RESUMO
Owing to the reduction of population density and/or the environmental changes it induces, selective logging could affect the demography, reproductive biology and evolutionary potential of forest trees. This is particularly relevant in tropical forests where natural population densities can be low and isolated trees may be subject to outcross pollen limitation and/or produce low-quality selfed seeds that exhibit inbreeding depression. Comparing reproductive biology processes and genetic diversity of populations at different densities can provide indirect evidence of the potential impacts of logging. Here, we analysed patterns of genetic diversity, mating system and gene flow in three Central African populations of the self-compatible legume timber species Erythrophleum suaveolens with contrasting densities (0.11, 0.68 and 1.72 adults per ha). The comparison of inbreeding levels among cohorts suggests that selfing is detrimental as inbred individuals are eliminated between seedling and adult stages. Levels of genetic diversity, selfing rates (â¼16%) and patterns of spatial genetic structure (Sp â¼0.006) were similar in all three populations. However, the extent of gene dispersal differed markedly among populations: the average distance of pollen dispersal increased with decreasing density (from 200 m in the high-density population to 1000 m in the low-density one). Overall, our results suggest that the reproductive biology and genetic diversity of the species are not affected by current logging practices. However, further investigations need to be conducted in low-density populations to evaluate (1) whether pollen limitation may reduce seed production and (2) the regeneration potential of the species.
Assuntos
Fabaceae/genética , Fluxo Gênico , Variação Genética , Genética Populacional , Polinização , Árvores/genética , África , Agricultura Florestal , Endogamia , Modelos Genéticos , Densidade Demográfica , Floresta ÚmidaRESUMO
Cycles of Quaternary climatic change are assumed to be major drivers of African rainforest dynamics and evolution. However, most hypotheses on past vegetation dynamics relied on palaeobotanical records, an approach lacking spatial resolution, and on current patterns of species diversity and endemism, an approach confounding history and environmental determinism. In this context, a comparative phylogeographical study of rainforest species represents a complementary approach because Pleistocene climatic fluctuations may have left interpretable signatures in the patterns of genetic diversity within species. Using 1274 plastid DNA sequences from eight tree species (Afrostyrax kamerunensis, A. lepidophyllus, Erythrophleum suaveolens, Greenwayodendron suaveolens, Milicia excelsa, Santiria trimera, Scorodophloeus zenkeri and Symphonia globulifera) sampled in 50 populations of Atlantic Central Africa (ACA), we averaged divergence across species to produce the first map of the region synthesizing genetic distinctiveness and standardized divergence within and among localities. Significant congruence in divergence was detected mostly among five of the eight species and was stronger in the northern ACA. This pattern is compatible with a scenario of past forest fragmentation and recolonization whereby forests from eastern Cameroon and northeastern Gabon would have been more affected by past climatic change than those of western Cameroon (where one or more refugia would have occurred). By contrast, southern ACA (Gabon) displayed low congruence among species that may reflect less drastic past forest fragmentation or a more complex history of vegetation changes. Finally, we also highlight the potential impact of current environmental barriers on spatial genetic structures.
Assuntos
Mudança Climática , Evolução Molecular , Variação Genética , Genética Populacional , Árvores/genética , África Central , DNA de Cloroplastos/genética , DNA de Plantas/genética , Haplótipos , Filogenia , Filogeografia , Dinâmica PopulacionalRESUMO
Gene flow within and between species is a fundamental process shaping the evolutionary history of taxa. However, the extent of hybridization and reinforcement is little documented in the tropics. Here we explore the pattern of gene flow between three sister species from the herbaceous genus Marantochloa (Marantaceae), sympatrically distributed in the understorey of the African rainforest, using data from the chloroplast and nuclear genomes (DNA sequences and AFLP). We found highly contrasting patterns: while there was no evidence of gene flow between M. congensis and M. monophylla, species identity between M. monophylla and M. incertifolia was maintained despite considerable gene flow. We hypothesize that M. incertifolia originated from an ancient hybridization event between M. congensis and M. monophylla, considering the current absence of hybridization between the two assumed parent species, the rare presence of shared haplotypes between all three species and the high percentage of haplotypes shared by M. incertifolia with each of the two parent species. This example is contrasted with two parapatrically distributed species from the same family in the genus Haumania forming a hybrid zone restricted to the area of overlap. This work illustrates the diversity of speciation/introgression patterns that can potentially occur in the flora of tropical Africa.
Assuntos
Fluxo Gênico , Marantaceae/genética , Filogenia , África , DNA de Plantas/genética , Florestas , Haplótipos , Hibridização Genética , Filogeografia , Análise de Sequência de DNARESUMO
Rapid geographic range expansions can have dramatic effects on the distribution of genetic diversity, both within and among populations. Based on field records collected over the past two decades in Western Europe, we report on the rapid geographic range expansion in Colletes hederae, a solitary bee species. To characterize how this expansion shaped the distribution of genetic diversity within and among populations, we performed a genetic analysis based on the sequencing of three nuclear loci (RNAp, CAD and WgL). We then simulated the evolution of DNA sequences under a spatially explicit model of coalescence to compare different hypotheses regarding the mode of colonization associated with this rapid expansion and to identify those that are most consistent with the observed molecular data. Our genetic analyses indicate that the range expansion was not associated with an important reduction in genetic diversity, even in the most recently colonized area in the United Kingdom. Moreover, little genetic differentiation was observed among populations. Our comparative analysis of simulated data sets indicates that the observed genetic data are more consistent with a demographic scenario involving relatively high migration rates than with a scenario based on a high reproduction rate associated with few migrants. In the light of these results, we discuss the factors that might have contributed to the rapid geographic range expansion of this pollen-specialist solitary bee species across Western Europe.
Assuntos
Abelhas/genética , Animais , Sequência de Bases , Europa (Continente) , Evolução Molecular , Feminino , Variação Genética , Masculino , Dados de Sequência Molecular , FilogeografiaRESUMO
The long generation time and large effective size of widespread forest tree species can result in slow evolutionary rate and incomplete lineage sorting, complicating species delimitation. We addressed this issue with the African timber tree genus Milicia that comprises two morphologically similar and often confounded species: M. excelsa, widespread from West to East Africa, and M. regia, endemic to West Africa. We combined information from nuclear microsatellites (nSSRs), nuclear and plastid DNA sequences, and morphological systematics to identify significant evolutionary units and infer their evolutionary and biogeographical history. We detected five geographically coherent genetic clusters using nSSRs and three levels of genetic differentiation. First, one West African cluster matched perfectly with the morphospecies M. regia that formed a monophyletic clade at both DNA sequences. Second, a West African M. excelsa cluster formed a monophyletic group at plastid DNA and was more related to M. regia than to Central African M. excelsa, but shared many haplotypes with the latter at nuclear DNA. Third, three Central African clusters appeared little differentiated and shared most of their haplotypes. Although gene tree paraphyly could suggest a single species in Milicia following the phylogenetic species concept, the existence of mutual haplotypic exclusivity and nonadmixed genetic clusters in the contact area of the two taxa indicate strong reproductive isolation and, thus, two species following the biological species concept. Molecular dating of the first divergence events showed that speciation in Milicia is ancient (Tertiary), indicating that long-living tree taxa exhibiting genetic speciation may remain similar morphologically.
Assuntos
Evolução Biológica , Especiação Genética , Moraceae/anatomia & histologia , Moraceae/genética , Filogenia , África , Sequência de Bases , Teorema de Bayes , Primers do DNA/genética , Evolução Molecular , Haplótipos/genética , Repetições de Microssatélites/genética , Modelos Genéticos , Dados de Sequência Molecular , Família Multigênica/genética , Filogeografia , Análise de Sequência de DNARESUMO
AFLP markers are often used to study patterns of population genetic variation and gene flow because they offer a good coverage of the nuclear genome, but the reliability of AFLP scoring is critical. To assess interspecific gene flow in two African rainforest liana species (Haumania danckelmaniana, H. liebrechtsiana) where previous evidence of chloroplast captures questioned the importance of hybridization and species boundaries, we developed new AFLP markers and a novel approach to select reliable bands from their degree of reproducibility. The latter is based on the estimation of the broad-sense heritability of AFLP phenotypes, an improvement over classical scoring error rates, which showed that the polymorphism of most AFLP bands was affected by a substantial nongenetic component. Therefore, using a quantitative genetics framework, we also modified an existing estimator of pairwise kinship coefficient between individuals correcting for the limited heritability of markers. Bayesian clustering confirms the recognition of the two Haumania species. Nevertheless, the decay of the relatedness between individuals of distinct species with geographic distance demonstrates that hybridization affects the nuclear genome. In conclusion, although we showed that AFLP markers might be substantially affected by nongenetic factors, their analysis using the new methods developed considerably advanced our understanding of the pattern of gene flow in our model species.
Assuntos
Análise do Polimorfismo de Comprimento de Fragmentos Amplificados/métodos , Fluxo Gênico , Variação Genética , Marantaceae/genética , África Central , Teorema de Bayes , DNA de Plantas/genética , Marcadores Genéticos , Haplótipos , Hibridização Genética , Marantaceae/classificação , Filogenia , Filogeografia , Reprodutibilidade dos TestesRESUMO
Understanding the history of forests and their species' demographic responses to past disturbances is important for predicting impacts of future environmental changes. Tropical rainforests of the Guineo-Congolian region in Central Africa are believed to have survived the Pleistocene glacial periods in a few major refugia, essentially centred on mountainous regions close to the Atlantic Ocean. We tested this hypothesis by investigating the phylogeographic structure of a widespread, ancient rainforest tree species, Symphonia globulifera L. f. (Clusiaceae), using plastid DNA sequences (chloroplast DNA [cpDNA], psbA-trnH intergenic spacer) and nuclear microsatellites (simple sequence repeats, SSRs). SSRs identified four gene pools located in Benin, West Cameroon, South Cameroon and Gabon, and São Tomé. This structure was also apparent at cpDNA. Approximate Bayesian Computation detected recent bottlenecks approximately dated to the last glacial maximum in Benin, West Cameroon and São Tomé, and an older bottleneck in South Cameroon and Gabon, suggesting a genetic effect of Pleistocene cycles of forest contraction. CpDNA haplotype distribution indicated wide-ranging long-term persistence of S. globulifera both inside and outside of postulated forest refugia. Pollen flow was four times greater than that of seed in South Cameroon and Gabon, which probably enabled rapid population recovery after bottlenecks. Furthermore, our study suggested ecotypic differentiation-coastal or swamp vs terra firme-in S. globulifera. Comparison with other tree phylogeographic studies in Central Africa highlighted the relevance of species-specific responses to environmental change in forest trees.
Assuntos
Clusiaceae/genética , Evolução Molecular , Árvores/genética , África , Clusiaceae/classificação , DNA de Cloroplastos/genética , Ecossistema , Variação Genética , Filogeografia , Árvores/classificaçãoRESUMO
The main factor of differentiation at six nuclear microsatellite and seven cpDNA loci in Salicornia from the Mediterranean and Atlantic coasts of France is cytotypic identity, suggesting the presence of a strong reproductive barrier among sympatric cytotypes. Within cytotypes, a substantial proportion of the differentiation between species is due to confounded phylogeographic signal. Conspecific individuals tend to be significantly more related than individuals from different species at the population scale, but mean kinship coefficients among pairs of conspecific and nonconspecific individuals from different populations are not significantly different, suggesting the absence of reproductive isolation among species of the same cytotype. The observed association between morphology and genetic variation within populations would thus result from the selfing mating system (F(is)) = 0.70) generating substantial linkage within the genome, linkage that would quickly disappear among unrelated individuals from different populations. Salicornia species thus function as a network of inbred populations, strongly challenging taxonomic concepts.
Assuntos
Evolução Biológica , Chenopodiaceae/classificação , Chenopodiaceae/genética , Variação Genética , Chenopodiaceae/fisiologia , DNA de Plantas/genética , Demografia , França , Repetições de Microssatélites , PoliploidiaRESUMO
African rainforests have undergone major distribution range shifts during the Quaternary, but few studies have investigated their impact on the genetic diversity of plant species and we lack knowledge on the extent of gene flow to predict how plant species can cope with such environmental changes. Analysis of the spatial genetic structure (SGS) of a species is an effective method to determine major directions of the demographic history of its populations and to estimate the extent of gene dispersal. This study characterises the SGS of an African tropical timber tree species, Distemonanthus benthamianus, at various spatial scales in Cameroon and Gabon. Displaying a large continuous distribution in the Lower Guinea domain, this is a model species to detect signs of past population fragmentation and recolonization, and to estimate the extent of gene dispersal. Ten microsatellite loci were used to genotype 295 adult trees sampled from eight populations. Three clearly differentiated gene pools were resolved at this regional scale and could be linked to the biogeographical history of the region, rather than to physical barriers to gene flow. A comparison with the distribution of gene pools observed for two other tree species living in the same region invalidates the basic assumption that all species share the same Quaternary refuges and recolonization pathways. In four populations, significant and similar patterns of SGS were detected. Indirect estimates of gene dispersal distances (sigma) obtained for three populations ranged from 400 to 1200 m, whereas neighbourhood size estimates ranged from 50 to 110.
Assuntos
Fabaceae/genética , Genes de Plantas , Variação Genética , Árvores/genética , Camarões , Fabaceae/classificação , Gabão , Filogenia , PopulaçãoRESUMO
The effect of selection on patterns of genetic structure within and between populations may be studied by contrasting observed patterns at the genes targeted by selection with those of unlinked neutral marker loci. Local directional selection on target genes will produce stronger population genetic structure than at neutral loci, whereas the reverse is expected for balancing selection. However, theoretical predictions on the intensity of this signal under precise models of balancing selection are still lacking. Using negative frequency-dependent selection acting on self-incompatibility systems in plants as a model of balancing selection, we investigated the effect of such selection on patterns of spatial genetic structure within a continuous population. Using numerical simulations, we tested the effect of the type of self-incompatibility system, the number of alleles at the self-incompatibility locus and the dominance interactions among them, the extent of gene dispersal, and the immigration rate on spatial genetic structure at the selected locus and at unlinked neutral loci. We confirm that frequency-dependent selection is expected to reduce the extent of spatial genetic structure as compared to neutral loci, particularly in situations with low number of alleles at the self-incompatibility locus, high frequency of codominant interactions among alleles, restricted gene dispersal and restricted immigration from outside populations. Hence the signature of selection on spatial genetic structure is expected to vary across species and populations, and we show that empirical data from the literature as well as data reported here on three natural populations of the herb Arabidopsis halleri confirm these theoretical results.
Assuntos
Arabidopsis/genética , Loci Gênicos , Polinização , Seleção Genética , Autofertilização , Simulação por Computador , Deriva Genética , Genótipo , Repetições de Microssatélites , Modelos GenéticosRESUMO
Genetic diversity and structure are described in the aquatic moss Platyhypnidium riparioides to assess its dispersal ability at a regional scale and to determine whether patterns of genetic differentiation correlate with environmental variation. Variation at six nuclear microsatellite loci from 50 populations in southern Belgium was investigated through Mantel tests, partial Mantel tests and spatial analysis of molecular variance. Overall patterns of genotypic variation showed strong differentiation among populations at a regional scale (F(ST) = 0.57). The high values of F(IS) observed within populations at both the ramet and genet levels, and the higher proportion of ramets with the same genotype than expected by chance, all point to a strongly clonal or selfing mating system. A genetic discontinuity was identified between northern and southern groups of populations. Within each group, F(ST) and geographical distances were significantly correlated. Partial Mantel tests suggest that genetic and ecological distances are significantly correlated in the southern group. The results point to strong dispersal limitation at the landscape scale and suggest that the southern and northern groups experienced different histories. Within the former, the correlation between genetic and ecological variation is suggestive of reproductive isolation among ecotypes.
Assuntos
Briófitas/genética , Fenômenos Ecológicos e Ambientais , Variação Genética , Água , Alelos , Bélgica , Frequência do Gene/genética , Geografia , Heterozigoto , Repetições de Microssatélites/genética , Ploidias , Dinâmica Populacional , Análise de Componente Principal , Tamanho da AmostraRESUMO
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.
Assuntos
DNA de Cloroplastos/genética , Filogeografia , Árvores/classificação , Árvores/genética , África Central , Biodiversidade , Fabaceae/classificação , Fabaceae/genética , Variação Genética/genéticaRESUMO
We analysed the spatial distribution of genetic diversity to infer gene flow for Baillonella toxisperma Pierre (Moabi), a threatened entomophilous pollinated and animal-dispersed Central African tree, with typically low density (5-7 adults trees/km(2)). Fifteen nuclear and three universal chloroplast microsatellites markers were used to type 247 individuals localized in three contiguous areas with differing past logging intensity. These three areas were within a natural forest block of approximately 2886 km(2) in Gabon. Expected heterozygosity and chloroplast diversity were He(nuc) = 0.570 and H(cp) = 0.761, respectively. F(IS) was only significant in one area (F(IS) = 0.076, P < 0.01) and could be attributed to selfing. For nuclear loci, Bayesian clustering did not detect discrete gene pools within and between the three areas and global differentiation (F(STnuc) = 0.007, P > 0.05) was not significant, suggesting that they are one population. At the level of the whole forest, both nuclear and chloroplast markers revealed a weak correlation between genetic relatedness and spatial distance between individuals: Sp(nuc) = 0.003 and Sp(cp) = 0.015, respectively. The extent of gene flow (σ) was partitioned into global gene flow (σ(g)) from 6.6 to 9.9 km, seed dispersal (σ(s)) from 4.0 to 6.3 km and pollen dispersal (σ(p)) from 9.8 to 10.8 km. These uncommonly high dispersal distances indicate that low-density canopy trees in African rainforests could be connected by extensive gene flow, although, given the current threats facing many seed disperser species in Central Africa, this may no longer be the case.
Assuntos
Estruturas Genéticas , Pólen/genética , Dispersão de Sementes , Sementes/genética , Árvores/genética , África Central , Animais , Cloroplastos/genética , Ecossistema , Fluxo Gênico , Variação Genética , Genética Populacional , Humanos , Endogamia , Repetições de Microssatélites , Polinização , Clima TropicalRESUMO
Species delimitation is a fundamental biological concept which is frequently discussed and altered to integrate new insights. These revealed that speciation is not a one step phenomenon but an ongoing process and morphological characters alone are not sufficient anymore to properly describe the results of this process. Here we want to assess the degree of speciation in two closely related lianescent taxa from the tropical African genus Haumania which display distinct vegetative traits despite a high similarity in reproductive traits and a partial overlap in distribution area which might facilitate gene flow. To this end, we combined phylogenetic and phylogeographic analyses using nuclear (nr) and chloroplast (cp) DNA sequences in comparison to morphological species descriptions. The nuclear dataset unambiguously supports the morphological species concept in Haumania. However, the main chloroplastic haplotypes are shared between species and, although a geographic analysis of cpDNA diversity confirms that individuals from the same taxon are more related than individuals from distinct taxa, cp-haplotypes display correlated geographic distributions between species. Hybridization is the most plausible reason for this pattern. A scenario involving speciation in geographic isolation followed by range expansion is outlined. The study highlights the gain of information on the speciation process in Haumania by adding georeferenced molecular data to the morphological characteristics. It also shows that nr and cp sequence data might provide different but complementary information, questioning the reliability of the unique use of chloroplast data for species recognition by DNA barcoding.
Assuntos
DNA de Cloroplastos/genética , DNA de Plantas/genética , Marantaceae/classificação , Marantaceae/genética , Filogenia , África Central , Código de Barras de DNA Taxonômico , Fluxo Gênico/genética , Filogeografia , Especificidade da EspécieRESUMO
In this study, we analysed spatial genetic structure (SGS) patterns and estimated dispersal distances in Milicia excelsa (Welw.) C.C. Berg (Moraceae), a threatened wind-pollinated dioecious African tree, with typically low density (approximately 10 adults/km(2)). Eight microsatellite markers were used to type 287 individuals in four Cameroonian populations characterized by different habitats and tree densities. Differentiation among populations was very low. Two populations in more open habitat did not display any correlation between genetic relatedness and spatial distance between individuals, whereas significant SGS was detected in two populations situated under continuous forest cover. SGS was weak with a maximum S(p)-statistic of 0.006, a value in the lower quartile of SGS estimates for trees in the literature. Using a stepwise approach with Bayesian clustering methods, we demonstrated that SGS resulted from isolation by distance and not colonization by different gene pools. Indirect estimates of gene dispersal distances ranged from sigma(g) = 1 to 7.1 km, one order of magnitude higher than most estimates found in the literature for tropical tree species. This result can largely be explained by life-history traits of the species. Milicia excelsa exhibits a potentially wide-ranging wind-mediated pollen dispersal mechanism as well as very efficient seed dispersal mediated by large frugivorous bats. Estimations of gene flow suggested no major risk of inbreeding because of reduction in population density by exploitation. Different strategy of seed collection may be required for reforestation programmes among populations with different extent of SGS.
Assuntos
Fluxo Gênico , Variação Genética , Genética Populacional , Moraceae/genética , Teorema de Bayes , Análise por Conglomerados , DNA de Plantas/genética , Ecossistema , Pool Gênico , Genótipo , Repetições de Microssatélites , Polinização , Densidade Demográfica , Análise de Sequência de DNA , Árvores/genética , VentoRESUMO
The extent and spatial patterns of genetic variation at allozyme markers were investigated within and between diploid and autotetraploid knapweeds (Centaurea jacea L. sensu lato, Asteraceae) at contrasted geographic scales: (1) among populations sampled from a diploid-tetraploid contact zone in the northeastern part of the Belgian Ardennes, and (2) within mixed populations from that zone where diploids and tetraploids coexist. Our data were also compared with a published dataset by Sommer (1990) describing allozyme variation in separate diploid and tetraploid knapweeds populations collected throughout Europe. Genetic diversity was higher in tetraploids. In the Belgian Ardennes and within the mixed populations, both cytotypes had similar levels of spatial genetic structure, they were genetically differentiated, and their distributions of allele frequencies were not spatially correlated. In contrast, at the European scale, diploids and tetraploids did not show differentiated gene pools and presented a strong correlation between their patterns of spatial genetic variation. Numerical simulations showed that the striking difference in patterns observed at small and large geographic scales could be accounted for by a combination of (1) isolation by distance within cytotypes; and (2) partial reproductive barriers between cytotypes and/or recurrent formation of tetraploids. We suggest that this may explain the difficulty of the taxonomic treatment of knapweeds and of polyploid complexes in general.
Assuntos
Asteraceae/enzimologia , Asteraceae/genética , Variação Genética , Bélgica , Evolução Biológica , Simulação por Computador , Diploide , Europa (Continente) , Genética Populacional , Modelos Genéticos , PoliploidiaRESUMO
In the northeastern part of Belgium, the Centaurea jacea complex shows extensive morphological variation and is represented by a diploid (2n = 22) and a tetraploid (2n = 44) cytotype. Polysomic inheritance of allozyme markers in the tetraploids, suggesting autopolyploidy, is here demonstrated for the first time. In order to test whether the two cytotypes occupy distinct habitats and possess different gene pools, patterns of allozymic and morphological variation were investigated in relation to ploidy level and site characteristics in 26 populations from the Belgian Ardennes. The two cytotypes showed a parapatric distribution, the diploids occurring at higher elevations (mostly above 500 m) than the tetraploids (mostly below 500 m). Three mixed populations were found near the contact zone of the two cytotypes. Within the mixed populations no triploid plant and no evidence for gene flow between cytotypes were found, despite widely overlapping flowering periods. The two cytotypes can be distinguished on the basis of morphological traits and enzymatic gene pools. The congruence of morphological and allozymic variation with chromosome numbers suggests a secondary contact between the two cytotypes with limited gene flow between them. The origin and persistence of the parapatric distribution are discussed.
RESUMO
Eight microsatellite loci from the aquatic moss Platyhypnidium riparioides were identified using the method of microsatellite-enriched libraries. Polymorphism was assessed in a sample of four populations of 20 individuals each from four streams of the Meuse hydrographic basin in southern Belgium. The markers amplified three to seven alleles per locus. Comparison of observed and expected heterozygosities as well as F-statistics (F(ST) = 0.62) reveals a significant genetic differentiation among populations. These markers will be useful for further investigation of population genetic structure and diversity at different nested spatial scales.
RESUMO
Oaks are model species for the study of natural introgressive hybridization. High interfertility among oak taxa might result in collective evolution, through transpecific spread of advantageous alleles, challenging the standard concept of species. Nine highly polymorphic microsatellite (nuSSR) loci were analysed in three mixed oak populations of Quercus pyrenaica and Quercus petraea (Montejo, Somosierra and Robregordo) with different density and hybridization rates. Both leaf morphology and molecular markers were used to assess individual admixture rates. Insights about the relative effect of density and hybridization rates on fine-scale spatial genetic structure (SGS) were obtained from autocorrelograms and Sp statistics. Differences in SGS among populations were higher than between species. These differences cannot be attributed solely to census densities but also relate to other factors, such as the spatial configuration of the population. Hybridization was an important factor shaping within-population spatial genetic structure, and an interspecific component of SGS was found in Somosierra. Indirect estimates of historical gene flow in Montejo were compared with actual values of gene dispersal assessed by parentage analysis in a former study. Similar values were found for current and historical gene flow in both species, which might reflect demographical stability.