Search

International audience

International audience

International audience; Most plant-pollinator networks are based on observations of contact between an insect and a flower in the field. Despite significant sampling efforts, some links are easier to report, while others remain unobserved. Therefore, visit-based networks represent a subsample of possible interactions in which the ignored part is variable. Pollen is a natural marker of insect visits to flowers. The identification of pollen found on insect bodies can be used as an alternative method to study plant-pollinator interactions, with a potentially lower risk of bias than the observation of visits, since it increases the number of interactions in the network. Here we compare plant-pollinator networks constructed (i) from direct observation of pollinator visits and (ii) from identification of pollen found on the same insects. We focused on three calcareous grasslands in France, with different plant and pollinator species diversities. Since pollen identification always yields richer, more connected networks, we focused our comparisons on sampling bias at equal network connectance. To do so, we first compared network structures with an analysis of latent blocks and motifs. We then compared species roles between both types of networks with an analysis of specialization and species positions within motifs. Our results suggest that the sampling from observations of insect visits does not lead to the construction of a network intrinsically different from the one obtained using pollen found on insect bodies, at least when field sampling strives to be exhaustive. Most of the significant differences are found at the species level, not at the network structure level, with singleton species accounting for a respectable fraction of these differences. Overall, this suggests that recording plant-pollinator interactions from pollinator visit observation does not provide a biased picture of the network structure, regardless of species richness; however, it provided less information on species roles than the pollen-based network.

International audience; Urbanization is one of the major threats to wild plants and pollinators, and its global increase demands a better understanding of the mechanism driving its negative impact. Urban warming and altered local environmental conditions have the potential to affect the timing of flowering and of pollinator activity. While previous evidence has shown that plant phenology tends to advance in urban areas, little is known about its effects on pollinator phenology. In this study we simultaneously assessed the response of the timing of flowering of native plants and of the flight period of wild pollinators to increased urbanization. We collected data from 12 sites along an urbanization gradient in northern France, a region under strong anthropogenic pressure. Overall, we recorded more than 70 plant species, and we sampled more than 4300 wild bees and hoverflies belonging to 154 species. Plant flowering showed a strong response to urbanization at the community level with a striking advancement of the flowering peak in sites at high urbanization. On the contrary, pollinator communities did not show any clear shift of their flight phenology along the gradient, neither regarding abundance nor diversity. Our results indicate that phenologies of plant and pollinator communities can respond differently along the same urbanization gradient. These asymmetric responses can drive modifications in the structure of plant–pollinator networks, and potentially negatively affect the fitness of both mutualistic partners.

International audience; For plant–pollinator interactions to occur, the flowering of plants and the flying period of pollinators (i.e. their phenologies) have to overlap. Yet, few models make use of this principle to predict interactions and fewer still are able to compare interaction networks of different sizes. Here, we tackled both challenges using Bayesian structural equation models (SEM), incorporating the effect of phenological overlap in six plant–hoverfly networks. Insect and plant abundances were strong determinants of the number of visits, while phenology overlap alone was not sufficient, but significantly improved model fit. Phenology overlap was a stronger determinant of plant–pollinator interactions in sites where the average overlap was longer and network compartmentalization was weaker, i.e. at higher latitudes. Our approach highlights the advantages of using Bayesian SEMs to compare interaction networks of different sizes along environmental gradients and articulates the various steps needed to do so.

International audience; Urbanisation is one of the main drivers of landscape modification, as vegetation andopen lands are replaced by paved surfaces, roads and buildings. These changes imply thephenomenon of urban heat islands – urban and suburban areasshow increased temperaturescompared to their rural surroundings. However, its influences on flowering period and polli-nator activity remains poorly investigated.We surveyed 12 meadows managed by local practitioners with standardised native seed mixesin the M ́etropole Europ ́eenne de Lille (northern France). Sites were selected according to anurban gradient (three urbanisation levels, based on the proportion of impervious areas withina 500m radius), and were visited bi-weekly from April 4th to June 29th 2017.We estimatedthe flowering abundance at all sites at any given observation period. At the same time, wesampled pollinators (bees and hoverflies) using pan traps and hand net. We estimated theeffects of sampling day and urbanisation level on flower cover and pollinator activity usingGLMMs.Urbanisation had a strong effect on average flowering phenology of the whole plant commu-nity. In medium and high urbanisation sites, flowering peak occurred respectively 7 and 30days earlier, compared to rural areas. Similar patterns were found for the most commonflowering species. Our results also suggest that flowering duration has increased in urbanareas. We observed phenological shifts of pollinators as urbanisation increased. Bumblebeesexperienced a similar advance of activity in moderately and highly urbanised sites, comparedto rural sites. Likewise, we observed an advancement of the activity of some solitary bees(Halictidae) in urban areas. We are currently analysing data regarding the other bee andhoverfly taxa found during the study. We will discuss the possible effects of phenology shiftson plants and pollinators, and their consequences in relation to temporal mismatches amongspecies

Arabidopsis halleri is a pseudometallophyte plant model hyperaccumulating zinc and cadmium. This study investigates which abiotic parameters may cause phenotypic divergence among accessions for hyperaccumulation traits. We studied 23 sites from a mining and industrial area in Italian Alps. Sites were characterized for altitude, topographic data, absolute humidity, and accompanying flora. Plant-soil couples were also sampled to measure shoot metal concentrations and soil elemental concentrations, particles size distribution, and pH. Using PLSR analyses, we investigated whether the natural variation in hyperaccumulation abilities could be explained by variation of abiotic parameters. Habitats heterogeneity was high, distinguishing metalliferous and non-metalliferous sites. However, heterogeneity was also observed for soil metal concentrations, particles size distribution and altitude, particularly among metalliferous habitats. This result was supported by floristic data. Soil zinc and cadmium concentrations showed the most contrasting effects on phenotypic divergence between metalliferous and non-metalliferous habitats. However, except for cadmium-related traits in non-metalliferous habitats, other abiotic parameters may affect the variation of zinc or cadmium hyperaccumulation within each habitat type. The classical dichotomous distinction between metalliferous and non-metalliferous habitats may hide the ecological diversity existing within each category for abiotic parameters. This study reveals abiotic parameters that may shape the natural variation of hyperaccumulation abilities.

International audience; At evolutionary equilibrium, ecological factors will determine the optimal combination of life-history trait values of an organism. This optimum can be assessed by assuming that the species maximizes some criterion of fitness such as the Malthusian coefficient or lifetime reproductive success depending on the degree of density-dependence. We investigated the impact of the amount of resources and habitat stability on a plant's age at maturity and life span by using an evolutionary optimization model in combination with empirical data. We conducted this study on sea beet, Beta vulgaris subsp. maritima, because of its large variation in life span and age at first reproduction along a latitudinal gradient including considerable ecological variation. We also compared the consequence in our evolutionary model of maximizing either the Malthusian coefficient or the lifetime reproductive success. Both the data analysis and the results of evolutionary modeling pointed to habitat disturbance and resources like length of the growing season as factors negatively related to life span and age at maturity in sea beet. Resource availability had a negative theoretical influence with the Malthusian coefficient as the chosen optimality criterion, while there was no influence in the case of lifetime reproductive success. As suggested by previous theoretical work the final conclusion on what criterion is more adequate depends on the assumptions of how in reality density-dependence restrains population growth. In our case of sea beet data R0 seems to be less appropriate than l.

Beyond the direct influence of climate change on species distribution and phenology, indirect effects may also arise from perturbations in species interactions. Infectious diseases are strong biotic forces that can precipitate population declines and lead to biodiversity loss. It has been shown in forest ecosystems worldwide that at least 10% of trees are vulnerable to extinction and pathogens are increasingly implicated. In Europe, the emerging ash dieback disease caused by the fungus Hymenoscyphus fraxineus, commonly called Chalara fraxinea, is causing a severe mortality of common ash trees (Fraxinus excelsior); this is raising concerns for the persistence of this widespread tree, which is both a key component of forest ecosystems and economically important for timber production. Here, we show how the pathogen and climate change may interact to affect the future spatial distribution of the common ash. Using two presence-only models, seven General Circulation Models and four emission scenarios, we show that climate change, by affecting the host and the pathogen separately, may uncouple their spatial distribution to create a mismatch in species interaction and so a lowering of disease transmission. Consequently, as climate change expands the ranges of both species polewards it may alleviate the ash dieback crisis in southern and occidental regions at the same time.

International audience; Aim: At a global scale, biodiversity changes are mainly driven by extinction, resulting in an overall decrease in species richness. At regional and local scales, although immigration often (over)compensates for local extinction, biodiversity changes have no clear trends. We tested the hypotheses that (1) at both regional and local scales, biodiversity changes result from the differential dynamics of local extinction and immigration, ultimately leading to increases in biodiversity following periods of transient surplus or deficit and (2) habitats are differentially affected by human activities, resulting in contrasting trends and dynamics that together shape the regional biodiversity budget. Location: Northern France and Belgian Flanders, north-western Europe. Methods: We analysed changes in plant species richness over one century in two adjacent and densely populated regions. Because local to regional changes are expected to be driven by species–environment interactions, and because species ecology and environmental change are largely embodied by the habitat, we assessed biodiversity budgets according to the type of habitat. Results: We observed major changes in species composition at the regional scale with about one of every five to six species becoming regionally extinct or newly naturalized. Immigration offset or exceeded losses, with local extinctions generally preceding gains. Overall, regional dynamics were driven by contrasting changes in a few habitat types, with either local extinction or immigration predominating. Transient biodiversity surpluses or deficits were observed at the regional scale and in certain habitat types. Main conclusions: Including habitat types bridges the gap between regional and local studies and provides a more accurate assessment of the biodiversity budget: integrating habitat type into regional analyses or meta-analyses can lead future research towards the understanding of the determinisms of biodiversity change across spatial scales.

Summary • Removal of developing fruits delays ageing in many annual plant species. This has long been seen as evidence that ageing is caused by resource depletion due to fruit development. Excision experiments can be interpreted from both evolutionary-ecology and resource-allocation viewpoints. Iteroparous (reproducing repeatedly) and annual plants may show different responses to excision. Furthermore, under the resource-depletion hypothesis, the more precocious the excision of reproductive organs, the greater the effect on allocation patterns. • A controlled-conditions experiment was set up involving six life cycles (from long-lived iteroparous to annual taxa of wild beets, Beta spp.) under three treatments of excision of reproductive-parts (buds, fruits and control). • Treatment effect was similar in semelparous and iteroparous beets, although effect on reproduction was more obvious in iteroparous ones. Flower-bud removal tended to induce resource allocation in new reproductive meristems, whatever the life cycle, without any effect on ageing in semelparous beets and probably none on survival in most iteroparous plants. Fruit removal had no effect. • These results and the occurrence of between-accessions variation are discussed in terms of the ecological significance of optimal allocation strategies.

Summary 1 Life span, an essential component of fitness, can be affected by traits such age at first reproduction, as well as by the mortality regime imposed by the environment. In a long-term experiment under controlled conditions, we studied life span in an iteroparous plant species, the sea beet Beta vulgaris ssp. maritima (L.), from 104 wild populations sampled in France, Belgium, Great Britain and the Netherlands. We estimated the intensity of selective pressures on life span and the specific effects of age at first reproduction (first or second year, depending on vernalization requirement) and of environmental disturbance. 2 Mean life span (measured in glasshouse conditions) increases with latitude from about 2 years in inland habitats of south-western France to at least 11 years in north Brittany, before decreasing to about 5 years in the northernmost populations. 3 Comparison of spatial autocorrelograms for life span and cytoplasmic neutral markers showed there is strong selection for life span. 4 Within populations the effect of age at first reproduction on life span proved to be marginal. No causal relationship was detected among populations, although, in the southern part of the study area, a strong association between reproduction in the first year and short life span suggests common selective pressures. 5 Life span appeared to be highly associated with habitat type, as defined by mortality regimes. Populations with the longest-lived individuals occurred in the most stable habitats and populations with the shortest-lived individuals in the most disturbed locations.

The Beta species complex shows a gradient of life histories from pronounced semelparity (big-bang reproduction) to pronounced iteroparity (repeated reproduction). Models assume a trade-off between investment in reproduction and survival. Reproductive effort is thought to increase with decreasing life span, and to be invariable in semelparous plants and susceptible to environmental conditions in iteroparous plants. These assumptions and hypotheses were verified by a greenhouse experiment testing six different life cycles at three contrasting nutrient levels. This study suggests that reproductive effort is negatively correlated with mean life span along the life-cycle gradient. Unlike semelparous beets, reproductive effort in iteroparous beets is extremely sensitive to nutrient level. Phenotypic correlation between allocation to reproduction and allocation to survival generally appeared significantly negative in the longest-lived iteroparous beets, nonsignificant in intermediate life histories and obviously positive in semelparous beets (no trade-off control).

Summary 1 Long-distance dispersal events are biologically very important for plants because they affect colonization probabilities, the probabilities of population persistence in a fragmented habitat, and metapopulation structure. They are, however, very difficult to investigate because of their low frequency. We reviewed the use of molecular markers in the population genetics approach to studying dispersal. With these methods the consequences of long-distance dispersal are studied, rather than the frequency of the dispersal events themselves. 2 Molecular markers vary, displaying different amounts of variation and different modes of inheritance: they may be either dominant or codominant, and may or may not be subjected to genetic recombination. Use of markers has inspired the development of maximum likelihood techniques that take the evolutionary history of alleles into account while estimating gene flow. 3 Inferring seed dispersal rates from indirect measurements of gene flow involves three steps: (i) quantifying genetic differentiation among populations and using this to estimate the rate of gene flow; (ii) producing a genetic dispersal curve by regressing geographical distance among populations against the amount of gene flow; and (iii) separating seed-mediated from pollen-mediated gene flow, by comparing differentiation in nuclear vs. cytoplasmic molecular markers. In this way, potentially very low levels of gene flow can be detected. 4 The indirect approach is based on a number of assumptions. The validity of each assumption should be assessed by independent methods or the estimates of gene flow and dispersal should be mainly used in a comparative context. In metapopulations, with frequent extinction and colonization, the relationship between genetic differentiation and gene flow is not straightforward, and other methods should be used. 5 Highly variable molecular markers, especially microsatellites, have facilitated a direct genetic approach to measuring gene flow, based on parental analyses. 6 The population genetic approach provides different information about dispersal than ecological methods. Thus population genetic and ecological methods may supplement each other, and together lead to a better insight into the dispersal process than either of the methods on its own.

International audience; Biscutella neustriaca is an isolated plant taxon with about three thousand known individuals distributed in several fragmented populations. Despite its status as an endangered plant subject to a European LIFE programme for its protection, no conclusive genetic analysis has been performed to help its conservation. We analysed the genetic variability and distribution of nuclear microsatellite markers in a large sample of the population, as well as of the MatK chloroplastic gene in a subsample. We showed, first, that both pollen and seed dispersal, as well as clonal reproduction are strongly limited, and the mating system is obligate outcrossing. Second, we detected two highly divergent chloroplast haplogroups, as well as two completely distinct nuclear gene pools suggesting an ancient isolation between two groups of populations. Intriguingly, a third group of populations appears to combine the nuclear gene pool of one group with the chloroplast haplotype of the other group, suggesting a more recent dramatic colonization and foundation event. Thanks to complementary geological and historical data, we propose a scenario for the evolutionary history of this metapopulation influenced by the dynamics of Seine meanders and human activities. Finally, we give some suggestions for future conservation actions.

International audience; Despite the increasing number of genomic tools, identifying the genetics underlying adaptive complex traits remains challenging in the model species Arabidopsis thaliana. This is due, at least in part, to the lack of data on the geographical scale of adaptive phenotypic variation. The aims of this study were (i) to tease apart the historical roles of adaptive and nonselective processes in shaping phenological variation in A. thaliana in France and (ii) to gain insights into the spatial scale of adaptive variation by identifying the putative selective agents responsible for this selection. Forty-nine natu- ral stands from four climatically contrasted French regions were characterized (i) phe- nologically for six traits, (ii) genetically using 135 SNP markers and (iii) ecologically for 42 variables. Up to 63% of phenological variation could be explained by neutral genetic diversity. The remaining phenological variation displayed stronger associations with ecological variation within regions than among regions, suggesting the impor- tance of local selective agents in shaping adaptive phenological variation. Although climatic conditions have often been suggested as the main selective agents acting on phenology in A. thaliana, both edaphic conditions and interspecific competition appear to be strong selective agents in some regions. In a first attempt to identify the genetics of phenological variation at different geographical scales, we phenotyped worldwide accessions and local polymorphic populations from the French RegMap in a genome- wide association (GWA) mapping study. The genomic regions associated with pheno- logical variation depended upon the geographical scale considered, stressing the need to account for the scale of adaptive phenotypic variation when choosing accession pan- els for GWAS.

International audience; † Background and Aims Knowledge of those traits that vary with latitude should be helpful in predicting how they may evolve locally under climate change. In the sea beet Beta vulgaris ssp. maritima, seed dormancy largely controls the timing of germination, is highly heritable and varies geographically; it is therefore thought to be selected by climate. The aim here was to characterize the variation in seed dormancy among sea beet populations across the French distribution area, as well as the ecological factors in situ that are correlated with and that could therefore select for seed dormancy. The relative importance of genetic inheritance vs. non-genetic variation is also evaluated. † Methods The proportions of dormant seeds from 85 natural populations encompassing different climates over the whole French distribution area were measured under controlled conditions. Germination phenology was observed in a common garden experiment. Dormancy variation of seeds collected in situ was compared with that of seeds collected on plants grown in the greenhouse. † Key Results The proportions of dormant seeds in the greenhouse were highly variable, covering almost the entire range from 0 to 1, and followed a geographical pattern from lower dormancy at high latitudes to high dormancy at low latitudes. The distribution of dormancy was positively correlated with yearly temperatures, especially summer temperatures. Minimum temperatures in winter did not significantly explain the trait variation. The genetic component of the total variation was significant and is probably completed by an important adjustment to the local conditions brought about by maternal adaptive phenotypic plasticity. †Conclusions Dormancy in sea beet could be interpreted as a way to limit summer germination and spread germination over the first autumn and spring or following autumns. This highly heritable trait has the potential to evolve in the relatively near future because of climate change.

In sea beet (Beta vulgaris ssp. maritima), germination occurs in autumn or spring and is mediated by dormancy which can be released by cold or dry periods. Environmental change such as current climate change may require evolutionary response in seasonal timing. Here, we explore the potential for such evolutionary change. Seed dormancy was studied in a composite population based on seeds from all over the species range in France together with several generations of reciprocal crosses. We found high, repeatable variability for dormancy rate among individuals under greenhouse conditions and confirmed its relevance for germination phenology in the field. Our data fitted best with an exclusively maternal determination of the dormancy phenotype. Narrow-sense heritability, h(2) approximately 0.5 in the composite population and approximately 0.4 in the original local populations, was such that rapid evolutionary change in the relative proportions of autumn and spring germination may be possible.

Liparis loeselii is a rare and endangered orchid occurring in Europe and north-east America. Genetic diversity and structure of this species in north-west France and the United Kingdom were investigated using amplified fragment length polymorphisms (AFLPs). Although clonality and autogamy are common in L. loeselii, we found moderate to important variability within populations. We observed a significant genetic differentiation between populations occurring in dune slacks and fens. This may be correlated with leaf shape as dune slack individuals are sometimes treated as the distinct variety L. loeselii var. ovata. Genetic differentiation between populations was generally low suggesting that gene flow can occur over long distances and possibly across the English Channel. These results show that populations from dune slacks and fens should be managed separately and that geographically distant populations may be equivalent.

DNA fingerprinting has been successfully applied to plants to develop genetic profiles. It has become an important tool in diverse fields of plant population research, e.g. the study of breeding systems (sexual versus asexual reproduction in clonal plant species, estimating of selfing rates, paternity and maternity analysis), genetic relatedness between or within species and populations, assessment of gene flow and gene identification.

The vast majority of perennial plants reproduce sexually and vegetatively at the same time. This may lead to important variation among clonal plant populations in their amount of genotypic diversity. In order to verify this assumption, we compare the clonal diversity of 10 natural populations of the aquatic clonal macrophyte Sparganium erectum in France. Diversity was quantified by DNA fingerprinting and allozyme electrophoresis for a sample of 10 shoots per population. Two DNA probes (CA)8 and (TAA)6TA, were selected among 10 synthetic oligonucleotide probes containing simple repeat motifs. Both allozymes and DNA fingerprints revealed different amounts of diversity among populations. Five populations consist of a single genotype, whereas two populations were genetically highly diverse. In four of the monomorphic populations, absence of fingerprints diversity was combined with uniformly heterozygous allozyme loci, suggesting that each population was composed of a single clone. In the highly diverse populations, the level of clonal diversity combined with the allele segregation of the two allozyme loci Lap and Est suggests frequent seedling recruitment. The origin of new genotypes remains unclear but the absence of widespread genotypes together with the discrete distribution of Sparganium erectum populations implies that new genotypes are locally produced through sexual reproduction.