draft assembly is close to the genome size estimated by k-mers distribution and covers 89% of complete and 4.3 % of partial Arabidopsis orthologous groups in BUSCO. Conclusion: The A. spinosa genome will be useful for assessing biodiversity leading to efficient conservation of this endangered endemic tree. Furthermore, the genome may enable genome-assisted cultivar breeding, and provide a better understanding of important metabolic pathways and their underlying genes for both cosmetic and pharmacological.

An Assessment of Genetic Diversity and Drought Tolerance in Argan Tree (Argania spinosa) Populations: Potential for the Development of Improved Drought Tolerance

The argan tree (Argania spinosa) occurs in a restricted area of Southwestern Morocco characterized by low water availability and high evapotranspirative demand. Despite the adaptation of the argan tree to drought stress, the extent of the argan forest has declined markedly due to increased aridity, land use changes and the expansion of olive cultivation. The oil of the argan seed is used for cooking and as the basis for numerous cosmetics. The identification of argan tree varieties with enhanced drought tolerance may minimize the economic losses associated with the decline of the argan forest and constrain the spread of desertification. In this study we collected argan ecotypes from four contrasting habitats and grew them under identical controlled environment conditions to investigate their response to drought. Leaf gas exchange analysis indicated that the argan ecotypes showed a high degree of adaptation to drought stress, maintaining photosynthetic activity at low levels of foliar water content and co-ordinating photosynthesis, stomatal behavior and metabolism. The stomata of the argan trees were highly sensitive to increased leaf to air vapor pressure deficit, representing an adaptation to growth in an arid environment where potential evapotranspiration is high. However, despite originating in contrasting environments, the four argan ecotypes exhibited similar gas exchange characteristics under both fully irrigated and water deficit conditions. Population genetic analyses using microsatellite markers indicated a high degree of relatedness between the four ecotypes; indicative of both artificial selection and the transport of ecotypes between different provinces throughout centuries of management of the argan forest. The majority of genetic variation across the four populations (71%) was observed between individuals, suggesting that improvement of argan is possible. Phenotypic screening of physiological responses to drought may prove effective in identifying individuals and then developing varieties with enhanced drought tolerance to enable the maintenance of argan production as climate change results in more frequent and severe drought events in Northern Africa.

Genetic structure and demographic history of Cycas chenii (Cycadaceae), an endangered species with extremely small populations

Geological activities and climate oscillations during the Quaternary period profoundly impacted the distribution of species in Southwest China. Some plant species may be harbored in refugia, such as the dry-hot valleys of Southwest China. Cycas chenii X. Gong & W. Zhou, a critically endangered cycad species, which grows under the canopy in subtropical evergreen broad-leaved forests along the upstream drainage area of the Red River, is endemic to this refugium. In this study, 60 individuals of C. chenii collected from six populations were analyzed by sequencing two chloroplast intergenic spacers (cpDNA: psbA-trnH and trnL-trnF) and two nuclear genes (PHYP and RBP-1). Results showed high genetic diversity at the species level, but low within-population genetic diversity and high interpopulation genetic differentiation. A Bayesian phylogenetic tree based on cpDNA showed that five chloroplast haplotypes were clustered into two clades, which corresponds to the division of the western and eastern bank of the Red River. These data indicate a possible role for the Red River as a geographic barrier to gene flow in C. chenii. Based on our findings, we propose appropriate in situ and ex situ conservation strategies for C. chenii.

Genetic consequences of selection cutting on sugar maple (Acer saccharum Marshall)

Selection cutting is a treatment that emulates tree-by-tree replacement for forests with uneven-age structures. It creates small openings in large areas and often generates a more homogenous forest structure (fewer large leaving trees and defective trees) that differs from old-growth forest. In this study, we evaluated whether this type of harvesting has an impact on genetic diversity of sugar maple (Acer saccharum Marshall). Genetic diversity among seedlings, saplings, and mature trees was compared between selection cut and old-growth forest stands in Québec, Canada. We found higher observed heterozygosity and a lower inbreeding coefficient in mature trees than in younger regeneration cohorts of both forest types. We detected a recent bottleneck in all stands undergoing selection cutting. Other genetic indices of diversity (allelic richness, observed and expected heterozygosity, and rare alleles) were similar between forest types. We concluded that the effect of selection cutting on the genetic diversity of sugar maple was recent and no evidence of genetic erosion was detectable in Québec stands after one harvest. However, the cumulative effect of recurring applications of selection cutting in bottlenecked stands could lead to fixation of deleterious alleles, and this highlights the need for adopting better forest management practices.

The phylogeography of Fagus hayatae (Fagaceae): genetic isolation among populations

The beech species Fagus hayatae is an important relict tree species in subtropical China, whose biogeographical patterns may reflect floral responses to climate change in this region during the Quaternary. Previous studies have revealed phylogeography for three of the four Fagus species in China, but study on F. hayatae, the most sparsely distributed of these species, is still lacking. Here, molecular methods based on eight simple sequence repeat (SSR) loci of nuclear DNA (nDNA) and three chloroplast DNA (cpDNA) sequences were applied for analyses of genetic diversity and structure in 375 samples from 14 F. hayatae populations across its whole range. Both nDNA and cpDNA indicated a high level of genetic diversity in this species. Significant fixation indexes and departures from the Hardy–Weinberg equilibrium, with a genetic differentiation parameter of R_st of 0.233, were detected in nDNA SSR loci among populations, especially those on Taiwan Island, indicating strong geographic partitioning. The populations were classified into two clusters, without a prominent signal of isolation‐by‐distance. For the 15 haplotypes detected in the cpDNA sequence fragments, there was a high genetic differentiation parameter (G_st = 0.712) among populations. A high G_st of 0.829 was also detected outside but not within the Sichuan Basin. Consistent with other Fagus species in China, no recent population expansion was detected from tests of neutrality and mismatch distribution analysis. Overall, genetic isolation with limited gene flow was prominent for this species and significant phylogeographic structures existed across its range except for those inside the Sichuan Basin. Our study suggested long‐term geographic isolation in F. hayatae with limited population admixture and the existence of multiple refugia in the mountainous regions of the Sichuan Basin and southeast China during the Quaternary. These results may provide useful information critical for the conservation of F. hayatae and other Chinese beech species.

The embryo and the endosperm contribute equally to argan seed oil yield but confer distinct lipid features to argan oil

In the perspective of studying lipid biosynthesis in the argan seed, the anatomy, ploidy level and lipid composition of mature seed tissues were investigated using an experimental design including two locations in Algeria and four years of study. Using flow cytometry, we determined that mature argan seeds consist of two well-developed tissues, the embryo and the endosperm. The lipid content of the embryo was higher than that of the endosperm, but the dry weight of the endosperm was higher. Consequently, both tissues contribute equally to seed oil yield. Considerable differences in fatty acid composition were observed between the two tissues. In particular, the endosperm 18:2 percentage was twofold higher than that of the embryo. The tocopherol content of the endosperm was also markedly higher than that of the embryo. In contrast, the endosperm and the embryo had similar sterol and triterpene alcohol contents and compositions.

Development and characterization of microsatellite loci for the Moroccan endemic endangered species Argania spinosa (Sapotaceae)

PREMISE OF THE STUDY

Microsatellite loci were developed for the Moroccan endemic endangered species Argania spinosa with a combination of a typical library enrichment procedure and a 454 GS FLX Titanium-based high-throughput sequencing approach. •

METHODS AND RESULTS

A genomic DNA library was enriched and further screened using (GA)15, (GTA)8, and (TTC)8 biotin-labeled probes coupled with chemi-luminescence detection. To increase simple sequence repeat (SSR) loci number, an ultra-high-throughput sequencing-based approach was used. Evaluation of all primer pairs was performed with labeled dUTP on an ABI 3130xl sequencer. Eleven polymorphic SSR loci were selected out of 79 SSR regions and extensively characterized on 150 individuals from eight populations. Total alleles ranged from six to 19 alleles per locus while expected heterozygosity ranged from 0.618 to 0.869. •

CONCLUSIONS

The SSRs developed here will be used to further characterize the genetic diversity of A. spinosa across its distribution range, mainly in the southern part of Morocco and southwestern Algeria. They may also be transferable to other Sapotaceae species.

Estructura genética y germinación de semillas en poblaciones portuguesas de

Cheirolophus uliginosus es una especie amenazada endémica de la costa atlántica de la península ibérica, donde ocupa unas pocas y reducidas localidades. En nuestro estudio, analizamos los patrones de variación de los haplotipos de ADN cloroplástico y el éxito reproductivo —capacidad germinativa— en siete poblaciones portuguesas de diferente tamaño. El éxito reproductivo de Ch. uliginosus se ha examinado en relación con la estructura genética y el tamaño de sus poblaciones. Los resultados indican una variabilidad intrapoblacional muy baja para los marcadores cloroplásticos utilizados. Nuestro estudio muestra una tasa de germinación significativamente reducida en las poblaciones pequeñas (< 50 individuos) respecto a aquellas de tamaño mediano (50-250 individuos) o grande (> 250 individuos). Para explicar este fenómeno, se deben tomar en consideración las limitaciones ecológicas y el aislamiento genético. Por otro lado, en las poblaciones de Ch. uliginosus de mayor tamaño (> 250 individuos) se ha observado una incidencia más acusada de la depredación de semillas antes de su dispersión, lo cual podría estar afectando a su respuesta reproductiva. Finalmente, las poblaciones más pequeñas —que presentan un reducido éxito reproductivo— contienen los haplotipos más distantes evolutivamente y su conservación debería ser, por tanto, prioritaria.

spads 1.0: a toolbox to perform spatial analyses on DNA sequence data sets

SPADS 1.0 (for 'Spatial and Population Analysis of DNA Sequences') is a population genetic toolbox for characterizing genetic variability within and among populations from DNA sequences. In view of the drastic increase in genetic information available through sequencing methods, spads was specifically designed to deal with multilocus data sets of DNA sequences. It computes several summary statistics from populations or groups of populations, performs input file conversions for other population genetic programs and implements locus-by-locus and multilocus versions of two clustering algorithms to study the genetic structure of populations. The toolbox also includes two MATLAB and r functions, GDISPAL and GDIVPAL, to display differentiation and diversity patterns across landscapes. These functions aim to generate interpolating surfaces based on multilocus distance and diversity indices. In the case of multiple loci, such surfaces can represent a useful alternative to multiple pie charts maps traditionally used in phylogeography to represent the spatial distribution of genetic diversity. These coloured surfaces can also be used to compare different data sets or different diversity and/or distance measures estimated on the same data set.

Insecticide-driven patterns of genetic variation in the dengue vector Aedes aegypti in Martinique Island

Effective vector control is currently challenged worldwide by the evolution of resistance to all classes of chemical insecticides in mosquitoes. In Martinique, populations of the dengue vector Aedes aegypti have been intensively treated with temephos and deltamethrin insecticides over the last fifty years, resulting in heterogeneous levels of resistance across the island. Resistance spreading depends on standing genetic variation, selection intensity and gene flow among populations. To determine gene flow intensity, we first investigated neutral patterns of genetic variability in sixteen populations representative of the many environments found in Martinique and experiencing various levels of insecticide pressure, using 6 microsatellites. Allelic richness was lower in populations resistant to deltamethrin, and consanguinity was higher in populations resistant to temephos, consistent with a negative effect of insecticide pressure on neutral genetic diversity. The global genetic differentiation was low, suggesting high gene flow among populations, but significant structure was found, with a pattern of isolation-by-distance at the global scale. Then, we investigated adaptive patterns of divergence in six out of the 16 populations using 319 single nucleotide polymorphisms (SNPs). Five SNP outliers displaying levels of genetic differentiation out of neutral expectations were detected, including the kdr-V1016I mutation in the voltage-gated sodium channel gene. Association tests revealed a total of seven SNPs associated with deltamethrin resistance. Six other SNPs were associated with temephos resistance, including two non-synonymous substitutions in an alkaline phosphatase and in a sulfotransferase respectively. Altogether, both neutral and adaptive patterns of genetic variation in mosquito populations appear to be largely driven by insecticide pressure in Martinique.

Phylogeography of sea oats (Uniola paniculata), a dune-building coastal grass in southeastern North America

We tested the hypothesis that Uniola paniculata populations are divided into eastern and western lineages, with the primary geographic break at the southern tip of Florida, as observed in codistributed animal taxa. We asked whether the geographic distribution of chloroplast DNA (cpDNA) variation in U. paniculata corresponds to 1) genetic structure in nuclear variation reported in previous studies, and 2) the geographic distribution of morphological adaptive traits reported in previous studies. We sampled 66 populations and performed phylogeographic analyses using sequence variations in maternally inherited cpDNA. We reconstructed the intraspecific phylogenetic network with TCS software and identified phylogeographic breaks in the species using Monmonier's algorithm. Analyses identified 6 cpDNA haplotypes and 2 major lineages: eastern (Atlantic) and western (Gulf), with a phylogeographic break at the southern tip of Florida. The data suggest U. paniculata survived the last glacial maximum (LGM) in southern refugia. Following the LGM, differential leading-edge recolonization explains the current distribution of haplotypes into 2 lineages. Populations containing a haplotype from outside its native range are likely due to human-mediated transplantation. The genetic structure of cpDNA variation has weak correlation with nuclear DNA variation, and there is partial concordance between the geographic distribution of cpDNA and morphological variation.

Plague circulation and population genetics of the reservoir Rattus rattus: the influence of topographic relief on the distribution of the disease within the Madagascan focus

BACKGROUND

Landscape may affect the distribution of infectious diseases by influencing the population density and dispersal of hosts and vectors. Plague (Yersinia pestis infection) is a highly virulent, re-emerging disease, the ecology of which has been scarcely studied in Africa. Human seroprevalence data for the major plague focus of Madagascar suggest that plague spreads heterogeneously across the landscape as a function of the relief. Plague is primarily a disease of rodents. We therefore investigated the relationship between disease distribution and the population genetic structure of the black rat, Rattus rattus, the main reservoir of plague in Madagascar.

METHODOLOGY/PRINCIPAL FINDINGS

We conducted a comparative study of plague seroprevalence and genetic structure (15 microsatellite markers) in rat populations from four geographic areas differing in topology, each covering about 150-200 km(2) within the Madagascan plague focus. The seroprevalence levels in the rat populations mimicked those previously reported for humans. As expected, rat populations clearly displayed a more marked genetic structure with increasing relief. However, the relationship between seroprevalence data and genetic structure differs between areas, suggesting that plague distribution is not related everywhere to the effective dispersal of rats.

CONCLUSIONS/SIGNIFICANCE

Genetic diversity estimates suggested that plague epizootics had only a weak impact on rat population sizes. In the highlands of Madagascar, plague dissemination cannot be accounted for solely by the effective dispersal of the reservoir. Human social activities may also be involved in spreading the disease in rat and human populations.

Phylogeography of Tibouchina papyrus (Pohl) Toledo (Melastomataceae), an endangered tree species from rocky savannas, suggests bidirectional expansion due to climate cooling in the Pleistocene

Many endemic species present disjunct geographical distribution; therefore, they are suitable models to test hypotheses about the ecological and evolutionary mechanisms involved in the origin of disjunct distributions in these habitats. We studied the genetic structure and phylogeography of Tibouchina papyrus (Melastomataceae), endemic to rocky savannas in Central Brazil, to test hypothesis of vicariance and dispersal in the origin of the disjunct geographical distribution. We sampled 474 individuals from the three localities where the species is reported: Serra dos Pirineus, Serra Dourada, and Serra de Natividade. Analyses were based on the polymorphisms at cpDNA and on nuclear microsatellite loci. To test for vicariance and dispersal we constructed a median-joining network and performed an analysis of molecular variance (AMOVA). We also tested population bottleneck and estimated demographic parameters and time to most recent common ancestor (TMRCA) using coalescent analyses. A remarkable differentiation among populations was found. No significant effect of population expansion was detected and coalescent analyses showed a negligible gene flow among populations and an ancient coalescence time for chloroplast genome. Our results support that the disjunct distribution of T. papyrus may represent a climatic relict. With an estimated TMRCA dated from ∼836.491 ± 107.515 kyr BP (before present), we hypothesized that the disjunct distribution may be the outcome of bidirectional expansion of the geographical distribution favored by the drier and colder conditions that prevailed in much of Brazil during the Pre-Illinoian glaciation, followed by the retraction as the climate became warmer and moister.

Not an ancient relic: the endemic Livistona palms of arid central Australia could have been introduced by humans

Livistona mariae is an endemic palm localized in arid central Australia. This species is separated by about 1000 km from its congener L. rigida, which grows distantly in the Roper River and Nicholson-Gregory River catchments in northern Australia. Such an isolated distribution of L. mariae has been assumed to have resulted from contraction of ancestral populations as Australia aridified from the Mid-Miocene (ca 15 Ma). To test this hypothesis at the population level, we examined the genetic relationships among 14 populations of L. mariae and L. rigida using eight nuclear microsatellite loci. Our population tree and Bayesian clustering revealed that these populations comprised two genetically distinct groups that did not correspond to the current classification at species rank, and L. mariae showed closest affinity with L. rigida from Roper River. Furthermore, coalescent divergence-time estimations suggested that the disjunction between the northern populations (within L. rigida) could have originated by intermittent colonization along an ancient river that has been drowned repeatedly by marine transgression. During that time, L. mariae populations could have been established by opportunistic immigrants from Roper River about 15 000 years ago, concurrently with the settlement of indigenous Australians in central Australia, who are thus plausible vectors. Thus, our results rule out the ancient relic hypothesis for the origin of L. mariae.

Significant demographic and fine-scale genetic structure in expanding and senescing populations of the terrestrial orchid Cymbidium goeringii (Orchidaceae)

PREMISE OF THE STUDY

Fine-scale genetic structure (FSGS) in plants is influenced by variation in spatial and temporal demographic processes. To determine how demographic structure and FSGS change with stages of population succession, we studied replicate expanding and senescing populations of the Asian terrestrial orchid Cymbidium goeringii.

METHODS

We used spatial autocorrelation methods (O-ring and kinship statistics) to quantify spatial demographic structure and FSGS in two expanding and two senescing populations, also measuring genetic diversity and inbreeding in each.

KEY RESULTS

All populations exhibited significant aggregation of individuals and FSGS at short spatial scales. In expanding populations, this finding was associated with high recruitment rates, suggesting restricted seed dispersal. In senescing populations, recruitment was minimal, suggesting alternative mechanisms of aggregation, perhaps including spatial associations with mycorrhizal fungi. All populations had significant evidence of genetic bottlenecks, and inbreeding levels were consistently high.

CONCLUSIONS

Our results indicate that different successional stages can generate similar patterns of spatial demographic and genetic structure, but as a consequence of different processes. These results contrast with the only other study of senescence effects on population genetic structure in an herbaceous perennial, which found little to no FSGS in senescing populations. With the exception of populations subject to mass collection by orchid sellers, significant FSGS is characteristic of the 16 terrestrial orchid species examined to date. From a conservation perspective, this result suggests that inference of orchid population history will benefit from analyses of both FSGS and demographic structure in combination with other ecological field data.

Nuclear and chloroplast microsatellites show multiple introductions in the worldwide invasion history of common ragweed, Ambrosia artemisiifolia

BACKGROUND

Ambrosia artemisiifolia is a North American native that has become one of the most problematic invasive plants in Europe and Asia. We studied its worldwide population genetic structure, using both nuclear and chloroplast microsatellite markers and an unprecedented large population sampling. Our goals were (i) to identify the sources of the invasive populations; (ii) to assess whether all invasive populations were founded by multiple introductions, as previously found in France; (iii) to examine how the introductions have affected the amount and structure of genetic variation in Europe; (iv) to document how the colonization of Europe proceeded; (v) to check whether populations exhibit significant heterozygote deficiencies, as previously observed.

PRINCIPAL FINDINGS

We found evidence for multiple introductions of A. artemisiifolia, within regions but also within populations in most parts of its invasive range, leading to high levels of diversity. In Europe, introductions probably stem from two different regions of the native area: populations established in Central Europe appear to have originated from eastern North America, and Eastern European populations from more western North America. This may result from differential commercial exchanges between these geographic regions. Our results indicate that the expansion in Europe mostly occurred through long-distance dispersal, explaining the absence of isolation by distance and the weak influence of geography on the genetic structure in this area in contrast to the native range. Last, we detected significant heterozygote deficiencies in most populations. This may be explained by partial selfing, biparental inbreeding and/or a Wahlund effect and further investigation is warranted.

CONCLUSIONS

This insight into the sources and pathways of common ragweed expansion may help to better understand its invasion success and provides baseline data for future studies on the evolutionary processes involved during range expansion in novel environments.

Chromosomal inversions, natural selection and adaptation in the malaria vector Anopheles funestus

Chromosomal polymorphisms, such as inversions, are presumably involved in the rapid adaptation of populations to local environmental conditions. Reduced recombination between alternative arrangements in heterozygotes may protect sets of locally adapted genes, promoting ecological divergence and potentially leading to reproductive isolation and speciation. Through a comparative analysis of chromosomal inversions and microsatellite marker polymorphisms, we hereby present biological evidence that strengthens this view in the mosquito Anopheles funestus s.s, one of the most important and widespread malaria vectors in Africa. Specimens were collected across a wide range of geographical, ecological, and climatic conditions in Cameroon. We observed a sharp contrast between population structure measured at neutral microsatellite markers and at chromosomal inversions. Microsatellite data detected only a weak signal for population structuring among geographical zones (F(ST) < 0.013, P < 0.01). By contrast, strong differentiation among ecological zones was revealed by chromosomal inversions (F(ST) > 0.190, P < 0.01). Using standardized estimates of F(ST), we show that inversions behave at odds with neutral expectations strongly suggesting a role of environmental selection in shaping their distribution. We further demonstrate through canonical correspondence analysis that heterogeneity in eco-geographical variables measured at specimen sampling sites explained 89% of chromosomal variance in A. funestus. These results are in agreement with a role of chromosomal inversions in ecotypic adaptation in this species. We argue that this widespread mosquito represents an interesting model system for the study of chromosomal speciation mechanisms and should provide ample opportunity for comparative studies on the evolution of reproductive isolation and speciation in major human malaria vectors.

CpDNA-based species identification and phylogeography: application to African tropical tree species

Despite the importance of the African tropical rainforests as a hotspot of biodiversity, their history and the processes that have structured their biodiversity are understood poorly. With respect to past demographic processes, new insights can be gained through characterizing the distribution of genetic diversity. However, few studies of this type have been conducted in Central Africa, where the identification of species in the field can be difficult. We examine here the distribution of chloroplast DNA (cpDNA) diversity in Lower Guinea in two tree species that are difficult to distinguish, Erythrophleum ivorense and Erythrophleum suaveolens (Fabaceae). By using a blind-sampling approach and comparing molecular and morphological markers, we first identified retrospectively all sampled individuals and determined the limits of the distribution of each species. We then performed a phylogeographic study using the same genetic data set. The two species displayed essentially parapatric distributions that were correlated well with the rainfall gradient, which indicated different ecological requirements. In addition, a phylogeographic structure was found for E. suaveolens and, for both species, substantially higher levels of diversity and allelic endemism were observed in the south (Gabon) than in the north (Cameroon) of the Lower Guinea region. This finding indicated different histories of population demographics for the two species, which might reflect different responses to Quaternary climate changes. We suggest that a recent period of forest perturbation, which might have been caused by humans, favoured the spread of these two species and that their poor recruitment at present results from natural succession in their forest formations.

A fast and cost-effective approach to develop and map EST-SSR markers: oak as a case study

Background

Expressed Sequence Tags (ESTs) are a source of simple sequence repeats (SSRs) that can be used to develop molecular markers for genetic studies. The availability of ESTs for Quercus robur and Quercus petraea provided a unique opportunity to develop microsatellite markers to accelerate research aimed at studying adaptation of these long-lived species to their environment. As a first step toward the construction of a SSR-based linkage map of oak for quantitative trait locus (QTL) mapping, we describe the mining and survey of EST-SSRs as well as a fast and cost-effective approach (bin mapping) to assign these markers to an approximate map position. We also compared the level of polymorphism between genomic and EST-derived SSRs and address the transferability of EST-SSRs in Castanea sativa (chestnut).

Results

A catalogue of 103,000 Sanger ESTs was assembled into 28,024 unigenes from which 18.6% presented one or more SSR motifs. More than 42% of these SSRs corresponded to trinucleotides. Primer pairs were designed for 748 putative unigenes. Overall 37.7% (283) were found to amplify a single polymorphic locus in a reference full-sib pedigree of Quercus robur. The usefulness of these loci for establishing a genetic map was assessed using a bin mapping approach. Bin maps were constructed for the male and female parental tree for which framework linkage maps based on AFLP markers were available. The bin set consisting of 14 highly informative offspring selected based on the number and position of crossover sites. The female and male maps comprised 44 and 37 bins, with an average bin length of 16.5 cM and 20.99 cM, respectively. A total of 256 EST-SSRs were assigned to bins and their map position was further validated by linkage mapping. EST-SSRs were found to be less polymorphic than genomic SSRs, but their transferability rate to chestnut, a phylogenetically related species to oak, was higher.

Conclusion

We have generated a bin map for oak comprising 256 EST-SSRs. This resource constitutes a first step toward the establishment of a gene-based map for this genus that will facilitate the dissection of QTLs affecting complex traits of ecological importance.

Genetic discontinuities and disequilibria in recently established populations of the plant pathogenic fungus Mycosphaerella fijiensis

Dispersal processes of fungal plant pathogens can be inferred from analysis of spatial genetic structures resulting from recent range expansion. The relative importance of long-distance dispersal (LDD) events vs. gradual dispersal in shaping population structures depends on the geographical scale considered. The fungus Mycosphaerella fijiensis, pathogenic on banana, is an example of a recent worldwide epidemic. Founder effects in this species were detected at both global and continental scale, suggesting stochastic spread of the disease through LDD events. In this study, we analysed the structure of M. fijiensis populations in two recently (∼1979-1980) colonized areas in Costa Rica and Cameroon. Isolates collected in 10-15 sites distributed along a ∼250- to 300- km-long transect in each country were analysed using 19 microsatellite markers. We detected low-to-moderate genetic differentiation among populations in both countries and isolation by distance in Cameroon. Combined with historical data, these observations suggest continuous range expansion at the scale of banana-production area through gradual dispersal of spores. However, both countries displayed specific additional signatures of colonization: a sharp discontinuity in gene frequencies was observed along the Cameroon transect, while the Costa Rican populations seemed not yet to have reached genetic equilibrium. These differences in the genetic characteristics of M. fijiensis populations in two recently colonized areas are discussed in the light of historical data on disease spread and ecological data on landscape features.

Population substructure in continuous and fragmented stands of Populus trichocarpa

Population substructure has important implications for both basic and applied genetic research. We used 10 microsatellite markers to characterize population substructure in two ecologically and demographically contrasting populations of the model tree Populus trichocarpa. The Marchel site was a continuous stand growing in a mesic habitat in western Oregon, whereas the Vinson site consisted of three disjunct and isolated stands in the high desert of eastern Oregon. A previous study revealed that pollen-mediated gene flow is extensive in both populations. Surprisingly, model-based clustering, principal components analysis and analyses of molecular variance provided overwhelming support for the existence of at least two intermingled sub-populations within the continuous Marchel population (F(ST)=0.026, P<0.001), which occupied an area with a radius of only about 250 m. Genets in these two sub-populations appeared to have different relative clone ages and phenologies, leading us to hypothesize that they correspond to different seedling cohorts, each established from seeds produced by relatively few mothers. As expected, substructure was stronger in the fragmented Vinson population (F(ST)=0.071, P=0.001), and this difference appeared to result from the more extensive family structure in this population. Using group-likelihood methods, we reconstructed multiple interconnected half-sib families in the Vinson population, with some genets having as many as eight putative siblings. Researchers involved in ongoing and future association studies in P. trichocarpa should account for the likely presence of subtle but practically significant substructure in populations throughout the range of this species.

Population genetic analysis of the Brahman cattle (Bos indicus) in Colombia with microsatellite markers

The Brahman is one of the most popular breeds for meat production in the Neotropics. However, genetic studies of the breed in Latin American countries have only recently begun. In total, 178 animals of the Brahman breed from 20 Colombian provinces were genotyped at 11 microsatellite markers with the aim of studying the genetic diversity of this population and its genetic relationships with zebuine and taurine breeds. The outcomes of multivariate analyses, Bayesian inferences and inter-individual genetic distances suggested that there is no genetic sub-structure in the population, because of the high rate of animal migration among provinces. The population shows a high degree of heterozygosity and allelic diversity compared with other breeds, reflecting its multibreed origin. The study of the genetic relationships among the breeds reveals that the Brahman breed belongs to the zebuine group. However, it is the population nearest to taurine breeds with high frequencies of taurine alleles. Intensive artificial selection may have favoured the taurine alleles after the breed was formed. There has also been some degree of mixture with local taurine breeds while the Brahman breed has evolved in Colombia.

Fine-scale spatial genetic structure in a predominantly selfing plant: role of seed and pollen dispersal

We present a study of fine-scale spatial genetic structure (SGS) and assess the impact of seed and pollen dispersal on the pattern of genetic diversity in the predominantly selfing Hordeum spontaneum. The study included (1) direct measurement of dispersal in a controlled environment, and (2) analyses of SGS and estimation of the ratio of pollen to seed flow in three natural populations sampled in linear transects at fixed increasing inter-plant distances. Analysis of SGS with 10 nuclear SSRs showed in all three populations a significant autocorrelation for the distance classes of 1 or 2 m and a negative linear relationship between kinship coefficients, calculated for pairs of individuals, and logarithm of geographical distance between members of the pairs. Major seed dispersal (95%) was found to be within 1.2 m from the mother plant. Pollen flow, estimated from the comparison of nuclear and chloroplast variation, was spatially limited as much as was seed dispersal, and tended to be overestimated when measured at spatial scales exceeding that of SGS. We conclude that combined effects of selfing, occasional outcrossing, localized seed dispersal and high plant density create an equilibrium between drift and gene flow in this species resulting in SGS at a very fine spatial scale.

Microsatellite genetic differentiation among populations of the Trinidadian guppy

Insight into the processes of evolutionary change can be obtained by studying the distribution of genetic diversity among populations. Such diversity can be shaped by historical colonization events, population connectivity and adaptation to local selection pressures. Here we examine genetic differentiation of Trinidadian guppies, Poecilia reticulata, by genotyping 373 individuals from 15 populations located in three drainages (northern coast, Caroni and Oropouche) with 7 microsatellite loci. Our data provide little evidence to support previous claims of two major genetic lineages of guppies in northern Trinidad but instead suggest a more complex pattern of gene flow among populations from different drainages. First, some of the populations in the Caroni drainage show genetic signatures similar to those in the Oropouche drainage. Second, the populations in the northern coast are all highly differentiated from those in either the Caroni or Oropouche drainages. Despite differing selection regimes owing to predation pressure, populations from upstream and downstream locales typically cluster together, albeit upstream populations consistently have less genetic variability than the corresponding downstream population. There is, however, no overall pattern of isolation by distance. We also find evidence that an artificially transplanted population from the Caroni drainage is successfully invading into other populations within the Oropouche system. Our analysis details the genetic and phylogeographic structure of Trinidadian guppies in the northern range and provides insight into evolutionary processes at different timescales that have shaped genetic heterogeneity in this fish.

The origin of clonal diversity and structure of Populus alba in Sardinia: evidence from nuclear and plastid microsatellite markers

BACKGROUND AND AIMS

Populus alba is a thermophilic forest tree present in the Mediterranean basin. Its habitat is highly fragmented and its distribution range has been subject to long-term human interference, resulting in debate surrounding whether certain populations are native or exotic in origin. In particular, populations from the islands of Corsica and Sardinia are of uncertain origin. While populations of P. alba mainly reproduce sexually, clonal reproduction is also common. The aims of this study were to locate and molecularly characterize the poorly studied island populations of P. alba and compare these with samples from various spatial scales, in order to provide information on the genetic structure and phylogeography of this species. This information will provide evidence on whether the species is native to Sardinia, which is important for the development of conservation strategies.

METHODS

DNA extracts were obtained from the following P. alba trees: 159 from Sardinia, 47 from Ticino regional park (northern Italy), 15 acquired from an Italian Germoplasm Bank (IRC; Italian Reference Collection) and 28 from the Mediterranean basin (MB). Genetic polymorphisms were revealed at nuclear and chloroplast DNA (cpDNA) microsatellite loci, both at the island scale (Sardinia) and at broader scales, for comparative assessment of the genetic and genotypic diversity and phylogeography.

KEY RESULTS

Based on nuclear microsatellite loci, Sardinian white poplar consists of a small number of genets (26), each of which is represented by several ramets. Despite the uniqueness of the Sardinian haplotypes and the very low value of genetic diversity at the cpDNA level (vK = 0.15), the HT (0.60) and the AR (3.61) values, estimated at the nuclear level for Sardinia, were comparable with those of the other populations and collections.

CONCLUSIONS

The uniqueness of the cpDNA haplotypes, the prevalence of clonality and the restricted number of genets recorded suggest that Sardinian white poplar could be a floristic relict of the native flora of the island, which has spread through available habitats on the island mainly by means of vegetative propagation and human activities.

Conserving the evolutionary potential of California valley oak (Quercus lobataNée): a multivariate genetic approach to conservation planning

California valley oak (Quercus lobata Née) is a seriously threatened endemic oak species in California and a keystone species for foothill oak ecosystems. Urban and agricultural development affects a significant fraction of the species' range and predicted climate change is likely to dislocate many current populations. Here, we explore spatial patterns of multivariate genotypes and genetic diversity throughout the range of valley oak to determine whether ongoing and future patterns of habitat loss could threaten the evolutionary potential of the species by eradicating populations of distinctive genetic composition. This manuscript will address three specific questions: (i) What is the spatial genetic structure of the chloroplast and nuclear genetic markers? (ii) What are the geographical trends in the distribution of chloroplast and nuclear genotypes? (iii) Is there any part of the species' range where allelic diversity in either the chloroplast or nuclear genomes is particularly high? We analysed six chloroplast and seven nuclear microsatellite genetic markers of individuals widespread across the valley oak range. We then used a multivariate approach correlating genetic markers and geographical variables through a canonical trend surface analysis, followed by GIS mapping of the significant axes. We visualized population allelic richness spatially with GIS tools to identify regions of high diversity. Our findings, based on the distribution of multivariate genotypes and allelic richness, identify areas with distinctive histories and genetic composition that should be given priority in reserve network design, especially because these areas also overlap with landscape change and little degree of protection. Thus, without a careful preservation plan, valuable evolutionary information will be lost for valley oak.

Spatial analysis of nuclear and cytoplasmic DNA diversity in wild sea beet (Beta vulgaris ssp. maritima) populations: do marine currents shape the genetic structure?

Patterns of seed dispersal in the wild sea beet (Beta vulgaris ssp. maritima) are predicted to be influenced by marine currents because populations are widely distributed along the European Atlantic coast. We investigated the potential influence of marine currents on the pattern of spatial genetic structuring in natural populations of sea beet. Populations were located along the French coasts of the Anglo-Norman gulf that features peculiar marine currents in the Channel. Thirty-three populations were sampled, among which 23 were continental and 10 were insular populations located in Jersey, Guernsey and Chausey, for a total of 1224 plants genotyped. To validate the coastal topography influence and the possibility of marine current orientated gene flow on the genetic features of sea beet populations, we assessed patterns of genetic structuring of cytoplasmic and nuclear diversity by: (i) searching for an isolation-by-distance (IBD) pattern using spatial autocorrelation tools; (ii) using the Monmonier algorithm to identify genetic boundaries in the area studied; and (iii) performing assignment tests that are based on multilocus genotype information to ascertain population membership of individuals. Our results showed a highly contrasted cytoplasmic and nuclear genetic differentiation and highlighted the peculiar situation of island populations. Beyond a classical isolation-by-distance due to short-range dispersal, genetic barriers fitting the orientation of marine currents were clearly identified. This suggests the occurrence of long-distance seed dispersal events and an asymmetrical gene flow separating the eastern and western part of the Anglo-Norman gulf.

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.