Juan F. Fernández-Manjarrés

Assortative mating and differential male mating success in an ash hybrid zone population.

BackgroundThe structure and evolution of hybrid zones depend mainly on the relative importance of dispersal and local adaptation, and on the strength of assortative mating. Here, we study the influence of dispersal, temporal isolation, variability in phenotypic traits and parasite attacks on the male mating success of two parental species and hybrids by real-time pollen flow analysis. We focus on a hybrid zone population between the two closely related ash species Fraxinus excelsior L. (common ash) and F. angustifolia Vahl (narrow-leaved ash), which is composed of individuals of the two species and several hybrid types. This population is structured by flowering time: the F. excelsior individuals flower later than the F. angustifolia individuals, and the hybrid types flower in-between. Hybrids are scattered throughout the population, suggesting favorable conditions for their local adaptation. We estimate jointly the best-fitting dispersal kernel, the differences in male fecundity due to variation in phenotypic traits and level of parasite attack, and the strength of assortative mating due to differences in flowering phenology. In addition, we assess the effect of accounting for genotyping error on these estimations.ResultsWe detected a very high pollen immigration rate and a fat-tailed dispersal kernel, counter-balanced by slight phenological assortative mating and short-distance pollen dispersal. Early intermediate flowering hybrids, which had the highest male mating success, showed optimal sex allocation and increased selfing rates. We detected asymmetry of gene flow, with early flowering trees participating more as pollen donors than late flowering trees.ConclusionThis study provides striking evidence that long-distance gene flow alone is not sufficient to counter-act the effects of assortative mating and selfing. Phenological assortative mating and short-distance dispersal can create temporal and spatial structuring that appears to maintain this hybrid population. The asymmetry of gene flow, with higher fertility and increased selfing, can potentially confer a selective advantage to early flowering hybrids in the zone. In the event of climate change, hybridization may provide a means for F. angustifolia to further extend its range at the expense of F. excelsior.

Temporal cline in a hybrid zone population between Fraxinus excelsior L. and Fraxinus angustifolia Vahl

The two closely related ash species Fraxinus excelsior L. (common ash) and Fraxinus angustifolia Vahl (narrow‐leaved ash) have a broad contact zone in France where they hybridize. However, little is known about the local structure of hybrid zone populations and the isolation mechanisms. We assessed the potential effect of floral phenology on the structure of a riparian ash hybrid zone population in central France. The distribution of flowering times was unimodal and lay between the flowering periods of the two species. Using microsatellite markers, we detected isolation by time, which has possibly originated from assortative mating. Multivariate analyses indicated that morphological variation is not distributed at random with respect to flowering times. Spatial autocorrelation analyses showed that temporal and spatial patterns were tightly linked. Interestingly, despite the fact that the population shows isolation by time, neighbourhood size and historical dispersal variance (σ̂ = 63 m) are similar to those detected in pure stands of F. excelsior where individuals flower rather synchronously and hermaphrodites are not the most frequent sexual type. Trees flowering at intermediate dates, which comprised the majority of the population, produced on average more flowers and fruits. We detected no significant differences in floral parasite infections relative to reproductive timing, although there was a tendency for late flowering trees to suffer from more gall attack. We discuss the impact of temporal variation in fitness traits and their possible role in the maintenance of the hybrid zone.

Environmental Heterogeneity Explains the Genetic Structure of Continental and Mediterranean Populations of Fraxinus angustifolia Vahl

Tree species with wide distributions often exhibit different levels of genetic structuring correlated to their environment. However, understanding how environmental heterogeneity influences genetic variation is difficult because the effects of gene flow, drift and selection are confounded. We investigated the genetic variation and its ecological correlates in a wind-pollinated Mediterranean tree species, Fraxinus angustifolia Vahl, within a recognised glacial refugium in Croatia. We sampled 11 populations from environmentally divergent habitats within the Continental and Mediterranean biogeographical regions. We combined genetic data analyses based on nuclear microsatellite loci, multivariate statistics on environmental data and ecological niche modelling (ENM). We identified a geographic structure with a high genetic diversity and low differentiation in the Continental region, which contrasted with the significantly lower genetic diversity and higher population divergence in the Mediterranean region. The positive and significant correlation between environmental and genetic distances after controlling for geographic distance suggests an important influence of ecological divergence of the sites in shaping genetic variation. The ENM provided support for niche differentiation between the populations from the Continental and Mediterranean regions, suggesting that contemporary populations may represent two divergent ecotypes. Ecotype differentiation was also supported by multivariate environmental and genetic distance analyses. Our results suggest that despite extensive gene flow in continental areas, long-term stability of heterogeneous environments have likely promoted genetic divergence of ashes in this region and can explain the present-day genetic variation patterns of these ancient populations.

Identifying refugia from climate change using coupled ecological and genetic data in a transitional Mediterranean-temperate tree species

Populations occurring in areas of overlap between the current and future distribution of a species are particularly important because they can represent “refugia from climate change”. We coupled ecological and range‐wide genetic variation data to detect such areas and to evaluate the impacts of habitat suitability changes on the genetic diversity of the transitional Mediterranean‐temperate tree Fraxinus angustifolia. We sampled and genotyped 38 natural populations comprising 1006 individuals from across Europe. We found the highest genetic diversity in western and northern Mediterranean populations, as well as a significant west to east decline in genetic diversity. Areas of potential refugia that correspond to approximately 70% of the suitable habitat may support the persistence of more than 90% of the total number of alleles in the future. Moreover, based on correlations between Bayesian genetic assignment and climate, climate change may favour the westward spread of the Black Sea gene pool in the long term. Overall, our results suggest that the northerly core areas of the current distribution contain the most important part of the genetic variation for this species and may serve as in situ macrorefugia from ongoing climate change. However, rear‐edge populations of the southern Mediterranean may be exposed to a potential loss of unique genetic diversity owing to habitat suitability changes unless populations can persist in microrefugia that have facilitated such persistence in the past.

Testing scenarios for assisted migration of forest trees in Europe

One approach to compensating for rapid climate change and protecting biodiversity is assisted migration (AM) of key tree species. However, tools for evaluating the sensitivity of target sites and identifying potential sources have not yet been developed. We used the National Forest Inventories of Spain and France to design scenarios for AM between and within both countries. We characterized sensitivity to climate change as the expected changes in volume and mortality of Pinus halepensis Miller and Pinus pinaster Aiton between the present and 2050. Target zones were selected from provenances with high sensitivity and seed zones from provenances with low sensitivity to climate change; the latter can be considered “seed refugia” as the climate changes. Three plausible scenarios for translocation to the target zone were developed on the basis of volume simulations calibrated with different planting strategies: (1) seeds only from foreign provenances; (2) foreign provenances plus local seeds; and (3) only local seeds. The results for both species show that models based on foreign “top-three” provenances always increased the standing volume of the target zone. Models run with only local seeds predicted increased volume for P. halepensis but not for P. pinaster. Our results suggest that volume and mortality trends are not always correlated with seed sources and targets, that projected provenances mortality do not follow always a southern–northern pattern and that seed refugia, if any, may be useful for compensating for the effects of climate change only in a subset of provenances.

Ecological coassociations influence species' responses to past climatic change: an example from a Sonoran Desert bark beetle

Ecologically interacting species may have phylogeographical histories that are shaped both by features of their abiotic landscape and by biotic constraints imposed by their coassociation. The Baja California peninsula provides an excellent opportunity to examine the influence of abiotic vs. biotic factors on patterns of diversity in plant‐insect species. This is because past climatic and geological changes impacted the genetic structure of plants quite differently to that of codistributed free‐living animals (e.g. herpetofauna and small mammals). Thus, ‘plant‐like’ patterns should be discernible in host‐specific insect herbivores. Here, we investigate the population history of a monophagous bark beetle, Araptus attenuatus, and consider drivers of phylogeographical patterns in the light of previous work on its host plant, Euphorbia lomelii. Using a combination of phylogenetic, coalescent‐simulation‐based and exploratory analyses of mitochondrial DNA sequences and nuclear genotypic data, we found that the evolutionary history of A. attenuatus exhibits similarities to its host plant that are attributable to both biotic and abiotic processes. Southward range expansion and recent colonization of continental Sonora from the Baja peninsula appear to be unique to this taxon pair and probably reflect influences of the host plant. On the other hand, abiotic factors with landscape‐level influences on a diverse suite of codistributed arid‐adapted taxa, such as Plio‐ and Pleistocene‐aged marine incursions in the region, also left genetic signatures in beetle and host plant populations. Superimposed on these similarities, bark beetle‐specific patterns and processes were also evident: our data revealed two secondarily sympatric, reproductively isolated genetic lineages, as well as a previously unrecognized mid‐peninsular warm desert refuge. Taken together, this work illustrates that the evolutionary history of species‐specific insect herbivores may represent a mosaic of influences, including—but not limited to—those imposed by the host plant.

Molecular and Morphological Characterization of Reciprocal F1 Hybrid Ash (Fraxinus excelsior × Fraxinus angustifolia, Oleaceae) and Parental Species Reveals Asymmetric Character Inheritance

Hybridization between Fraxinus excelsior and Fraxinus angustifolia is common. However, identifying hybrids in natural populations is difficult because the closely related parental species share many morphological characters and the inheritance pattern of these characters in hybrids is unknown. We evaluated how morphological characters are inherited and whether morphological and molecular markers can efficiently discriminate artificial first-generation hybrids. Reciprocal F1 hybrids of F. excelsior with F. angustifolia were examined using six microsatellite DNA marker loci and 14 morphological characters. Plants were divided into four groups (F. angustifolia, F. excelsior, the F1 hybrid with F. angustifolia as the maternal parent, and the F1 hybrid with F. excelsior as the maternal parent). The F1 hybrids showed intermediate morphology in most characters, and the range of variation overlapped with the parental species. Canonical discriminant analysis using only the morphological traits separated the four groups without any overlap between the two parental species. F1 hybrids from different maternal parent species could therefore be distinguished. A further analysis that combined molecular and morphological traits allowed clear separation of the four groups and strongly confirmed the a priori defined groups. Our results suggest that intermediate characters can be expected in F1 hybrids of ash but differences may be observed due to maternal/paternal effects.

What sampling is needed for reliable estimations of genetic diversity in Fraxinus excelsior L. (Oleaceae)

Abstract• Sample size is a critical issue for genetic diversity studies and conservation programs. However, sample size evaluation requires previous knowledge of allele frequencies estimated with precision and this is not often the case.• Here, we evaluated sample size requirements for accurate genetic diversity in adult trees and family arrays in a 12 ha plot of Fraxinus excelsior L. (Oleaceae) in a community forest in central France. Data consisted of 579 adult trees and 480 offspring from 24 families genotyped at four nuclear microsatellites.• Mean square errors (MSE) estimates performed on Monte Carlo simulations of resampled data indicated that several adult individuals (> 300) are necessary for accurate measures of allele richness. However, expected heterozygosity requires smaller samples (< 30). Seeds captured about 90% of adult allelic diversity requiring a sampling effort roughly 50% larger than that of adult trees (480 seeds vs. 300 adults) suggesting that seed sampling is heavily penalized for allele counts. Nevertheless, gene diversity of seeds was essentially identical to that of the adult population.• Extrapolation of these results to other ash tree populations appears feasible because of similar levels of diversity reported in the literature but it is not granted for species with significant selfing or high genetic structure.Résumé• La taille d’un échantillonnage est un paramètre difficile à estimer a priori pour les études de diversité génétique ainsi que pour les programmes de conservation. En général, l’évaluation de cette taille nécessite au préalable une connaissance fine des fréquences alléliques ce qui n’est pas toujours le cas.• Dans cette étude, nous avons évalué sur une parcelle de 12 ha de Fraxinus excelsior L. (Oleaceae), dans un forêt domaniale, la taille de l’échantillonnage nécessaire pour estimer de façon fiable la diversité génétique des arbres adultes ainsi que de leurs descendants. Un échantillon de 579 individus adultes et 480 graines issues de 24 arbres-mères ont été génotypés grâce à quatre marqueurs microsatellites nucléaires.• Des analyses d’erreurs quadratiques moyennes obtenues dans le cadre de simulations de type Monte Carlo indiquent que plus de 300 individus adultes sont nécessaires pour obtenir des mesures alléliques fiables. Par contre, l’hétérozygotie espérée est obtenue pour des échantillons plus petits (< 30). Les graines capturent 90 % de la diversité allélique des adultes indiquant que l’échantillonnage des graines doit être deux fois celui des adultes pour obtenir la même information (480 graines vs. 300 adultes). Par ailleurs, la diversité génétique est identique pour les deux échantillonnages.• L’extrapolation de ces résultats à d’autres espèces de frêne est possible compte tenu des niveaux de diversité observés dans la littérature mais n’est pas garantie pour des espèces qui s’autofécondent et qui ont des populations très structurées génétiquement.

Trees on the move: using decision theory to compensate for climate change at the regional scale in forest social-ecological systems

The adaptation of social-ecological systems such as managed forests depends largely on decisions taken by forest managers who must choose among a wide range of possible futures to spread risks. We used robust decision theory to guide management decisions on the translocation of tree populations to compensate for climate change. We calibrated machine learning correlational models using tree height data collected from five common garden tests in France where Abies alba provenances from 11 European countries are planted. Resulting models were used to simulate tree height in the planting sites under current and 2050 climates (regional concentration pathway scenarios (RCPs) 2.6, 4.5, 6.0 and 8.5). Our results suggest an overall increase in tree height by 2050, but with large variation among the predictions depending on the provenance and the RCPs. We applied maximin, maximax and minimax decision rules to address outcomes under five uncertain states of the world represented by the four RCPs and the present climate (baseline). The maximin rule indicated that for 2050, the best translocation option for maximising tree height would be the use of provenances from Northwest France into all target zones. The maximax and minimax regret rules pointed out the same result for all target zones except for the ‘Les Chauvets’ trial, where the East provenance was selected. Our results show that decision theory can help management by reducing the number of options if most decision rules converge. Interestingly, the commonly suggested recommendation of using multiple provenances to mitigate long-term maladaptation risks or from ‘pre-adapted’ populations from the south was not supported by our approach.