Morphological and RAPD analysis of hybridization between Quercus affinis and Q. laurina (fagaceae), two Mexican red oaks

Nuclear and chloroplast DNA phylogeography reveals high genetic diversity and postglacial range expansion in Quercus mexicana

PREMISE

Phylogeographical studies are fundamental for understanding factors that influence the spatial distribution of genetic lineages within species. Population expansions and contractions, distribution shifts, and climate changes are among the most important factors shaping the genetic compositions of populations.

METHODS

We investigated the phylogeography of an endemic oak, Quercus mexicana (Fagaceae), which has a restricted distribution in northeastern Mexico along the Sierra Madre Oriental and adjacent areas. Nuclear and chloroplast DNA microsatellite markers were used to describe the genetic diversity and structure of 39 populations of Q. mexicana along its entire distribution area. We tested whether population expansion or contraction events influenced the genetic diversity and structure of the species. We also modeled the historical distributional range of Q. mexicana (for the Mid Holocene, the Last Glacial Maximum, and the Last Interglacial) to estimate the extent to which climate fluctuations have impacted the distribution of this oak species.

RESULTS

Our results revealed high genetic diversity and low genetic structure in Q. mexicana populations. Ecological niche models suggested historical fluctuations in the distributional range of Q. mexicana. Historical range changes, gene flow, and physical barriers seem to have played an important role in shaping the phylogeographic structure of Q. mexicana.

CONCLUSIONS

Our study indicates that the genetic structure of Q. mexicana may have been the result of responses of oak trees not only to heterogeneous environments present in the Sierra Madre Oriental and adjacent areas, but also to elevational and latitudinal shifts in response to climate changes in the past.

Strategies to mitigate shifts in red oak (Quercus sect. Lobatae) distribution under a changing climate

Red oaks (Quercus sect. Lobatae) are a taxonomic group of hardwood trees, which occur in swamp forests, subtropical chaparral and savannahs from Columbia to Canada. They cover a wide range of ecological niches, and many species are thought to be able to cope with current trends in climate change. Genus Quercus encompasses ca. 500 species, of which ca. 80 make up sect. Lobatae. Species diversity is greatest within the southeastern USA and within the northern and eastern regions of Mexico. This review discusses the weak reproductive barriers between species of red oaks and the effects this has on speciation and niche range. Distribution and diversity have been shaped by drought adaptations common to the species of sect. Lobatae, which enable them to fill various xeric niches across the continent. Drought adaptive traits of this taxonomic group include deciduousness, deep tap roots, ring-porous xylem, regenerative stump sprouting, greater leaf thickness and smaller stomata. The complex interplay between these anatomical and morphological traits has given red oaks features of drought tolerance and avoidance. Here, we discuss physiological and genetic components of these adaptations to address how many species of sect. Lobatae reside within xeric sites and/or sustain normal metabolic function during drought. Although extensive drought adaptation appears to give sect. Lobatae a resilience to climate change, aging tree stands, oak life history traits and the current genetic structures place many red oak species at risk. Furthermore, oak decline, a complex interaction between abiotic and biotic agents, has severe effects on red oaks and is likely to accelerate species decline and fragmentation. We suggest that assisted migration can be used to avoid species fragmentation and increase climate change resilience of sect. Lobatae.

Efficient regeneration of mature castanopsis hystrix from in vitro stem explants

Castanopsis hystrix is one of the main timber trees grown in China. However, severe shortage of natural seeds and the difficulty of explant regeneration has limited seedling supply. As such, there is a need for research on asexual multiplication of C. hystrix. This study established a rapid propagation technology system for C. hystrix genotypes, including explant treatment, proliferation, and rooting. HZ (a modified MS medium) supplemented with 4.4 μM BA and 0.5 μM IBA was found to be the optimal medium for shoot sprouting. The maximum proliferation coefficient and the number of effective shoots was obtained on HZ medium supplemented with 2.6 μM BA and 1.0 μM IBA, were 3.00 and 5.63, respectively. A rooting rate of 83.33% was achieved using half-strength HZ medium supplemented with 3.2 μM NAA. Adding vitamin C (80 mg⋅l-1) for 7 days in a dark environment reduced the browning rate, while increasing the proliferation rate. Additionally, through cytological observation, we established how and where adventitious roots occur. The survival rate of transplanted plantlets was > 90%. This is the first report of an in vitro regeneration technique that uses stem segments of mature C. hystrix as explants.

Leaf shape and size variation in bur oaks: an empirical study and simulation of sampling strategies

PREMISE

Leaf shape and size figure strongly in plants' adaptation to their environments. Among trees, oaks are notoriously variable in leaf morphology. Our study examines the degree to which within-tree, among-tree, and among-site variation contribute to latitudinal variation in leaf shape and size of bur oak (Quercus macrocarpa: Fagaceae), one of North America's most geographically widespread oak species.

METHODS

Samples were collected from four sites each at northern, central, and southern latitudes of the bur oak range. Ten leaf size traits were measured, and variance in these traits and eight ratios based on these traits was partitioned into tree and population components. Population means were regressed on latitude. We then parameterized a series of leaf collection simulations using empirical covariance among leaves on trees and trees at sites. We used the simulations to assess the efficiency of different collecting strategies for estimating among-population differences in leaf shape and size.

RESULTS

Leaf size was highly responsive to latitude. Site contributed more than tree to total variation in leaf shape and size. Simulations suggest that power to detect among-site variance in leaf shape and size increases with either more leaves per tree (10-11 leaves from each of 5 trees) or more trees per site (5 leaves from each of 10+ trees).

CONCLUSIONS

Our study demonstrates the utility of simulating sampling and controlling for variance in sampling for leaf morphology, whether the questions being addressed are ecological, evolutionary, or taxonomic. Simulation code is provided as an R package (traitsPopSim) to help researchers plan morphological sampling strategies.

Genomic landscape of the global oak phylogeny

The tree of life is highly reticulate, with the history of population divergence emerging from populations of gene phylogenies that reflect histories of introgression, lineage sorting and divergence. In this study, we investigate global patterns of oak diversity and test the hypothesis that there are regions of the oak genome that are broadly informative about phylogeny. We utilize fossil data and restriction-site associated DNA sequencing (RAD-seq) for 632 individuals representing nearly 250 Quercus species to infer a time-calibrated phylogeny of the world's oaks. We use a reversible-jump Markov chain Monte Carlo method to reconstruct shifts in lineage diversification rates, accounting for among-clade sampling biases. We then map the > 20 000 RAD-seq loci back to an annotated oak genome and investigate genomic distribution of introgression and phylogenetic support across the phylogeny. Oak lineages have diversified among geographic regions, followed by ecological divergence within regions, in the Americas and Eurasia. Roughly 60% of oak diversity traces back to four clades that experienced increases in net diversification, probably in response to climatic transitions or ecological opportunity. The strong support for the phylogeny contrasts with high genomic heterogeneity in phylogenetic signal and introgression. Oaks are phylogenomic mosaics, and their diversity may in fact depend on the gene flow that shapes the oak genome.

Influence of a climatic gradient on genetic exchange between two oak species

PREMISE

In plant groups with limited intrinsic barriers to gene flow, it is thought that environmental conditions can modulate interspecific genetic exchange. Oaks are known for limited barriers to gene flow among closely related species. Here, we use Quercus as a living laboratory in which to pursue a fundamental question in plant evolution: Do environmental gradients restrict or promote genetic exchange between species?

METHODS

We focused on two North American oaks, the rare Quercus dumosa and the widespread Q. berberidifolia. We sampled intensively along a contact zone in California, USA. We sequenced restriction site-associated DNA markers and measured vegetative phenotype. We tested for genetic exchange, the association with climate, and the effect on phenotype.

RESULTS

There is evidence for genetic exchange between the species. Admixed plants are found in areas of intermediate climate, while less admixed plants are found at the extremes of the climatic gradient. Genetic and phenotypic patterns are out of phase in the contact zone; some plants display the phenotype of one species but are genetically associated with another.

CONCLUSIONS

Our results support the hypothesis that a strong climatic gradient can promote genetic exchange between species. The overall weak correlation between genotype and phenotype in the contact zone between the species suggests that genetic exchange can lead to the breakdown of trait combinations used to define species. This incongruency predicts ongoing problems for conservation of Q. dumosa, with implications for conservation of other oaks.

Exploring the potential of gametic reconstruction of parental genotypes by F1 hybrids as a bridge for rapid introgression

Interspecific hybridization and genetic introgression are commonly observed in natural populations of many species, especially trees. Among oaks, gene flow between closely related species has been well documented. And yet, hybridization does not lead to a "melting pot", i.e., the homogenization of phenotypic traits. Here, we explore how the combination of several common reproductive and genomic traits could create an avenue for interspecific gene flow that partially explains this apparent paradox. During meiosis, F₁ hybrids will produce approximately (½)ⁿ "reconstructed" parental gametes, where n equals the number of chromosomes. Crossing over would introduce a small amount of introgressive material. The resulting parental-type gametophytes would probably possess a similar fertilization advantage as conspecific pollen. The resulting "backcross" would actually be the genetic equivalent of a conspecific out-cross, with a small amount of heterospecific DNA captured through crossing over. Even with detailed genomic analysis, the resulting offspring would not appear to be a backcross. This avenue for rapid introgression between species through the F₁ hybrid will be viable for organisms that meet certain conditions: low base chromosome number, conserved genomic structure and size, production of billions of gametes/gametophytes during each reproductive event, and conspecific fertilization advantage.

A framework phylogeny of the American oak clade based on sequenced RAD data

Previous phylogenetic studies in oaks (Quercus, Fagaceae) have failed to resolve the backbone topology of the genus with strong support. Here, we utilize next-generation sequencing of restriction-site associated DNA (RAD-Seq) to resolve a framework phylogeny of a predominantly American clade of oaks whose crown age is estimated at 23-33 million years old. Using a recently developed analytical pipeline for RAD-Seq phylogenetics, we created a concatenated matrix of 1.40 E06 aligned nucleotides, constituting 27,727 sequence clusters. RAD-Seq data were readily combined across runs, with no difference in phylogenetic placement between technical replicates, which overlapped by only 43-64% in locus coverage. 17% (4,715) of the loci we analyzed could be mapped with high confidence to one or more expressed sequence tags in NCBI Genbank. A concatenated matrix of the loci that BLAST to at least one EST sequence provides approximately half as many variable or parsimony-informative characters as equal-sized datasets from the non-EST loci. The EST-associated matrix is more complete (fewer missing loci) and has slightly lower homoplasy than non-EST subsampled matrices of the same size, but there is no difference in phylogenetic support or relative attribution of base substitutions to internal versus terminal branches of the phylogeny. We introduce a partitioned RAD visualization method (implemented in the R package RADami; http://cran.r-project.org/web/packages/RADami) to investigate the possibility that suboptimal topologies supported by large numbers of loci--due, for example, to reticulate evolution or lineage sorting--are masked by the globally optimal tree. We find no evidence for strongly-supported alternative topologies in our study, suggesting that the phylogeny we recover is a robust estimate of large-scale phylogenetic patterns in the American oak clade. Our study is one of the first to demonstrate the utility of RAD-Seq data for inferring phylogeny in a 23-33 million year-old clade.

Morphological and molecular evidence for natural hybridization in sympatric population of Roscoea humeana and R. cautleoides (Zingiberaceae)

The sympatric occurrence of some species in Roscoea is very common, but little information is available on natural hybridization. However, some intermediate individuals were found on the sympatric population of Roscoea humeana and R. cautleoides at Ganhaizi population in northwestern Yunnan Province, China. We suspected that these intermediate individuals were the hybrids of R. humeana and R. cautleoides from the previous evidence, but could not confirm them. In this study, morphometric analysis was followed by examination of HAT-RAPD polymorphisms to determine the occurrence of natural hybridization between sympatric R. humeana and R. cautleoides. The results showed that most morphological characters of the putative hybrids were found to be intermediate between those of R. humeana and R. cautleoides. Meanwhile, molecular analysis confirmed that the morphological intermediates were derived from hybridization between the two species. From the analysis of the NewHybrids, the hybridization individuals were mainly F₁s. These results indicated that interspecific hybridization between R. humeana and R. cautleoides indeed occurred in sympatric population.

Genetic evidence for hybridization in red oaks (Quercus sect. Lobatae, Fagaceae)

PREMISE OF THE STUDY

Hybridization is pervasive in many plant taxa, with consequences for species taxonomy, local adaptation, and management. Oaks (Quercus spp.) are thought to hybridize readily yet retain distinct traits, drawing into question the biological species concept for such taxa, but the true extent of gene flow is controversial. Genetic data are beginning to shed new light on this issue, but red oaks (section Lobatae), an important component of North American forests, have largely been neglected. Moreover, gene flow estimates may be sensitive to the choice of life stage, marker type, or genetic structure statistic.

METHODS

We coupled genetic structure data with parentage analyses for two mixed-species stands in North Carolina. Genetic structure analyses of adults (including F(ST), R(ST), G'(ST), and structure) reflect long-term patterns of gene flow, while the percentage of seedlings with parents of two different species reflect current levels of gene flow.

KEY RESULTS

Genetic structure analyses revealed low differentiation in microsatellite allele frequencies between co-occurring species, suggesting past gene flow. However, methods differed in their sensitivity to differentiation, indicating a need for caution when drawing conclusions from a single method. Parentage analyses identified >20% of seedlings as potential hybrids. The species examined exhibit distinct morphologies, suggesting selection against intermediate phenotypes.

CONCLUSIONS

Our results suggest that hybridization between co-occurring red oaks occurs, but that selection may limit introgression, especially at functional loci. However, by providing a source of genetic variation, hybridization could influence the response of oaks and other hybridizing taxa to environmental change.

Conservation biogeography of red oaks (Quercus, section Lobatae) in Mexico and Central America

PREMISE OF THE STUDY

Oaks are dominant trees and key species in many temperate and subtropical forests in the world. In this study, we analyzed patterns of distribution of red oaks (Quercus, section Lobatae) occurring in Mexico and Central America to determine areas of species richness and endemism to propose areas of conservation.

METHODS

Patterns of richness and endemism of 75 red oak species were analyzed using three different units. Two complementarity algorithms based on species richness and three algorithms based on species rarity were used to identify important areas for conservation. A simulated annealing analysis was performed to evaluate and formulate effective new reserves for red oaks that are useful for conserving the ecosystems associated with them after the systematic conservation planning approach.

KEY RESULTS

Two main centers of species richness were detected. The northern Sierra Madre Oriental and Serranías Meridionales of Jalisco had the highest values of endemism. Fourteen areas were considered as priorities for conservation of red oak species based on the 26 priority political entities, 11 floristic units and the priority grid-cells obtained in the complementarity analysis. In the present network of Natural Protected Areas in Mexico and Central America, only 41.3% (31 species) of the red oak species are protected. The simulated annealing analysis indicated that to protect all 75 species of red oaks, 12 current natural protected areas need to be expanded by 120000 ha of additional land, and 26 new natural protected areas with 512500 ha need to be created.

CONCLUSIONS

Red oaks are a useful model to identify areas for conservation based on species richness and endemism as a result of their wide geographic distribution and a high number of species. We evaluated and reformulated new reserves for red oaks that are also useful for the conservation of ecosystems associated with them.

Exploring species limits in two closely related Chinese oaks

BACKGROUND

The species status of two closely related Chinese oaks, Quercus liaotungensis and Q. mongolica, has been called into question. The objective of this study was to investigate the species status and to estimate the degree of introgression between the two taxa using different approaches.

METHODOLOGY/PRINCIPAL FINDINGS

Using SSR (simple sequence repeat) and AFLP (amplified fragment length polymorphism) markers, we found that interspecific genetic differentiation is significant and higher than the differentiation among populations within taxa. Bayesian clusters, principal coordinate analysis and population genetic distance trees all classified the oaks into two main groups consistent with the morphological differentiation of the two taxa rather than with geographic locations using both types of markers. Nevertheless, a few individuals in Northeast China and many individuals in North China have hybrid ancestry according to Bayesian assignment. One SSR locus and five AFLPs are significant outliers against neutral expectations in the interspecific FST simulation analysis, suggesting a role for divergent selection in differentiating species.

MAIN CONCLUSIONS/SIGNIFICANCE

All results based on SSRS and AFLPS reached the same conclusion: Q. liaotungensis and Q. mongolica maintain distinct gene pools in most areas of sympatry. They should therefore be considered as discrete taxonomic units. Yet, the degree of introgression varies between the two species in different contact zones, which might be caused by different population history or by local environmental factors.

Interspecific gene flow in a multispecies oak hybrid zone in the Sierra Tarahumara of Mexico

BACKGROUND AND AIMS

Interspecific gene flow can occur in many combinations among species within the genus Quercus, but simultaneous hybridization among more than two species has been rarely analysed. The present study addresses the genetic structure and morphological variation in a triple hybrid zone formed by Q. hypoleucoides, Q. scytophylla and Q. sideroxyla in north-western Mexico.

METHODS

A total of 247 trees from ten reference and 13 presumed intermediate populations were characterized using leaf shape variation and geometric morphometrics, and seven nuclear microsatellites as genetic markers. Discriminant function analysis was performed for leaf shape variation, and estimates of genetic diversity and structure, and individual Bayesian genetic assignments were obtained.

KEY RESULTS

Reference populations formed three completely distinct groups according to discriminant function analysis based on the morphological data, and showed low, but significant, genetic differentiation. Populations from the zone of contact contained individuals morphologically intermediate between pairs of species in different combinations, or even among the three species. The Bayesian admixture analysis found that three main genetic clusters best fitted the data, with good correspondence of reference populations of each species to one of the genetic clusters, but various degrees of admixture evidenced in populations from the contact area.

CONCLUSIONS

The three oak species have formed a complex hybrid zone that is geographically structured as a mosaic, and comprising a wide range of genotypes, including hybrids between different species pairs, backcrosses and probable triple hybrids.

Molecular, morphological, and ecological niche differentiation of sympatric sister oak species, Quercus virginiana and Q. geminata (Fagaceae)

The genus Quercus (the oaks) is notorious for interspecific hybrization, generating questions about the mechanisms that permit coexistence of closely related species. Two sister oak species, Quercus virginiana and Q. geminata, occur in sympatry in Florida and throughout the southeastern United States. In 11 sites from northern and southeastern regions of Florida, we used a leaf-based morphological index to identify individuals to species. Eleven nuclear microsatellite markers significantly differentiated between the species with a high correspondence between molecular and morphological typing of specimens. Nevertheless, Bayesian clustering analysis indicates interspecific gene flow, and six of 109 individuals had mixed ancestry. The identity of several individuals also was mismatched using molecular markers and morphological characters. In a common environment, the two species performed differently in terms of photosynthetic performance and growth, corresponding to their divergent ecological niches with respect to soil moisture and other edaphic properties. Our data support earlier hypotheses that divergence in flowering time causes assortative mating, allowing these ecologically distinct sister species to occur in sympatry. Limited gene flow that permits ecological differentiation helps to explain the overdispersion of oak species in local communities.

Contrasting nuclear and cytoplasmic exchanges between phylogenetically distant oak species (Quercus suber L. and Q. ilex L.) in Southern France: inferring crosses and dynamics

Gene flow is particularly frequent in the genus Quercus (oaks), especially between closely related species. We focus here on Quercus ilex and the cork-producing Quercus suber, which occasionally hybridize although they are phylogenetically markedly separated. Morphological observations were combined with both allozymic and chloroplastic diagnostic markers to characterize hybridization and introgression and to infer their dynamics in two French regions (French Catalonia and Provence), which are separated by several hundred kilometres. Some hybrids were found in both regions, indicating recent hybridization events. As expected from previous studies, most hybrids resulted from female symbol Q. ilex x male symbol Q. suber crosses, but our data showed that the reciprocal cross is also possible. Partial independence between nuclear and chloroplastic introgression was observed in the two species. Nuclear introgression was limited in both species and both regions, with no preferred direction. In Provence, chloroplastic introgression was very rare in both species. Conversely, all Q. suber individuals from French Catalonia were introgressed by Q. ilex chlorotypes. This might be explained by introgression in the Iberian Peninsula antedating the first occurrence of the two species in French Catalonia. We also observed a new chlorotype that was created locally, and was exchanged between the two species. However, the two species still remain genetically differentiated. The dynamics and complexity of exchanges and the factors determining them (including human management of Q. suber) are discussed.

Detection of hybrids in nature: application to oaks (Quercus suber and Q. ilex)

Powerful and accurate detection of first-generation (F1) hybrids and backcrosses in nature is needed to achieve a better understanding of the function and dynamics of introgression. To document the frequency of ongoing interspecific gene exchange between two Mediterranean evergreen oaks, the cork oak (Quercus suber) and the holm oak (Q. ilex), we analyzed 1487 individuals originating from across the range of the two species using eight microsatellite loci and two Bayesian clustering approaches (implemented in the programs STRUCTURE and NEWHYBRIDS). Simulated data were used to assess the differences between the two clustering methods and to back up the choice of the threshold value for the posterior probability to discriminate admixed from pure individuals. We found that the use of STRUCTURE resulted in the highest power to detect hybrids, whereas NEWHYBRIDS provided the highest accuracy. Irrespective of the approach, the two species were clearly distinguished as independent genetic entities without any prior information. In contrast with previous reports, we found no evidence for unidirectional introgression. The overall hybridization rate was very low (<2% of introgressed individuals). Only two individuals were identified as F1 hybrids and five as early backcrosses. This work shows that the combined application of the two complementary Bayesian approaches and their systematic validation with simulations, fit for the case at hand, helps gain resolution in the identification of admixed individuals.

Genetic and morphological patterns show variation in frequency of hybrids between Ipomopsis (Polemoniaceae) zones of sympatry

Variation in rates of hybridization among zones of sympatry between a pair of species provides a useful window into the effect of local conditions on the evolution of reproductive isolation. We employed floral morphological traits and neutral genetic markers to quantify the frequency of individuals intermediate to the two parental species in two zones of sympatry between Ipomopsis aggregata and I. tenuituba, using clustering methods that make no a priori assumptions about population structure. The sites differed not only in the frequency of intermediate individuals, but also in climate, pollinator abundance and behavior and spatial structure of plant populations. Both floral traits, which are likely to be under natural selection and molecular markers, which are quasi-neutral, indicated more population structure at one site than the other, the pattern being more pronounced for floral morphology. One likely explanation for this difference between sites is that local ecological conditions, particularly pollinator choice of flowers, have promoted different rates of hybridization between these species. Hence, the evolution of reproductive isolation might depend in part on local conditions, and thus differ among populations of the same pair of species.

Evidence for hybridization and introgression within a species-rich oak (Quercus spp.) community

Background

Analysis of interspecific gene flow is crucial for the understanding of speciation processes and maintenance of species integrity. Oaks (genus Quercus, Fagaceae) are among the model species for the study of hybridization. Natural co-occurrence of four closely related oak species is a very rare case in the temperate forests of Europe. We used both morphological characters and genetic markers to characterize hybridization in a natural community situated in west-central Romania and which consists of Quercus robur, Q. petraea, Q. pubescens, and Q. frainetto, respectively.

Results

On the basis of pubescence and leaf morphological characters ~94% of the sampled individuals were assigned to pure species. Only 16 (~6%) individual trees exhibited intermediate morphologies or a combination of characters of different species. Four chloroplast DNA haplotypes were identified in the study area. The distribution of haplotypes within the white oak complex showed substantial differences among species. However, the most common haplotypes were present in all four species. Furthermore, based on a set of 7 isozyme and 6 microsatellite markers and using a Bayesian admixture analysis without any a priori information on morphology we found that four genetic clusters best fit the data. There was a very good correspondence of each species with one of the inferred genetic clusters. The estimated introgression level varied markedly between pairs of species ranging from 1.7% between Q. robur and Q. frainetto to 16.2% between Q. pubescens and Q. frainetto. Only nine individuals (3.4%) appeared to be first-generation hybrids.

Conclusion

Our data indicate that natural hybridization has occurred at relatively low rates. The different levels of gene flow among species might be explained by differences in flowering time and spatial position within the stand. In addition, a partial congruence between phenotypically and genetically intermediate individuals was found, suggesting that intermediate appearance does not necessarily mean hybridization. However, it appears that natural hybridization did not seriously affect the species identity in this area of sympatry.

Fine-scale spatial genetic structure in mixed oak stands with different levels of hybridization

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.

Natural hybridization origin of Rhododendron agastum (Ericaceae) in Yunnan, China: inferred from morphological and molecular evidence

The natural hybridization that occurs between two sympatric species of Rhododendron subgenus Hymenanthes in Yunnan, China, was investigated. The assumed parents, Rhododendron delavayi Franch. and R. decorum Franch., are morphologically distinct, and the putative hybrid species, R. agastum Balf. f. et W. W. Smith, has an intermediate morphology. We used the main morphological characters, sequences of the nuclear ribosomal DNA ITS region, and the chloroplast DNA trnL-F intronspacer to analyze the three species, and compared these morphological and molecular data with an artificial hybrid between R. decorum (female symbol) x R. delavayi (male symbol). From the results, we conclude that R. agastum is a natural hybrid between a female R. delavayi and a male R. decorum.

Genetic variation and differentiation of populations within the Quercus affinis – Quercus laurina (Fagaceae) complex analyzed with RAPD markers

The population genetics of two hybridizing Mexican red oaks, Quercus affinis Schweid. and Quercus laurina Humb. & Bonpl., was investigated with 54 randomly amplified polymorphic DNA (RAPD) markers scored in 415 individuals from 16 populations representing the distribution area of the two species and a probable secondary hybrid zone. Genetic relationships among populations, depicted in a unweighted pair group method with arithmetic averaging (UPGMA) dendrogram, were largely incongruent with the morphological classification of populations as Q. affinis-like or Q. laurina-like that was obtained in previous studies. In contrast, the two main population clusters in the UPGMA dendrogram corresponded to the location of populations in two distinct geographical areas: southwestern and northeastern. A Mantel test confirmed a significant association between geographic and genetic distances among populations. Analyses of molecular variance (AMOVA) indicated that most genetic variation is contained within populations (84%), while 10.5% (P < 0.0001) is among populations, and 5.1% (P = 0.007) is between the two morphological groups. Differentiation between the southwestern and northeastern geographical groups (as recognized by the UPGMA), was 7.8% (P < 0.0001). The incongruence between genetic and phenotypic patterns suggests that introgression of neutral markers has been considerable between the two species in the hybrid zone, while morphological differentiation has remained comparatively stable.Key words: hybridization, population genetics, Quercus, RAPD markers.

Chloroplast DNA variation in the Quercus affinis-Q. laurina complex in Mexico: geographical structure and associations with nuclear and morphological variation

The geographical distribution of chloroplast DNA (cpDNA) variation in 39 populations of two hybridizing Mexican red oaks, Quercus affinis and Q. laurina, was investigated using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Six haplotypes were identified. Of these, two (H1 and H4), separated by four mutations, had high frequencies (58 and 23% of the individuals, respectively) and were present across the whole geographical range of both species, often co occurring in the same populations. The other four haplotypes were rare, geographically restricted, and are probably derived from the two frequent haplotypes. Latitudinal or other clinal patterns in diversity levels or haplotype composition of populations were not apparent. The pattern of haplotype distribution was characterized by some mosaicism, with contrasting populations often situated in proximity. Average within-population diversity (hS=0.299) and population differentiation (GST=0.499) were, respectively, higher and lower than values reported in previous studies of oak species. There was evidence for phylogeographical structure, as indicated by NST (0.566) being significantly higher than GST. Haplotypic variation was largely species-independent, although some very weak associations were detected between haplotypes H1 and H4 and morphological and nuclear molecular variation correspondingly characterizing Q. affinis and Q. laurina. These oaks probably did not experience a marked restriction to one or a few particular subregions of their present range during the last glacial cycle. It is more likely that substantial populations persisted throughout several episodes of climatic change, but experienced recurrent latitudinal and altitudinal migrations which may have caused the widespread distribution of haplotypes H1 and H4 and frequent intermixing of populations.