Résultat de la recherche
Transcriptomic analysis of deceptively pollinated Arum maculatum (Araceae) reveals association between terpene synthase expression in floral trap chamber and species-specific pollinator attraction
Deceptive pollination often involves volatile organic compound emissions that mislead insects into performing nonrewarding pollination. Among deceptively pollinated plants, Arum maculatum is particularly well-known for its potent dung-like volatile organic compound emissions and specialized floral chamber, which traps pollinators—mainly Psychoda phalaenoides and Psychoda grisescens—overnight. However, little is known about the genes underlying the production of many Arum maculatum volatile organic compounds, and their influence on variation in pollinator attraction rates. Therefore, we performed de novo transcriptome sequencing of Arum maculatum appendix and male floret tissue collected during anthesis and postanthesis, from 10 natural populations across Europe. These RNA-seq data were paired with gas chromatography–mass spectrometry analyses of floral scent composition and pollinator data collected from the same inflorescences. Differential expression analyses revealed candidate transcripts in appendix tissue linked to malodourous volatile organic compounds including indole, p-cresol, and 2-heptanone. In addition, we found that terpene synthase expression in male floret tissue during anthesis significantly covaried with sex- and species-specific attraction of Psychoda phalaenoides and Psychoda grisescens. Taken together, our results provide the first insights into molecular mechanisms underlying pollinator attraction patterns in Arum maculatum and highlight floral chamber sesquiterpene (e.g. bicyclogermacrene) synthases as interesting candidate genes for further study.
Elevational gradients in constitutive and induced oak defences based on individual traits and their correlated expression patterns
Elevational gradients are useful ecological settings for revealing the biotic and abi-otic drivers of plant trait variation and plant–insect interactions. However, most work focusing on plant defences has looked at individual traits and few studies have assessed multiple traits simultaneously, their correlated expression patterns, and abiotic fac-tors associated with such patterns across elevations. To address this knowledge gap, we studied elevational variation in direct (phenolic compounds) and indirect (volatile organic compounds) constitutive defences and their inducibility after feeding by a spe-cialist beetle Altica quercetorum in saplings of 18 wild populations of Quercus pyrenaica. We tested for: 1) clines in each defensive trait individually, 2) their patterns of corre-lated expression and 3) associations between any such clines and climatic factors. We found that constitutive direct defences (lignins and hydrolysable tannins) decreased with increasing elevation. We observed no elevational gradient for constitutive indi-rect defences (volatile organic compounds) or the inducibility of direct or indirect defensive traits when looking at groups of compounds. However, at individual tree-level, increased induction of two monoterpenes (α-fenchene and camphene) at higher elevation was shown. Furthermore, we show a significant pattern of co-expression of constitutive and induced phenolics across populations, which weakened with increas-ing elevation. Finally, we found no evidence that climatic factors were associated with either individual or correlated trait expression patterns across elevations. Overall, these findings call for moving beyond elevational clines in individual plant defences, and argue that assessing elevational shifts in trait correlated expression patterns and their underlying mechanisms can increase our understanding of plant defence evolution and plant–herbivore interactions along environmental gradients.
On the chemical and molecular ecology of deceptively pollinated "Arum maculatum" (Araceae)
La pollinisation est à la base de la diversité de tous les écosystèmes terrestres, et il existe de nombreuses évidences démontrant que les pollinisateurs sont l’un des principaux mécanismes de spéciation des angiospermes (plantes à fleurs). La spéciation peut souvent être liée à la sélection naturelle des caractères reproductifs : la couleur, la forme, la taille et l'odeur des fleurs ont toutes été considérées comme des adaptations essentielles aux pollinisateurs locaux. Cependant, les communautés de pollinisateurs sont rarement homogènes dans l'espace et dans le temps, ce qui peut produire des scénarios complexes d'adaptation ou de maladaptation, d'évolution unilatérale ou de coévolution. Par conséquent, comprendre comment des caractéristiques complexes à multiples fonctions - telles que l'odeur florale - se diversifient sur de vastes distances géographiques est difficile. Les interactions de pollinisation obligatoire spécialisée, au sein desquelles une plante attire un nombre limité d'espèces de pollinisateurs, impliquent généralement l'émissions de composés organiques volatils (COVs) sous forte sélection naturelle. Ces systèmes constituent un modèle utile pour étudier la problématique du maintien d’une interaction spécialisée. Ici, nous avons étudié Arum maculatum (Araceae) comme modèle de notre recherche. Arum maculatum fascine les botanistes depuis longtemps en raison de sa pollinisation par duperie qui mime des sites de ponte : les pollinisateurs sont attirés par une odeur semblable à celle d'une bouse ou de la matière en décomposition et sont temporairement piégés pendant le processus de pollinisation, sans aucune récompense. Des recherches antérieures ont démontré que les deux principaux pollinisateurs de A. maculatum, les moucherons diptères (Psychodidae) Psychoda phalaenoides et P. grisescens, sont majoritairement piégées dans deux régions géographiques d’Europe, ce qui reflète largement les deux clusters génétiques trouvés chez A. maculatum. Dans chapitre 2, nous avons cherché à savoir si ce fonctionnement était le résultat d'adaptations locales de l'odeur des fleurs. En utilisant une analyse comparative à l'échelle européenne, de la variation des COV floraux ainsi qu'une expérience de transplantation, nous avons découvert que la plupart des populations d'A. maculatum présentent des émissions de COVs très variables. Ils sont donc généralement capables d'attirer à la fois P. phalaenoides et P. grisescens. Des données sur les pollinisateurs, avec réplication temporelle, ont révélé que des changements interannuels et décennaux dans la composition des espèces de pollinisateurs sont présents, et peuvent expliquer pourquoi une variation considérable des COV est maintenue au sein des populations. Dans le chapitre 3, nous avons cherché à étudier les bases génétiques de cette variation intraspécifique de l'odeur florale en utilisant le séquençage du transcriptome entier de deux tissus floraux émettant des COV de A. maculatum : l'appendice et les étamines. Ces données nous ont permis d'identifier des transcriptions candidates pour plusieurs COVs inhabituels d'A. maculatum, et ont fourni des informations sur la spécificité du tissu floral de leur production. Les analyses de co-inertie entre les modèles d'expression des transcriptions et les taux d'attraction des pollinisateurs ont également identifié une corrélation entre les émissions de sesquiterpènes par les étamines situées dans la chambre florale à la base de l’inflorescence et l'attraction des pollinisateurs spécifique à l'espèce. Finalement, dans le chapitre 4, nous avons étendu notre échantillonnage des COVs et de génétique pour inclure d’autres espèces d’Arum. Ici, nous avons cherché à caractériser les impacts de la sélection exercée par les pollinisateurs par rapport aux contraintes phylogénétiques sur la variation de l'odeur florale, à des échelles microévolutives (c.-à-d. au sein d’A. maculatum) et macroévolutives (c.-à-d. à l'échelle du genre). En utilisant des méthodes de génotypage par séquençage à haut débit, nous avons identifié des milliers de loci putativement neutres ; ces données ont été utilisées pour confirmer que les COVs sont labiles du point de vue de l'évolution à la fois au sein d'A. maculatum et à l'échelle du genre. Cette adaptabilité chimique peut être la clé pour tromper une gamme large et variable d'espèces de diptères. Même si nous avons identifié les COVs associés à l'attraction des pollinisateurs spécifiques à l'espèce, ainsi que les gènes qui les supportent, la sélection naturelle n'a pas encore fixé ces gènes au sein des populations. Les communautés variables de pollinisateurs dans le temps semblent être un facteur important dans ce fonctionnement, favorisant potentiellement des bouquets d'odeurs floraux diversifiés afin de s'assurer qu'un plus grand nombre d'espèces Psychoda puisse être trompé. Une haute diversité d'odeurs florales est également apparente au niveau du genre, bien que la variation intraspécifique chez d'autres espèces d'Arum doive encore être étudiée. Pris ensemble, les résultats de cette thèse soulignent comment et pourquoi la variation de traits floraux peut persister au sein des populations, même lorsque les interactions de pollinisation sont spécifiques et obligatoires.
Abstract
Pollination is foundational to the diversity of all terrestrial ecosystems, and there is substantial evidence that pollinators are the main driver of angiosperm (flowering plant) speciation rates. Speciation can often be linked to natural selection on reproductive traits: floral color, shape, size, and scent all may represent key adaptations to local pollinator guilds. However, pollinator communities are rarely consistent through space or time, which can lead to tangled patterns of adaptation versus maladaptation, and evolution or coevolution. Consequently, understanding how highly-dimensional traits with multiple functions – such as floral scent – diversify across wide geographic ranges remains a major challenge. Specialized obligate pollination systems, where plants attract a limited number of pollinator species using volatile organic compound (VOC) emissions putatively under strong selection (i.e. with few trade-offs present), are a useful model to address this problem. Here, we used Arum maculatum (Araceae) as the model for our work. Arum maculatum has long fascinated botanists due to its deceptive pollination via brood-site mimicry: pollinators are attracted by a dung-like scent and are temporarily trapped during the pollination process, without any reward. Previous research has demonstrated that the two main pollinators of A. maculatum, the dipteran (Psychodidae) moth flies Psychoda phalaenoides and P. grisescens, were respectively trapped in two main geographic zones in Europe, which mirror the two population genetic clusters present in A. maculatum. In Chapter 2, we investigated whether this pattern was a result of local adaptations in floral scent. Using a combination of Europe-wide surveys of floral VOC variation and pollinator attraction patterns and a transplant experiment, we found that most A. maculatum populations have highly variable VOC emissions, and consequently are capable of attracting both P. phalaenoides and P. grisescens. Temporally replicated pollinator data revealed that inter-annual and decadal changes in pollinator species composition are present, and may explain why considerable VOC variation is maintained within populations. In Chapter 3, we further aimed to investigate the genetic bases of intraspecific variation in floral scent using whole transcriptome sequencing of two VOC-emitting floral tissues in A. maculatum (i.e. the appendix and male florets). These data identified candidate transcripts for several unusual A. maculatum VOCs, and provided insights into the tissue-specific nature of their production. Co-inertia analyses between transcript expression patterns and pollinator attraction rates further identified a correlation between male floret terpene synthases and species-specific pollinator attraction. Finally, in Chapter 4, we expanded our VOC and genetic sampling to include species from across the genus Arum. Here, we aimed to characterize the impacts of pollinator-mediated selection versus phylogenetic constraints on floral scent variation, at microevolutionary (i.e. within A. maculatum) and macroevolutionary (i.e. genus-wide) scales. Using high throughput genotyping-by-sequencing methods, we identified thousands of putatively neutral loci; these data were used to confirm that VOCs are evolutionarily labile both within A. maculatum and across the genus. This adaptability may be key to deceiving a wide and variable range of dipteran species. Even though we identified VOCs associated with species-specific pollinator attraction, and the genes underlying them, natural selection has not fixed these genes within populations yet. Temporally variable pollinators appear to be an important factor in this pattern, potentially favoring diverse floral scent bouquets to ensure that a wider range of Psychoda species can be deceived. High floral scent diversity is also apparent at the genus level, although the extent of intraspecific variation in other species of Arum needs to be studied further. Taken together, the results of this thesis highlight how and why trait variation may persist within populations, even when species interactions are specific and obligate.
A mirage of cryptic species: Genomics uncover striking mitonuclear discordance in the butterfly Thymelicus sylvestris
Mitochondrial DNA (mtDNA) sequencing has led to an unprecedented rise in the identification of cryptic species. However, it is widely acknowledged that nuclear DNA (nuDNA) sequence data are also necessary to properly define species boundaries. Next generation sequencing techniques provide a wealth of nuclear genomic data, which can be used to ascertain both the evolutionary history and taxonomic status of putative cryptic species. Here, we focus on the intriguing case of the butterfly Thymelicus sylvestris (Lepidoptera: Hesperiidae). We identified six deeply diverged mitochondrial lineages; three distributed all across Europe and found in sympatry, suggesting a potential case of cryptic species. We then sequenced these six lineages using double‐digest restriction‐site associated DNA sequencing (ddRADseq). Nuclear genomic loci contradicted mtDNA patterns and genotypes generally clustered according to geography, i.e., a pattern expected under the assumption of postglacial recolonization from different refugia. Further analyses indicated that this strong mtDNA/nuDNA discrepancy cannot be explained by incomplete lineage sorting, sex‐biased asymmetries, NUMTs, natural selection, introgression or Wolbachia‐mediated genetic sweeps. We suggest that this mitonuclear discordance was caused by long periods of geographic isolation followed by range expansions, homogenizing the nuclear but not the mitochondrial genome. These results highlight T. sylvestris as a potential case of multiple despeciation and/or lineage fusion events. We finally argue, since mtDNA and nuDNA do not necessarily follow the same mechanisms of evolution, their respective evolutionary history reflects complementary aspects of past demographic and biogeographic events.
Spatial and temporal heterogeneity in pollinator communities maintains within-species floral odour variation
Flowering plants emit complex bouquets of volatile organic compounds (VOCs) to mediate interactions with their pollinators. These bouquets are undoubtedly influenced by pollinator-mediated selection, particularly in deceptively-pollinated species that rely on chemical mimicry. However, many uncertainties remain regarding how spatially and temporally heterogeneous pollinators affect the diversity and distribution of floral odour variation. Here, we characterized and compared the floral odours of ten populations of deceptively-pollinated Arum maculatum (Araceae), and inter-annual and decadal variation in pollinator attraction within these populations. Additionally, we transplanted individuals from all sampled populations to two common garden sites dominated by different pollinator species (Psychoda phalaenoides or Psycha grisescens), and compared pollinator attraction rates to investigate whether populations maintained odour blends adapted to a specific pollinator. We identified high within- and among-population variation in a common blend of VOCs found across the range of A. maculatum. We also observed shifts in pollinator community composition within several populations over 1–2 years, as well as over the past decade. Common garden experiments further revealed that transplanted inflorescences generally attracted the dominant local pollinator species in both transplant sites. However, one population (Forêt du Gâvre, France) appears to exclusively attract P. grisescens, even when transplanted to a P. phalaenoides-dominated site. Together, our results suggest that maintaining diverse floral odour bouquets within populations may be advantageous when pollinator communities vary over short timescales. We propose that temporally-replicated ecological data are one potential key to understanding variation in complex traits such as floral odour, and in some cases may reveal resiliency to shifting pollinator communities.
Bacterial communities within Phengaris (Maculinea) alcon caterpillars are shifted following transition from solitary living to social parasitism of Myrmica ant colonies
Bacterial symbionts are known to facilitate a wide range of physiological processes and ecological interactions for their hosts. In spite of this, caterpillars with highly diverse life histories appear to lack resident microbiota. Gut physiology, endogenous digestive enzymes, and limited social interactions may contribute to this pattern, but the consequences of shifts in social activity and diet on caterpillar microbiota are largely unknown. Phengaris alcon caterpillars undergo particularly dramatic social and dietary shifts when they parasitize Myrmica ant colonies, rapidly transitioning from solitary herbivory to ant tending (i.e., receiving protein‐rich regurgitations through trophallaxis). This unique life history provides a model for studying interactions between social living, diet, and caterpillar microbiota. Here, we characterized and compared bacterial communities within P. alcon caterpillars before and after their association with ants, using 16S rRNA amplicon sequencing and quantitative PCR. After being adopted by ants, bacterial communities within P. alcon caterpillars shifted substantially, with a significant increase in alpha diversity and greater consistency in bacterial community composition in terms of beta dissimilarity. We also characterized the bacterial communities within their host ants (Myrmica schencki), food plant (Gentiana cruciata), and soil from ant nest chambers. These data indicated that the aforementioned patterns were influenced by bacteria derived from caterpillars’ surrounding environments, rather than through transfers from ants. Thus, while bacterial communities are substantially reorganized over the life cycle of P. alcon caterpillars, it appears that they do not rely on transfers of bacteria from host ants to complete their development.