Maria Mayol

Genetic effects of chronic habitat fragmentation revisited: Strong genetic structure in a temperate tree, Taxus baccata (Taxaceae), with great dispersal capability

Tree species are thought to be relatively resistant to habitat fragmentation because of their longevity and their aptitude for extensive gene flow, although recent empirical studies have reported negative genetic consequences, in particular after long-term habitat fragmentation in European temperate regions. Yet the response of each species to habitat loss may differ greatly depending on their biological attributes, in particular seed dispersal ability. In this study, we used demographic and molecular data to investigate the genetic consequences of chronic habitat fragmentation in remnant populations of Taxus baccata in the Montseny Mountains, northeast Spain. The age structure of populations revealed demographic bottlenecks and recruitment events associated with exploitation and management practices. We found a strong genetic structure, both at the landscape and within-population levels. We also detected high levels of inbreeding for a strictly outcrossing species. Chronic forest fragmentation resulting from long-term exploitation in the Montseny Mountains seems the most plausible explanation for the strong genetic structure observed. Our results support the view that, contrary to some predictions, tree species are not buffered from the adverse effects of habitat fragmentation, even in the case of species with a high dispersal potential.

Spatial genetic structure of Taxus baccata L. in the western Mediterranean Basin: Past and present limits to gene movement over a broad geographic scale

English yew (Taxus baccata L., Taxaceae), a Tertiary relict, provides a seminal example of a widespread albeit locally endangered (often close to extinction) tree species. In order to gain detailed insights into the evolutionary dynamics of the species on a broad geographical scale, over 1000 trees from 91 populations of English yew in the western Mediterranean were analyzed using seven nuclear microsatellite markers. Our results revealed contrasting patterns of genetic structure at different spatial scales: genetic variation was highly structured at the local scale, while only a low proportion of the observed variation was attributed to regional differences. We also found a geographic gradient of decreasing diversity and increasing population divergence from northwest (central Europe and northern Iberian Peninsula) to southeast (Mediterranean Iberia and North Africa). The patterns revealed in this study probably reflect the combined effects of Quaternary climatic changes and recent impact of human activities, and potentially also more ancient events dating back to the Tertiary. Both climatic and anthropogenic factors seem to have conducted to a long history of population isolation, which may have contributed significantly to enhance population divergence through restricted gene flow and genetic drift.

Isolation and characterization of polymorphic nuclear microsatellite loci in Taxus baccata L.

Seven polymorphic nuclear microsatellite markers for Taxus baccata L. (English yew) were developed using an enriched-library method. An additional polymorphic SSR was obtained by testing eight primer pairs from the congeneric species Taxus sumatrana. Mendelian inheritance for the seven Taxus baccata SSRs was proved by genotyping 17 individuals and 124 megagametophytes (conifer seed haploid tissue). A total of 96 individuals from 5 different populations (10–26 samples per population) were used to estimate genetic diversity parameters. High levels of genetic diversity, with values ranging from 0.533 to 0.929 (6–28 alleles per SSR) were found. No linkage disequilibrium between pairs of loci was detected. All loci but one showed significant departures from Hardy–Weinberg equilibrium. Excess of homozygosity was probably due to high inbreeding in English yew populations, an outcome of low effective population size and/or fragmented distribution. Highly polymorphic SSRs will be used to conduct population genetic studies at different geographical scales and to monitor gene flow.

Adapting through glacial cycles: insights from a long-lived tree (Taxus baccata).

Despite the large body of research devoted to understanding the role of Quaternary glacial cycles in the genetic divergence of European trees, the differential contribution of geographic isolation and/or environmental adaptation in creating population genetic divergence remains unexplored. In this study, we used a long-lived tree (Taxus baccata) as a model species to investigate the impact of Quaternary climatic changes on genetic diversity via neutral (isolation-by-distance) and selective (isolation-by-adaptation) processes. We applied approximate Bayesian computation to genetic data to infer its demographic history, and combined this information with past and present climatic data to assess the role of environment and geography in the observed patterns of genetic structure. We found evidence that yew colonized Europe from the East, and that European samples diverged into two groups (Western, Eastern) at the beginning of the Quaternary glaciations, c. 2.2 Myr before present. Apart from the expected effects of geographical isolation during glacials, we discovered a significant role of environmental adaptation during interglacials at the origin of genetic divergence between both groups. This process may be common in other organisms, providing new research lines to explore the effect of Quaternary climatic factors on present-day patterns of genetic diversity.

Past and current gene flow in the selfing, wind-dispersed species Mycelis muralis in western Europe

The distribution of genetic diversity in Mycelis muralis, or wall lettuce, was investigated at a European scale using 12 microsatellite markers to infer historical and contemporary forces from genetic patterns. Mycelis muralis has the potential for long‐distance seed dispersal by wind, is mainly self‐pollinated, and has patchily distributed populations, some of which may show metapopulation dynamics. A total of 359 individuals were sampled from 17 populations located in three regions, designated southern Europe (Spain and France), the Netherlands, and Sweden. At this within‐region scale, contemporary evolutionary forces (selfing and metapopulation dynamics) are responsible for high differentiation between populations (0.34 < FST < 0.60) but, contrary to expectation, levels of within‐population diversity, estimated by Neis unbiased expected heterozygosity (HE) (0.24 < HE < 0.68) or analyses of molecular variance (50% of the variation found within‐populations), were not low. We suggest that the latter results, which are unusual in selfing species, arise from efficient seed dispersal that counteracts population turnover and thus maintains genetic diversity within populations. At the European scale, northern regions showed lower allelic richness (A = 2.38) than populations from southern Europe (A = 3.34). In light of postglacial colonization hypotheses, these results suggest that rare alleles may have been lost during recolonization northwards. Our results further suggest that mutation has contributed to genetic differentiation between southern and northern Europe, and that Sweden may have been colonized by dispersers originating from at least two different refugia.

Recent population decline and selection shape diversity of taxol-related genes.

Taxanes are defensive metabolites produced by Taxus species (yews) and used in anticancer therapies. Despite their medical interest, patterns of natural diversity in taxane‐related genes are unknown. We examined variation at five main genes of Taxus baccata in the Iberian Peninsula, a region where unique yew genetic resources are endangered. We looked at several gene features and applied complementary neutrality tests, including diversity/divergence tests, tests solely based on site frequency spectrum (SFS) and Zeng’s compound tests. To account for specific demography, microsatellite data were used to infer historical changes in population size based on an Approximate Bayesian Computation (ABC) approach. Polymorphism‐divergence tests pointed to positive selection for genes TBT and TAT and balancing selection for DBAT. In addition, neutrality tests based on SFS found that while a recent reduction in population size may explain most statistics’ values, selection may still be in action in genes TBT and DBAT, at least in some populations. Molecular signatures on taxol genes suggest the action of frequent selective waves with different direction or intensity, possibly related to varying adaptive pressures produced by the host–enemy co‐evolution on defence‐related genes. Such natural selection processes may have produced taxane variants still undiscovered.

Patterns of genetic variability and habitat occupancy in Crepis triasii (Asteraceae) at different spatial scales: insights on evolutionary processes leading to diversification in continental islands.

BACKGROUND AND AIMS Archipelagos are unique systems for studying evolutionary processes promoting diversification and speciation. The islands of the Mediterranean basin are major areas of plant richness, including a high proportion of narrow endemics. Many endemic plants are currently found in rocky habitats, showing varying patterns of habitat occupancy at different spatial scales throughout their range. The aim of the present study was to understand the impact of varying patterns of population distribution on genetic diversity and structure to shed light on demographic and evolutionary processes leading to population diversification in Crepis triasii, an endemic plant from the eastern Balearic Islands. METHODS Using allozyme and chloroplast markers, we related patterns of genetic structure and diversity to those of habitat occupancy at a regional (between islands and among populations within islands) and landscape (population size and connectivity) scale. KEY RESULTS Genetic diversity was highly structured both at the regional and at the landscape level, and was positively correlated with population connectivity in the landscape. Populations located in small isolated mountains and coastal areas, with restricted patterns of regional occupancy, were genetically less diverse and much more differentiated. In addition, more isolated populations had stronger fine-scale genetic structure than well-connected ones. Changes in habitat availability and quality arising from marine transgressions during the Quaternary, as well as progressive fragmentation associated with the aridification of the climate since the last glaciation, are the most plausible factors leading to the observed patterns of genetic diversity and structure. CONCLUSIONS Our results emphasize the importance of gene flow in preventing genetic erosion and maintaining the evolutionary potential of populations. They also agree with recent studies highlighting the importance of restricted gene flow and genetic drift as drivers of plant evolution in Mediterranean continental islands.

Random amplified polymorphic DNA (RAPD) markers detect a single phenotype in Lysimachia minoricensis J.J. Rodr. (Primulaceae), a wild extinct plant

Lysimachia minoricensis is a Mediterranean (Balearic Islands) endemic that is extinct in the wild but extant in botanical gardens. Previously, no variation at 22 isozyme loci was revealed in more than 150 analysed plants. Random amplified polymorphic DNA (RAPD) analysis was used to examine genetic variation among five individuals from each of eight botanical garden accessions (40 plants). No polymorphisms were detected at 201 amplified bands. This is the first report of RAPD monomorphism in a nonapomictic vascular plant. The lack of detectable genetic variation suggests that an extremely reduced gene pool was recovered in the field before its extinction. Although the screening of other genomic markers is feasible, it is suggested that the knowledge of biological and autoecological features should be prioritized before new re‐introductions are attempted.

HYBRIDIZATION STUDIES IN SILENE SUBGEN. PETROCOPTIS (CARYOPHYLLACEAE)

Silene subgenusPetrocoptis comprises sexual diploid taxa and it is restricted to calcareous cliffs in the Iberian peninsula. Artificial crosses involvingSilene pyrenaica (Bergeret)Mayol etRosselló (≡Petrocoptis pyrenaica (Bergeret)Walp.),Silene laxipruinosaMayol etRosselló andS. montserratii subsp.crassifolia (Rouy)Mayol etRosselló (≡P. crassifoliaRouy) were attempted to assess the extent of barriers developed within the subgenusPetrocoptis. Usually, intraspecific crosses involving allopatric populations were successful, suggesting that geographically isolated populations are not genetically isolated. Cross-compatibility was noted among the polymorphicS. pyrenaica, which developed fertile F1 hybrids. All other interspecific crosses failed due to cross- or seed-incompatibility. Crossing results agree with available evidence supporting the merging of segregates ofS. pyrenaica within a single taxon.

INTERPOPULATION ESTERASE VARIATION IN THE GENUSPETROCOPTIS A. BRAUN (CARYOPHYLLACEAE)

The electrophoretic patterns of a- and b-isozyme esterases from seeds of 44 populations belonging to twelve Petrocoptis taxa were compared in order to clarify taxonomic relationships within the genus. Electrophoretic zymograms showed the presence of 20 electromorphs, which yield 24 a-esterase and 22 b-esterase phenotypes. Most of the bands and the electrophoretic profiles were restricted to a few localities and taxa. Correlation between isozyme similarity and geographic distance among populations was not detected, as revealed by the Mantel test (r = 0.007;p = 0.5811), suggesting that there was no greater phenetic similarity among neighboring populations than among distant populations. Multivariate analysis of the data(Factorial Correspondence Analysis and Multidimensional Scaling) did not depict a consistent picture of the taxonomic relationships within the genus, probably due to the high intraspecific variability present in some taxa. Given this polymorphism within some Petrocoptis taxa, esterase isozyme...