Using Metabarcoding to Investigate the Strength of Plant-Pollinator Interactions From Surveys of Visits to DNA Sequences

The ongoing decline in pollinators and increasing concerns about pollination services require a better understanding of complex pollination networks, particularly their response to global climate change. While metabarcoding is increasingly used for the identification of taxa in DNA mixtures, its reliability in providing quantitative information on plant-pollinator interactions is still the subject of debate. Combining metabarcoding and microscopy, we investigated the relationships between the number and composition of sequences and the abundance and composition of pollen in insect pollen loads (IPL) and how the two are linked to insect visits. Our findings confirm that metabarcoding is more effective than microscopy in identifying plant species in IPL. For a given species, we found a strong positive relationship between the amount of pollen in IPL and the number of sequences. The relationship was stable across species even if the abundance of co-occurring species in IPL (hereafter “co-occurring pollen”) tended to reduce the sequence yield (number of sequences obtained from one pollen grain) of a given species. We also found a positive relationship between the sequence count and the frequency of visits, and between the frequency and the amounts of pollen in IPL. Our results demonstrate the reliability of metabarcoding in assessing the strength of plant-pollinator interactions and in providing a broader perspective for the analyses of plant-pollinator interactions and pollination networks.

Experimental quantification of pollen with DNA metabarcoding using ITS1 and trnL

Although the use of metabarcoding to identify taxa in DNA mixtures is widely approved, its reliability in quantifying taxon abundance is still the subject of debate. In this study we investigated the relationships between the amount of pollen grains in mock solutions and the abundance of high-throughput sequence reads and how the relationship was affected by the pollen counting methodology, the number of PCR cycles, the type of markers and plant species whose pollen grains have different characteristics. We found a significant positive relationship between the number of DNA sequences and the number of pollen grains in the mock solutions. However, better relationships were obtained with light microscopy as a pollen grain counting method compared with flow cytometry, with the chloroplastic trnL marker compared with ribosomal ITS1 and with 30 when compared with 25 or 35 PCR cycles. We provide a list of recommendations to improve pollen quantification.

Pollinator specialization increases with a decrease in a mass-flowering plant in networks inferred from DNA metabarcoding

How native mass-flowering plants affect the specialization of insects at individual and species levels and the consequences for pollination networks have received much less attention than for mass-flowering crops or alien species and basically remain unexplored.Using existing DNA metabarcoding data on the pollen loads of 402 flower-visiting insects, we assessed the effects of a native mass-flowering plant of high reward quality, the shrub Rhododendron ferrugineum, on pollination networks by investigating: (a) the food niches of individual pollinators and pollinator species and (b) the structure of individual and species networks in subalpine heathland patches with extremely contrasted densities of R. ferrugineum.Relative to its high abundance in high-density patches, the shrub was greatly underrepresented and did not dominate individual's or species' generalized networks, rather individual and species specialization increased with a decrease in R. ferrugineum density. Furthermore, individuals of the more generalist dipteran Empididae species tended to extend exclusive interactions with rare plant species in low-density networks. The same trend was observed in the more specialist Apidea but toward rare species in high-density networks. Our results reveal a quite paradoxical view of pollination and a functional complementarity within networks. Niche and network indices mostly based on the occurrence of links showed that individual pollinators and pollinator species and networks were highly generalized, whereas indices of link strength revealed that species and above all individuals behave as quite strict specialists. Synthesis. Our study provides insights into the status of a native mass-flowering plant in individual's and insect species' food niches and pollination networks. It revealed that a generalist pollinator species can be highly specialized at the individual level and how rare plant species coexisting with mass-flowering plants may nevertheless be visited.

Contribution of plant species to the high N retention capacity of a subalpine meadow undergoing elevated N deposition and warming

While numerous studies have examined the effect of N deposition on ecosystem N retention, few have analyzed the involvement of plant species and climate warming in this process. We experimentally investigated the effects of increasing N deposition (N_exo) and climate warming on the fate of N_exo in a subalpine meadow and established the involvement of plant species. Using ¹⁵N tracer, we tracked N_exo sprayed on the vegetation in belowground and aboveground plant biomasses (AGB) and in bulk soil over three growing seasons. We assessed the N_exo absorption capacity of plant species and the contribution of N_exo to their AGB N pool. The meadow retained a large proportion of N_exo (≈65%, mostly in AGB) for depositions up to four times the background N rate. N_exo present in the meadow compartments in year 2 was still present in year 3, suggesting that the ecosystem was unsaturated after three years of high N input. N_exo retention resulted more from an increase in N concentration in plant tissues than from the increase in AGB. The species-specific N_exo absorption capacity was inversely related to their AGB N concentration. N_exo accounted for up to 40% of total AGB N depending on the species and the N treatments. The contribution of species to ecosystem N_exo retention more contingent on their AGB than on their relative cover in the community, ranked as follows: C. vulgaris (14.0%) > N. stricta (7.0%) > other Poaceae = C. caryophyllea (2.5%) > other Eudicotyledons (1.5%) > non-vascular species = P. erecta > Fabaceae (0.8-0.2%). Climate warming increased AGB and decreased tissue N concentration. No warming-N_exo interaction was observed. Thus, Pyrenean subalpine meadows that have not undergone a decline in plant species richness in recent decades paradoxically display a high potential to sequester atmospheric N deposition.

Pollen transfer in fragmented plant populations: insight from the pollen loads of pollinators and stigmas in a mass-flowering species

Pollinator and/or mate scarcity affects pollen transfer, with important ecological and evolutionary consequences for plant reproduction. However, the way in which the pollen loads transported by pollinators and deposited on stigmas are affected by pollination context has been little studied. We investigated the impacts of plant mate and visiting insect availabilities on pollen transport and receipt in a mass-flowering and facultative autogamous shrub (Rhododendron ferrugineum). First, we recorded insect visits to R. ferrugineum in plant patches of diverse densities and sizes. Second, we analyzed the pollen loads transported by R. ferrugineum pollinators and deposited on stigmas of emasculated and intact flowers, in the same patches. Overall, pollinators (bumblebees) transported much larger pollen loads than the ones found on stigmas, and the pollen deposited on stigmas included a high proportion of conspecific pollen. However, comparing pollen loads of emasculated and intact flowers indicated that pollinators contributed only half the conspecific pollen present on the stigma. At low plant density, we found the highest visitation rate and the lowest proportion of conspecific pollen transported and deposited by pollinators. By contrast, at higher plant density and lower visitation rate, pollinators deposited larger proportion of conspecific pollen, although still far from sufficient to ensure that all the ovules were fertilized. Finally, self-pollen completely buffered the detrimental effects on pollination of patch fragmentation and pollinator failure. Our results indicate that pollen loads from pollinators and emasculated flowers should be quantified for an accurate understanding of the relative impacts of pollinator and mate limitation on pollen transfer in facultative autogamous species.

Using metabarcoding to reveal and quantify plant-pollinator interactions

Given the ongoing decline of both pollinators and plants, it is crucial to implement effective methods to describe complex pollination networks across time and space in a comprehensive and high-throughput way. Here we tested if metabarcoding may circumvent the limits of conventional methodologies in detecting and quantifying plant-pollinator interactions. Metabarcoding experiments on pollen DNA mixtures described a positive relationship between the amounts of DNA from focal species and the number of trnL and ITS1 sequences yielded. The study of pollen loads of insects captured in plant communities revealed that as compared to the observation of visits, metabarcoding revealed 2.5 times more plant species involved in plant-pollinator interactions. We further observed a tight positive relationship between the pollen-carrying capacities of insect taxa and the number of trnL and ITS1 sequences. The number of visits received per plant species also positively correlated to the number of their ITS1 and trnL sequences in insect pollen loads. By revealing interactions hard to observe otherwise, metabarcoding significantly enlarges the spatiotemporal observation window of pollination interactions. By providing new qualitative and quantitative information, metabarcoding holds great promise for investigating diverse facets of interactions and will provide a new perception of pollination networks as a whole.

Subalpine Pyrenees received higher nitrogen deposition than predicted by EMEP and CHIMERE chemistry-transport models

Deposition of reactive nitrogen (N) from the atmosphere is expected to be the third greatest driver of biodiversity loss by the year 2100. Chemistry-transport models are essential tools to estimate spatially explicit N deposition but the reliability of their predictions remained to be validated in mountains. We measured N deposition and air concentration over the subalpine Pyrenees. N deposition was found to range from 797 to 1,463 mg N m(-2) year(-1). These values were higher than expected from model predictions, especially for nitrate, which exceeded the estimations of EMEP by a factor of 2.6 and CHIMERE by 3.6. Our observations also displayed a reversed reduced-to-oxidized ratio in N deposition compared with model predictions. The results highlight that the subalpine Pyrenees are exposed to higher levels of N deposition than expected according to standard predictions and that these levels exceed currently recognized critical loads for most high-elevation habitats. Our study reveals a need to improve the evaluation of N deposition in mountains which are home to a substantial and original part of the world's biodiversity.

Relative impact of mate versus pollinator availability on pollen limitation and outcrossing rates in a mass-flowering species

Plant mating systems are driven by several pre-pollination factors, including pollinator availability, mate availability and reproductive traits. We investigated the relative contributions of these factors to pollination and to realized outcrossing rates in the patchily distributed mass-flowering shrub Rhododendron ferrugineum. We jointly monitored pollen limitation (comparing seed set from intact and pollen-supplemented flowers), reproductive traits (herkogamy, flower size and autofertility) and mating patterns (progeny array analysis) in 28 natural patches varying in the level of pollinator availability (flower visitation rates) and of mate availability (patch floral display estimated as the total number of inflorescences per patch). Our results showed that patch floral display was the strongest determinant of pollination and of the realized outcrossing rates in this mass-flowering species. We found an increase in pollen limitation and in outcrossing rates with increasing patch floral display. Reproductive traits were not significantly related to patch floral display, while autofertility was negatively correlated to outcrossing rates. These findings suggest that mate limitation, arising from high flower visitation rates in small plant patches, resulted in low pollen limitation and high selfing rates, while pollinator limitation, arising from low flower visitation rates in large plant patches, resulted in higher pollen limitation and outcrossing rates. Pollinator-mediated selfing and geitonogamy likely alleviates pollen limitation in the case of reduced mate availability, while reduced pollinator availability (intraspecific competition for pollinator services) may result in the maintenance of high outcrossing rates despite reduced seed production.

Microsatellite marker analysis reveals the complex phylogeographic history of Rhododendron ferrugineum (Ericaceae) in the Pyrenees

Genetic variation within plant species is determined by a number of factors such as reproductive mode, breeding system, life history traits and climatic events. In alpine regions, plants experience heterogenic abiotic conditions that influence the population's genetic structure. The aim of this study was to investigate the genetic structure and phylogeographic history of the subalpine shrub Rhododendron ferrugineum across the Pyrenees and the links between the populations in the Pyrenees, the Alps and Jura Mountains. We used 27 microsatellite markers to genotype 645 samples from 29 Pyrenean populations, three from the Alps and one from the Jura Mountains. These data were used to estimate population genetics statistics such as allelic richness, observed heterozygosity, expected heterozygosity, fixation index, inbreeding coefficient and number of migrants. Genetic diversity was found to be higher in the Alps than in the Pyrenees suggesting colonization waves from the Alps to the Pyrenees. Two separate genetic lineages were found in both the Alps and Pyrenees, with a substructure of five genetic clusters in the Pyrenees where a loss of genetic diversity was noted. The strong differentiation among clusters is maintained by low gene flow across populations. Moreover, some populations showed higher genetic diversity than others and presented rare alleles that may indicate the presence of alpine refugia. Two lineages of R. ferrugineum have colonized the Pyrenees from the Alps. Then, during glaciation events R. ferrugineum survived in the Pyrenees in different refugia such as lowland refugia at the eastern part of the chain and nunataks at high elevations leading to a clustered genetic pattern.

Massive floral display affects insect visits but not pollinator-mediated pollen transfer in Rhododendron ferrugineum

Fragmentation of natural vegetation creates one of the largest threats to plant-pollinator interactions. Although fragmentation impacts on plant populations have been explored in many, mainly herbaceous, species, the response of wild mass-flowering species is poorly known. Here, we studied 28 heathland patches dominated by the mass-flowering shrub Rhododendron ferrugineum, each presenting different R. ferrugineum floral display sizes (total inflorescence number per patch) and patch isolation (median distance to the three nearest patches). We assessed the impacts of these two factors on (i) heathland patch visitor assemblage (considering R. ferrugineum versus surrounding community) and (ii) R. ferrugineum flower visitation rate and pollen transfer limitation (comparing seed set from emasculated to pollen-supplemented flowers). We found that diversity and abundance of bees visiting R. ferrugineum in heathland patches significantly decreased with decreasing R. ferrugineum floral display, while overall visitor density per patch and flower visitation rate increased. Moreover, a decrease in massive floral display and increase in patch isolation resulted in reduced visitor density in the surrounding community. Even in patches with few individuals, we found disproportionate visitor abundance in R. ferrugineum compared to the surrounding community. Finally, pollen transfer limitation in R. ferrugineum was neither affected by visitation rate nor by patch attributes. By disproportionally attracting pollinators from co-flowering species, and probably promoting geitonogamous pollen transfer, the mass-flowering trait appears adequate to compensate, in terms of conspecific pollen transfer, for the decrease in visitor diversity and abundance and in mate availability, which usually result from population fragmentation.

Isolation and characterization of microsatellite loci in Rhododendron ferrugineum (Ericaceae) using pyrosequencing technology

PREMISE OF THE STUDY

Microsatellite primers were developed for Rhododendron ferrugineum (Ericaceae) to evaluate genetic diversity, population genetic structure, and mating system of this self-compatible species.

METHODS AND RESULTS

The new-generation 454 FLX Titanium pyrosequencing technology was used to isolate 102 novel microsatellite loci. Two multiplex PCR sets were optimized to genotype nine polymorphic microsatellite loci. The level of genetic diversity was assessed in two populations from the Pyrenees (France). The mean number of alleles per locus ranged from 1.5 to 7. The mean observed and expected heterozygosities ranged from 0 to 0.76 and from 0.03 to 0.66, respectively. Cross-species amplification was successful for 13 other Rhododendron species and two additional genera of Ericaceae, with an average of seven pairs of primers amplifying per species.

CONCLUSIONS

These markers will facilitate further studies on the evolutionary history of the large Ericaceae family and Rhododendron in particular.

Conservation genetics of the rare Pyreneo-Cantabrian endemic Aster pyrenaeus (Asteraceae)

BACKGROUND AND AIMS

Aster pyrenaeus (Asteraceae) is an endangered species, endemic to the Pyrenees and Cantabrian Mountain ranges (Spain). For its long-term persistence, this taxon needs an appropriate conservation strategy to be implemented. In this context, we studied the genetic structure over the entire geographical range of the species and then inferred the genetic relationships between populations.

METHODOLOGY

Molecular diversity was analysed for 290 individuals from 12 populations in the Pyrenees and the Cantabrian Mountains using inter simple sequence repeats (ISSRs). Bayesian-based analysis was applied to examine population structure.

PRINCIPAL RESULTS

Analysis of genetic similarity and diversity, based on 87 polymorphic ISSR markers, suggests that despite being small and isolated, populations have an intermediate genetic diversity level (P % = 52.8 %, H(E) = 0.21 ± 0.01, genetic similarity between individuals = 49.6 %). Genetic variation was mainly found within populations (80-84 %), independently of mountain ranges, whereas 16-18 % was found between populations and <5 % between mountain ranges. Analyses of molecular variance indicated that population differentiation was highly significant. However, no significant correlation was found between the genetic and geographical distances among populations (Rs = 0.359, P = 0.140). Geographical structure based on assignment tests identified five different gene pools that were independent of any particular structure in the landscape.

CONCLUSIONS

The results suggest that population isolation is probably relatively recent, and that the outbreeding behaviour of the species maintains a high within-population genetic diversity. We assume that some long-distance dispersal, even among topographically remote populations, may be determinant for the pattern of genetic variation found in populations. Based on these findings, strategies are proposed for genetic conservation and management of the species.

Leaf life span optimizes annual biomass production rather than plant photosynthetic capacity in an evergreen shrub

*Owing to nitrogen (N) translocation towards new leaves, the shedding of old leaves can increase the whole-plant carbon gain. It occurs when their photosynthetic nitrogen use efficiency (PNUE) declines below a given threshold. *Here, we investigated variations in net photosynthetic capacity (A(max)), N resorption and PNUE in populations of Rhododendron ferrugineum presenting different mean leaf life spans (LLS). *Both populations had comparable annual leaf surface area production and A(max) across leaf-age cohorts. Branch photosynthetic capacity was up to 95% higher in the population with the longer LLS mainly because of the high contribution of old leaves to the total leaf area. Despite lower N concentrations, old leaves maintained relatively high A(max) and consequently PNUE that were higher than or similar to the values found in current-year leaves. *As the ratio of PNUE in old to PNUE in new leaves was always higher than the fraction of leaf N resorbed during leaf shedding, we concluded that leaf shedding did not improve plant photosynthetic capacity. We suggest that in R. ferrugineum, leaf shedding is mainly controlled by the leaf storage function and, therefore, that models aiming to explain LLS should not only consider the leaf carbon assimilation function, particularly in nutrient-poor habitats.

Complex interactions between a legume and two grasses in a subalpine meadow

Interactions between plants are a complex combination of positive and negative interactions, with the net outcome depending on environmental contexts. The more frequent association of Trifolium alpinum (legume) with Festuca eskia than with Nardus stricta (grasses) in many Pyrenean subalpine meadows suggests a differential ability to use nitrogen (N) derived from N(2) fixation. In the field, we investigated the interactions between the legume and grasses and, in the glasshouse, the transfer of (15)N from the legume to the grasses. In one grass-Trifolium mixture, the legume had a strong positive effect on the biomass and N content of the grass as compared to pure grass stands. When both grasses grew together with the legume, only Festuca benefited from the presence of Trifolium but, surprisingly, the benefit decreased with increasing Trifolium abundance. Leaf labeling experiments with (15)N-NH(4)(+) revealed a higher transfer of (15)N from Trifolium to Festuca than to Nardus, suggesting a more direct N pathway between the two species. This more direct pathway could prevent Nardus from benefiting from the legume N in the three-species mixtures. Thus, the positive interactions between N-fixers and nonfixers appear to be largely species-specific and to depend strongly on the species in the plant assemblage.

Endogenous sink-source interactions and soil nitrogen regulate leaf life-span in an evergreen shrub

How the balance between exogenous and endogenous nitrogen for shoot growth varies with soil nitrogen availability, and its consequences on leaf life-span, have rarely been studied within a single species in the field. In this study, we investigated two Rhododendron ferrugineum populations with contrasting leaf life-span. Soil nitrogen availability and nitrogen resorption of different leaf age classes were assessed, as were the interactions between plant compartments, using (15)N labelling and sink organ suppression. The population growing on poorer soil had a shorter leaf life-span (17.9 vs 21.5 months) and a higher net contribution of leaf reserves to shoot growth (32% vs 15%), achieved by faster nitrogen resorption and greater shedding of young nitrogen-rich leaves. For both populations, wood contributed to over 40% of shoot nitrogen demand. Both the negative relationship between current-year shoot mass and the percentage of 1-yr-old attached leaves and the delay of leaf shedding after bud removal suggest that shoot development has a strong effect on leaf life-span. Our results suggest that, contrary to the evolutionary response, plastic response to low soil nitrogen could reduce leaf life-span in evergreen plants. In addition, leaf life-span seems to be strongly influenced by the discrepancy between shoot nitrogen demand and soil nitrogen uptake rather than nitrogen demand alone.

Complementarity in mineral nitrogen use among dominant plant species in a subalpine community

The underlying mechanisms that enable plant species to coexist are poorly understood. Complementarity in resource use is among the major mechanisms proposed that could favor species coexistence but is insufficiently documented. In alpine soil, low temperatures are a major constraint for the supply of plant nitrogen. We carried out (15)N labeling of soil mineral N to determine to what extent four major species of a subalpine community compete for N, or develop ionic (NH(4)(+) vs. NO(3)(-)) or temporal complementarity. The Poaceae took up much more (15)N per soil area unit than the ericaceous species, and all species displayed three major strategies in exploiting (15)N: (1) uptake mainly early in the growing season (Vaccinium myrtillus), (2) uptake at a slow and similar rate throughout the growing season (Rhododendron ferrugineum), and (3) uptake at high rates over the growing season (Festuca eskia and Nardus stricta). However, while F. eskia used (15)NH(4)(+) mainly early and (15)NO(3)(-) mainly late in the growing season, the reverse was observed for N. stricta. Taking into account (15)N dilution in soil NH(4)(+) and NO(3)(-) pools, we calculated that NH(4)(+) provided more than 80% of the mineral N uptake in Ericaceae and about 60% in grasses. Together, such ionic and temporal complementarity would reduce competition between species and could be a major mechanism promoting species diversity.

Nitrogen resorption and photosynthetic activity over leaf life span in an evergreen shrub, Rhododendron ferrugineum, in a subalpine environment

Here, the advantages for a shrub of having long vs short-lived leaves was investigated in Rhododendron ferrugineum by following nitrogen(15N) and carbon(14C) resorption and translocation, and photosynthetic capacity over the life span. Mean leaf life span was 19 months. Nitrogen (N) resorption in attached leaves occurred mainly in the first year (23%) and reached a maximum of 31% in the second. Although, resorption was similar in attached and fallen 1-yr-old leaves, it was on average one-third higher in fallen than in attached older leaves. Final N resorption of a leaf compartment reached 41%, half occurring from healthy leaves during the first year. Photosynthetic capacity decreased slightly during the life span. Before shoot growth, plant photosynthesis was mainly supported by 1-yr-old leaves, although the contribution of the 2-yr-old leaves was nonnegligible (15% of the capacity and higher carbon transfer toward the roots). After shoot growth, the current-year leaves made the greatest contribution. Our results suggest that short-lived leaves (half of the cohort) are mainly involved in a photosynthetic function, having a high photosynthetic capacity and drawing most of their resorbed N towards current-year leaves; and long-lived leaves are also involved in a conservative function, increasing N resorption and mean residence time (MRT).

Uncoupling nitrogen requirements for spring growth from root uptake in a young evergreen shrub (Rhododendron ferrugineum)

•   Internal cycling of nitrogen (N) was investigated in a subalpine field population of the evergreen shrub Rhododendron ferrugineum during spring growth. •   The foliar nitrogen of 5-yr-old-plants was directly labeled with ¹⁵ N and subsequently traced to all plant compartments. In addition, ¹⁵ N-ammonium uptake was estimated in glasshouse experiments. •   Before shoot growth, redistribution of ¹⁵ N occurred in the plant without net N transfer. During spring development, the decreases in both leaf ¹⁵ N and total N were almost identical in terms of percentage, and most of the ¹⁵ N withdrawn from the leaf compartments was recovered in the growing shoots. Net changes in the N contents of the various leaf and woody compartments indicate that internal remobilization (especially from 1-yr-old leaves) could have met most of the N needs of new shoot growth. Simultaneously, the rate of mineral N uptake was very low. •   Thus, leaves in young plants provide N for new shoots (by contrast with old individuals) and allow, with woody tissues, almost complete uncoupling of N requirement for spring growth from root uptake.

Stamen dimorphism in Rhododendron ferrugineum (Ericaceae): development and function

The function of stamen dimorphism in the breeding system of the alpine shrub Rhododendron ferrugineum was studied in two populations in the French Alps. This species has pentameric flowers with two whorls of stamens: an inner whorl of five long stamens and an outer whorl of short stamens. We studied the development of stamens from buds to mature flowers (measurement of the filament, anther, and style lengths at five successive phenological stages) and compared the size and position of reproductive organs at maturity in control and partially emasculated flowers (removal of long-level stamens) to determine whether the presence of long-level stamens constitutes a constraint for the development of the short-level ones. Stamen dimorphism can be observed early in stamen development, from the bud stage of the year prior to flowering. At this early stage, meiosis had already occurred. Emasculation of the long-level stamens induced the short-level ones to grow longer than in normal conditions. We also performed seven pollination treatments on ten randomly chosen individuals in each population, and the number of seeds following each treatment was recorded. Results from these treatments showed that R. ferrugineum produced spontaneous selfed seeds in the absence of pollinators. However, no seed was produced when short-level stamens were emasculated and pollinators excluded, suggesting that long-level stamens are not responsible for selfing in the absence of pollinators and that reproductive assurance is promoted by short-level stamens.

Dynamics of genotypic structure in clonal Rhododendron ferrugineum (Ericaceae) populations

Two populations of Rhododendron ferrugineum growing at subalpine level in the Pyrenees (France) were studied in two sites (Bethmale and Mourtis). Identification and delimitation of genets were inferred from amplified fragment length polymorphism (AFLP) markers, along a closure gradient (from meadow to more closed heath) in each site. Surface and age of genets, genotypic diversity (Simpson's index D), 'proportion distinguishable' genotypes and genetic relationships between genets were then estimated. Amplification of the 312 DNA samples with three selective primer pairs gave a mean of 98 detectable peaks (i.e. bands) per sample, with size ranging from 60 to 300 bp. In total 60% (Bethmale) and 70% (Mourtis) of the peaks were polymorphic, and a total of 31 and 23 multilocus genotypes were identified, in Bethmale and Mourtis, respectively. We inferred that pioneer genotypes began arriving 110 years ago mainly over a 40-year period in the Mourtis meadow, and began about 130 years ago over a 100-year period in the Bethmale meadow. After this pioneer stage, populations extended vegetatively. Two different patterns of genotypic dynamics can be identified. At Bethmale, population closure could have led to a dramatic loss of genets and to the selection of highly genetically related genotypes. In contrast, at Mourtis, genotypic diversity and genet density did not change fundamentally along the closure gradient. However the range of genetic diversity diminished from the open to the closed situation, suggesting that thinning could have occurred in the past.

Breeding system in an alpine species: Rhododendron ferrugineum L. (Ericaceae) in the French northern Alps

The breeding system of the alpine shrub Rhododendron ferrugineum was studied at two sites of a population (site L and site H) in the French Alps. The aims were to estimate the degree of selling and (or) outcrossing and to determine if floral traits (including floral morphology, pollen to ovule ratio, stigma receptivity) may be related to the breeding system of the shrub. First, floral traits (corolla length and anther, and style heights) were measured, the number of pollen tetrads and ovules were counted, and the date of stigma receptivity and anther dehiscence were recorded. Second, five pollinations treatments (unmanipulated flowers, natural selfing, hand selfing, natural outcrossing, and hand outcrossing) were performed on 35 randomly chosen individuals in each site, and after each treatment the number of seeds per fruit was recorded. The results demonstrate that Rhododendron ferrugineum is self-compatible (self-compatibility index = 0.95 and 0.97; auto-fertility index = 0.92 and 0.98; selfing rate = 0.24 and 0.11 at sites L and H, respectively). Indices values are not significantly different between the two sites and facultatively xenogamous (pollen to ovule ratio = 669.5 ± 280.5 and 845.2 ± 246.9 at sites L and H, respectively). Key words: breeding system, Rhododendron ferrugineum, floral morphology, phenology, P:O ratio.

Action mechanism of retinoid-synergistic dibenzodiazepines

4-[5H-2,3-(2,5-Dimethyl-2,5-hexano)-5-methyldibenzo[b,e][1,4 ]diazepin-11-yl]benzoic acid (HX600), as well as its oxa- (HX620) and thia- (HX630) analogs, enhanced the activity of retinoic acid and a receptor alpha (RAR alpha)-selective agonist Am80 in HL-60 cell differentiation assays. HX600 synergizes with Am80 by binding to, and transactivating through, the RXR subunit of the RXR-RAR heterodimer. HX600 exhibited RXR pan-agonist activity in transient transfections with a DR1-based reporter gene and synergized with RA-bound RAR alpha and RAR beta in inducing transcription from a DR5-based reporter. In addition, all three compounds at high concentrations acted as RAR pan-antagonists in stably transfected RAR "reporter cells." These efficient synergists bind only weakly with RXRs in vitro, suggesting that they are RXR-RAR heterodimer-selective activators. These HX retinoids exhibited dual functionality, since they affected signalling through both retinoid receptor families (RARs and RXRs).

Cell-specific inhibitory and stimulatory effects of Fos and Jun on transcription activation by nuclear receptors

We investigated the effect of c-Fos and/or c-Jun co-expression on transcription activation by the progesterone (PR), glucocorticoid (GR) or androgen (AR) receptors using three different reporter genes and four different cell lines. We found that c-Fos could only inhibit, while c-Jun could either inhibit or further stimulate receptor-induced transcription. All these effects were receptor, promoter, and cell type specific, and, importantly, the steroid receptors had non-reciprocal effects on the transactivation ability of c-Jun in the presence or absence of c-Fos. Collectively, these results argue against heterodimer formation as a mechanism to explain the phenomena. Transactivation by the endogenous PR in T47D cells could be inhibited by increasing the intracellular c-Fos level with forskolin as well as by co-expressing c-Fos; no such effect was seen in MCF-7 cells. The inhibition by c-Fos of PR-induced transcription involves a competitive mechanism, which requires the presence of the intact c-Fos leucine zipper and is directed mainly at the transcription activation function (TAF) located in the PR and GR hormone binding domains (TAF-2). However, the co-expression of c-Fos did not alter the 'squelching/transcriptional interference' by the PR of estrogen receptor (ER)-induced transcription. Multiple mechanisms are discussed which may be involved in the crosstalk between the two signal transduction pathways.

Agonistic and antagonistic activities of RU486 on the functions of the human progesterone receptor

RU486 induced the binding to a palindromic progestin responsive element (PRE) in vitro of homo- and heterodimers of the human progesterone receptor (hPR) isoforms A and B, present in T47D breast cancer cells or in HeLa cells transiently expressing the recombinant proteins. The resulting complexes were indistinguishable from those induced with the agonist R5020 with respect to specificity, affinity and stability. Ligand exposure was a necessary prerequisite to observe PR/PRE complexes. Antagonist-induced complexes migrated more rapidly during electrophoresis than agonist-induced ones, and no 'mixed' PR/RU486-PR/R5020 complexes were observed, suggesting that the dimerization interfaces of agonist- and antagonist-bound molecules are non-compatible. The analysis of a series of deletion mutants and chimeric receptors revealed the presence of two transcription activation functions (TAFs), located in the N-terminal region A/B (TAF-1) and the hormone binding domain (TAF-2). In the presence of agonists, both TAFs were active in HeLa cells. In the presence of RU486 TAF-2 was inactive, while TAF-1 within the hPR form B/RU486 complex activated transcription from a reporter gene containing a single palindromic PRE. We consider this to be the most convincing evidence that the receptor/RU486-complex does in fact bind to PREs in vivo. No transcriptional activation was observed in the presence of RU486 from a reporter gene containing the complex MMTV-LTR PRE. In contrast to hPR form B, form A was not able to activate transcription from PRE/GRE-tk-CAT in the presence of RU486. In vivo competition between hPR/RU486 and either cPR/R5020 or the human glucocorticoid receptor/dexamethasone (hGR/Dex) complex further supported that hPR/RU486 bound in vivo to its cognate responsive element. Indeed, the observed inhibition of transcription was shown to be due to competition for the MMTV PRE, since no transcriptional interference by the hPR/RU486 was observed, and since no heterodimers were formed between hPR/RU486 and cPR/R5020 or hGR/Dex. That the ligand-free hPR, however, was unable to compete, demonstrated that ligand binding is the prerequisite for DNA binding of hPR in vivo.